Subgrade Design KJJ

8/10/2019 Subgrade Design KJJ

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Hitchhikers Guide to Pavement Engineering: Prof Kim Jenkins 1

4. Subgrade : Foundation Considerations

1. Introduction

The performance of any structure is strongly dependent on the foundations. Roadpavements are no exceptions and their foundations need to be fundamentallymodelled using sound geotechnical engineering principles. Thus, it is very importantto have sufficient information about the subgrade, which is obtained throughinvestigation and accumulation of test results. In this way the risks regarding thesubgrade can be estimated. Variation in the subgrade directly below the pavementcannot be prevented (this is a fact of nature), so it is important not to overload thesubgrade at any place. The primary structural task of the pavement is to prevent toohigh stresses in the subgrade as shown in Figure 4.1.

Figure 4.1 Load spreading abiliti es of di fferent pavements

Every pavement design method requires some knowledge of the subgrade quality as

an important input parameter in analysis procedure. Until recently the most commondesign methods were based on the CBR value of the materials in the pavement, asdetermined in the CBR test. Based on CBR values of succeeding layers designcharts were developed. This is called the CBR cover design method, as it aim toidentify the thickness and bearing capacity of different layers above the subgradethat are needed to provide enough load-spreading to protect the subgrade. Examplesof the CBR cover design are provided in the figures below.

Surfacing

GranularBase

Poor pavement

High E

High E

Sound pavement

Low E

Low E

HIGHstressesin S/G

Subgrade

LOWstressesin S/G


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Figure 4.2 CBR Cover Design Curves adapted for 80kN axles (Freeme, Maraisand Walker)

The procedure for calculating the CBR is shown below. Material is first compacted todensities that are representative of field density and soaked for 4 days before testing.The name CBR comes from Californian Bearing Ratio because a high qualityCalifornian gravel was used as the reference material. Ironically, California does notuse CBR any longer (but many other countries still do).

Figure 4.3. CBR Test ProcedureCBR (%) = F0.1/F0.1 Refx 100

Typical values : Sand CBR = 10 to 15% Crushed stone CBR = 40 to 100%

ESALs (80kN x106)E0


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Researchers have tried to characterise the subgrade stiffness by using a correlationwith CBR values. Shell, for example, tested in situ stiffness of sands by measuringwave propagation velocity through the sand and comparing that to laboratorymeasured stiffness, as shown in the Figure below. They developed an averagerelation as follows:

E [MPa] = 10 x CBR [%]

Such a correlation not very reliable, as can be seen by the variability of therelationship. In addition, it is material specific and cannot be applied to othergradations.

Figure 4.3 Relationship between CBR and Dynamic Modulus (EDINAMIES) forsands (Shell)

2. Material depth

In TRH4 in South Africa (Technical Recommendations for Highways: Structuraldesign of Flexible Pavements for Interurban and Rural Roads), the subgrade isshown to play a very important role in the structural design of a road (Paragraph 6.2 -6.8). The catalogue design method in the TRH4 can be used only if CBR of thesubgrade is at least 15% for a depth of 150mm and decreasing gradually with depth.

This is the depth below which very little influence of the loads on the subgrade canbe expected.


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Table 4.1 Subgrade Classification for Structural Design (TRH4)

Class Subgrade CBR (%) Comment

SG1 >15 Good quality material, just rip and recompact

SG2 7 to 15 Moderate quality, needs 150mm of CBR>15 above

SG3 3 to 7 Fair quality, needs 150mm of CBR>15 + 150mm of

CBR>7 aboveSG4 < 3 Poor material, special treatment

Note: It should be remembered that the standard CBR is determined after specimenshave been soaked for 4 days (quite extreme but necessary to ensure that dramaticfailure doesnt occur when a material is exposed to moisture).

