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Geotechnical Engineering Report on Subgrade Strength of Existing Highway (Balkh ~ Andkhoy) >>>


Fore word

As a Design Team Leader, the Writer had

participated in one of the Afghanistan

Highway Rehabilitation Projects which runs

from Balkh to Andkhoy in length of a

180km. The pavement rehabilitation work

are comprised of i) scarifying existing

asphaltic pavement layers, ii) regulating

the remaining base course of crushed

stone to make use for the subbase course

of new pavement structure, iii) construct

other new pavement courses such as

crushed stone base course, black base,

wearing course and shoulder.

Highway rehabilitation design are com-

prised of two major aspects such as

improvement of highway geometry and

pavement structure, the latter of which is

most crucial course in the job. Meanwhile,

according to the Conditions of Contract

and their Design Specification of the

Afghanistan Highway Rehabilitation

Project, the Designer should perform i) the

Falling Weight Deflectometer test to inves-

tigate Subgrade Strength in terms of

MR(Resilient Modulus) of the existing high-

way under service, ii) refine the pavement

thickness design presented in the bidding

김 민 웅2)

Geotechnical Engineering Report onSubgrade Strength of Existing Highway

-Balkh~Andkhoy-

1. Introduction

2. Data Acquisition for Subgrade Strength

3. Strength of Existing Subgrade

4. Conclusion 홍 륜1)

1) 감리본부 전무·토질 및 기초기술사 ([email protected])

2) 건설기술연구소 차장([email protected])

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334 | 유신기술회보 제13호

기술정보및출장보고서등

document, which had been based on IRC-

37-2001 Code using CBR developed from

DCPT, iii) pavement design shall be carried

out in compliance with IRC-37-2001 Code

but their results should be compared with

the AASHTO design.

In fact, the two design codes ie IRC,

AASHTO are entirely different ones espe-

cially in the classification system to define

the existing subgrade strength. To over-

come such incompatibility, it had been

tried to find out any correlation between

MR and CBR in field and further their

results were applied in actual design.

1. Introduction

In pavement rehabilitation design, it is

much delicate work for designer to esti-

mate field strength of existing subgrade of

sealed asphalt pavement road under ser-

vice. The highway from Balkh to Andkhoy

are extended over a 180km in length,

which is sealed with asphalt pavement

except a 20km of end stretch near

Andkhoy. As for tools used in estimating

strength of existing subgrade, a few kinds

of tests were assigned in this project ie ;

1) Laboratory Soaked CBR adjusted to

field density

2) Dynamic Cone Penetration Test

(DCPT)

3) FWD-MR( Resilient Modulus by Falling

Weight Deflectometer)

Each test has its own particulars but it

can be said in general, CBR and DCPT are

categorized as sorts of Destructive Test

Methods while FWD-MR as a Non-

Destructive one. Further, the first two tests

either lab.CBR or DCPT present a decisive

value of individual test point, while the

FWD-MR present merely arbitrary values in

continuous mode. Accordingly, in assess-

ing in-situ strength of existing subgrade, it

is indispensable for designer to examine

the output resulted from the different types

of tests simultaneously with a proper cor-

relation between each other.

Detailed procedures are introduced in

relevant sections of this report.

In the meantime, those tests had yielded

various soil properties of subgrade listed as

following with the interval not farther than

2km.

i) Index properties

-Sieve Analysis, #4, #200

-Atterberg Limit(LL, PI)

-Soil Classification(AASHTO, USCS)

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ii) Field Compaction

- Natural Moisture Content

- Optimum Moisture Content

- Field Dry Density(FDD)

- MDD for 27 blows, 30 blows

- Compaction Level

iii) CBR

- Soaked CBR for 95% compaction

- Unsoaked CBR for 95% compaction

- Field CBR adjusted for FDD

iv) DCPT

- DCPT during at hot-dry season

- DCPT during at wet season

- Adjusted for seasonal variation

v) Falling Weight Deflectometer test

every 50m in both way during dry sea-

son.

2. Data Acquisition for SubgradeStrength

As pavement design in the project is

based on IRC-37-2001 Code, subgrade

strength should be defined in terms of

soaked CBR. Field strength of existing sub-

grade were determined based on the vari-

ous test results deposited throughout the

steps listed as the following.

1) Final pavement thickness design

report prepared based on Geotechnical

Report

2) Additionally, in the 1st week of April

2005, performed lots of test pit run

together with lab. CBR to check exist-

ing pavement profile as well as sub-

grade strength.

