The Role of Enhanced Research in Geotechnical Engineering for Pragmatic Infrastructure Development...

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The Role of Enhanced Research in Geotechnical Engineering for Pragmatic Infrastructure Development within the Vision 2030.

John MUKABI1 1Kensestu Kaihatsu Consultants Ltd. dr.mukabi@kensetsu.co.ke

Abstract: This paper summarizes some of the State of the Art technologies and advances made recently in the Eastern Africa region based on Research and Development (R&D), tailored particularly for developing countries. It introduces newly developed scientific and engineering theories, concepts, techniques, technologies as well as various analytical, design and construction methods. Practical examples of the prevalent engineering challenges and Geotechnologies that would provide pragmatic solutions for sustainable infrastructure development are proposed. Further discussions are made on the most appropriate and suitable approach for the Engineer to practically adopt in order to realize the Kenya Vision 2030 Objectives by introducing some of the achievements made by post World War II Japan through examples of some mega structures designed and constructed on the basis of R&D Oriented Techniques.

1. INTRODUCTION

Infrastructure development is the heart and key to any visionary and pragmatic socio-economic growth of a country. The Kenya Vision 2030 aims at maintaining a sustained economic growth of 10% p.a. over the next 25 years through the direction that the Vision Strategy be accompanied with realistic and concrete action plans upon expiry of the Economic Recovery Strategy (ERS) in December 2007. The overarching component of the Vision is that Kenya transforms into a globally competitive and prosperous nation with a high quality of life by 2030. Nevertheless, the major question still remains; how can this vision actually be achieved in reality? In order for the Vision to be realized within the designated time-frame, it is imperative that the targeted economic growth is achieved through rapid industrialization, enhanced agricultural production and agro-industry development, booming tourism, advanced education (science and technology) among other factors. Such goals can only be achieved through rapid infrastructure development based on innovative techniques, methods and technologies as a primary driving factor. In this Paper, innovative methods that can realize the practical achievements of such advancement are also discussed in terms of cost-effectiveness, performance and environmental considerations. In order to achieve these fundamental goals, the paper also emphasizes the need to enhance capacity building programmes through the development of strong Young Engineers Programmes (YEPs) for public, private and academic institutions through technology transfer, technical training and R&D activities. The versatility of advanced research based Consolidation and Shear Stress Ratio (CSSR) Functions in the prediction of ground, pavement and foundation behaviour is also demonstrated. It further proposes that the application of CSSR Functions in F.E analysis or other constructive models may reduce the complexity of the models and/or number of parameters required in such modelling. Further demonstration on the application of CSSR Functions in relation to the design of appropriate testing, experimental and research regimes through the conceptual correlation of loading rates, reconsolidation, aging, geomaterial characteristics, ground structure and re-constitution in relation to multi-stage construction of embankments and foundations, precise determination of bearing capacity factors numerical computerized modelling and prediction of ground and foundation behaviour, as well as the overall enhancement of engineering parameters.

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The Paper also introduces and discusses some recently developed research oriented geotechnical engineering solutions to problems related to tropical problematic soils and recommends appropriate methods of design and construction that would ensure the application of such geomaterials. The ongoing research regarding this topic is also introduced. The strength and deformation characteristics derived from the interaction of geogrids, geotextiles and tropical geomaterials are discussed from a scientific and engineering perspective. Recently developed techniques and geotechnical and engineering concepts for ground improvement, OPMC Stabilized retaining walls and enhancements of design, construction and maintenance engineering aspects are also introduced. The paper demonstrates and concludes that for purposes of achieving the Kenya Vision 2030, sustainable development and maintenance, Research and Development (R&D) is absolutely necessary.

2. INTRODUCING SOME EXAMPLES OF POST WORLD WAR II RAPID DEVELOPMENTS Japan is usually cited as a typical example of one of the countries that has made the most rapid technological and economic growth and development in the post World War II era. This has made Japanese technology become the focus and model for the Tigers and developing countries. Sections 2.1 ~ 2.11 demonstrate some of the technological advances in civil engineering that Japan has achieved based on enhanced Research and Development (R&D). All the examples cited were either fully or partly constructed by Kajima Corporation, while the Author of this Paper participated in the research and design studies of most of the projects during his graduate and post graduate studies. The harbour and railway structures were basically designed by Katahira and Engineers International.

2.1 Highways and Bridges Figs 2.1.1 ~ 2.1.4 show examples of the magnificent bridges that have already been constructed in Japan. The Akashi Kaikyo Bridge is the longest suspension bridge in the world with a centre span of 1991 metres (approximately 2km) and an overall length of 3,911 metres (approximately 4km). Initially, this bridge was designed to have a centre span of 2,000m (2km). However, during construction, the great Kansai earthquake prevailed causing the bridge to experience a total shift of 9metres. Nevertheless, the bridge stayed intact without experiencing or exhibiting any critical technical problems.

Fig 2.1.1 Overview of Kurishima Kaikyo Bridge stretching across Kirishima Straits. Total length 4.105km

(4,105m) – (Kajima Corporation)

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Fig 2.1.2 Akashi Kaikyo Bridge, 3-span 2-hinged truss – Stiffened Suspension Bridge with 6 Lanes. Longest Suspension Bridge in the world with centre Span of 1991m (Approx 2km) overall length of 3,911m (approx 4km)

Fig 2.1.3 Akashi Kaikyo Bridge – Anchorages, Structural Elements and Profile

Fig 2.1.4 Akashi Kaikyo Bridge – Cross section of strata, stiffening girder Techniques and construction

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2.2 Airports

Due to the lack of ample existing land space in Japan, most airports and other mega civil engineering structures have been constructed on reclaimed land. Figs 2.2.1 are some of such examples.

Fig 2.2.1 Tokyo International Airport (Haneda) and Kansai International Airport

Fig 2.2.2 New Tokyo Int. Airport (Narita) and Colombo Int. Airport, Sri Lanka on the right

2.3 Dams Figs 2.3.1 and 2.3.2 show rock-fill arch dams and gravity dams respectively. The rock-fill dam is simulated after Kajima’s 3D-Dam-CAD Techniques which are applied in optimizing the construction, quality and material control vis a vis cost reduction, while the 480m height Miyagase dam was constructed by employing the RCD Method, also developed by Kajima Corporation for the construction of large dams.

Fig 2.3.1 Dams that have been applied kajima’s 3D-Dam-CAD Technique, ensuring High Quality Structures vis a vis, cost reduction

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Fig 2.3.2 Gravity Dam – Miyagase dam constructed employing the RCD Method developed for construction of large dams

2.4 Tunnels As a mountainous country, tunnels are a common geotechnical engineering feature. Furthermore, Japan is one of the pioneer countries that developed under sea tunnels. Fig 2.4.1 is a depiction of the Tokyo Wan Aqualine under sea tunnel along the Trans Tokyo Bay Highway which is a 15.1km route connecting Kawasaki and Kisarazu man made islands. The route traverses mainly for 4.4km above sea vide bridges, for 9.4km under sea via tunnels and two man-made islands.

Fig 2.4.1 Trans Tokyo Bay Highway – a 15.1km route connecting Kawasaki and Kisarazu.

2.5 Buildings Some of the mega building structures in Tokyo are shown in Figs 2.5.1. The 240m (80 stories) building is the headquarters of the Nippon Telephone and Telegraphic DOCOMO in Yoyogi, while the JR Osaki train station on Tokyo’s Yamanote line has a floor space of more than 80 acres (320,000m2) and aesthetically integrates business and amenity within limited space.

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Fig 2.8.1 Land Reclamation

2.6 Railway Systems

After the end of the 2nd World War, Japan embarked on a technological mission and developed the Shinkansen (bullet train), a supersonic speed electric train just before the Tokyo Olympics in 1964. Along with this development became the necessity to construct high-tech railway lines and systems.

Currently, Japan has one of the most advanced railway network, underground (subway) and Mass Transit (MT) systems in the world.

2.7 Harbours

As an island country, Japan is wholly surrounded by sea. Ports and Harbours development is therefore a key prerequisite.

An example is depicted in Fig 2.7.1

2.8 Land Reclamation As stated earlier, Japan is not only a small country (approximately half the size of Kenya), but it is also so mountainous so much so that less than 20% of its land area is habitable. Furthermore it has a

population of more than 120million people. These are the reasons why Japan is considered as one of the countries with the highest population density in the world. Consequently, the development of land reclamation technology became one of the primary components of realizing reasonable urban development.

Fig 2.8.1 is a basic example of one of such technologies.

Fig 2.6.1 Railway Systems

Fig 2.7 Ports and Harbors Development

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2.9 Amenity Facilities

Figs 2.9.1 and 2.9.2 are examples of some of the amenity facilities that have been developed on the basis of Japanese technology.

