Mitigation of Risks Associated with Deep Excavations: State of the Art Review

Sayed M. Ahmed1 & Ayman L. Fayed2 Structural Engineering Dept., Ain Shams University

1 Al-Sarayat St., Abdo Basha Square, Abbasya, Cairo, Egypt 1sayed_mohamed@eng.asu.edu.eg 2 ayman_fayed@eng.asu.edu.eg

AbstractDeep excavations inevitably initiate lateral and

vertical ground deformations due to the stress relaxation and bottom heave associated with the excavation process. Thus, adjacent buildings and utilities become kinematically loaded by the induced ground deformations. To date, the ground displacements induced by deep excavations and their associated risks cannot be truthfully evaluated utilizing only systematic engineering calculations for many reasons including the need to account for the natural variability of geomaterials and the uncertainties in soil properties, the ground constitutive behavior, modeling of construction stages, three-dimensional effects of deep excavations, time-dependent natures of the ground deformations as well as the crucial needs to incorporate human factors such as workmanship in the predicting models. The aforementioned aspects require comprehensive knowledge and vast experience not only in deep excavations and their effects on structures and utilities but also in all geotechnical engineering aspects. In this article, a state-of-the-art review of the powerful approaches in quantifying risks associated with deep excavations and their contemporary mitigation methods are highlighted.

Key wordsDeep excavations, risk quantification, risk mitigation, settlement, horizontal tensile strain, building damage, monitoring

I. INTRODUCTION There is an increasing National demand to utilize the

underground space in the developments of the urban congested areas for different purposes such as transportation tunnels, underground parking garages, basements and utilities. El-Nahhas [1] highlighted many plans to utilize the underground space in Egypt. One of the most ambitious plans that were detailed by El-Nahhas [1] is the utilization of the underground space is the construction of transportation tunnels and underground garages under main Cairo streets such as Gamat Aldoul Alarabia as demonstrated in Fig. 1.

Fig. 1. Utilization of the underground space under Gamat Aldoul Alarabya in

Cairo Vision 2050 (after El-Nahhas [1])

Such ambitious developments call for deep vertical excavations and underground tunneling that are frequently close to existing structurally-sensitive buildings and utilities. The induced deformations depend in magnitude and direction on the building proximity to the excavations as schematically demonstrated in Fig. 2. It is well-acknowledged that the control of ground movements and protection of adjacent or overlying structures is a major element in the design and construction of deep excavations and tunneling in urban areas.

Fig. 2. Ground and building deformations induced by a deep excavation

(after Hsiao [2])

It is a common practice to support deep excavations by continuous walls in urban areas to limit the induced movements and consequently the associated risks. The excavation support systems for deep excavations consist of two main components: a wall, and its supporting measures. Many types of walls and supports have been used in deep excavations. Walls supporting deep excavation may be classified into the following three major categories according to the form of supporting measures provided for them:

1. Cantilevered wall (usually for shallow excavation);

2. Strutted/braced wall; and 3. Tied-back or anchored wall

Under each of the above support category, the following wall types may be utilized:

a) Sheet pile wall;

SayedTypewritten TextCitation: Ahmed, S.A. & Fayed, A.L. (2015) "Mitigation of Risks Associated with Deep Excavations: State of the Art Review", Industry Academia Collaboration (IAC 2015), Cairo, Egypt, 6-8 April, 2015.

b) Soldier pile and lagging wall (Berliner wall); c) Contiguous bored piles wall; d) Secant piles wall; e) Diaphragm wall; and f) Soil-mixing walls

Puller [3] described the aforementioned systems and other less widely used support systems in considerable details. The excavation-induced deformations may be affected by a large number of factors such as: wall stiffness, ground conditions, groundwater condition and control measures, excavation depth, construction sequences and workmanship. The following sections address some of the important factors that profoundly affect the induced deformations and hence the associated buildings' damage.

II. RISKS ASSOCIATED WITH DEEP EXCAVATIONS Ground deformations are the inevitable devils awaked by

deep excavations. The horizontal stress relaxation by the excavation induces horizontal movement of the retaining wall towards the excavation side accompanied by vertical deformations for the soil around the excavation. The vertical deformations are mostly downward deformations (settlement); yet, sometimes upward deformations (heave) are noticed adjacent to the retaining wall or at far distances from the wall. Settlement may be associated with the instability of the excavation base in clayey soils. Deformations may also occur due to the increases in the effective stresses during lowering groundwater table.

To date, failures of structures or roadways adjacent to excavations occur despite the recent advances made in assessing the stability of excavations and the effects of excavations on nearby properties. Fig. 2 shows a very recent example of a failure case history of a collapsed 13-floor building by toppling in Minhang District of Shanghai, China. The failure, which happened in 2009, was due to a nearby deep excavation that overloaded the piles of the collapsed building. Chai et al. [4] indicated that the failure was initiated by lateral overloading on the pile foundation due to excavation near one side of the collapsed building and stockpiling the excavation at another side of the building. The unbalanced excavation and fill induced lateral loads on piles were also accompanied by unforeseen soil softening due to a rain event.

Fig. 3. Failure of a building in China in 2009 initiated by a nearby deep

excavation

Another very well-known recent failure is the failure of Nicoll Highway in Singapore, Fig. 4, which occurred due to insufficient site investigations, misinterpretation of the observations, faults in design of the bracing system and utilization of unsuitable method for wall strutting by jet grouting (Whittle & Davies [5]; Lee [6]).