<Table 1> Tests for Subgrade Strength(Paved Area Length:180km)

Investigationphase Period

Test quantity

Test pit Remarks

Pavementdesign stage Aug. 2004

1st additionaltest

1st week ofApri, 2005

2nd additionaltest

End ofApri, 2005

3rd additionaltest

sum

End ofJuly, 2005

33

72

16

25

146 52 49 56

LabCBR

17

23

11

1

FieldDensity

12

-

8

29

DCPT

15

-

16

25

FWD

22㎞~202㎞

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3) 2nd additional subgrade investigation

was conducted in the last week of

April, 2005 focusing on the areas

where present low FWD-MR and are

badly defected of pavement.

4) 3rd additional subgrade investigation

was executed on/around end of July

2005 to acquire lots of in-situ tests for

existing subgrade inside of carriage-

way. In this phase, plenty of in-situ

data was acquired such as field densi-

ty, Proctor test to get Compaction

Level and DCPT, which enabled to

establish a reliable correlation with

others so that they can be applied in

the project road.

The following Table. 1 shows a summary

of all the test types and quantities executed

in the project for design purpose.

3. Strength of Existing Subgrade

3.1 Determination of strength

Strength of existing subgrade stated in

the Final Pavement Design Report was esti-

mated based on limited field information as

the highway with asphalt sealed pavement

had been occupied by traffic service. This

necessitates a series of field complimentary

test on the scarified surface during con-

struction phase. However, considering the

limited construction period of this emer-

gency infrastructure rehabilitation project, it

is most indispensable in design stage to

determine reliable strength of existing sub-

grade even prior to scarification of the

asphalt surfacing course of highway. In

order to resolve this matter, FWD-MR are

assigned to provide very useful information

to complement the gap without field test

data. Following process was applied in

determining existing subgrade strength.

Step 1. Sort out the stretches where pre-

sent low FWD-MR and pavement condi-

tions are poor by visual inspection.

Step 2. Conduct field density test together

with DCPT in the test pit inside carriage-

way.

Step 3. Establish correlation between

lab.CBR, DCPT and FWD-MR for deciding

reasonable field CBR.

Step 4. Final engineering judgment was

made on examining and comparing each

strength parameters obtained by different

sources of tests.

Step 5. Recommend acceptance criteria

for subgrade strength to be applied during

construction stage.

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3.2 Strength based on Lab.CBR withcorrection for field compactionlevel

Rationally, in order to simulate the worst

environmental condition to be faced by

highway during its design life, CBR for

existing subgrade must be obtained by in-

situ CBR test with 4 days soaking.

However, this is impractical in many

aspects. Instead, field CBR was obtained

using laboratory soak-CBR at 95% MDD

with due adjustment for actual degree of

compaction of existing subgrade. In fact,

this method has some problems to be

taken into account for evaluating field

strength.

1) In case of coarse grained soil and/or

non-plastic silt, lab.CBR with 95 %

MDD usually yields high values

regardless of actual field density of

existing subgrade.(refer to lab CBR at

the hilly stretches)

2) Field compaction level is highly

dependent on the moisture content of

roadbed soil with plasticity.

3) Further, it happens often to take a test

sample from dampened roadbed to

ease test pit digging in damaged

asphalt surface. That is why judgment

should be carefully decided, compar-

ing the values resulted from different

test methods. At this jobsite, evidences

show, if compaction level of existing

subgrade is not less than 95% MDD,

field CBR shall be more than design

CBR. Field CBR can be derived from

lab. soak CBR with adjustment for

field compaction level.

3.3 Field CBR from DCPT

Some of the field dry density test are

resulted in low compaction level far from

expectation. This might be presumably

affected by several factors such as stress

release during sampling and/or local damp

by surface water percolation. The demerit

of the adjusted lab.CBR method caused by

low measured field dry density can be

overcome in the DCPT mostly. DCPT

applied CBR value was calculated follow-

ing the TRRL recommended formulae.

According to observation during the test,

penetration resistance varied by points

even in same test pit. This may come from

either different moisture content or in-situ

stress field by locality. DCPT was carried

by 3 phases. The 1st and 2nd run during

spring seasons respectively while the 3rd

one was done in dry summer season(end

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of July, 2005). According to the test result

review, the DCPT done in summer season

reveal high increased values compared

with others, which necessitate due correc-

tion for seasonal effect. Seasonal effect

could be accounted by adopting a 74% to

that measured in hot and dry summer sea-

son except the case of DCPT less than 20%

on which damp effect are already reflected.