2.10 Example of Modern Urban Development Incorporating Futuristic Components Minato Mirai 21 (MM21), first conceived as an Idea almost 3 decades ago, was Yokohama's vision of the future. It was practically realized in 1997 once, only dockyards, the city is turning this harbour front area into a world class business / recreation complex with one of the most advanced information infrastructure. The main crown is the Landmark Tower - at 296m, the tallest building in East Asia. It was meant to be higher, but flight restrictions at the Haneda airport prevented it. It boasts the latest in computerized anti-earthquake and anti-motion equipment and the fastest elevator in the world ascending at a speed of 45km/hr (12.5m/s).

Intended to be a cultural cosmopolitan and information city of the 21st Century with superior environment, the Intelligent City is a manifestation of the success of Public and Private Sector joint partnership.

Figs 2.10.1 ~ 2.10.9 depict various aspects and components of the Minato Mirai 21 in Yokohama City, Japan.

Some of the major concepts for the Nairobi metropolitan Development for Vision 2030 can be modelled on the MM21 Yokohama City.

Fig 2 9 2 Amenity Facilities – Practical use of land and time

Fig 2.10.1 Partial Scenery of the sea front of MM21

Fig 2.10.2 Mode of Realizing a visionary Urban Development

Fig 2.10.3 Land Use Map of Minato Mirai (MM21)

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Fig 2.10.4 Satellite Image of Minato Mirai (MM21)

Fig 2.10.5 A General View of Minato Mirai 21 (MM21)

Fig 2.10.7 Various Perspectives of the MM21

Fig 2.10.8 MM21 Intelligent Transport System

Fig 2.10.6 Various Perspectives of the MM21

Fig 2.10.7 MM21 Intelligent Transport System

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Fig 2.10.9 Demonstration Test of ITV in MM21

2.11 Examples of Mega Floating Structures

With a large sea area, Japanese marine and geotechnical engineering technology has also been strongly geared towards the development of mega floating structures.

Figs 2.11.1~2.11.9 give examples of such floating structures. Fig 2.11.2 depicts the main components within a mega floating system.

Fig 2.11.2 Components of a Mega-Float System

Fig 2.11.1 General View of Very Large Floating Structure

Fig 2.11.3 Examples of Mega-Float Structures

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Fig 2.11 5 Floating Pier at Ujina, Japan

Fig 2.11.8 Osaka Focus A by Japanese Society of Steel Construction

Fig 2.11.7 Marine Uranus by Nishimimatsu Corporation

Fig 2.11.4 Examples of Mega-Float Structures

Fig 2.11.6 Proposed Floating Runway at Tokyo International Airport (Haneda)

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3. EXAMPLES OF INNOVATIVE TECHNIQUES FOR ADVANCED GEOTECHNICAL INVESTIGATION For the successful design and construction of any civil engineering structure, comprehensive geotechnical engineering investigations are a definite prerequisite. On the other hand, due to lack of sufficient funds, human resources and technical capacity, the Engineer in developing countries, particularly in Africa, is faced with situations whereby they have to either adopt the existing sub-standard and/or outdated techniques or innovate methods that optimize the use of the available equipment, human resources and technical capacity. This section presents examples of some of the recently developed innovative geotechnical engineering testing and analytical techniques that form a concrete basis in realizing the design and construction of sound civil engineering structures. 3.1 Example of Innovative In-situ Testing Method

Fig 3.1 Mode of Achieving Objectives

Objectives of Study

Necessity for Innovative Modification

Fig 3.2 Load, Speed and Pavement Structure Effects on Deflection Measurements

Fig 3.3 Innovative NDT Method – Theoretical Considerations

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Fig 3.4 Innovative NDT Method – Theoretical Considerations Resilient Deviation

Fig 3.5 Need for Innovation

3.2 Example of Innovative In-situ Analytical Techniques

Fig 3.3 Major Objective

Fig. 3.5 Effects of Pre-loading and Load Intensity on Deflection

Fig. 3.6 Correlation of Pr-loading and Deflection Basin Concept

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Fig. 3.7 Theory of Damped Oscilatory Motion Impacted by Layer Stifness

Fig. 3.8 Influence of Confining Stress on Deflection

Measurement

Fig 3.9 Influence of Confining Stress on Deflection Basin Characteristics

Fig 3.10 Influence of seasonal Changes on Deflection Measurement

Fig. 3.11 Significance of Temparature Effects on the Elastic Modulus of Asphalt

Concrete 3.3 Innovative Methods of Analyzing Environmental Factors Environmental factors are known to highly affect the concepts of design, actual construction and ultimate performance of highway pavement structures. In this study, some comprehensive methods that may be effective for evaluating the impact of these factors are proposed. A new concept of evaluating the deterioration of the structural thickness as a result of infiltration of underlying material to the upper layers is also introduced. Application of these concepts and methods show that the impact of environmental factors over a given period of time can be more detrimental than commonly considered in most cases

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The main objective of undertaking this research therefore was to develop new quantitative analytical concepts and methods of effectively evaluating the impact of environmental factors such as geology, topography and climate (seasonal changes) on the performance of highway pavement structures.

The major environmental factors considered which highly depend on topographic, geographical, geological, climatic and other changes are depicted in Figure 3.12 and 3.13.

3.3.1 Evaluation of The Effects of Moisture~Suction Variation The effect of moisture changes on the current strength, durability and bearing capacity of the pavement and roadbed materials is evaluated on the basis of three concepts predominantly related to saturation levels, swelling and variation in the design moisture content.

The detrimental effects of moisture~suction variation on strength and deformation resistance are depicted in Figs. 3.14.

(1) Effect of Saturation Level

Fig 3.12 Effect of Dynamic Loading on Strength Characteristics of Tropical Soils

Proposed by Mukabi et al. (2001c) for Tropical Soils with a PI<43

where, ∆mc = 0.53 Cs = Swelling Index, CC=Compression Index

Fig 3.13 Effect of Moisture~Suction Variation on BCS

For purposes of evaluating the actual deterioration in pavement strength, durability and bearing capacity as a consequence of variations in saturation levels, the chart in Figure 3.15 and the equation therein proposed by Savage et al. (1999) and modified by Mukabi (2001e), are adopted.

Figure 3.14 Impact of Moisture ~ Suction Variation on Tropical Soils

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Figure 19 Effect of Saturation Level

Figure 3.17 Swell vs. Surcharge Pressure Characteristics

(2) Effect of Swelling

Figure 3.16 and 3.17 show the compound effects of compaction, surcharge pressure and monotonic loading-unloading-reloading cycles on expansive soils. The swell related equations derived from the generalized equation proposed by Mukabi et al. (1999c and 2003d) are also presented.

( )( ) ( )

iSRfSS H•

ααα max

Fig 3.16 Swell vs. Soaking Time Characteristics

(3) Effect of Variation in Design

Moisture Content The selection of an appropriate design moisture content and density condition is critical to the design analysis. The moisture content at which subgrade strength should be assessed is that which can be expected to be exceeded only rarely. Pronounced exceedance of this factor is known to have adverse effects on the pavement structure. Fig. 3.18 shows the coupled effects of seasonal changes and plasticity index on the design moisture content, while Fig.

3.19 introduces equations that can be applied in correcting for seasonal effects for reconstruction and overlay design.

Fig. 3.15 Effect of Saturation Level

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Figure 3.20 Seasonal and Soaking Effects

Dmc = -0.0093PI2 + 1.1745PI + 3.2

Dmc = -0.0063PI2 + 0.9384PI + 5.4

0 10 20 30 40 50 60 70 80

Plasticity Index(PI)

Wet Season Dry Season Poly. (Wet Season) Poly. (Dry Season)

Wet Season

Dry Season

Average of 10 dataPoints

Figure 3.18 Influences of Seasonal Changes and Plasticity on DMC

ωPIeAPI PBpd =

Correcting for Seasonal Effects for Reconstruction Design

03.0,10 ===

BADeAD

02.0,12 ===

BAPIeAPI

Bmmc DeAD m=ω

Correcting for Seasonal effects for Overlay Design

( )sfEP

Df tSdfxxR

TTI ∆∆+= 1

[ ]fIdmc

wmc eDeAD −+= 1

[ ]fIw

BPPd ePIeAPI −−= 1

Figure 3.19 Influences of Seasonal Changes and Plasticity on DMC

(4) Seasonal and Soaking Condition Effects on Bearing Capacity The combined effects of seasonal changes and soaking conditions on the bearing capacity of some subgrade materials is depicted in Figure 3.20, while the equation that can be applied to correct for this effect is also presented in the same figure.

3.3.2 Intrusion of Native subgrade Material into Upper Layers of Pavement Structure Various research undertaken by Mukabi (2001) and Mukabi et al. (2003) indicate that Intrusion of native subgrade material into the overlying layers of the pavement usually results in the ultimate degradation of the layers. Depending on the nature of the subgrade, topography of environment and seasonal changes, intrusion of native subgrade material into overlying layers of the pavement structure, as depicted in Figure 3.21, can be rampant and extremely detrimental.

