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directly below the cap to a depth of 6 ft below the base of the displacement curves for Cap 3 during Test 5 after treatment
pile cap. The fill was flush with the edge of the pile cap on one with flowable fill compared to Cap 1 during Test 2 in untreated
side but extended 5 ft beyond the pile cap on the other side to virgin soil. In both cases, the soil adjacent to the pile cap was
evaluate the effect of improvement width in front of the pile excavated. The ultimate resistance for the cap with flowable fill
groups. The flowable fill zone was originally intended to be was once again about 30 kips greater than that for the pile cap
deeper, but the depth had to be reduced to prevent failure of in untreated clay. This represents an increase of about 10% rel-
the excavation in the weak clay layers. Although the flowable ative to the pile cap in untreated clay. These results indicate
fill was designed to have an unconfined compressive strength that excavating the weak clay and replacing it with the weakly
of about 100 psi, only one of the six test cylinders was intact cemented sand provided only minimal increases in lateral
enough to be tested and that test cylinder only had a compres- resistance.
sive strength of 30 psi. Therefore, the flowable fill was proba- Figure 3-34 provides a comparison of the lateral load-
bly closer to a weakly cemented sand at a medium relative displacement curves for Cap 3 for Test 10 where the flowable
density. For Test 3, the two pile groups were pushed apart but fill extended to the top of the pile cap and for Test 12 where a
for Test 5 the pile groups were pulled together. 1-ft wide slot was excavated to the base of the pile cap imme-
Because of the lower than expected compressive strength of diately adjacent to the cap. The flowable fill wall increased the
the original flowable fill zone, a second set of lateral load tests lateral resistance at a displacement of 1.85 in. by about 150 kips.
was subsequently performed after constructing a flowable fill This represents an increase in lateral resistance of about 50%
wall adjacent to the pile cap. This technique would represent with relatively little cost or effort.
an approach for improving lateral pile group resistance after Figure 3-35 provides a comparison of the lateral load-dis-
construction. Plan and profile drawings for this case are pro- placement curve for Cap 3 after excavation of the slot rela-
vided in Figure 3-31. The flowable fill zone was only 6 ft deep, tive to the curve for Cap 1 in untreated clay after excavation
12 ft wide, and extended 6 ft in front of the pile cap. The flow- adjacent to the cap. The load-displacement curves for both
able fill was designed to have a compressive strength of 150 psi; cases are relatively comparable, suggesting that the increase
however, the average of four test cylinders was 137 psi. For Test in resistance was achieved when the pile cap impacted the
10, the pile groups were pushed apart but for Test 12 the pile flowable fill wall and caused it to move into the surround-
groups were pulled together. ing ground. As the wall moved laterally, both passive force
Some concern has been expressed about long-term strength on the back of the wall and adhesive resistance on the side of
loss of flowable fill in saturated conditions with groundwater the wall could produce increased lateral resistance. When the
flow. Therefore, three flowable fill cylinders were kept in a fog slot was excavated next to the cap, the cap did not impact the
room and tested 700 days after placement. The test results for wall and the resistance was about the same as that for the cap
these cylinders were very consistent and yielded an average in untreated clay.
compressive strength of 57 psi, which represents a 56% The load test results for the flowable fill wall are very simi-
decrease in strength over 2 years' time. Visual observations of lar to those obtained for the soil mixed wall and suggest that
the test cylinders did indicate that indeed some of the cemen- the mechanism of increased resistance is produced by passive
titious material had leached out. Leaching was observed as force and adhesive shear on the side walls as the wall is pushed
white streaks on the outside of the samples. The leaching into the surrounding soil rather than by increased lateral pile-
soil resistance. The results also suggest that the treated zone
occurred because water was able to flow into the flowable
may only need to have an unconfined compressive strength
fill. If the flowable fill had higher cement content the leaching
of 140 psi to effectively behave as a "rigid wall" in developing
would have been reduced. Also, if the water did not flow over
increased lateral resistance. To produce a more readable report,
the flowable fill, the leaching would have been reduced or
additional plots, similar to those presented for the pile group
possibly eliminated.
in virgin clay are not presented here but are available in Miner
(2009, Appendix 3).
