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TRANSCRIPT
Technical Report
Long-Term In Situ Characteristics of Lime Stabilized Soil
Tommy C. Hopkins Kentucky Transportation Center
University of Kentucky College of Engineering 508 Administration Dr.
282 Oliver Raymond Bldg. Lexington, KY 40506
2
© Carmeuse Lime & Stone 2008
Long-Term In Situ Characteristics Of Lime Stabilized Soil
TABLE OF CONTENTS: Introduction........................................................................................................................... 3
Objectives and Scope of Study ............................................................................................ 3
Roadway Test Sections and Coring Techniques ................................................................. 4
Field and Laboratory Test Methods..................................................................................... 4
Results and Analysis............................................................................................................. 4 In Situ CBR Values.................................................................................................. 4 Soil Classification ................................................................................................... 7 Clay Fractions ......................................................................................................... 7 Moisture Contents.................................................................................................. 7
Untreated Subgrades............................................................................... 8 Lime Stabilized Subgrades...................................................................... 8
Summary and Conclusions .................................................................................................. 9 References............................................................................................................................ 9
Appendix—Coring Technique and Field Testing Procedures ............................................10
ABSTRACT Chemical admixtures, such as lime, have been used for stabilizing soil subgrades in Kentucky since the mid-1980’s. Although short-term observations showed that lime was successful in improving subgrade strengths, a need existed to assess long-term strengths and benefits of lime stabilization. Questions concerning long-term bearing strength, longevity, and durability are addressed in a research study (1), funded by the Kentucky Transportation Cabinet and Federal Highway Administra-tion and conducted by the Kentucky Transportation Center, University of Kentucky.
In this study (1), field and laboratory studies were performed at seven flexible pavement sections constructed on soil subgrades mixed with hydrated lime. Percentage of hydrated lime treatment ranged from 4 to 6 percent by dry weight. At the time of the study, the ages of the pavement sections ranged from about 8 to 13 years. Relative bearing strengths of untreated and lime stabilized clayey subgrades are compared using the results of in situ CBR tests. At individual study sections, average values of in situ CBR tested at the top of the lime treated subgrades were some 8 to 34 times greater than the average in situ CBR values measured at the top of the untreated subgrades located directly below the lime sta-bilized layer. These differences show that the lime stabilized soil has maintained strength after 8 to 13 years of service as pavement subgrades on these roadways. Additionally, the lime changed the soil properties by significantly reducing the clay content of the soil (particles finer than 0.002 mm).
As shown by moisture content data, a “soft” layer of soil frequently exists at the top of un-treated subgrades that does not occur at the top of lime stabilized subgrades. The most economical and effective means of mitigating the effects of the soft zone is to use chemical stabilization, such as lime. Using lime stabilization is a good, durable and economical tech-nique for improving clayey subgrade strength and pavement performance.
Carmeuse Lime & Stone 3600 Neville Road Pittsburgh, Pa. 15225 412-777-0700
Technical Report No. 5
This information is intended for the use of personnel competent to evaluate it. The characteristics of the soil tested are not neces-sarily the same as the character-istics of other soils. Any technical information furnished is believed to be reliable, but Carmeuse makes no warranty. The implied warranties of merchantability and fitness for a particular purpose are expressly disclaimed. The user of this information releases Carmeuse from any liability aris-ing out of the use of this informa-tion.
The contents of this report reflect the views of the authors, who are responsible for the facts and accuracy of the data herein. The contents do not necessarily re-flect the official views or policies of the University of Kentucky, Kentucky Transportation Cabinet, nor the Federal Highway Admini-stration. This report does not constitute a standard, specifica-tion, or regulation.