Fig. 4. Failure of Nicoll Highway in Singapore initiated by a nearby deep

excavation and other geotechnical factors

Serviceability problems associated with the substantial foundation settlement and lateral deformations induced by deep excavations are much more widespread than failures. Structure may experience distresses such as cracking of structural or architectural elements, uneven floors, or inoperable windows and doors due to the induced deformations. Fig. 5 shows an example of a cracked external wall due to a nearby excavation. The amount of the tolerable deformations and the severity of the excavation-related damages depend on the building type, configuration and stiffness as well as the characteristics of the excavation support, the ground geotechnical conditions and the construction sequence. Both geotechnical and structural engineers are required to collaborate in quantifying the amount of building settlement, assess the possible structural damages and set up the counter measures and risk mitigations to avoid such damages.

Fig. 5. A masonry wall suffered from severe cracking due to ground

deformations (after Vatovec et al. [7])

The effect of deformations associated with deep excavation depends on the geotechnical characteristics of the soils. The less strength and more compressible the soils have, the more pronounced effects and deformations are anticipated. Awkwardly, most of the deep excavations are in urban areas characterized by deltaic soils originating from rivers and oceans and comprising sediments such as silts, clay and sands under shallow groundwater table. Such deltaic soils are often encountered in the most densely populated areas in the world. This fact emphasizes the need to predict, control and mitigate the deformations resulting from deep excavations.

III. GEOTECHNICAL AND GEOLOGICAL ASPECTS Peck [8] showed that settlements next to deep excavations

correlate to soil type as illustrated in Fig. 6. He proposed three zones of settlement profiles based on soil conditions. In general, larger wall deflection and ground deformations are induced due to excavations in soils with lower strength and stiffness.

Fig. 6. Effect of soil type on the settlement induced by deep excavation

(after Peck [8])

The effect of soil type on the defamations induced by deep excavations was further demonstrated by many subsequent research efforts (e.g., Goldberg et al. [9]; Clough & ORourke [10]; Bentler [11]; and others). Bentler [11] showed that the average maximum horizontal wall deflection for excavations in sand or hard clays is 0.19% H and for soft to stiff clays 0.45% H, where H is the depth of excavation. The average of the maximum settlement is 0.22% H in sands/hard clays and 0.55% H in soft-stiff clays. The ratio between the maximum vertical settlement and the maximum wall deformation is mostly ranged between 0.5 and 1.

Nationally, most of the developments that need deep excavations in Egypt are located in the Greater Cairo area which is characterized by recent Nile alluviums with shallow groundwater table. Geologically, the Nile developed its course in this area through the down faulting of the limestone extending between the El-Muqattam cliff and the Pyramids plateau and deposited recent alluviums of alternating layers of cemented silty sand, clayey sand and medium to coarse sand underlain by Pliocene very stiff plastic clay that rests on the Upper Eocene limestone marine formations as illustrated in Fig. 7 (Said [12]; El-Sohby & Mazen [13]; El-Ramli, [14]; El-Nahhas [15]; others).

Fig. 7. Typical formations in the Greater Cairo area

(after El-Sohby and Mazen [8])

The geotechnical conditions of the Nile alluviums are considered problematic for deep excavations particularly as the expected deformations impose risks on the adjacent structures and utilities including possible loss of support to existing foundations and structurally distressing buildings, pavements and utilities surrounding the excavation. Abdel Rahman & El-Sayed [16] [17] and [18] and El-Sayed & Abdel Rahman [19] concluded the following regarding the deformations of shallow and deep foundations associated with excavation supported by diaphragm walls in Nile Alluviums: The maximum settlement associated with trenching is

equal to 0.045% for both shallow and deep foundations. The maximum settlement due to pit excavation is about

0.11% of the excavation depth for shallow foundations and 0.03% of the maximum depth of excavation for pile foundations

The extent of the settlement troughs was found to reach up to a distance equivalent to 3.5 of the depth of excavation in alluvial soils for both shallow and deep foundations.

Most of the settlement of buildings on pile foundations occurs during the trenching stage

IV. FACTORS AFFECTING GROUND DEFORMATIONS

A. Ground and wall deformation patterns Goldberg et al. [9] identified different settlement patterns

associated with the wall lateral deformations modes as shown in Fig. 8. They showed that the settlement model do not only depend on the soil type but also on the wall lateral deformations as well.

Fig. 8. Settlement patterns associated with different wall deformation modes (after Goldberg et al. [9])

Clough and ORourke [10] explained the lateral walls deformations according to the method of construction in two modes: cantilever mode, and bulging mode. The settlement troughs associated with each mode are different as shown in Fig. 9. Boone [20] and Boone & Westland [21] concluded the same effect of wall deformation on the surficial settlement trough as shown in Fig. 10.

Fig. 9. Modes of deformation of the wall (after Clough and ORourke [10])

Fig. 10. Lateral and vertical displacement patterns: concave on left, spandrel

on right (after Boone [20]; Boone & Westland [21]).

Ou et al. [22] presented a tri-linear settlement profile called spandrel-type settlement based on 10 case histories of deep excavations in soft clays from Taipei and Taiwan. The maximum settlement is located at the wall face when the wall deforms as a cantilever. The settlement trough is shown in Fig. 11.

Fig. 11. Spandrel-type settlement trough (Ou et al. [22])

Hsieh & Ou [23] presented a concave settlement profile for the bulging mode of walls based on the analysis of 9 case histories. The maximum settlement is assumed to occur at 0.5 He, where He is the excavation depth. The settlement at the wall is approximated to 50% of the maximum settlement as shown in Fig. 12.