Uniform sections

In South Africa Road Categories are used to designate the importance of the roadand the level of reliability for the design i.e. Categories A, B, C and D. In order toidentify the classification of the subgrade one should first identify the uniform

sections. There are two methods that one can use in identifying the uniform sections:

a) Graphical method

This method uses a graphical plot of CBR values over the centreline distance of theroad. Visual distinction can help to identify the uniform sections as shown in thefigure below.

Figure 4.4 Graphical Identification of Uniform Sections

b) Cumulative Sum Method

This method uses the equation below to calculate the cumulative sum value for theCBR data over the length of the road. The cumulative sum values are then plottedagainst centreline distance and the limits of the uniform sections are identified as theinflection points of the lines.

Si = x i x + Si-1

WhereSi = cum sum of deviations of mean CBR values

xi= CBR at point ix = mean CBRSi-1= cum sum of previous point

CBR(%)

Centreline Distance (m)

Section 1 Section 2 Section 3


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Figure 4.4 Cumulative Sum Identification of Uniform Sections

Design Reliability

For each of the uniform sections, the CBRdesign value must be calculated using thereliability of the road category. There are two basic methods that can be followed forsuch analyses:

a) Standard normal distribution method

This method uses the assumption that the CBR results in a uniform section arenormally distributed. This is a reasonable assumption for most cases of subgradeconditions. Thereafter the method uses a basic statistical approach.

CBRdesign= x k. S

WhereCBRdesign = CBR design value given the reliability of the uniform sectionx = average CBR value of the subgrade in the uniform section

k= statistical coefficient for a given level of reliabilityS = standard deviation for the CBR values in the uniform section

Table 4.2 Design reliability for Roads in South Africa (TRH4)

Category A B C D

Description Majorinterurbanfreeways

Interurbancollectors andmajor rural rds

Lightly trafficrural roads &strategic rds

Lightpavements,rural access

Approx designreliability (%)

95 90 80 50

k 1.695 1.282 0.842 0

b) Cumulative less than frequency method

This method uses the distribution of only the values in the uniform section beingevaluated with fitting any standard normal distribution. The reliability value can beread of a plot cumulative frequency versus CBR value, as shown in the examplebelow for a Category B road.

Example: CBR values in the uniform section: 5, 6, 7, 7, 8, 5

CumSumCBR

(%)

Centreline Distance (m)

Section 1 Section 2 Section 3


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Figure 4.5 Cumulative Less than Frequency Plot (example)

From such a plot the CBRdesign would be identified as 4.4% for the given uniformsection.

Seasonal Changes

The CBR test makes provision for extreme conditions i.e. 4 days of saturation. Inreality, the subgrade in the outer wheelpath changes in moisture content with timei.e. from the dry season to the wet season, as shown in the figure below. Theinfluence of climatic effects will be discussed in a separate chapter.

Figure 4.6 Seasonal variations in moisture and CBR under a road (TRH4)

CBR (%)

Cum Frequency(%)

10

20

30

40

50

60

5 6 84

0

Cat B

CBR Freq Cum (%)

4

5

6

7

8

0

2

1

3

1

2/7 = 28.5

3/7 = 42.8

0

6/7 = 85.7

7/7 = 100

CBRdesign

dryseason

wetseason


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3. Compaction

In all aspects of material behaviour, the compaction plays a very important role. Theprocess of compaction increases the density of a soil by packing the particles closer

together with a reduction in the volume of air. Compaction influences stiffness (loadspreading), shear strength, bearing capacity, permeability, porosity etc of a material.How is the compactibility of a material tested and which parameters are important?

There are various standard compaction methods used in different parts of the world.Two commonly used methods are Proctor Compaction and Modified AASHTO (orModified Proctor) Compaction (see method A7 in TMH 1 of SA). These methods areused in a laboratory to provide a benchmark for the levels of compaction that can beachieved in the field. Each method uses a standard amount of energy that isimparted on a material in a special way (a falling weight over a known height for acertain number of blows, and compaction of 5 layers in the mould). From theModified AASHTO method, density requirements for the material in the field after

compaction are determined. One of the reasons for the evolution from Proctorcompaction (lower energy) to Modified AASHTO compaction (higher energy) was tokeep pace with the developments and improvements in roller technology. In this waythe specifications were kept more realistic.