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Fig. 3.21 Intrusion of Subgrade Material Fig. 3.22 Impact of Inferior Material Intrusion

The consequences of such a physical action are the deterioration of bearing capacity, cohesion intercept (c) and internal friction (φ) as well as mechanical stability.

Some of the results of the quantitative analysis of this factor are presented in Fig.3.22 and further discussed for expansive soils in Mukabi and Gono (2007f, This Conference) and Mukabi et al. (2003c).

In this series of experimental testing, materials with varying qualities and properties were infiltrated into high quality crushed aggregate base course material mechanically stabilized at varying ratios (0~40mm:0~5mm aggregate).

Fig.3.22 clearly indicates that: 1) inferior material intrusion into the upper layers drastically reduces the bearing strength. 2) The magnitude and rate of reduction in bearing capacity is a direct function of the quality of material and batching ratio. 3) The threshold of the CBR reduction is at approximately PI=35%. 4) The effect of subgrade material intrusion ceases after the PI reaches the threshold value.

The results indicate therefore, that it is absolutely imperative to take this fact into consideration during the structural design and analysis of a pavement.

3.3.3 Evaluation of Variation in Quality of Pavement Layer Materials The quantitative assessment of deficiency in the physical properties of pavement materials with time through the intrusion of fines to upper pavement layers is undertaken by employing the following equation: [ ] USmmS CBRPIBACBR −= (3.1) where, CBRs = soaked CBR, CBRus = unsoaked CBR, Am and Bm are material related constants which were generally determined as 0.97 and 0.027 respectively for materials tested in this study. This equation enables the evaluation of the effect of increased Fines Contents (FC) and PI on the bearing capacity.

3.3.4 Deterioration of Pavement Structural Thickness

Examples of the quantitative changes in pavement structural thickness, which is defined as the effective thickness that acts structurally are discussed extensively in Mukabi (2002a). The deterioration of pavement structural thickness occurs mainly due to cyclic action of increased axle loading, water infiltration and intrusion of subgrade fines to upper layers as briefly introduced in the preceding sections. The Intensity Factor If, proposed by Mukabi et al. (2002a) is expressed as follows :

( )sdff

Df txSxR

TTI ∆∆+= 1

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where, TD = design thickness, TEP = measured thickness of the existing pavement, Rf

= roughness factor expressed as 25.0

Rf given, Rf = roughness factor Ri =

initial roughness value, Rt = terminal roughness value, ∆Sdf = rate of surface distress depreciation factor, ∆ts = time lapsed since the previous study or survey was undertaken . 3.4 Innovative Material Characterization Techniques Prompted by the lack of suitable subbase material due to weathering and high plasticity in most areas of the Addis Ababa ~ Goha Tsion Trunk Road in Ethiopia, innovative tests to characterize various physical and chemical properties of the available materials were carried out. Analyses of the test results were done in respect to the intrinsic plasticity characteristics of the geomaterials under the influence of chemical and physical changes on the one hand, and the effect of the resulting variation and magnitude of the consistency limit values on the bearing capacity of the geomaterial, on the other.

Influence of clay content - The relationship between clay content and plasticity characteristics is mostly dependent on the geological origin and mineralogy of a geomaterial. Typical results of the soils tested from the Project area exhibited a Liquidity Limit (LL) of approx. 50% (47~52) for various conditions. Plotting the results from the tests conducted in this study in Fig.3.23 indicated that the level of enhanced clay activity was only 6.4%. This implied that the clay content activity influencing the variation of plasticity characteristics was very low, hence low potential to swell (ref. to Fig.3.24). Furthermore, due to the low clay content of the Project material tested, it was considered that the main cause for the initial high plasticity index is the low degree of leaching and laterization due to the low amounts of mineral coatings by sesquioxides which act to suppress the activity of the clay minerals. Since this effect is probably mainly due to the titration of clay particles from the overburden material and weathering, the subsequent geological effect in the post inter-particle state was to increase the sesquioxide influence.

Fig.3.23 Typical Colloidal Activity Fig. 3.24 Typical Swell Potentional

Due to the foregoing discussion therefore, it was considered that the clay content and activity effect on the variation of the plasticity characteristics and in particular the plasticity index of the Project geomaterials designated for use as subbase materials was quite negligible.

Influence of nature of soil material -The contribution of the nature of soil materials to the plasticity characteristics highly depends on the shape and structure of the soil minerals in relation to the surface area in contact. In other words, the soil particle orientation and micro-aggregate cluster formation become increasingly important in consideration of the magnitude of consistency limits. The global effect is considered to be related to the ultimate magnitude of the surface of activity of clay minerals in reference to the interaction of sesquioxides.

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Interpretation of this influence would therefore be related to the genesis, degree of weathering and clay mineralogy. The results from the study showed that:

Contribution of the chemical composition of the colloid - The practical significance of the liquid and plastic limits lies in their ability to reflect on the types and amounts of clay minerals present in the fine fraction (Skempton, 1953). For natural soils, the plasticity index has been found to increase in proportion to the amount of clay size particles present whereby the relationship is practically linear and passing through the origin as shown in Fig.3.25. As can be noted from the same figure, very different relationships between the plasticity index and the percent clay-fraction size are obtained for three clay minerals namely kaolinite, illite and montmorilonite for some temperate-zone clay soils. Wu (1966) suggested that the slope of the lines indicate the relative magnitudes of the surface forces which are representative of the colloidal activity. The active clay characterized by large colloidal activity exhibit plastic properties over a wide water content range. This is generally considered to be the result of the strong interaction between the surface forces and water molecules.

The results from this investigation showed that although the material exhibited plasticity indices greater than 15 (the Specified value), the subsequent variation in plasticity notwithstanding varying conditions was fairly low. This may be attributed to the presence of mica in the silt fraction of the soil as suggested by Ruddock (1967).

This substantiates the fact that the geomaterial along the Project Road was mainly influenced by Kaolinite clay fractions and as a consequence, its plasticity characteristics are hardly influenced by the local history of large seasonal movements.

Influence of exchangeable cations – According to Hough’s (1957) proposal, Sodium (Na), Potassium (K), Calcium (Ca), Hydrogen (H) and Magnesium (Mg) ions in the montmorillonite mineral exhibit the highest values to Atterberg limits, while those of the Kaolinite mineral exhibit the lowest values. For all minerals, however, the sodium ion is the most exchangeable. In this investigation, sodium chloride (NaCl) and hydrated lime in the form of CaCo3 were adopted as catalistic agents to study to presence and effect of exchangeable ions. The addition of these agents had little influence on the variation of the plasticity values of this material. It was therefore considered that the available exchangeable ions were quite limited and were virtually in an inert state.

i) Virtually negligible variation existed between the plasticity parameters of oven dried and air dried samples. This implied that the clay mineral surfaces do not orient in full contact position as this should have enhanced the degree of saturation and intrinsic localized suction stresses analogous to surcharge stresses whereby the plasticity index of the oven dried sample should have been much lesser than the air dried sample.

ii) The addition of sodium chloride did not seem to have considerable impact in the plasticity behaviour of

the clay minerals tested in this study. This may imply that due to the nature and the structure of the clay minerals, the osmotic suction stress levels were not affected to such an appreciable extent.

iii) Although addition of hydrated lime caused reduction in the plasticity limits and index to a level below

the lower threshold of PI=16, the effect of drying, water, and temperature in relation to the chemical reaction prompted by addition of CaCo was not apparent. This implied that the nature of the clay minerals was appreciably stable hence drastic variation in the plasticity characteristics was not expected.

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( )( ){ } 012

1****1 =−−−= Dijijijijy Rxxf ηη (3.3)

0 20 40 60 80 100

- 2m CLAY CONTENT - %

PLASTICITY CHARACTERISTICS OF LATERITE SOILS

London ClayllliteHorton ClayKaoliniteCo MontmorilloniteData from this study

Data from

Fig.3.25 Clay Content and Plasticity Index Fig.3.26 Impacts of Environmental Factors

Following this study therefore, it was concluded that the materials could be adopted for sub base purposes despite their relatively high PI values. Analysis of their vital engineering properties such as strength, bearing capacity and deformation resistance determined from both laboratory tests and field trial sections indicated high values well beyond the specified ones. 3.5 Innovative Method for Back-Analysis of distressed Pavement Structure

The Constitutive model on cyclic plasticity for geomaterials based on non-linear kinematic hardening theory proposed by Yashima et al. (1994) is adopted in attempting to back analyze the deformation history of the pavement structure. This model was chosen because of it’s incorporation of the non-linear kinematics hardening rule. When incorporated into an overstress type of model, it is found to be effective in expressing

the changes in retardation in the strain rate direction upon a corresponding change in the direction of the stress. Furthermore this model is found to reproduce to an appreciable extent, the plastic damage during cyclic or repeated loading. By taking into account the effects of sub grade layer material into the sub base, the constitutive model for clay is adopted in simulating the composite yield characteristics of these layers, while the distress behaviour of the upper pavement consisting of the unbound crushed aggregate base course and the asphalt concrete, are analyzed by modifying the theories in the constitutive model for soft rock. A representation of the results of this concept is given in Fig.3.27.