Fig. 12. Concave settlement profile (after Hsieh & Ou [23])

B. Wall Stiffness and Excavation Stability Stability and deformations are interrelated. For walls with

large factor of safety against collapse, strains around the excavation will be small and ground deformations will be limited. Conversely, if the factor of safety is small, strains around the excavation will be large and ground deformations will also be high. Additionally, the wall stiffness greatly affects the induced ground movements. Goldberg et al. [9] showed using finite element and measured data that the maximum lateral deformations for deep excavations in clays can be estimated using the stability number of the excavation H/cu (where is the soil unit weight, H is the depth of the excavation and cu is the undrained shear strength) and the stiffness of the supporting system EwIw/h4 (where Ew is the Youngs modulus of the wall, Iw is the moment of inertia of the wall per linear meter and h is a representative unsupported length of the wall such as the average distance between struts). Figure 7 illustrates the findings of Goldberg et al. [9].

Fig. 13. Effect of wall stiffness and soil stability number on the wall

deformations in clays (Goldberg et al. [9])

Mana & Clough [24] utilized the finite element and the field measurements to relate the maximum wall movements with the factor of safety against basal heave in clays as shown in Fig. 14. The quasi-constant non-dimensional movement at high safety factor is an indication of an elastic response. The rapid increase in movements at lower factor of safety is a result of plastic soil deformations at low factors of safety.

Fig. 14. Effect of the basal heave stability on the wall deformations induced

by deep excavations in clays (after Mana & Clough [24])

Clough & ORourke [10] utilized the nonlinear finite elements and field measurements to determine the effect of the wall stiffness on the maximum lateral wall movement in clays that is induced by excavation. They introduced a system stiffness factor, similar to Goldberg et al. [9], for estimating wall stiffness of unit thickness (plane strain) which depends on wall material, section properties and support spacing; this factor is giving by:

4avewh

EIk

= (1)

where: k = Dimensionless system stiffness E = Youngs modulus of wall system I = Moment of inertia of wall system have = average vertical distance between tiebacks/struts w = unit weight of water = 9.81 kN/m3

The results of their analyses are shown in Fig. 15.

Fig. 15. Effect of the basal heave stability and the system stiffness on the wall

deformations induced by deep excavations in clays (after Clough & ORourke [10])

C. Excavation Geometry and Three-Dimensional Effects Ou et al. [25] performed parametric three-dimensional

finite element analyses to investigate the features of three-dimensional deep excavation behaviors. They found that close relationships exist between the aspect ratio of the excavation geometry (B/L) and the wall deformation. B and L are the excavation dimensions in horizontal plane in the direction of lateral wall measurements and the perpendicular direction, respectively. Increasing the B/L decreases the wall deformation. Additionally, the wall deformation of a deep excavation is directly related to the smallest distance from the corner (d). The smaller is the value of d, the less is the wall deformation.

Ou et al. [25] defined a ratio called the Plane Strain Ratio (PSR). PSR is defined as the ratio of the maximum wall deformation of the cross section at a distance (d) from the excavation corner to the maximum wall deformation in the plane strain conditions of the same geometry. They established the relationship between (PSR), (B/L) & (d) based on the results of parametric studies, as shown in Fig. 16.

Fig. 16. Plane strain ratio (PSR) as a function of the aspect ratio B/L and distance from the corner d (Ou et al., 1996)

Finno & Roboski [26] and Roboski & Finno [27] studied

deep excavations in soft to medium clays based on the settlements that were observed using optical survey around a 12.8 m deep excavation in Chicago. The excavation was supported by a flexible sheet pile wall and three levels of re-groutable anchors. They suggested a parallel distribution for the deformation to account for the corner effect. They found that the complementary error function (erfc) can be used to define the three-dimensional settlement distributions of ground movement around excavation of finite length.

(2)

Where, max can be either the maximum settlement or the maximum lateral movement, L is the length of the excavation, and He is the height of the excavation as presented in Fig. 17.

Fig. 17. Three-dimensional distribution of settlement and lateral movement around finite deep excavation

(after Finno & Roboski [26] and Roboski & Finno [27])

D. Wall Installation Effect The wall installation process can cause significant

movements in the surrounding ground. The assumption of negligible deformations associated with wall installation may lead to a substantial underestimation of excavation-related lateral movements. In a survey of the problematic deep excavations in The Netherlands carried out between years 2007-2012, Korff & Tol [28] noted that many problematic deep excavation cases occurred due to ignoring the installation effects of the walls.

Morton et al. [29], Budge-Reid et al. [30], Cowland & Thorley [31], and Thorley & Forth [32] reviewed the settlements induced by the construction of the diaphragm walls in Hong Kong, particularly for the Mass Transit Railway project where soils are generally fill, marine deposits and alluviums underlain by decomposed granite. Settlement values up to 150mm were reported for shallow foundations while less settlement was reported for deep foundations as shown in Fig. 18, 19 & 20.

Fig. 18. Settlement associated with trenching in Hong Kongs MTR

(after Morton et al. [29])

Fig. 19. Maximum building settlements due to slurry trench excavation for

diaphragm walls as a function of foundation depth in Hong Kongs MTR (after Cowland & Thorley [31])

Fig. 20. Building settlement due to diaphragm wall installation in Hong

Kongs MTR (after Budge-Reid et al. [30])

Clough & ORourke [10] showed that significant

settlement may occur behind a diaphragm wall due to the installation process (up to 0.15% of the trench depth) as shown in Fig. 21. Deep trenches in Hong Kongs marine and alluvial deposits controlled the data presented by Clough and ORourke [10]; therefore, it is anticipated that Fig. 19 overestimates the ground movements for most cases.