3
INTRODUCTION
Most pavements in Kentucky are constructed on fine-grained clays and silts. The majority of highway subgrades are constructed on compacted clays. Statistically, some 85 percent of Kentucky soils are comprised of clays, which usu-ally have sizeable bearing strengths when first compacted. As shown by past research (2, 3), CBR strengths of soil sub-grades immediately after compaction, typically, range from 15 to 40. However, shortly after the pavement is placed and the clayey subgrade is exposed to moisture, CBR strengths decrease to a range of about 1 to 6 (1, 3). Obvi-ously, low CBR strengths can affect pavement perform-ances. Subgrade strength is an important factor that deter-mines pavement thickness and performance. Past studies show that low bearing strengths can cause premature fail-ures of pavements and point to the need to stabilize soil subgrades (2, 4, 5, 6, and 7).
Pavement subgrades must be stable during construction and perform throughout the design life of the pavement. Usually the subgrade is the weakest member of the pave-ment structure and is an important factor influencing pave-ment performance. The subgrade must be sufficiently sta-ble during construction to prevent rutting, pushing, and shoving. The subgrade must also provide a sound “working platform” so that the various pavement layers can be effec-tively and efficiently placed and compacted. Even when construction of the pavement is successful, the bearing strength decreases significantly with the passage of time and exposure to moisture; this adversely affects the behav-ior of the pavement. The subgrade must possess sufficient strength so that large permanent deformations do not accu-mulate over a long period of time and affect the perform-ance of the pavement.
Although compaction of clayey soils increases shear strength, compaction alone will not, necessarily, insure that a subgrade will act properly throughout pavement life. Sub-grades are subjected to the infiltration of water from sur-face runoff and subsurface seepage. Compacted clayey subgrades absorb water, swell, and lose strength. More-over, use of drainage measures, although desirable, will not prevent the development of this situation because the sub-grade will be exposed to water during some period of the pavement's life. Although water may drain outward from a drainage layer, it also drains downward where it can be ab-sorbed by the top of the soil subgrade. Therefore, compac-tion and drainage measures used alone will not totally in-sure good performance of clayey subgrades and pave-ments.
Before the mid-1980’s, the primary method of stabilizing soil subgrades in Kentucky was by mechanical means, such as compaction. Other methods included undercutting the soft soil and backfilling with aggregate or mixing aggregate into the subgrade. Laboratory studies and field experience show that the latter technique was largely ineffective (8) in clayey soils. Only a few chemically treated subgrade stabili-zation projects had been constructed in Kentucky prior to the mid-1980’s. Little well-documented information existed regarding the performances of those past projects.
In the mid-1980’s, a major program was initiated in Ken-tucky to stabilize highway soil subgrades with chemical ad-mixtures, such as lime (9). This alternative form of sub-grade stabilization was based on a recommendation (3, 10) that the low bearing strengths of subgrade soils in Kentucky needed improvement to avoid pavement failures during and after construction and that using chemical admixtures pro-vided a good means of achieving this purpose. Although more than 100 roadway sections have been treated chemi-cally in Kentucky since that time, there remained some lin-gering questions concerning long-term durability, bearing strengths, and pavement performances. A study (1) was undertaken as a effort to address those questions.
OBJECTIVES AND SCOPE OF STUDY
Many immediate benefits, as well-documented in the litera-ture, are obtained from subgrade stabilization, especially chemical admixture stabilization. For example, by improv-ing the bearing strength and stiffness of the subgrade, a good working platform is established for supporting con-struction traffic and for compacting paving materials. Sub-grade soils that have poor engineering properties may be used effectively in-place when chemical stabilization is used. Therefore, construction can continue efficiently. From a long-term aspect, the use of chemical stabilization appears to increase the long-term cohesive strength of the subgrade. This large cohesive strength of the subgrade tends to resist large excess pore pressures in the subgrade caused by heavy vehicular traffic stresses. This aids in pre-venting the creation of voids below the pavement.
Although short-term benefits of subgrade stabilization were readily apparent in earlier studies (2 and 3), more informa-tion regarding long-term benefits was desirable. Research (1) was initiated as an effort to organize and provide well-documented case studies so that the long-term benefits of lime stabilization could be evaluated.