3.5.1 Constitutive model applied for lower pavement layers The viscoplastic model for over consolidated clay extended to a cyclic model by Oka (1988) is applied. The static yield functions that account for changes in the stress ratio are given as follows:

where, 1DR = parameter defining the elastic region and *

ijx =the kinematics hardening tensor. By introducing the non linearity of the kinematics hardening, *

ijx can be written as

Fig.3.27 Results From Back Analysis

00.10.20.30.40.50.60.70.80.9

0 0 0 0 0 0

ion δ r

Deflection Basin, DB (m)

Lower Pavement Layers

Distressed

Virgin

Constitutive Model for Lower Pavement Layers

Constitutive Model for Upper Pavement Layers

Estimation of Consolidation and Shear Stress History

( )( ){ } 012

1****1 =−−−= Dijijijijy Rxxf ηη

( ){ } ( ) ''/'exp1 '0

*' dZZZZT ij

ZOij δτδ −−∫=

NCNCooc

CSRAKKqKq

... max

maxφ−

CSRAKKKq '

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ijvpijij dxdeABdx γ−= *

( ) 21vp

ijvpij

vp deded =γ (3.5)

( )( ){ } 0')1(

'1*~21****

+−−= mamnMijxijijxijg σσηη

( ) ( ) 01~ ')1(

'**0 =+= mamnmb Mf σση (3.7)

( )''*~mcmnM σση −= (3.8)

φφδφ BA CSR +=′∆ (3.9)

SRSRSR Β∆Α= /'φ (3.10)

( )( )CSRK CSRI

σφη

ψ′∆−

=′ (3.11)

In which *

1A and *1B are the material constants

and *ijde is the increment of the viscoplastic

deviatoric strain. The second invariant of the increment of the plastic deviatoric strain is derived as:

For the first yield function, the plastic potential is assumed to be:

where, ')1(maσ = material parameter and

*~M is the stress ratio when the layers are under maximum compression condition: Considering the over consolidated boundary surface between the NC and OC zones to be expressed as:

In the NC Zone ( )0≥bf , *~M is kept constant i.e., *~M = *~mM

region, it is defined as: ( )0<bf , *~M is defined as: whereas in the OC

where, the current stress ratio ( ) 21***

ijijηηη = and ''mbmc σσ = exp

( ( )**

0 / mMη )

3.5.2 Estimation of Consolidation and Shear Stress Paths The input parameters for the constitutive model introduced in the preceding section were derived from the following theories and concepts. As the repeated loading progresses, the cumulative effects are back analyzed by applying the concepts of consolidation and shear stress ratio functions under normally consolidated (NC) conditions introduced by Mukabi and Tatsuoka (1996) and Mukabi (2001d). In so doing, the initial stresses are computed from the experimental results of full scale trial sections (Mukabi, 2002; Gono et al., 2003, this conference) .The cumulative stresses are then derived by considering the average loading rate and cumulative repeated loading over a given period of time. Once the maximum deviator and mean effective stresses are determined, the stress ratio functions, defined from the following expressions proposed by Mukabi and Tatsuoka (1999b) and Mukabi (2001d) are applied.

Where, φA and φB are material properties, and the consolidation

function CSRδ , which is independent of the effects of loading rate, is stress ratio

derived from the relation max

~ qCSR

φδ , whereby 'φ∆ = function of normalized angle of internal

friction expressed as IQ

A ∆∆=∆ /' φφ (A: An isotropic I: Isotropic) and maxq = maximum deviator stress. 'φ can be determined from the quasi-empirical equation (Mukabi, 2001d) expressed in general form as:

Where, ASR and BSR are stress ratio constants and ( )'pqSR =∆ is the invariant stress ratio variable.

The antistrophic stress path is derived from the isotropic one by introducing a modifier proposed by Mukabi and Tatsuoka (1999b) expressed as:

where, maxη = (q/p’) at qmax, KI=1 and CSR= consolidations

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NCNCooc

CSRAKKqKq

... max

maxφ−

= (3.12)

CSRAKKKq '

( ){ } ( ) ''/'exp1 '0

*' dZZZZT ij

ZOij δτδ −−∫=

(3.15)

( )( ){ } 01~'

1****1 =

+−−=bb

Mxxgmb

mnijijijij σσ

ηη (

( ) 01~'

+=bbMf

mnmb σση

stress ratio. The modifier is applied in the relation pq ′′=ψ . On the other hand, the invariant stresses and angle of internal friction under over

consolidated (OC) condition were derived from the flowing correlations proposed by Mukabi (2001d).

where, =OCOxK

'sin fOCRK OCOx

φ• and fOCOxK 'sin1 φ−= .

The corresponding mean effective stress, OCfp ' and angle of

internal friction OCf'φ are given by:

(3.13) and ,

fNCOCO

f CSRAKKK '

(3.14)

3.5.3 Constitutive Model Applied for Upper Pavement Layers Adachi and Oka (1992) proposed that the stress history tensor is a function of the effective stress history with respect of the strain measure. This history tensor, *'O

ijδ is given by

where, dz= ( ) Zdede ijij ,21 = strain measure, T=material

parameter which controls the strain-hardening and strain-softening phenomena and deij is the increment of deviator strain tensor. The plastic potential is assumed to be:

(3.16)

The OC boundary is given as :

(3.17)

The OC region is therefore defined as:

−=bbM

σση

(3.18) 4. APPLICATION OF CONSOLIDATION AND SHEAR STRESS RATIO CONCEPTS Laboratory tests are primarily carried out for purposes of obtaining engineering parameters which can be directly applied to conditions in the field. Such an exercise would not only provide parameters for design and construction quality control but also an insight into the fundamental processes which affect the field behaviour. In developing countries, both affordability and accessibility to high quality testing equipment are major curtailing factors to realizing this aim. This situation therefore necessitates the development of empirical methods that can aid in providing estimated parameters that are reasonable enough for the design and modelling of foundations bearing civil engineering structures. In this paper, unique methods derived as Consolidation and Shear Stress Ratio (CSSR) Functions that were recently developed, providing solutions on how to circumvent these problems, are presented.

23 | P a g e

4.1 Brief background of developing CSSR Concepts Laboratory tests can for example, be employed to investigate how strength and stiffness develop during large strain consolidation and how this behaviour is dependent on various factors such as loading rate and direction, principal stress rotation in relation to location within the foundation etc. However, the precision of adopting these results involves an analytical approach that would be appropriate in simulating as accurately as possible, the actual field conditions. Furthermore, precise determination of such parameters for natural clays usually requires high quality sampling and testing techniques for a reliable laboratory investigation. This translates to high costs and long time durations for performing the tests. The method that is described in this paper is based on that proposed by Mukabi and Tatsuoka (1999b) which modified some aspects of the Critical State Soil Mechanics (CSSM) theories. This was prompted by their (Mukabi and Tatsuoka 1992) investigation into the effects of consolidation stress ratio and strain rate on the peak stress ratio of clay which concluded that the shear stress ratio (q/p’)max, increases as the consolidation stress ratio ''

acK σσ= decreases based on high-precision automated CD/CU triaxial compression and extension tests performed on high-quality undisturbed samples of various natural soft to very stiff clays, related to prediction of ground displacement in actual construction projects. For control purposes, commercially produced Kaolin which contains appreciable quantities of mica and quartz was also used. Their study also confirmed that the shearing stress ratio at failure Kf is a function of the initial consolidation stress ratio and that it decreased proportionally with decreasing Kc parameters. Furthermore, having characterized the effects of loading rate into a generalized state, a method of unifying the behaviour of anisotropically consolidated clay into a coherent form was considered. A κ-function which relates the φ’ determined from various tests performed by applying different CSRs was defined as κ=φ’/ ηqm. This relationship was found to be virtually constant and related closely to a reference line considered to be analogous to a modified CSL (i.e.,φ’ constant when Kc = 1). In order to compute φ’, a relation between the invariant stress ratio (∆SR) and the angle of internal friction was derived from linear regressional analysis of experimental data on various clays. Based on the foregoing fundamental theories, versatile functions and parameters related to the concepts of loading rate, SHANSEP consolidation, ageing and reconstitution that can be applied effectively during multi-stage construction of geostructures such as deep excavations, tunnels, embankments and foundations, precise determination of bearing capacity factors, numerical modelling and prediction of ground and foundation behaviour, as well as the overall enhancement of engineering parameters. 4.2 Derivation of Application Functions The derivation of the CSSR application functions are presented in Mukabi (2007c). 4.3 Application of CSSR Functions in simulating Field Conditions

4.3.1 Functions and parameters based on SHANSEP consolidation As was discussed by Mukabi and Tatsuoka (1999a and 1999b) and Mukabi (2001d), the “intact” specimen exhibits much more superior engineering properties in comparison to the specimens reconsolidated applying the SHANSEP method. It was also derived that the higher the stress level of the consolidation stress ratio ηc=(qc/p’c), the more the structure is destroyed through remoulding. This implies that specimens reconsolidated by applying the SHANSEP method can not be representative or correctly simulate the in-situ conditions. Consequently, correction factors have to be applied on the parameters determined adopting such a method . Based on the concepts of consolidation and shear stress ratio functions, the following correlations for qmax, pf” and φf” were derived for these purposes.