Fig. 21. Settlement due to installation of a diaphragm wall

(after Clough and ORourke [10])

Finno et al. [33] observed that 25% of the total lateral

movement occurs after installation of secant piles wall in soft to medium Chicago clay, as can be shown in Fig. 22. It was concluded that lateral movements of this magnitude cannot be neglected and must be taken into account when designing support systems, especially when sensitive structures are nearby.

Fig. 22. Lateral deformation associated with trenching for secant piles installed in Chicago Clay (after Finno et al. [33])

CIRIA report 580 (Gaba et al. [34]) summarizes the horizontal and vertical wall movements due to installation of diaphragm walls and bored pile walls in stiff clays as shown in Fig. 23. While Clough & ORourke [10] predicted that the maximum settlement could reach 0.15% of the trench depth, Gaba et al. [34] found out that the maximum settlement is 0.04-0.05% of the trench depth and the maximum lateral

deformation is about 0.04 to 0.08% of the maximum trench depth.

Fig. 23. Vertical deformations due to diaphragm wall installation

(after Gaba et al. [34])

E. Building Stiffness and Weight There is a mutual influence between a building located

close to deep excavations and the induced deformations. Both stiffness and weight of the building affect the final shape of the deformations. The building stiffness tends to flatten the deformations distribution across the building, while the building weight increases the deformations especially in the locations close to the deep excavation. Goh [35] and Goh & Mair [36] presented design charts that allow considering the effect of the buildings stiffness on the induced deformations.

F. Time-Dependent Effects For excavations in clay, longer durations before installing

the strut or constructing the floor slab may cause larger wall deflection due to the occurrence of consolidation or creep of clay. Studies that addressed that aspect by assessing the soil consolidation, as one of the components of the wall and ground deformations, were carried out based on finite element analysis since it is not possible to separate the consolidation deformation component out of the total deformations from the field data.

Osaimi & Clough [37], Yong et al. [38], and Ou & Lai [39] showed that significant consolidation can take place during the construction of a deep excavation in clay and that the effects of consolidation are significant. Consolidation and swelling during excavation result in changes in the shear strength of soils and time-dependent deformations. The negative water pressure, generated by the excavation at the base, dissipates with time causing loss of some passive resistance that occurred immediate after excavation. This leads to time-dependent deformations in the wall and the soil behind the wall.

G. Workmanship Workmanship can be considered as the human and/or experience factor which plays an important role in the success or failure of a certain project. It was initially introduced by Peck [8] as one of the main controlling factors of the ground and wall movements in deep excavation projects. This factor has never been thoroughly defined in the literature despite its impact is important and well-acknowledged in the final outcomes of the geotechnical projects. In fact, deep excavations are very special projects as they need the Designer and the Contractor to be well-acquainted with the technical and constructional aspects of the site as well as the structural nature of the adjacent buildings. Methods to enhance the workmanship include documentation of the performance and encountered problems in deep excavation projects and transfer the gained knowledge to other contractors and other personnel

V. RISKS OF BUILDING DAMAGE Deformation of the ground may cause noticeable damage

to the structure. This damage does not depend only on the induced ground deformations but also on the structural aspects of the affected building. The most settlement sensitive buildings to ground deformations are masonry load bearing walls or frames with masonry in-fill walls especially when they are located perpendicular to deep excavations tending to become distorted with shear strain and lateral deformations.

A purely theoretical approach to estimating building response to excavation-related deformations is not practical due to the variability and complexity of the factors that contribute to the response. Consequently, building response is estimated utilizing simplified structural approximations to provide limiting criteria/threshold against unacceptable damage. The ground deformation components as defined in Figs. 24 & 25 and explained hereafter, may affect the structural performance of buildings and/or utilities:

1. Settlement (S) is the vertical movement of a point. The maximum settlement is denoted by (Smax).

2. Differential or relative settlement (S) is the difference between two settlement values. The maximum differential settlement is denoted by (Smax).

3. Rotation or slope () describes the change in gradient of the straight horizontal line defined by two reference points embedded in the structure with respect to their initial horizontal orientation. The maximum rotation is denoted by (max).

4. Angular distortion () is an angle that produces sagging (or upward concavity) when it is directed downward from the building tilted as a rigid body, or hogging (or downward concavity) when it is directed upward from tilted rigid body building. The maximum angular distortion is denoted by (max). The mode of cracking of a distressing building affected by excessive settlement depends on the

mode of deflection (hogging and sagging) as they induce different damages.

5. Deflection Ratio (DF=/L) is defined as the quotient of relative defection () and the corresponding length (L).

6. Tilt () describes the rigid body rotation of the whole superstructure or a well-defined part of it. In certain cases also rigid body tilt can cause substantial damage, although this is not commonly acknowledged, especially when several rigid bodies are connected.

7. Average horizontal strain h develops as a change in horizontal length over the corresponding length; i.e., h = (L2 L1)/L.

Fig. 24. Definition of the deformations affecting the building

(after Burland et al. [40] and others)

Fig. 25. Definition of sagging and hogging deformation modes

(after Burland et al. [40] and others)

A. The maximum angular distortion criterion Skempton and MacDonald [41] correlated the damage of

buildings under the effect of ground deformations with the angular distortion (). They established the following limiting angular distortions for aesthetic and structural damages:

1. Cracking of panels in frame buildings or walls in load bearing wall structures is likely to occur if () exceeded 1/300.