As for the seasonal correction, the AASH-

TO method was quoted from the Final

Pavement Thickness Design Report at engi-

neering judgment. (refer to following page)

A correlation between field soak-CBR vs

effective DCPT are shown in Figure.1 and

Table 2. This correlation simulates sub-

grade weakening by “swell”when sub-

grade soil is soaked. In this spread sheet,

data with large deviation were excluded to

achieve a better correlation. For the high-

way stretches after Chainage 158km espe-

cially in cutting areas, several numbers of

unexpectedly high DCPT values are noted

notwithstanding high compaction level,

which might be attributed to the predomi-

nant soil components of Calcite mineral in

that region. Therefore, CBR from DCPT in

that area, have to be regarded less reliable

and hence preferably CBR from FWD-MR

should be accounted with priority to the

DCPT. The correlation of field soak-CBR

from effective DCPT can be defined by

the following formula for fine grained

soils with plasticity, which is subjected to

swell by soaking. Meanwhile, non-plastic

silt or coarse grained soil with low

swelling potential prevail in the hilly areas

from Chainage 168km to 181km. In the

case, DCPT-CBR is nearly equivalent to

field soak-CBR without applying adjust-

ment factor as shown in table 3. In result,

correlation between field CBR and DCPT

can be summarized as the following for-

mulae.

Fine grained soils ;

Effective CBR-DCPT = 2 times field CBR + 0.3STD (Standard Deviation)

Apply as, effective field CBR = DCPT-CBR / 2.3

Coarse grained soils or non-plastic silt ;

Effective DCPT-CBR = field soak-CBR

This formula can be applicable for the acceptance criteria in the field construction.

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<Table 2> Tests for Subgrade Strength(Fine Grained Soil)

<Figure 1> Soak-CBR vs DCPT

Test PitChainage

(km)

1) FieldSoak_CBR

2) EffectiveDCPT_CBR 2) / 1) Chainage 1) Field

Soak_CBR2) EffectiveDCPT_CBR

2) / 1)

32.50 12.8 25.9 2.02 116.00 12.0 21.9 1.83

42.50 14.0 22.9 1.64 118.00 8.5 19.6 2.31

51.50 14.4 24.8 1.72 122.50 13.0 19.6 1.51

55.01 9.5 18.8 1.98 128.00 14.4 31.2 2.17

64.00 12.2 20.8 1.70 146.00 21.8 39.8 1.83

68.00 7.0 18.2 2.60 154.00 22.0 39.6 1.80

72.50 16.8 37.7 2.27 168.00 38.6 65.1 1.69

111.00 8.7 14.4 1.66Average = 1.94, Standard Deviation = 0.31

113.50 8.4 18.9 2.25

<Table 3> Tests for Subgrade Strength(Coarse Grained Soil)

Test PitChainage(km)

SieveAnanysis LL(%) PI(%) Class Compaction

Level(%)

field-CBRadjusted for

FDD(%)

field-CBRbased onDCPT(%)

DCPT/CBR

# 4 # 200

172.00 - - - - sand 100.40 42.7 26.2 0.6

175.60 100.0 85.8 NP NP sand 95.13 27.5 26.6 1.0

176.00 100.0 47.3 NP NP A4 99.98 45.5 20.0 0.4

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Application for Figure. H.3 in the AASHTO Guide then gives effective

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Considering the particulars of the field

CBR derived from various test methods,

their results should be examined/compared

carefully in order to accomplish the most

reasonable strength of existing subgrade.

The distribution of FWD-MR for the overall

stretches of the highway are shown on

Figure 2. The MR values illustrated in the

Figure 2, are FWD-MR with due treatment

to achieve a coefficient of variation within

15%. Those values were calculated by

applying 0.33 as denominator to the MRC

presented in the Final Pavement Thickness

Design Report. Meanwhile correlation of

field CBR vs FWD-MR are presented in

Figure 3. Their data sheets are also shown

on the Table 5 below.

3.4 Field CBR from FWD-MR

As explained briefly in the preceding

chapter of this report, lab CBR has many

demerits in engineering aspect even after

adjustment to field density. Particulars of

lab-CBR and FWD-MR are compared in the

following Table 4.

<Table 4> Comparison of Various Subgrade Tests

Factors CBR (in laboratory) FWD-MR DCPT(in field)

Depend onsampling factorsor test points

Yes(FDD, NMC,

Test point selectionNo Yes

Represent soilproperties

Yes(swell by soaking) No Yes

Simulation of in-situconditions Less effective Effective Effective

ContinualData acquisition

Individual,Independent

Review overallStretch, Collective Data

IndividualIndependent

Decisive indicationon strength Yes Arbitrary &

need calibrationArbitrary &

need calibration

Natural moisture con-tent effect No Yes Yes

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<Table 5> Field CBR and FWD MR Data(Fine Grainde Soil)