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( )mrRa ttAe

dtd /αε

= (4.4)

( ) tlAe nR

αεε =− (4.5) ( )0/ ttlC na αε =∆ (4.6)

NCSNCSc

OCScNCS

NCSOCS

qKqmax

max0max

φηη

••

NCSNCSc

OCScNCS

NCSOCSf P

••

φηη

••

NCSNCSc

OCScNCS

NCSOCSf

φηη

where, subscript f denotes failure, superscript OCS and NCS denote Over Consolidated and Normally Consolidated under the SHANSEP method. 4.3.2 Functions and parameters related to the concept of ageing Ageing is considered to constitute mainly of two components; namely secondary

consolidation associated with creep ( )0' =∂∂ taε and thixotropy defined as a gain in strength at constant water content. Creep is basically caused by a continuing re-arrangement of the soil particles after the overburden pressure is fully supported by the soil skeleton, whereby the excess pore pressure has dissipated. Kuhn and Mitchell (1993) proposed that creep deformation is due to sliding between particles and that although the sliding is thought to occur at solid contacts, it is visco-frictional in nature and the sliding velocity at each contact depends on the ratio of tangential to normal components of contact force. Whether the creep strains in triaxial tests accelerate or not depends principally on the magnitude of the deviator stress compared to the strength or compressibility of the sample. Mitchell (1976) proposed the following general creep equation:

where R=qt/qf delineates the deviator stress level, tr is a reference time and A and α are solid constant parameters. When m=1 the

strain rate continues to decrease with time, while the strain rate accelerates towards failure when m<1. Considering m=1 and integrating Equation 26 with εa = εa α at t = 1 then,

is obtained. This is similar to the expression,

representative of a one dimensional creep.

Mukabi (1995), Mukabi and Tatsuoka, (1999a) reported results on the effects of ageing in reconsolidation on deformation characteristics of various natural clays. The comprehensive research showed that time plays an important role in the stress-strain time history of clays. Furthermore, it was also shown that only the laboratory Gmax values extrapolated by an ageing period of about 20 years were comparable with the field values Gf Based on comprehensive analysis of such results and considering that creep, which is predominantly associated with secondary consolidation, contributes more significantly to the strength development of clay in comparison to thixrotropy and further assuming that ∆εa is purely a function of consolidation properties, then the following generalized relations were derived which can simulate long term consolidation effects from laboratory tests conducted on the basis of short term consolidation.

( )[ ]STCn

STCSTCLTC

CSRAttKqKq

•−•

='/1 00

max0max φ (4.7)

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( ) ( )

••∆∆−=

STCfca

STCLTCf CSRAtttK

φεφ

( ) [ ]( ) 1/

−•••−

••=

pqCSRAK

ffφµ

φηµφ

(4.12)

while,

( )[ ] STC

LTCSTC

STCLTCf

qqCSRAttK

••−

φ (4.8)

where superscript LTC and STC denote long term and short term consolidation respectively whereas t : LTC

time and to : STC time., for OC conditions (∆εa/∆t)fc

STC=1. 4.3.3 Functions and parameters of reconstituted clays The adverse effects of reconstitution of clays was briefly discussed in the preceding section 4.3.2 of this paper. From the analysis of various data based on the concepts of consolidation and shear stress ratio, the following correlations that can be useful in computing qmax, pf’ and φf’ from CUTC tests on reconstituted clays were derived.

( )( )( )RR

e CSRAK

••−

••=

ηµ /

max (4.10) ( )

( )( )RRe

e CSRAKpp

••−

••=

ηµ / (4.11)

where superscripts I and R denote “intact” and “reconstituted” respectively and µR

c=(q/p’)fR,

ηc=(q/p’)c, and KRcf = (σ’r/σ’a)R

4.4 Application of Consolidation and Shear Stress Ratio (CSSR) Functions in Estimating and/or Predicting Consolidation Stress History Discussions from the preceding sections of this paper as well as Fig.4.1~4.3 clearly demonstrate the importance of predicting and/or retracing (back-analyzing) the consolidation and shear stress path and history of ground foundation or geostructure subjected to loading or otherwise to

be construc

ted. For deep braced excavations in soft ground for example, the movement of the soil surrounding an excavation must be taken into account with due consideration of the its interaction with the retaining system or structure. The application of CSSR concepts in undertaking such estimation or in F.E analysis and other constitutive models is one of the methods that can be effective and appreciably precise. Due to the underestimation of the elastic modulus mainly due to sample disturbance for

Fig.4.1-Stress paths during loading and unloading

Fig.4.2-Emax OCR relations(D70-3)

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Fig.4.3-Effect of OCR and LTC on elastic strain

ηη εη BA SRan += 1max (4.13)

ηη εε

ω BA RSRa

RSRASRmax

1max ηωη •= −

(4.15)

( ) ( )

( ){ } (%)max

ELSa +=

εφε

ε (4.16)

ζφ xEE

50 , (4.17)

example, most prediction overestimates the lateral movement. Since CSSR concepts can be applied in simulating the actual ground and field conditions to an appreciable level of accuracy, this problem can be circumvented. In Kenya, Ethiopia and Southern Sudan, the concepts have been successfully applied in the design and construction of various geostructures Including deep braced excavations, pad foundations, embankments, slope stability and

pavement structures. Various practical examples of such application have also been given in other publications by Mukabi et al. in this Conference. A discussion regarding the back analysis of distressed pavement deformation history has been made by Mukabi et al. (2007h). 4.5 Application of CSSR Functions during Multi-stage Construction 4.5.1 Functions and parameters related to the concept of loading rate Due to the importance of incorporating the analysis of the effects of loading on foundations and embankments of clayey geomaterials during modelling and design, Mukabi and Tatsuoka (1999b) developed a relation between the stress ratio at failure ηmax (q/p’m) and axial strain rate (εa) expressed in a generalized state as:

where constants AηI and Bη were determined as AηA =

0.037, BηA = 0.858 and AI

η=0.043, BηI=0.76

(Superscripts denote; A: Anistopic; I: Isotropic; SR: Strain Rate). Based on comprehensive analysis of various clays subjected to different axial strain rates and also applying Equation 35, the following co-relations were derived:-

(4.14)

where ωSR is a strain rate function and superscripts ASR and RSR denote “Applied Strain Rate” and “Reference

Strain Rate” respectively. 4.5.2 Application of the Elastic Limit Strain As can be derived from the preceding discussions, the determination of the linear elastic range of geomaterials defined as the region of the initial yield surface within which the behaviour of the geomaterial is virtually linear elastic and recoverable, is of paramount importance for various reasons. Consequently, based on long-term research undertaken since 1991, Mukabi (1995) proposed the following equation expressing the Elastic Limit Strain for estimating the linear elastic range.

Where ESLφ is a function of the level of max)( aε and A is a constant depending on the physical properties of the

geomaterial. For most clays φ is defined as,

Based on empirical relations for most clayey geomaterials, A = 603 and ξ = 462 are good estimates as constants where the curve

is considered to be positive in all quadrants.

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The theory of the Elastic Limit Strain has since been applied in the control of loading imposition during staging construction and excavation particularly when dealing with soft and problematic soils in the eastern Africa region. Figs. 4.4-4.6 are a demonstration of how this theory can be applied in combination with the CSSR concept on stress path, in monitoring the behaviour of geostructures in the field under various loading and unloading conditions.