2. Structural damage to columns and beams is likely to occur if () exceeded 1/150.

Bjerrum [42] presented data relating angular distortion to building performance based on additional data and the Skempton and MacDonalds [41] data. He suggested more levels of serviceability damage based on the angular distortion of the building as shown in Fig. 26.

Fig. 26. Damage criteria based on angular distortion (after Bjerrum [42])

B. Maximum angular distortion criterion Polshin and Tokar [43] studied the effect of the building

geometry based on the ratio (L/H) where L is the length between two joints in the building and H is the building height. They considered the deflection ratio (/L) as a structural criterion related to the curvature, and they used 0.05% as the limiting tensile strain for brick unreinforced walls using an analytical approach. They concluded the following limits of the deflection ratio for unreinforced load bearing walls:

Sagging mode: (L/H3) (/L)max = 1/3300 to 1/2500 Sagging mode: (L/H 5) (/L)max = 1/2000 to 1/1400 Burland & Wroth [44] & [45] assumed that the onset of

visible cracking in a given material may be linked to a limiting tensile strain similar to Polshin and Tokar [43]. The building is modeled as a beam deforming in the same shape as the settlement trough as shown in Fig. 27. Cracking may occur due to horizontal tensile strains from bending or diagonal tensile strains from shear.

Fig. 27. Beam model (after Burland and Wroth [44] & [45])

Burland and Wroth [44] & [45] suggested a strain value equal to 0.075% for the onset of cracking. Burland et al. [46] correlated the limiting tensile strains for unreinforced masonry walls and the crack width. Generally, the maximum strains that cause failure in common building materials vary widely as a function of material and mode of deformation (Boone [47]). Figs. 28 & 29 show the ratio /(L.c), where c is the limiting strain, as a function of L/H for sagging and hogging modes, respectively, based on the work of Burland & Wroth [44] & [45].

Fig. 28. Threshold of damage for sagging of load bearing walls

(after Burland and Wroth [44] & [45])

Fig. 29. Threshold of damage for hogging of load bearing walls

(after Burland and Wroth [44] & [45])

C. Effect of the tensile horizontal strains Buildings sited adjacent to excavations are generally less

tolerant to excavation-induced differential settlements than similar structures settling under their own weight. This is attributed to the lateral strains that develop in response to most excavations. These strains add to the strains imposed by the vertical movements associated with the excavation.

Boscardin and Cording [48] developed a damage criterion for buildings adjacent to excavations in form of multi-dimensional relationship between the angular distortion , the horizontal strain h and the expected tensile strain/degree of severity as shown in Fig. 30. The criterion was based on the state of strain of a simple deep beam with L/H=1, E/G=2.6 and neutral axis at the bottom of the beam, where E is Youngs modulus and G is the shear modulus. The critical tensile strains for different damage levels were determined considering the field observations of damage associated with deep excavations and tunnels.

Fig. 30. Relationship of Damage to Angular Distortion and Horizontal

Extension Strain (after Boscardin & Cording [48])

Son [49] and Son & Cording [50] provided analysis for the empirical criteria presented by Boscardin and Cordings [48] by estimating the principle tensile strain due to angular distortion and lateral strain as shown from Fig. 31. Furthermore, they presented an envelope of constant critical tensile strain (c). They also modified Boscardin & Cordings [48] envelopes, as shown in Fig. 31, based on the

presented above simple analysis. Burland [51] included the lateral horizontal strain based on the work of Boscardin and Cording [48] in the beam representation with L/H = 1.The results are shown in Fig. 33.

Fig. 31. Tensile strain components due to horizontal strain, angular distortion

and tilting for wall with L/H=1 & E/G =2.6 (after Son & Cording [50])

Fig. 32. Damage zones with different critical tensile strains

( after Son & Cording [50])

Fig. 33. Damage criterion according to Burland [51]

D. Effect of grade beams Boscardin and Cording [48] investigated the effect of grade

beams to reduce the greenfield horizontal tensile strain gh to less strain h as shown in Fig. 34, where EgA is the stiffness and area of the grade beam foundation, Es is the soil stiffness, H is the height of excavation or the length of the section of the foundation being strained, and S is the spacing between grade beams.

Fig. 34. Effect of grade beams on the horizontal tensile strain

(after Boscardin and Cording [48])

E. Assessment of the induced building damage Mair et al. [52] and Son & Cording [50] provided a

systematic procedure for damage assessment of buildings. The design approach consists of three stages:

1. Preliminary assessment 2. Second stage assessment 3. Detailed evaluation. The three phases are shown schematically in Fig. 35 and

elaborated in the following sections.

Fig. 35. Three-phases damage assessment flow chart

(after Mair et al. [52] and Son & Cording [50])

Primary assessment In the primary assessment, the greenfield settlement trough

is evaluated. Buildings which are located within the zone with 1/500 & Smax 10mm are assumed to experience negligible damages. The above values of maximum slope and settlement may need to be reduced when assessing the risk for structures of higher sensitivity (i.e., building with stone or glass claddings and important aesthetical features that should be maintained); however, for most structures the abovementioned damage criterion can be utilized.

If the settlement and/or the slope for a building exceeded the maximum slope and settlement stated above, a second stage assessment has to be carried out.