The relationship between the moisture content and achievable density duringcompaction is of particular importance. Based on the results of the Mod AASHTOtest, the optimum moisture content necessary to achieve a certain required drydensity for the material in the road, is determined. The achievable density andresulting material properties such as stiffness and shear strength etc, will determinethe behaviour of the material during service life in a road pavement.

Density is specified as dry(dry density) in kg/m3so that it is independent of moisturecontent. But bulk density (aggregate plus moisture) is what is measured, so themoisture content needs to be removed mathematically from the calculation. This canbe achieved using the formula below, where w = moisture content represented as afraction.

dry= bulk / (1+w)


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Figure 4.7 Moisture versus density relationship

The achievable compaction is dependent on numerous factors including aggregategrading, particle shape, particle angularity, plasticity, moisture content, compactionenergy etc. If the compaction energy is increased, not only will the dry densityincrease but the optimum moisture content reduces. A family of curves can bedeveloped for a specific material that shows the changes in the moisture-densityrelationship relative to the compaction energy, as shown in the figure below.

The influence of material gradation on compaction can be shown in a similar fashion.In Figure 4.9 it is apparent that the highest density is achieved for well graded gravel(GW), followed by well graded sand (SW), then low plasticity silt (ML), followed bylow plasticity clay (CL) and then high plasticity clay (CH).

Dry Density

d (kg/m3)

Moisture Content (%)

OMC=x%

Max dProctor-compaction of5 layers of equal blows


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Figure 4.8 Influence of Compaction Energy on the Moisture-Densityrelationship (Craig)

From the relationship given in Figure 4.8, it can be seen that the dry density is afunction of the aggregate specific gravity (Gs) and moisture content (w) when the airvoids are zero (Va = 0%).

Figure 4.9 Influence of Compaction Energy on the Moisture-Densityrelationship (Craig)

DD=(1-Va/V).1000/(1/Gs+w)

Improve gradingCoarser grained


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It can be shown using the compaction moisture curve in an elementary fashion thatwith increasing density it is possible to maintain the strength of a plastic soil materialin certain service conditions. Using Figure 4.10, it can be seen that 2 samples of thesame material compacted at 17% moisture content with different compaction efforts,the material at lower density (less compaction) exceeds the plastic limit whensaturated and will lose strength-supporting properties. The same material compactedto a higher level will probably remain stable. This also indicates that relationships canbe found between CBR, compaction level and void content. Dry density requirementsare so very important for practice, therefore.

A similar situation can be ascribed to asphalt mixes. Instead of water, bitumen isused in these materials. The bitumen cannot evaporate or leach out of the materialas is the case with water in granular materials and soils. However, the sameprinciples in terms of fluids will be used in order to understand what happens duringcompaction. This is dealt with in another chapter.

Figure 4.10 Advantage of higher compaction in terms of material plasticity(Oglesby)

Field Compaction

Achieving optimum compaction in the field requires judicious selection of rollers. Theroller selection (weight and type) is primarily judged on the gradation of the materialand the depth of the layer to be compacted. Figure 4.11 provides a guideline for rollerselection. In addition to this selection, rolling techniques for subgrade usually involvehigh amplitude, low frequency initial compaction to achieve greater depth ofpenetration followed by low amplitude, high frequency compaction to densify theupper part of the layer.

DryDensity

Moisture Content

ZAV

17% 25%

HighCompaction

LowCompaction

PL

Wetting after compactionto saturation

PL Plastic Limit


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Figure 4.11 Guide to Rol ler Selection (Wirtgen)

References

Oglesby. Highway Subgrade Structure

Wirtgen. Cold Recycling Manual.

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Subgrade Design KJJ