5. NECESSITY TO RECONSIDER SOME ASPECTS OF CRITICAL STATE SOIL MECHANICS The existing theories that define lateral pressures, under the framework of classical soil mechanics assume that failure for normally consolidated clayey geomaterials occurs at the Critical State Line (CSL) irrespective of drain condition, loading rate and stress path traversed towards the CSL. On the other hand Critical State Soil Mechanics has been developed on the basis of Rendulics generalized principle of effective stress which states that for a soil in an initial state of stress and stress history there exists a unique relationship between voids ratio (e) and effective stress for changes in stress, (Δσa,' or Δp'). Most of the existing theories for deformation and strength characteristics of clays therefore assume this principle. Within this context, it is presumed that for a given normally consolidated clay, failure occurs at a unique line called the Critical State Line (CSL) defined by q=Mp', without allowing the stress paths to locate above it

at all stages irrespective of drain conditions, strain rate and stress path traversed towards the CSL. Most finite element based analytical tools and simulation of such cases as multi-stage embankment design and construction widely employ Critical State models. While studying the influence of initial shear on undrained behaviour of normally consolidated kaolin, Ampadu (1988) concluded that the existing theory of Critical State Soil Mechanics alone cannot adequately explain the differences in behaviour between isotropically and anisotropically consolidated samples of the

Figure 5.1 Proposed Modification of The CSSM Theory

Fig.4.4 Stress states for DC1 specimens

Fig.4.5-Effect of pressure level on elastic limit strain

Fig.4.6-Effect of pressure level on elastic

limit strain (DC1-4)

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Table 5.1 Summary of results calculated using proposed equations

kaolin tested. It has also been reported by various researchers that the shapes and magnitudes of yield envelopes are influenced mainly by the composition, anisotropy and stress history of the clay features which have been inadequately modelled on the basis of the Critical State Soil Mechanics theory. Rigorous examination however, of the behaviour of clay that has been subjected through various stress ratios other than isotropic during consolidation and the corresponding relations is yet a subject to be exhausted. As can be seen from Fig.5.1, Mukabi and Tatsuoka (199b) proposed some modification of certain aspects of the existing theory of Critical State Soil Mechanics. Reference of the details on the background can be made from subsection 6.5.1 of this report. The relationship of the function ( ) and , a function of the normalized angle of internal friction ( ) in reference to that of developed from Fig. 6.87(a), is represented by a linear equation in the form , where and mean value in this case) are constants. In normalizing, the was determined for the two

respective strain rates from the linear regression of the NC line of the semi-log plot of shown in Fig. 5.2. The linear relation in Fig. 6.87(b) is virtually

similar and shows no dependency on the various strain rates . This suggests that this equation uniquely relates and and may be applied in developing further mathematical relations that may aid in redefining the basic parameters related to the

without particular reference to strain rates effects.

Fig. 5.2(a) Effect of CSR on Angle of Internal friction

Fig. 5.2 (b) Effect of CSR on Angle of Internal friction

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Fig. 5.3 Effects of CSR on k function

Fig. 5.4(a) CSR factor

Fig. 5.4(b) λa measured λa calculated relations

Fig. 5.5 (qmax) measured vs. (qmax) calculated

Fig. 5.6 (qmax) measured vs. (qmax) calculated

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Fig.5.7 Comparison of Critical State Lines based on calculated values compared to measured values

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6. NECESSITY TO RECONSIDER SOME ASPECTS OF CONVENTIONAL DESIGN PRINCIPLES The importance of applying advanced and appropriate analytical design techniques can be observed from Fig.6.1. Based on comprehensive research undertaken under JICA funding, it was found that the equation proposed by the Asphalt Institute tends to over-estimate the structural capacity of the existing pavement structure as a result of the lack of considering various factors related to environmental and structural depreciation with time, as input parameters in the equation. Based on elastic moduli results from advanced testing of various geomaterials and by applying environmental and structural depreciation components as integral in-put parameters that would characterize the elastic behaviour of the respective geomaterial, Mukabi (2000) proposed the modified equation expressed in Equation 6.1.

Figure 6.1 Proposed Method of Estimating Resilient Modulus

( ) ( ) ( ) 775.0623.00012.0102 263

−+−= − CBRCBRCBRcorr MrMrxMrxM (6.1)

where,

CBRMr = 10.3xCBR proposed by the Asphalt Institute for the Full-Depth overlay design method. Results from Case Study Analysis undertaken for road projects designed by applying the proposed method indicate that substantial cost savings can be realized relative to the design life of the road pavement structure. 7. MUKABI’S THEORY OF LATERAL EARTH PRESSURES Based on the foregoing concepts, Mukabi (2008d) modified the lateral earth pressure equations as subsequently defined. • Active Pressure for smooth surfaces

For cases assuming no friction is exhibited between the soil and the retaining structure,

where, (7.2)

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• Passive Pressure for smooth surfaces

The expressions for passive pressure assuming a retaining structure with a smooth surface. (7.4)

where,

• Active Thrust for frictional surfaces

In cases where friction between the soil and retaining structure is considered, the following equations may be applied.

(7.6) where,

• Passive thrust for frictional surfaces

The passive thrust is computed as follows under the framework of the modified theory. (7.8)

where,

8. SOME RECENTLY DEVELOPED RESEARCH AND DEVELOMENT (R&D) TECHNOLOGIES FOR ENHANCING GEOMATERIAL PROPERTIES 8.1 Problematic Soils

8.1.1 Rerap Methods

Fig. 8.2 – Quality Control normograph

Fig.8.1 - BRI Vs. Voids Ratio

Voids ratio e reduces as Batching Ratio Index tends to an Optimum Value. This increase shear Modulus i.e., Gmax =αm(γa).βm.(Am-e)2/(1+e.(σ0)m

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( ) ( )[ ][ ]P

dPdPPbP

DNCBRCCBRBAt

/logloglog 2+−=

[ ] 5.0//1 edCBRSP eS α= (8.3)

( ) cneeeMLLVCB

ee eA −=α (8.5)

Determination of appropriate counter-measures 1) Replacement Method - Tables 2 and 3 as well as Figs. 8.1 and 8.2 show the design and QC criteria developed on the basis of research and adopted for the construction of the Addis Ababa ~ Goha Tsion Trunk Road Project. In determining the necessary thickness tCL to replace the expansive soil, the following equations proposed in this study were adopted.

{ } SPbpPCL xStTt −=

The total pavement thickness TP is expressed as:

vfbPP txRtT +=

And the coefficient of subgrade structural performance SSP is computed from:

On the other hand, the basic pavement thickness tPb from Eq. (3.10) is computed from the following equation.

Where the roughness factor ( )[ ] 25.02 itif RRRR −= : Ri is the initial roughness factor and Rt is the terminal roughness factor, tV is the positive value of the

specified tolerance for pavement thickness, AP=219, BP=211, CP=58 and DP=120. The parameter αe is defined as:

where Ae=0.23, Be=0.54, Ce=0.08 are constants and Ve=Annual Average Evapotranspiration in m/year (ref. to Mukabi et al. (2003c), LL=liquid Limit in percentage and

Mcn=Natural Moisture Content of the subgrade material expressed in percentage form. All thickness are calculated in mm. Continuous assessment and evaluation of the performance of the sections already constructed by adopting this criteria indicates that the method has so far been quite successful.

Table 8.1 - Determining Required Capping Layer Thickness (cm)

Plasticity and Swell

Condition

Required Thickness for

Different Subgrade Bearing

Capacity

A PI>45 Sm>10 140 90 70 60 30 B 35<PI<45 Sm<10 110 75 60 50 20 C PI<35 Sm<5 70 65 55 50 20

Table 8.2 - Determining Required Capping Layer Thickness

Notes ♦ Where the results are on the Boundary Limit or

within its vicinity, the Criteria of Clay Activity (Ac) expressed as Ac = 3.6R-2.35 (R=LL/PI) may be adopted or otherwise as directed by the Consultant. For example, should the PI > 45 and Sm < 10, if Ac <1.0 then option B may be adopted instead of Option A or vice versa.

♦ If Ac < 0.75 then maximum swell values of Sm = 15% can be allowed for Options B and C.

♦ Sm represents maximum swell measured uniaxially after 4 days soak and under a standard surcharge pressure of 298kg/m2.

♦ For materials that exhibit excessively high initial rates of swell, the Consultant shall be consulted for further analysis prior to characterizing the expansive subgrade material

34 | P a g e

CBR of Imported material

Required Thickness for Different Native Subgrade Bearing Capacity (cm) for RE 1

Required Thickness for Different Native Subgrade Bearing Capacity

(cm) RE2 Type CBR = 1 CBR = 2 CBR = 3 CBR = 4 CBR = 5 CBR = 6 CBR = 7

15 70 65 55 50 40 30 20 20 60 55 45 40 30 25 20 25 55 45 40 35 25 20 15 30 50 40 35 30 20 15 10 40 45 35 30 25 15 10 10 50 40 30 25 20 10 10 10 60 35 25 20 15 10 10 10

Fig.8.3 - Layer thickness (1<CBRd<4) Fig.8.4 - (4<CBRd<8)

8.1.2 MC Technique

The importance of moisture control is clearly demonstrated in the preceding Figures 8.3 and 8.4, which are nomographs that depict the influence of moisture~suction variation on the bearing strength of varying geomaterials. The variation in the nature of geomaterials is simulating through the application of varying plasticity characteristiccs and magnitudes. The inset equations also indicate that the modulus of deformation or deformation resistance is susceptible to moisture~suction variations, a fact that has also been discussed in Section 4 as well as the preceding sections of this section. The on–going research or this subject intends to develop a technique of controlling the moisture content of a subgrade of an expansive nature by systematically and technically imbedding sand columns in predetermined areas or zones. Figures 8.5 ~ 8.6 present part of the preliminary results that have been obtained in the initial stages of testing. Although definite conclusions can not be derived from these results yet, the trends exhibited from these graphs are distinctly clear. In other words, imbedment of sand interface layers seams to be effective in reducing swell and increasing the bearing capacity notwithstanding the magnitude of the surcharge pressure.