4
ROADWAY TEST SECTIONS AND CORING TECHNIQUES
Seven roadway flexible pavement sections constructed on soil subgrades that had been mixed with hydrated lime were selected for detailed field and laboratory studies. At the time of the study, the ages of the roadway sections and lime-treated subgrades ranged from about 8 to 13 years.
Core holes were drilled approximately every 0.25 mile within each roadway test section. Special techniques were devel-oped to avoid using water during coring. By using com-pressed air, instead of water, as the drilling fluid, soaking and softening of the tops of the lime stabilized and un-treated subgrades at each hole were avoided. The tops of the lime stabilized and untreated subgrades as they existed in their natural setting were preserved and undisturbed. At each site within the study section, a cluster of 4 or 5 core holes were drilled so that the exact depths of the tops of the lime stabilized and untreated subgrades were located prior to performing in situ CBR testing. Standard Penetration Tests and thin-wall sampling tubes were used to recover samples for laboratory testing and obtaining thickness measurements of pavement components. More details of the coring techniques and sampling program are given in the Appendix.
FIELD AND LABORATORY TEST METHODS
A variety of laboratory tests were performed, as summarized in Table 1. Tests were performed in accordance with AASHTO (American Association of State Highway and Trans-portation Officials) (11) or ASTM (American Society for Test-ing Materials) procedures. Laboratory tests included mois-ture content, liquid limit, plastic limit, specific gravity, parti-cle-size analysis, and Unified and AASHTO soil classifica-tions. Generally, index tests (liquid limit, plastic limit, spe-cific gravity, and particle size analysis) were performed on the split-spoon samples and thin-walled specimens. Proce-dures of AASHTO T 193 were generally adapted in perform-ing in situ CBR tests. However, procedures were developed for performing CBR testing in the field. Equipment set up and discussion of the approach used to perform field CBR tests of the lime stabilized and untreated subgrades are given in the Appendix.
RESULTS AND ANALYSIS
In Situ CBR
Long-term in situ values of CBR of the untreated and lime stabilized subgrades at the seven highway sites are summa-rized and compared in Table 2. Average values of un-treated and lime stabilized in situ CBR values at each site are shown and compared in Figure 1.
Average values of in situ CBR of the untreated subgrades at individual sites ranged from only 2 to 6. The overall aver-age was 3.2. In contrast, average long-term values of in situ CBR of the lime stabilized subgrades ranged from 26 to 101. The overall average was 53. At the individual sites, the average in situ CBR values of the lime stabilized subgrades ranged from about 8 to 34 times greater than the average in situ values of CBR of the untreated subgrade located directly below the lime stabilized layer. The overall average of in situ CBR of the lime stabilized layers was about 17 times greater than the in situ CBR of the untreated sub-grades.
TABLE 1 Field and laboratory geotechnical tests
*Field setup devised; AASHO T 193 followed during testing.
Type of Test Test Designation
CBR (Laboratory) In Situ CBR (Field)
AASHTO T 193 *
Standard Penetration Test (SPT) AASHTO T 206
Thin-Walled Sampling AASHTO T 207
Moisture Content AASHTO T 265
Liquid Limit AASHTO T 89 Plastic Limit AASHTO T 90 Specific Gravity AASHTO T 100 Particle Size Analysis AASHTO T 88
Soil Classification: AASHTO System Unified System
AASHTO M 145 ASTM D2487
FIGURE 1. Comparison of in situ CBR values of untreated and lime stabilized subgrade
5
Loc
atio
n (U
ntre
ated
Soi
l) In
Situ
CBR
Cou
nty
Rou
te
Num
ber
Dat
e Bu
ilt
Age
at
the
Tim
e of
Stu
dy
(Yea
rs)
Mile
post
Pl
astic
ity
Inde
x (P
I) (%
)
AASH
TO S
oil
Cla
ssifi
catio
n
Stab
ilize
d La
yer
Thic
knes
s
(Inch
es)
Perc
ent
of
Hyd
rate
d Li
me
(Unt
reat
ed
(Tre
ated
)
Ande
rson
U
S127
19
91
11
9.1
*
* 12
4
1.3
19.0
An
ders
on
US1
27
1991
11
9
.5
12.7
A-
6 12
4
2.5
29.0
An
ders
on
US1
27
1991
11
9
.8
21.5
A-7-
6 12
4
2.5
55.0
An
ders
on
US1
27
1991
11
10
.4
5.8
A-
4 12
4
1.5
60.0
Av
erag
e =
2.