Chainage(km) sample MR sample

CBR' ratio Chainage(km)

sampleMR

sampleCBR ratio

32.50 12,179 13 951 72.50 20,797 17 1,253

35.03 12,179 15 840 111.00 8,636 9 993

38.00 12,179 18 680 113.50 8,636 8 1,028

42.50 12,179 14 870 116.00 11,667 12 972

46.00 12,179 13 916 118.00 11,667 9 1,373

48.00 12,179 18 696 122.50 11,667 13 897

51.50 18,212 14 1,265 126.00 11,667 14 810

55.01 9,933 10 1,046 128.00 11,667 14 810

56.00 9,933 8 1,197 133.50 11,000 9 1,222

58.00 9,933 8 1,242 136.01 7,152 7 1,100

63.00 17,736 14 1,267 156.00 11,000 10 1,089

64.00 12,839 12 1,052 156.01 11,000 11 982

68.00 12,839 12 1,035 Average = 1,023, Standard Deviation = 186

<Figure 2> Distribution of MR along the Highway(Fine Grained Soil)

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From the Figure 3, Table 5 and Table 6,

correlation between field CBR and FWD-

MR could be derived in a good quality.

They are :

Test PitChainage

(km)

SieveAnanysis LL(%) PI(%) Class

Compaction Level(%)

field-CBRadjusted for

FDD(%)

FWD-MR finalpavement

design report(psi)

MR/CBR Remarks

# 4 # 200

166.00 100.0 41.3 NP NP A4 99.70 22.20 15000 676

168.00 - - - - sand 99.04 38.6 15000 389

172.00 - - - - sand 100.40 42.7 15000 351

175.60 100.0 85.8 NP NP sand 95.13 27.5 15000 545

176.00 100.0 47.3 NP NP NP A4 99.98 45.5 15000 330

181.13 95.5 35.5 NP NP NP A4 106.46 30.2 15000 497

mean 465

std 133

<Table 6> Field CBR vs FWD-MR(Coarse Grained Soil)

<Figure 3> Distribution of Field CBR vs FWD-MR.

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As can be seen from the data log sheet,

either CBR or DCPT present sometimes

abnormally high or low values apart from

its strength due to several factors as stated

in previous page. In such case, FWD-MR

provides useful and rather reliable infor-

mation.

3.5 Evaluation of existing SubgradeStrength in PavementRehabilitation

Referring to IRC-37-2001 design code,

acceptance was initially examined for the

field CBR gained through lab.CBR with due

correction for compaction level of existing

subgrade.

DCPT result was applied with due corre-

lation where lab. CBR data not available.

Estimated field strength correlated with

FWD-MR present reasonable values close

to others and thus can be applied as basis

of acceptance for existing subgrade where

other field test data not available. Final

acceptance for existing subgrade was

decided after examining/ comparing care-

fully with others.

In order to check the strength of sub-

grade in the pavement rehabilitation, fol-

lowing guidelines are recommended for

field construction control :

1) Case 1 : Field dry density exceed 95%

of MDD

- In this case, it may be deemed that

field CBR exceed design CBR.

2) Case 2 : Field dry density less than

95% of MDD

- Method #1 : Make due correction to a

CBR at nearest location by applying

degree of compaction

- Method # 2 : Find CBR from DCPT for

Chainage Correlation

22km - 148km FWD-MR(psi)=1200 Field soak-CBR

148km -157km FWD-MR(psi)=1000 Field soak-CBR

158km - 181km FWD-MR(psi)= 1200 Field soak-CBR

168km - 181.3km( only for cutting areas)

FWD-MR(psi)= 600 Field soak-CBR

181km - 202km New Construction

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subgrade (upto depth of 500mm) esti-

mated after TRRL formula, then con-

vert it by dividing a constant of 2.3 to

get field CBR.

3) Acceptance will be decided by the

Engineer based on the field test values

mentioned above.

4. Conclusion

i) Subgrade Soils in arid region of the

North-Western part of Afghanistan show

unusually large dry densities as well as

CBR value even though they are classified

as fine grained soil, which may be due to

its soil particles of Calcite origin.

ii) The subgrade were found to be in the

degree of compaction not less than 95% in

most part of the highway. However, in

some stretches where were in poor pave-

ment condition, they show low leveled

compaction degree which had to be affect-

ed by weathering and swelling during the

past. Accordingly the laboratory CBR at the

95% degree of compaction should be duely

corrected for field dry density in pavement

rehabilitation design.

iii) On integrating all the test results, it

came to notice that FWD-MR= (1000~1200)

×Field CBR for a fine grained soil with

low plasticity while FWD-MR= (600~750)×

Field CBR for either a coarse grained soil

or non-plastic silt. Further it is expected to

get the correlation formular, MR = 1500×

CBR introduced in AASHTO code without

Applying the adjustment for C.V.(coeffi-

cient of variation, 15%) to the raw FWD-

MR for the fined grained soil in this region.

iv) In construction phase, it is believed

that DCPT will be a very useful and effi-

cient tool for evaluating strength of existing

subgrade in terms of Field-CBR derived

after the TRRL formular.

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