Fig.8.5 - Free swell soaking

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8.1.3 Suction Stress Method Research for purposes of developing this method is still in the initial stages. The basic idea is to develop a technique of constructing a subsurface drainage layer underlain by a layer compacted to a higher degree in order to induce high but varying suction stresses. The layer is intended to facilitate in directing any excess moisture away from the pavement structure. Reference can be made to Mukabi (2004a). Fig.8.7 and 8.8 are a representation of how this technique was used by incorporating a suction stress column in the design of an OPMC Stabilized retaining wall and maintenance along the Addis Ababa ~ Debre Markos International Trunk Road, the all important

northern corridor that connects Sudan and Eritrea.

Fig.8.7 - Use of a Suction Column for OPMC Stabilized Retaining Wall.

tABC=15cm

tAf=12.5cm

tBf=20cmNatural Gravel Boulders Filter Course

Crushed Aggregate Filter Course 0~40 only

Crushed Aggregate Base Course M.S @ 3:2→0.5 : 0~40

Asphalt Concrete Constructed to Specifications

Carriageway

Shoulder

Stepped & Compacted to higher degree to achieve high suction stresses

Subgrade

10cm 25cm 75cm

tAS=7.5cm

Crushed Aggregate Base Course M.S @ 3:2→0.5 : 0~40

tAf=7.5cm

tAf2=7.5cm

Fig.8.8 – Suction-stress method

8.2 Development and Application of Optimum Batching Ratio Method (OBRM) and Optimum Mechanical and Chemical Stabilization (OPMC) Techniques 8.2.1 Basis for Development of OBRM and OPMC Techniques The necessity to develop the Optimum Batching Ratio Method (OBRM) Optimum Mechanical and Chemical Stabilization (OPMC) prevailed due to the prevalence of the 1997~1998 El-Nino floods coupled with the lack of suitable road construction materials along the B3 Malindi-

Fig.8.6 – Soaking period for CBR

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Garissa Trunk Road. Details can be referenced from Mukabi et al (2003d) and Mukabi et al (1997).

1. Upon undertaking Case Study Analysis and analytical review of the post-El-Nino hydrological conditions, it was realized that additional hydraulic structures such as bridges and major culverts of appreciable dimensions would be necessary. These structures would necessitate additional funding totalling to almost 30-40% of the total Project cost. However, the economic and financial analysis revealed that investment of such magnitude would not be cost beneficial. Consequently, a cost reduction plan was embarked upon.

2. In order to reduce the costs, a plan to design and construct bridge approach abutments out of high reinforced soil embankments and reciprocal protection works, was proposed. Nevertheless, due to the non-availability of suitable geomaterials within the project area the preliminary design revealed the cost of reinforcement and protection material would be quite high due to the additional strength and stability required.

Important factor Objective of study

Develop a method of determining

optimum Mixing ratios for geomaterials with different grading characteristics in

order to achieve;

Better Compaction characteristics

Greater resistance to wear

Enhanced resilience properties

(f) (h) (g)

(a) (b)

Fig. 8.9 Mode of Characterizing Particle Motion

Fig. 8.10 Theoretical and Engineering basis presented at the 14th IRF World Road Congress, Paris 2001

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Fig. 8.11 Mechanical Stabilization Effects

Fig. 8.12 Graphical Representation of Proposed Batching Ratio Method

Fig. 8.13 Proposed Optimum Batching Ratio Method

Fig. 8.14 Effect of OPMC on Strength and Deformation Resistance

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8.2.2 Application of OBRM and OPMC Techniques in Reducing Cost and Environmental Impact of Rural Road Construction include: Some of the aspects that were considered Application Of A New Mechanical Stabilization And Other Techniques In Reducing The Cost And Impact Of Rural Road Construction The major objectives of undertaking this Study included:

- Reduced volume of materials used by 40% - Sustained dust reduction - Cost-effective - Environmental friendly - Less disturbance of land for borrow pits - Reduced amounts of disposable soil during construction - Enhancement of engineering properties of geomaterials - Reduced risk of collapse of civil engineering structures. - Enhancement of disaster avoidance and management

The Study undertook comprehensive appraisals and environmental assessments that would lead to sustainable development with minimal negative environmental impacts. A summary of the approach, considerations and contribution of OBRM and OPMC from a geotechnical engineering perspective are summarized in Figs. 8.20 to 8.28.

Fig. 8.15 OPMC Stabilization Technique Fig. 8.16 OPMC Stabilization Technique

Fig. 8.18 OPMC Stabilization Technique Fig. 8.19 Quality Control Normograph

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Fig. 8.20 Example of Negative Impacts of Rural Road Transport Infrastructure on Ecology and Wildlife in Eastern Africa

Fig. 8.21 Transport sector in industrialization and economic activities

Fig. 8.22 Road system effects on individual animals and the wildlife population

Fig. 8.23 (a) Example of positive impacts of rural road transport infrastructure on socio-economic development in Eastern Africa

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Fig. 8.23 (b) Example of positive impacts of rural road transport infrastructure on socio-economic development in E. Africa

Fig. 8.23 (c) Improved Livestock Health and Markets

Fig. 8.24 Importance of enhanced Research and development

Fig. 8.25 Application of SA&SEA using Technology and Techniques for Environment Impact

Fig. 8.26 OPMC: Drastic Reduction of Volume of Materials

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8.3 Strength and Deformation Characteristics of OPMC Stabilized Geomaterials and Geogrids Interaction The fundamental concept of incorporating geogrids for purposes of stabilizing and/or reinforcing soils is illustrated in Figs. 8.29 to 8.30.

Fig. 8.27 some vital aspects of OPMC

Fig. 8.29(a) Fundamental Concept of incorporating Geogrids

Fig. 8.29 (b) Fundamental Concept of incorporating Geogrids

Fig. 8.28 Construction Planning based on Comprehensive Research and Technology

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Fig. 8.30 Effect of Interlocking and Grid Apertures

Fig. 8.32 Drawing to illustrate inclusion of Tensar Geogrid. Geogrid was placed at 1/3 the height of the sample as shown

Fig. 8.32 shows a schematic drawing illustrating the placement of the geogrids, while Figs. 8.33 and 8.34 depict the post-failure states of the specimens with OPMC and geogrid compared to OMC alone for varying geomaterials and positioning of the geogrids. The fact that the geogrids contribute largely to the tensile stresses and tensile strain resistance while OPMC contributes immensely to compressive stresses and compressive strain resistance can be clearly derived from these figures.

Fig. 8.29 (c) Fundamental Concept of incorporating Geogrids

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Some of the typical results determined from this study are presented in Figs. 8.35 to 8.40. The fact that both OPMC Stabilization Technique and the application of geogrids greatly enhances the performance of geomaterials by increasing their strength, bearing capacity and deformation resistance can be clearly seen from these figures as well as Figs. 10.1~10.3 under section 10.

Fig. 8.34 (a) and Fig. 8.34(b) Comparison of Geogrid and OPMC effects for varying Geomaterials and Modes of Stabilization and/or Reinforcement

Fig. 8.35 Effects of Various Geosynthetic Locations and OPMC Stabilization on Axial Stress

Fig. 8.36 Effects of Various Geosynthetic Locations and OPMC Stabilization on Emax

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9. APPLICATION OF SOME NEWLY PROPOSED QUALITY CONTROL METHODS Measured and field data collection would certainly serve no purpose if appreciable accuracy and confidence levels are not achieved. Accurate and precise definition of the boundary limits of specification control can prove to be costly if they are not properly considered or tailored for a specific project.

Fig. 8.37 Effects of Various Geosynthetic Locations and OPMC Stabilization on Elastic Limit Strain

Fig. 8.38 Effects of Various Geosynthetic Locations and OPMC Stabilization on Angle of Internal Resistance

Fig. 8.39 Effects of Various Geosynthetic Locations and OPMC Stabilization on Ea(ELS)

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( ) 15.1log −= t

tSC Nxff (10.2)

The basic principles of some of the main quality control methods developed by Mukabi (2001a) and Mukabi et al. (2003) previously on other projects modified to suit the design and construction specification requirements for various Projects are introduced by Mukabi (2005a). Research on various other QC methodologies is still on-going. 10 ANALYSES AND PREDICTION OF STRUCTURAL CAPACITY OF PAVEMENT STRUCTURE The basic objectives of undertaking this Study were to develop formulae that would enable the prediction of structural capacity of any pavement structure notwithstanding type and configuration at any particular time mainly for maintenance purposes (also refer to section 11 in this paper). It is imperative, when undertaking the evaluation of the structural capacity of flexible-pavement structures, to consider factors such as subgrade characteristics, pavement layer strength and conditions, load and traffic parameters, environmental conditions 1) Initial Structural Capacity Some of the major factors that affect the status or condition of a pavement structure include the Relative Damaging Effect (RDeff.) introduced by Mukabi (2002c), which is related to the ESAL, variation in quality of materials prompted by environmental factors, deterioration in pavement layer thickness through loss of aggregates and infiltration of inferior lower quality materials into the upper layers of the pavement structure. As proposed by Mukabi (2002c), the concept of remaining life can be transposed or defined in terms of the existing structural capacity by application of the following equation.