Second stage assessment In this stage of the risk assessment, the building is

represented as an elastic deep beam whose foundation is assumed to follow the ground movement trough. The strain within the beam is evaluated. Categories of damage, defined in previous sections, can then be obtained from the magnitude of strain.

Although this approach is more detailed than the preliminary assessment it is still conservative as the building is assumed to follow the greenfield settlement trough. The category of damage obtained from this assessment shall only be considered for aesthetical damage (i.e., a maximum diagonal tensile strain of 0.15-0.167%).

Detailed assessment In this stage, details of the building and of the deep

excavation should be taken into account using advanced modeling such as:

Geotechnical conditions, sub-surface profile and groundwater conditions.

The three-dimensional aspects of the deep excavation construction.

The building stiffness and weight. The building orientation with respect to the deep

excavation. Building features such as the foundation type and

structural continuity as well as any previous movement a building may have experienced in the past.

Sensitivity of the building. If the risk of damage remains high after the detailed

assessment, necessary protective measures are to be considered in the form of risk mitigation plans.

VI. RISKS ASSOCIATED WITH GROUNDWATER Settlements are generated by the groundwater table

lowering as the soil is passing from a submerged to a saturated unit weight which leads to an increase of the effective stress as shown in Fig. 36. The settlement value depends on the drawdown of the water table and the soil stiffness. In sands, excessive pumping out the groundwater from a deep excavation results in a significant drop of the groundwater table within the surrounding areas with possible excessive settlement of the adjacent buildings and other structures and

piping if the exist hydraulic gradient at the bottom of excavation exceeded the safe value.

Fig. 36. Influence of the dewatering works on the ground settlements

Examples of groundwater-related failures and problems

occurred to deep excavations due to improper groundwater considerations in design and construction are as follows:

1. The collapse of a deep excavation for an underground metro station in Cologne, Germany in 2009, Fig. 37 & 38, which in-turn caused the collapse of the historical City Archive Building. This failure is anticipated to be a piping failure induced by the groundwater high velocity that was not considered during the design of the dewatering system, Rowson [53].

2. A diaphragm wall leaked during the construction of a deep exaction for a new underground station of the North-South Train Line in Amsterdam, the Netherlands. This leakage caused washing of sand below the foundations of surrounding buildings and a subsequent subsidence of 23 cm as shown in Fig. 39. The predicted costs have gone up from 1.5 to 3 billion euros and the project completion was shifted from 2011 to 2017, Van Tol [54] and Van Baars [55].

3. In 2005, a diaphragm wall leaked and surrounding houses started to subside in a deep excavation for a garage in Middelburg, The Netherland. To stop the subsidence, the pit was filled with water until 2009, Fig. 40, till new walls were placed in the pit and the pit was filled with 13,350 m3 of concrete with a loss of almost half the volume of the parking space, Van Baars [55].

4. In 2007, a well-known failure of the diaphragm for the Infinity Tower in Dubai occurred due to piping by seepage through a diaphragm wall joint as shown in Fig. 41.

Fig. 37. Collapse of City Archive Building in Cologne (Germany) due soil

piping induced by dewatering (after Rowson [53])

Fig. 38. The collapsed City Archive Building in Cologne (Germany)

(after Rowson, [53])

Fig. 39. Damage due to Subsidence along an underground station of the

North-South Train Line in Amsterdam (after Van Baars [55]).

Fig. 40. Leakage and damage at the building pit in Middelburg, the

Netherland (Van Baars [55])

Fig. 41. Failure of a diaphragm wall in The Infinity Tower in Dubai in 2007.

Chronological sequence of the failure is (a) to (d)

To avoid problems associated with groundwater and to

minimize the effect of groundwater lowering on the adjacent buildings, the concrete diaphragm walls in the Greater Cairo Metro was extended deeper without reinforcement and a low permeability grouted plug was provided at their toes as shown on Figure 42-a to avoid the possible effects of the large groundwater drawdown as schematically shown in Figure 42-b. The grouting materials were injected in two stages: bentonite-cement slurry and soft-silica gel, in order to reduce the permeability of the sand to 10-6 m/s. Thickness of the grouted plug and its elevation are selected to satisfy a safe limit of the average hydraulic gradient within the plug.

(a) With plug (utilized in Greater Cairo

Metro)

(b) without plug causing large drawdown (not utilized in the

Greater Cairo Metro) Fig. 42. Schemes for groundwater control in a deep excavation

(after El-Nahhas [15]).

VII. OBSERVATIONAL METHOD AND MONITORING Precise prediction of the deformations associated with deep

excavations using advanced numerical analysis is practically unfeasible due to the highly variable nature of geomaterials. Therefore, there are always uncertainties about the assessed deformations associated with excavations. Consequently, the risks of distressing adjacent buildings due to the deformations induced by deep excavation cannot be waived by any pre-construction analyses alone.

A. The observational Approach To address uncertainties in geotechnical design and the

associated risks, Peck [56] proposed to utilize the observational approach as an effective tool in the geotechnically related projects. The following definition of the observational method is quoted from CIRIA 185 (Nicholson at al. [57]): The Observational Method in ground engineering is a continuous, managed, integrated, process of design, construction control, monitoring and review that enables previously defined modifications to be incorporated during or after construction as appropriate. All these aspects have to be demonstrably robust. The objective is to achieve greater overall economy without compromising safety. The objective of the observational method is to achieve greater overall economy without compromising safety. The benefits of the observational method are schematically shown in Fig. 43.