0
4
0.5
Boyl
e U
S127
19
90
12
3.40
NBL
*
* 12
5
2.4
40.8
Bo
yle
US1
27
1990
12
3.
65 N
BL
32.6
A-
7-6
12
5 3.
7 29
.3
Boyl
e U
S127
19
90
12
4.1
NBL
*
* 8
5 1.
0 16
.5
Boyl
e U
S127
19
90
12
4.3
NBL
40
.9
A-7-
5 8
5 2.
1 64
.3
Boyl
e U
S127
19
90
12
4.6
NBL
24
.1
A-7-
6 8
5 2.
7 50
.0
A
vera
ge =
2.4
40.
2
Faye
tte
US
125
19
94
8 17
.14
(NBL
) 44
.7
A-7-
5 8
5 6.
9 9
Faye
tte
US
125
19
94
8 17
.50
(NBL
) *
* 8
5 1.
2 26
Fa
yette
U
S 1
25
1994
8
17.9
0 (N
BL)
19.2
A-
7-6
8 5
4.5
12
Faye
tte
US
125
19
94
8 18
.10
(SBL
) *
* 8
5 2.
5 32
Fa
yette
U
S 1
25
1994
8
18.2
5 (S
BL)
* *
10
5 3.
0 33
Fa
yette
U
S 1
25
1994
8
18.3
0 (S
BL)
16.0
A-
7-6
10
5 0.
5 78
Av
erag
e =
3
.1
31.
7
* N
ot d
eter
min
ed.
TABL
E 2.
In
situ
CBR
val
ues
of u
ntre
ated
and
trea
ted
subg
rade
s an
d ot
her a
ttrib
utes
of h
ighw
ay te
st s
ectio
ns
6
Loca
tion
(Unt
reat
ed S
oil)
Cou
nty
Rou
te
Num
ber
Dat
e Bu
ilt
Age
at
the
Tim
e of
Stu
dy
(Yea
rs)
Mile
post
Pl
astic
ity
Inde
x (P
I)(%
)
AAS
HTO
Soi
l C
lass
ifica
tion
Stab
ilized
La
yer
Thic
knes
s (In
ches
)
Perc
ent
of
Hyd
rate
d Li
me
In S
itu
CB
R
(Unt
reat
ed)
In S
itu
CBR
(T
reat
ed)
Har
din
US
62
1989
13
12
.00
(EBL
) 10
.9
A-6
11
6 5.
0 10
8 H
ardi
n U
S 62
19
89
13
12.2
0 (W
BL)
* *
8 6
4.8
51
Har
din
US
62
1989
13
12
.45
(WBL
) *
* 8
6 2.
5 15
7 H
ardi
n U
S 62
19
89
13
12.8
0 (W
BL)
* *
10
6 2.
0 9
5 H
ardi
n U
S 62
19
89
13
12.9
0 (E
BL)
* *
8 6
1.4
49
Har
din
US
62
1989
13
13
.70
(WBL
) *
* 16
6
1.9
103
Har
din
US
62
1989
13
13
.75
(EBL
) 32
.6
A
-7-6
10
6
3.5
153
Har
din
US
62
1989
13
13
.75
(WBL
) *
*
8 6
3.0
95
Ave
rage
=
3.0
1
01.4
O
wen
U
S 12
7 19
90
12
14.3
(NBL
) 18
.4
A-7
-6
8 5
5.2
30
Ow
en
US
127
1990
12
14
.7 (N
BL)
14.0
A-
6 5
5 3.