Where Re

SCf represents the existing structural capacity,

RLf = Remaining Life Factor, ReSCf = Structural

Capacity Factor of a newly constructed or reconstructed pavement structure in which case ReSCf =1

and .effRD = 0.298 is the damaging factor while rf∆ = defines contribution of a multitude of factors affecting the magnitude of the damaging effect.

2) Deterioration in Structural Capacity with Time Some of the major factors that contribute to the deficiency of the structural capacity and serviceability level with time of an existing pavement structure were mentioned in the preceding sections. This deterioration with time is known to grossly affect the performance of a highway pavement structure. The deterioration with time of the structural capacity factor t

SCf after Nt = 2.2 year can be defined by Equation 44 below proposed by Mukabi (2002c).

Given that Nt > 2.2 years and applying the above equations, the deterioration with time, of the structural capacity of a road

in Ethiopia with varying AC thicknesses, was computed. Further basic formulae developed and adopted are presented below, while Figs. 10.1~10.3 depict some of the results derived through the adoption of theses formulae.

[ ]rfeffSCRLe

SC xRDfff ∆−== .Re (10.1)

46 | P a g e

11. PROPOSED METHOD OF DETEMINING PERIOD AND LEVEL OF MAINTENANCE Effective maintenance is a prerequisite in realizing an efficient road network in any country. In order to achieve an effective road maintenance system, it is imperative that the anomaly between the actual needs and available resources for road maintenance is resolved and subsequent implementation of appropriate measures undertaken accordingly. For purposes of achieving this, it is imperative that plausible road maintenance sceneries are constructed objected towards assessing the existence and magnitude of the need gap in order to draw up proposals that are both comprehensive and pragmatic in nature to integrate the relevant road maintenance components. This is demonstrated in Figure 11.1.

The following facts can be derived from Fig.11.2. 1. Traffic loading conditions

and environmental factors reciprocally influence each other with time.

2. As the pavement structure deteriorates due to loading, the impact of environmental factors becomes greater

3. Road surface type has direct influence on both the structural capacity and serviceability level of a road.

11.1 Choice of Effective Method

of Analysis (1) Theories And/Or

Concepts Considered

The choice of an effective analytical method depends predominantly on the choice of the backbone engineering theories, principles and concepts and the extent to which they translate to pragmatic application. For these purposes, the theories and concepts applied are based on fundamental theories, principles and concepts introduced by Mukabi (2002c).

Fig.11.1 Approach To Realize An Effective Road Maintenance System

Fig.11.2 Major Factors Influencing Road Conditions and their Reciprocal Interaction

47 | P a g e

[ ]vmsecidfc tePPtfR αφ ∆∆∂= ,,,,, (11.1)

[ ]oijyi

ocfdh fqpf δφψφε ,,',,',',' Σ= (11.2)

The generalized equation of the existing road conditions can be expressed as a function of loading conditions, pavement type (structurally), pavement layer quality, structural thickness as well as intrinsic material properties depicted in Equation 11.1.

Where,

cR = road condition, dfφ = dynamic load factor, it∂ =

response mode factor of layer of the pavement structure, cP = pavement configuration, eP = pavement

layer quality, et∆ = structural thickness, vmsα∆ = parameter delineating moisture – suction variation

On the other hand, the extent of distress of deformation can be derived based on the theories introduced in the preceding sections applied for carrying out back analysis of the deformation history of a distressed pavement structure. In a generalized state, this can be expressed as shown in Equation 45.

where,

dhε = parameter delineating deformation history 'φ = consolidation stress ratio, 'ψ = modifier between Isotropic and Anisotropic stress paths,

ocf qp ,' = invariant stress under over consolidation conditions, ,

fφ = Angle of Internal Friction within the failure zone Details of the mathematical computation of these parameters are presented by Mukabi (2007e).

(2) Proposed Method of Determining Appropriate Maintenance Schedule The proposed approach of evaluating the structural capacity of a pavement structure under section 10 has been applied as an effective method of determining the maintenance works and the respective levels of maintenance required. The three serviceability and structural capacity characteristic curves depicted in Fig.11.3, which proved to be quite precise, were applied in determining the appropriate maintenance schedule and level for a road in Ethiopia. Application of this method realized appreciable maintenance cost savings on long term basis. Mukabi (2005) presents more details on this subject matter.

Fig. 11.3 Depiction of Determining Period and Level of Maintenance 12. PROPOSED MODE OF EMHANCING RESEARCH AND DEVELOMENT (R&D) Fig. below is a depiction of the importance of upholding a three-tier system in developing pragmatic maintenance policies and effective management techniques, standards and design engineering of road infrastructure assets for ensuring appropriate and sustainable development. The practical maintenance techniques are presented in section 3 of this paper.

48 | P a g e

Maintenance Policy

Application of Appropriate Technology

Contractual & Implementation

Policy

Private Sector Academic

Institution

Government Institutions

Achievement of a pragmatic and Advanced

Road Sector Implementation Policy

Maintenance Policy

Application of Appropriate Technology

Contractual & Implementation

Policy

Private Sector Academic

Institution

Government Institutions

Achievement of a pragmatic and Advanced

Road Sector Implementation Policy

Fig. 12.1 Essence of the Three-tier System in Achieving Appropriate Maintenance Policy Making

Examples of Capacity Building Programs developed within this Region (Particularly Ethiopia) • Young Engineers Programmes for Public, Private and Academic Institutions • Technology Transfer for Engineers

– Direct technology Transfer During Construction – Sponsored Programmes for Overseas Studies

• Technical Training Programmes for Technicians – Direct technology Transfer During Construction – Sponsored Programmes for Overseas Studies

• Expansion of Capacity Building Institutes 13. ONGOING RESEARCH Currently the ongoing research is mostly related to the following topics. Other Topics in Relation to the New Technologies Incorporating Tensar and OPMC - Research Related to Black Cotton or Expansive Soil - Research Related to Intrusion of Underlying Material - Research Related To Temperature in Seasonal Cycle Effects on The Bearing Capacity and

Resilient Properties - Research Related to NDT/DT Testing for Evaluation of the Existing Condition of a Flexible

Pavement Structure - Proposed Research Related to Consolidation and Shear Stress Functions for Foundation

Design and Construction - Proposed Research to Relate Road Surface Distress Condition and Deformation and Failure

49 | P a g e

CONCLUSIONS Comprehensive testing and analytical methods were employed in this Study in order to realize the most Value Engineering based solution for the pavement structure of the Juba River Port Access Road in Juba Town, Central Equatoria State of Southern Sudan.

From both the laboratory and field tests results analyzed and discussed in Chapter 5 and summarized in Chapter 7 of the Engineering Report No. SST1 of May 2007, it can be concluded that the design proposed in this Engineering Report is adequate for the project pavement structure provided that the construction is undertaken in accordance with the Standard, Technical, and Particular Specifications as well as the stipulations in the Method of Construction.

Furthermore, in all the cases considered, and as can also be observed from the concluding tables and figures presented in the various chapters, it was derived that the Optimum Mechanical and Chemical (OPMC) stabilizing method was effective in enhancing the vital engineering properties of the geomaterials adopted as well as the composite pavement structure. Fundamentally, this method was quite effective in;

1. Retaining a substantial proportion of their strength even with increased saturation levels.

2. Reducing tremendously the surface deflection of the layers under loading. 3. Increasing resistance to erosion due to the scouring effect of water flow. 4. Increasing resistance to contamination by materials in underlying or

supporting layers that are not stabilized. 5. Increasing the effective elastic moduli of the composite pavement

structure. 6. Realizing an acceptable cost-effective design.

The OPMC stabilization technique, developed on the basis of a new approach, was determined to be the most cost-effective and value engineering based method in respect to all prevalent conditions considered. It is envisaged to be an interesting structure in terms of Case Study Analysis and Research for further development as an effective countermeasure for landslides, foundations, slope stability, embankment and pavement structure design and construction. ACKNOWLEDGEMENTS The author is highly indebted to the contributions of Professor Fumio Tatsuoka and the University of Tokyo. Sincere appreciation is also expressed to the Japan International Cooperation Agency (JICA), Japan Bank of International Cooperation (JBIC), Construction Project Consultants Inc., Kajima Corporation and Kajima Foundation for funding the subsequent part of the study conducted in Africa. The paper would certainly not have been completed without the crucial support of Ms. Piera Cesaroni, and the input of Sylvester Kotheki, Kenneth Wambugu, Ms. Zekal Ketsella, Joram Okado, Paul Kinyanjui, Bryan Otieno, Walter Okello, and Mr. Anthony Ngigi. It is also important to mention the cooperation and assistance extended by the Ethiopian Roads Authority as well as the Ministry of Roads, Public Works and Housing, Kenya. MAIN REFRENCES Ampadu, S.K (1988): The influence of initial shear on undrained Behavior of normally consolidated Kaolin,

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