Fig. 43. Potential benefits of the OM according to CIRIA 185

(after Nicholson at al. [57])

(a) (b)

(c) (d)

Peck [56] suggested that design is to be initiated based on the most probable conditions and utilizing monitoring as a tool to update the geotechnical related aspects as the construction proceeds. As such, Monitoring is considered the nucleus and the most important aspect in the observational method. It assists in managing a safe work place and helps to mitigate the risks associated with variability in geological conditions and the inappropriate interpretation of geotechnical data.

Nowadays, monitoring ground and support system response, recording construction activities, and learning from measured data to extract underlying soil behavior becomes an important component in all deep excavations and tunneling projects. Instruments often are installed to monitor and control the performance of excavations. If the observed performance of the excavation shows intolerable deformations, changes in the design and construction procedure of excavation is made.

It is to be noted that the observational approach is not suitable for brittle behaviors in the structure or rapid deteriorations that do not allow sufficient warning to implement any planned modifications such as rapid deteriorations of soils caused by groundwater or non-ductile failures of structural members (struts/waling connections) in multi-propped basements, Patel et al. [58].

B. Geotechnical monitoring An instrumentation program is a comprehensive approach

that assures that all aspects of instrumentation from planning and design through maintenance and rehabilitation are commensurate with the overall purpose. To be fruitful, such monitoring programs must be carried out for well-defined purposes, be well planned, and be supported by competent staff through completion and implementation of results from the monitoring program.

Most instrumentation measurement methods consist of three components: a transducer, a data acquisition system, and a linkage between these two components. A transducer is a module that translates a physical change into analogous electrical signals whilst data acquisition systems are the portable readout units.

Generally, the extent of the utilization of instrumentation (e.g., number and spacing of different types of measurements) depends on the variability of site conditions along and normal to the different sides of the excavation, Karlsrud [59].

Geotechnical instrumentation for deep excavation projects may be classified into two main types namely: the deformation-measuring instruments, and the stress-measuring instruments. The deformation instruments are used to assess the ground displacement fields. The stress measuring instruments are used to measure the pore water pressure, the soil pressure and stresses in wall.

C. Trigger Levels for Monitoring Trigger levels (response values or hazard warning values)

are defined as pre-defined values of the measured parameters. If an instrument reading is higher than the trigger value, then a pre-defined action is carried out. It is common to use two or more trigger values during monitoring of construction to denote different levels of response, given the magnitude of the

reading and urgency or significance of the required response. Commonly, the traffic light system is adopted (viz., Green, Amber and Red trigger levels). The following trigger zones are commonly defined, Devriendt [60]:

Green: OK, proceed Amber (Threshold, Alert, Review, or Warning):

Monitor more frequently, review calculations and start implementing contingency measures if trends indicate the Red trigger may shortly be reached.

Red (Limit, Maximum, Action, Response, or Tolerable limit): Implement measures to cease movements and stop work

The above trigger zones are separated by two trigger levels (Amber and Red) which can be considered as two separate unrelated scales; one related to calculated movements and one relating to tolerable movements. As such, the values of the triggers for deep excavations can be defined as follows, Devriendt [60] and Patel et al. [58]:

Amber trigger is set close to the calculated displacement from analysis (usually at 75 or 80% of the calculated settlement;

Red trigger is based on a tolerable damage or deformation criteria. It can be considered as a conservative estimate of when a serviceability limit state is likely to be exceeded.

An example for setting the trigger levels for a deep excavation for monitoring building deformations and triggering remedies for damage is shown in Fig. 44.

Fig. 44. Setting trigger levels for a building subject to settlement from a deep

excavation

VIII. RISK MANAGEMENT AND MITIGATIONS Many sources of risks are associated with the construction

of deep excavations including: Ground movements, groundwater control, and improper quality of construction. Some of the commonly-acknowledged risk categories are shown in Table 1. The major sources for the aforementioned

risks are the uncertainness in the soil properties and the construction procedure. Table 1. Examples of uncertainty in the geotechnical works (after Patel et al. [58])

No. Geotechnical Uncertainty Example 1 Geological Complex geology & hydrogeology

2 Parameter and modeling Undrained soil verses drained behavior

3 Ground treatment Grouting, dewatering 4 Construction Complex temporary work Risk management in deep excavations can be performed by

identifying the different risk sources and carrying out risk analyses using the following procedure (Ahuja [61]; Abdel-Rahman [62]; Lee at al. [63]):

1. Estimating the probability of occurrence of the undesirable event;

2. Estimating the magnitude of consequences; 3. Identifying options to accommodate the risks,

including: o Reducing the probability of the cause; o Mitigating the consequence; and o Reducing the escalation from cause to

consequence. 4. Prioritize risk management efforts based on:

o Level of risk (probability and consequence); o Status of risk control and risk management

activities; and o Optimum timescale for risk control action.

Risk control could be always ensured through the following:

1. Incorporating a design with adequate safety factor and reasonable ground movements that could be safely tolerated by the surrounding structures.

2. Incorporating an inclusive quality control program during construction.

3. Performing a pre-construction dilapidation survey to verify the conditions of the surrounding structures and their safety conditions when subjected to the predicted ground movements.

4. Adopting an elaborate monitoring system that suit the risk sources associated with the execution of the deep excavation.