2 45
O
wen
U
S 12
7 19
90
12
15.1
(NB
L)
20.5
A-7-
6 5
5 2.
9 27
O
wen
U
S 12
7 19
90
12
15.3
(SBL
) *
* 8
5 0.
8 63
Ave
rage
=
3.0
4
1.3
She
lby
K
Y 55
19
92
10
8.15
17
.3
A-6
8 5
6.4
52.0
S
helb
y
KY
55
1992
10
8.
30
21.0
A
-7-6
8
5 4.
2 18
.8
Shel
by
KY 5
5 19
92
10
8.60
*
* 8
5 2.
7 17
.5
She
lby
K
Y 55
19
92
10
8.65
25
.6
A-7
-6
8 5
1.0
16.5
Sh
elby
KY
55
1992
10
8.
85
* *
5
1.1
24.5
Ave
rage
=
3.1
2
5.9
Trig
g U
S 68
19
93
9 21
.5
22.9
A
-7-6
12
5
1.9
46
Trig
g U
S 68
19
93
9 22
.1
* *
11
5 3.
0 45
Tr
igg
US
68
1993
9
22.7
14
.2
A-6
10
5 9.
2 18
4 Tr
igg
US
68
1993
9
23.0
19
.2
A-6
8 5
7.5
99
Trig
g U
S 68
19
93
9 23
.6
* *
11
5 8.
3 29
Tr
igg
US
68
1993
9
24.2
*
* 12
5
6.0
147
A
vera
ge =
6
.0
91.
7
*
Not
det
erm
ined
.
TABL
E 2
(Con
tinue
d).
In s
itu C
BR v
alue
s of
unt
reat
ed a
nd tr
eate
d su
bgra
des
and
othe
r attr
ibut
es o
f hig
hway
test
sec
tions
7
In terms of the 85th percentile test value (PTV) of CBR data collected at the seven sites (12, 13), the in situ CBR of the untreated and lime stabilized subgrades were about 1.8 and 27, respectively, as shown in Figure 2. The in situ CBR of the lime stabilized layer at the 85th PTV was about 15 times the CBR of the untreated layer.
At the 50th percentile test value, the in situ CBRs of the un-treated and lime treated subgrades were about 3.5 and 51, respectively.
Soil Classification
Initially, about 72 percent of the untreated subgrade soils in the hydrated lime-soil sections were classified according to the Unified Soil Classification System as CL, or clays of low to medium plasticity (Figure 3). About 12 % of the soils were classified as CH (fat clays of high plasticity) or MH (silty clay—liquid limit > than 50 %). Hence, about 84 % of the soils were clayey soils. The other soil types, or 16 %, were ML (silts), SM (sandy silts), or SC (sandy clay). Based on the AASHTO Classification System, the untreated soils were classified as A-4, A-6, A-7-5, or A-7-6. After treatment with lime, the subgrade soils were classified mainly as ML (75%), SM (22%), or MH (3%). The clayey soils were mainly transformed into silts and sandy silts.
Clay Fractions
The percentile test value is shown in Figure 4 as a function of the clay fraction of the lime treated and untreated soils. Clayey fraction is defined here as the percentage of parti-cles in the soil matrix that is finer than the 0.002 mm size. Soils containing large clayey fractions generally have very poor engineering properties.
Lime causes a significant reduction in the clay fraction of soils. By reducing the clayey fraction, engineering proper-ties, such as shear strength and bearing capacity, improve.
