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