Contingency plans are used in the event of emergency

response, back-up operations, and disaster recovery for construction projects which carry a large element of risk. The contingency plan shall therefore focus upon ways in which certain events identified through completion of project risk assessments can be militated against using a set of pre-identified procedures. The plan shall be fit-for-purpose and undergo the following key tests prior to its release:

1. Is the plan achievable in reality, should this be required?

2. Are the trigger mechanisms for actual activation of the plan clear and realistic?

3. Does the plan address anticipated situations in a timely, affordable, effective, consistent manner?

Puller [3] listed the following contingency measures to reduce the deformation induced with the deep excavation and hence

reduce the risks of affecting nearby buildings: 1. Use of construction methods such as the top-down

system or preloading of temporary struts may achieve reductions in settlements below nearby buildings.

2. Strengthening the ground by means of cement or chemical grout injection, mix-in-place or pin piles. In extreme cases, freezing of the subsoil may prove an effective solution in granular, water-bearing soils.

3. Temporary or permanent strengthening the affected building by means of vertical support and horizontal ties to resist horizontal tensile strains imposed by the soil deformation. Shear stiffness of the building may also be improved by temporarily filling window and door openings in facades and cross-walls with brickwork or blockwork of requisite strength.

4. Structural jacking applied progressively as the deep excavation is made to counteract vertical settlement, possibly with improvements of temporary strengthening to the structure.

5. Compensation and fracture grouting may be applied progressively as deep excavation is made. Compaction grouting applied to both granular and cohesive subsoils can provide a means of lifting structures to counteract the effects of vertical settlements. Successive injections of compensation and fracture grouting may be carried out from tubes-a-manchette drilled in arrays from positions both inside and outside the affected structure.

Abdel-Rahman [62] illustrated the applicability of the risk management approach to mitigate the risks of affecting structures nearby a deep excavation for a multi-story underground garage in Al-Tahrir square, Cairo, Egypt. He studied in details the risks associated with the deep excavation in this project and presented a contingency plan of actions that was prepared to meet the unforeseen conditions as summarized in Table 2. Table 2. Contingency plans for deep excavation (after Abdel-Rahman [62])

Risk source Contingency plan of action

Excessive lateral movement of the wall and ground settlement

Increase the number of lateral supports

Instability of the grout plug

Refill the excavation pit with water up to the level that adequately re-stabilize the situation, or perform heavy dewatering to lower the water table as needed.

Insufficient drawdown to the water below excavation level

Increase the number of wells

Lateral leaking from the support system

Inject grout columns behind the leaking locations

IX. SUMMARY AND CONCLUSION Deep excavations occasionally cause failures of adjacent

structures yet they often produce serviceability problems to nearby buildings in form of wall cracks, tilting and impairments to windows and doors due to the ground deformations associated with deep excavations. With the increasing demands for deep excavation in urban areas having soft deltaic soils such as the Greater Cairo, it becomes increasingly important to have well-designed support systems for deep excavations that do not only ensure the stability of the excavation itself but also warrant that the excavation will not cause damage to the adjacent buildings and utilities due to potentially excessive ground deformations.

The deformations patterns associated with deep excavations depend of the mode of the wall deformations. Two basic patterns of the settlement troughs are commonly acknowledged: spandrel settlement trough (associated with the wall cantilever deformations) and concave settlement trough (associated with the wall bulging deformations). The cumulative settlement is a function of the relative ratio between the wall bulging and cantilever deformations.

Many damage criteria have been set to assess the effect of the ground deformations induced by deep excavations on building. The main common aspect of these approaches is the inclusion of the effect of the horizontal deformation caused by deep excavations.

Monitoring programs and risk management are powerful tools in the observational approach to allow construction to proceed smoothly in the face of the abundant risks associated with deep excavation projects, particularly the risks associated with unforeseen geotechnical conditions or construction problems. A proper prepared risk mitigation plans with well-set monitoring trigger levels become a necessity in deep excavations especially in urban areas. The results of the monitoring are made available to all concerned parties through modern communication means such as the Internet and cell phones.

ACKNOWLEDGMENT The authors would like to acknowledge the pioneering

work done by the Prof. Fathalla M. El-Nahhas (Ain Shams University, Faculty of Engineering, Egypt) in the fields of tunneling and deep excavations. Prof. El-Nahhas studies and researches inspired and motivated us during the preparation of this article. The authors would also like to express their truthful gratefulness to Prof. Sherif W. Agaiby (Dar Al-Handasah Consultants Shair & Partners), Prof. Ahmed Hosny Abdel-Rahman (The National Research Center, Egypt) and Prof. Ali A. Abdelfattah (Ain Shams University, Faculty of Engineering, Egypt) for their kind sharing of their engineering expertise with the authors in deep excavations and in other areas of the Geotechnical Engineering.

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I. IntroductionII. Risks Associated with Deep ExcavationsIII. Geotechnical and Geological AspectsIV. Factors Affecting Ground DeformationsA. Ground and wall deformation patternsB. Wall Stiffness and Excavation StabilityC. Excavation Geometry and Three-Dimensional EffectsD. Wall Installation EffectE. Building Stiffness and WeightF. Time-Dependent EffectsG. WorkmanshipV. Risks of Building DamageA. The maximum angular distortion criterionB. Maximum angular distortion criterionC. Effect of the tensile horizontal strainsD. Effect of grade beamsE. Assessment of the induced building damagePrimary assessmentSecond stage assessmentDetailed assessmentVI. Risks Associated with GroundwaterVII. OBSERVATIONAL METHOD AND MONITORINGA. The observational ApproachB. Geotechnical monitoringC. Trigger Levels for MonitoringVIII. Risk Management and MitigationsIX. Summary and ConclusionAcknowledgmentReferences