Based on tests performed on samples obtained from the lime-stabilized subgrades after 8 to 13 years, large reduc-tions in clay fractions of the subgrades at the study sites are very significant, as illustrated in Figure 4. At the 50th percentile test value, the clay fraction of the untreated soil is about 30 %. After treatment with lime, the clay fraction of the clays at the 50th percentile test value was reduced to 13%.
Moisture Contents
During field operations, moisture contents were obtained at all locations where in situ CBR tests were performed on tops of the lime stabilized and untreated subgrades. Mois-ture contents of the subgrades were also obtained at depths below the tops of the in situ CBR testing locations in
FIGURE 2. Percentile test value as a function of in situ CBR of the untreated and lime stabilized subgrade.
FIGURE 3. Comparison of soil classifications of untreated and lime stabilized subgrades
FIGURE 4. Percentile test value as a function of clay fraction.
8
the lime stabilized and untreated subgrades. Relative sampling positions of moisture content at a given pave-ment profile are depicted in Figure 5.
Untreated Subgrades
Moisture contents of untreated subgrades measured in previous research (1, 2, 3, 5), show that, oftentimes, a thin soft zone of soil exists in the top portion of untreated soil subgrades. To test this observation, moisture contents were determined at the tops of the untreated subgrades where in situ CBR tests were performed. Moisture con-tents at depths below the tops of the untreated subgrades were also determined.
As shown in Figure 6, moisture contents were compared as a function of the percentile test value. Between the 85th and 15th percentile test values, the moisture contents at the in situ CBR locations (tops of untreated subgrades) ranged from about 3.5 to 5.5 percent, respectively, higher than the moisture contents of samples obtained from loca-tions below the tops of the untreated subgrades.
Previous research (1, 3) has shown that field and labora-tory CBR values of Kentucky clayey soils, when first com-pacted (unsoaked), range from about 15 to 40. However, after soaking, CBR values of the same specimens after soaking decreased to values ranging from about 1 to 6. As shown in Tables 1 and 2, the average values of in situ CBR of the untreated subgrade layers (located below the lime stabilized subgrades) ranged from only 2 to 6. The larger moisture contents, and the measurements of small
FIGURE 5. Moisture content locations.
FIGURE 6. Percentile test value as a function of moisture contents measured at the tops and points below the tops of the untreated sub-grades
values of in situ CBR (2 to 6), generally show that a “soft zone” of soil developed at the tops of the untreated sub-grades.
Lime Stabilized Subgrades Moisture contents obtained at the very tops of the lime sta-bilized subgrades were compared to moisture contents measured at the tops of the untreated subgrades. Those locations represented points where in situ CBR tests were performed. The relationships, in the form of percentile test values as a function of the two different sets of moisture contents, are compared in Figure 7. The relationships are nearly identical.
9
SUMMARY AND CONCLUSIONS Seven flexible pavement roadway sections ranging in ages from 8 to13 years and constructed on lime stabilized soil subgrades were examined in detail to determine the long-term, in situ CBR strengths of lime stabilized layers and un-treated soil subgrades located below the lime stabilized layers. The following conclusions are made:
• Clay subgrades stabilized with lime have significantly long-term durability, longevity, and bearing strengths. The long-term values of in situ CBR of the lime stabi-lized clayey subgrades were much larger than the in situ CBR of the untreated layers located below the lime stabilized layers. The CBR values were 8 to 34 times greater. The overall average CBR for the lime stabilized soil was 17 times greater than the untreated layer soil.
• Lime transforms clays and fatty clays to silts and silty sands and vastly improves their engineering properties. This transformation is long lasting.
• A “soft zone” of soil frequently exists at the tops of un-treated subgrades. The soft zone was not observed on lime stabilized subgrades. One of the most economical means of mitigating the damaging effects of the soft zone is to use lime stabilization. Average values of in situ CBR at the tops of the untreated subgrades ranged from 2 to 6 while the average values of in situ values of CBR of the hydrated lime-soil subgrades ranged from 32 to 101. By mixing the soil subgrade with lime, the soft zone is relocated to a greater depth where the in-fluence of heavy wheel stresses diminishes.
• Using lime stabilization is a good, durable and economi-cal technique for improving clayey subgrade strength and pavement performance.
REFERENCES
1. Hopkins, T. C. (June 2002), Beckham, T. L., Sun, L., Ni, B., and Butcher, B., "Long-Term Benefits of Stabilizing Soil Subgrades," Research Report KTC-02-19/SPR-196-99-1F, University of Kentucky Transportation Center, College of Engineering, Lexington, Kentucky.
2. Hopkins, T. C. (1991). "Bearing Capacity Analyses of Pavements," Research Report KTC-91-8, University of Kentucky Transportation Center, College of Engineering, Lexington, Kentucky.
3. Hopkins, T. C. (1995), Beckham, T. L., and Hunsucker, D. Q. "Modification of Highway Soil Subgrades," Research Report KTC-94-11, University of Kentucky Transportation Center, College of Engineering, Lexington, Kentucky.
FIGURE 7. Percentile test value as a function of moisture content of treated and untreated subgrades.
Although in situ moisture contents at the tops of the lime stabilized subgrades are nearly identical (in terms of PTV) to the in situ moisture contents of the tops of the untreated subgrades, in situ CBR values at the tops of the lime stabi-lized subgrades are much larger than the in situ CBR values measured at the top of the untreated subgrades. As noted earlier (Tables 2 and 3; Figure 1), average values of in situ CBR measured at the tops of the lime stabilized subgrades of the study sites ranged from 26 to 101. The overall aver-age of the in situ CBR of the lime stabilized subgrade was 53 compared to 3.2 for the untreated layers. Basically, the top portion of the untreated layer decreased in CBR strength from the initial compaction stage while the CBR strength of the soil mixed with lime increased significantly and has remained very large with the passage of time. In both cases, the time of exposure of the lime stabilized layer to moisture and the time of exposure of the untreated layer to moisture located below the lime stabilized layer are the same. However, the CBR strength of the lime treated layer was much larger than the CBR strength of the untreated layer located below the lime stabilized layer.
The soft zone of soil did not develop at the top of the lime stabilized subgrades. By using lime stabilization, the dam-aging effect of the “soft zone” of soil on pavements is elimi-nated, or mitigated, because the soft zone is positioned at a lower level in the subgrade where the harmful effects of traffic stresses diminish.
10
4. Hopkins, T.C. (1994). "Case Studies of Flexible Pavement Failures during Construction," Proceedings, The 4th In-ternational Conference on the Bearing Capacity of Roads and Airfields, Vol.1, Minneapolis, Minnesota.
5. Hopkins, T. C., Beckham, T. L., Sun, L., (2006) "Characteristics and Engineering Properties of the Soft Soil Layer in Highway Soil Subgrades,” Research Report KTC-06-13/SPR 270-03-1F, University of Kentucky Transportation Center, College of Engineering, Lexington, Kentucky
6. Hopkins, T. C. and Slepak, M. E.; (1998). “Estimated Fac-tors of Safety of the AASHO Road Test Flexible Pave-ment Sections Based on Limiting Equilibrium Methods, ” Proceedings, Fifth International Conference on the Bear-ing Capacity of Roads, Railroads, and Airfields, Trond-heim, Norway.
7. Hopkins, T.C. (1994). "Minimum Bearing Strength of Soil Subgrades Required to Construct Flexible Pavements," Proceedings, The 4th International Conference on the Bearing Capacity of Roads and Airfields, Vol. 1, Minnea-polis, Minnesota.
8. Hopkins, T. C. and Beckham, T.L. (2000). “Influence of Clay Fraction and Moisture on the Behavior of Soil-Aggregate Mixtures,” Proceedings of the Fifth Interna-tional Symposium on Unbound Aggregates in Roads, UNBAR 5, University of Nottingham, United Kingdom, A. A. Balkema/Rotterdam/ Brookfield.
9. Hopkins, T.C. and Allen, D. L. (1986). "Lime Stabilization of Pavement Subgrade Soils of Section AA-19 of the Al-exandria-Ashland Highway," Research Report 86-24, University of Kentucky Transportation Center, College of Engineering, Lexington, Kentucky.
10. Hopkins, T.C. Hunsucker, D. and Sharpe, G.W. (1988). "Highway Field Trials of Chemically Stabilized Soil Sub-grades” Proceedings of the Ohio River Valley Soils Seminar XIX, Lexington, Kentucky.
11. American Association of State Highway and Transporta-tion Officials (2000). Standard Specifications for Trans-portation Materials and Methods of Sampling and Test-ing, Part II-Tests, II-1015-1029, 20th edition, Washing-ton, D.C., USA.
12. Yoder, E.J. (1969), "Selection of Soil Strength Values for the Design of Flexible Pavements,” Highway Research Board, Highway Research Record 276.
13. Yoder, E.J. Witczak, M.W.; (1975), “Principles of Pave-ment Design,” John Wiley & Sons, Inc. New York, New York.
Appendix — Coring Technique and Field Testing Procedures
Core holes were drilled approximately every 0.25 of a mile within each study section. Special coring techniques were developed to avoid using water. Compressed air, instead of water, was used to advance the drill down to the top of the subgrade of each section. By using compressed air as the drilling fluid, soaking and softening of the top of the sub-grade at each hole was prevented. Hence, the subgrade as it exists in its natural setting was preserved and undis-turbed.
Typically, four holes were drilled at each location. The first core hole was drilled to measure the thicknesses of the as-phalt, aggregate base, and stabilized subgrade layers of the flexible pavement section. After removing and measuring the thickness of the asphalt core, the base aggregate was removed by hand to expose the top of the stabilized sub-grade (or in some cases the top of the untreated subgrade). The depth, or thickness, of the aggregate base was noted. Then a standard penetration test (SPT) was performed on the stabilized subgrade to obtain a split spoon specimen of the stabilized subgrade. Phenolphthalein was applied along the length of the split spoon specimen to determine the portion of the specimen that had been stabilized. The stabi-lized portion of the core turns to a reddish color when phe-nolphthalein is applied. Thickness of the stabilized sub-grade was noted.
At the same location, a second hole was drilled. After au-guring through the flexible pavement and aggregate base and exposing the top of the stabilized subgrade, an in situ CBR test was performed, as shown in Figure A-1. The in situ testing arrangement is shown in Figure A-2. The equip-ment was designed to mount on the back of a drill rig. It consisted of a crank for advancing the CBR plunger, a dial gage for measuring delection, and a proven ring with a dial gage for measuring the applied force. The weight of the drill rig is used as a reactionary force. CBR calculations are per-formed using equations given in AASHTO T 193.
After completing the CBR test, a moisture content specimen was obtained at the top of the stabilized subgrade. Augur-ing continued down through the stabilized subgrade to the top of the untreated subgrade below the stabilized layer. A second in situ CBR test was performed on the untreated subgrade and a moisture content was obtained at the top of the untreated subgrade. The SPT and in situ tests were performed according to test designations listed in Table 1. A third hole was advanced through the asphalt layer and aggregate base and a thin-walled undisturbed sample or a core specimen was obtained of the stabilized subgrade. Thin-walled tube samples of the stabilized subgrades could
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not be obtained in many cases. In such cases, core speci-mens were obtained. A fourth hole was augured down through the asphalt layer, the aggregate layer, and stabi-lized layers to expose the untreated layer below the stabi-lized layer. A thin-walled tube sample was obtained of the nonstabilized subgrade for laboratory testing.
FIGURE A-1 Field testing sequence and sample recovery
FIGURE A-2. Field CBR testing equipment.