Load Test Results
Figure 3-32 shows the load-displacement curves for Cap 3 3.10 Pile Group Load Tests
during Test 3 after treatment with flowable fill compared to Involving Excavation and
Test 1, Cap 2 in untreated virgin soil with soil to the top of the Replacement
pile cap. In these tests, the pile caps were both in contact with
Excavation and Replacement
the adjacent soil. Both curves have the same general hyperbolic
with Compacted Fill
shape; however, the flowable fill treatment increased the resis-
tance by about 20 to 30 kips or about 10% relative to the pile Plan and profile drawings showing the layout of Pile Cap
cap in untreated soil. Figure 3-33 provides a plot of the load- 4 with compacted fill are provided in Figure 3-36. Tests on this
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Figure 3-31. Plan and profile views of Cap 3 (left) and Cap 2 (right) during Tests 10 and 12. Test 10 performed with flowable fill adjacent to
pile cap and Test 12 performed after excavation of flowable fill adjacent to cap.
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350
300
250
Load (kips)
200
150
100
Weaker flowable fill beneath the cap with passive T3 Cap 3
50
untreated clay with passive T1 Cap 2
0
0 0.5 1 1.5 2
Displacement (in)
Figure 3-32. Load vs displacement results comparing Test 3 on
Cap 3 (weak flowable fill below the cap) to Test 1 on Cap 2
(untreated clay).
350
300
250
Load (kips)
200
150
100
untreated clay w / o passive T2 Cap 1
50
Weaker flowable fill beneath the cap w / o passive T5 Cap 3
0
-0.25 0.25 0.75 1.25 1.75 2.25
Displacem e nt (in)
Figure 3-33. Load vs displacement curves for Test 5 on Cap 3 (weak
flowable fill below the cap excavated to base of cap) to Test 2 on Cap 1
(untreated clay excavated to base of cap).
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450
400
350
300
Load (kips) 250
200
150
100
Flowable fill behind cap w/o passive T12 Cap 3
50
Flowable fill behind cap with passive T10 Cap 3
0
0 0.5 1 1.5 2
Displacement (in)
Figure 3-34. Load vs displacement results comparing Tests 12 and 10.
pile group were designed to determine the increased strength in-place dry density of 104.2 lb/ft3, which is 93.7% of the mod-
that could be provided by excavating the soft clay and replac- ified Proctor density (d max = 111 lbs/ft3). Plans originally called
ing it with compacted sand. Prior to pile driving, clay was exca- for excavation and replacement to greater depth; however, cav-
vated to a depth of 6.25 ft and replaced with compacted fill up ing of the soft clay precluded deeper excavation. When the piles
to the base of the pile cap. Clean concrete sand, meeting ASTM were installed, the ground heaved and, in order to maintain the
C-33 specifications, was used as the backfill material. The sand correct pile cap thickness, approximately 0.75 ft of backfill had
was compacted in 6- to 8-in. lifts using a hydraulic plate com- to be removed, leaving approximately 3 ft of sand under the
pactor attached to the end of a trackhoe. Based on nuclear den- cap. The sand fill extended 5 ft beyond the cap face on one side
sity measurements, the sand was compacted to an average to evaluate the increased pile-soil resistance from extending the
Figure 3-35. Load vs displacement results comparing Test 2 on Cap 1 to
Test 12 on Cap 3.
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Figure 3-36. Plan and profile views of Pile Caps 3 and 4 after excavation and replacement with compacted fill around Pile Cap 4 and
placement of flowable fill under Pile Cap 3.
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44
sand fill. Lateral load tests were performed in both directions. Greater improvement could potentially have been achieved
Comparison with the pile caps in Tests 1 and 2 allow a deter- if the compacted fill could have extended deeper; however, this
mination of the increased resistance for sand fill. would have required flatter excavation slopes to prevent cav-
ing and more backfill material, which would increase the cost.
Finite element studies conducted by Weaver and Chitoori
Test Results for Compacted Fill
(2007) suggest that most of the benefit from compacted fill
A comparison of the load-displacement curves for Tests 1 around a pile occurs for fill materials extending five pile dia-
and 5 is provided in Figure 3-37. Test 1 involves the pile cap meters below the ground surface based on FEM analysis. In this
in untreated clay; compacted sand was placed directly below case, the fill extended about three pile diameters.
the pile cap for Test 5. The comparison shows an increase in Figure 3-39 provides a comparison of the load-displacement
lateral resistance of about 23 kips at a displacement of 1.5 in. curves for Test 3 and Test 4. The only difference between the
resulting from placing compacted fill directly below the pile two tests is that for Test 4 sand was compacted adjacent to the
cap. This represents an 8% increase in resistance relative to pile cap extending 5 ft beyond the cap. Therefore, the differ-
the total resistance from soil-pile interaction and passive ence between the two tests represents the passive force that the
force or a 10% increase in resistance relative to soil-pile resis- 5-ft wide and 2.5-ft thick layer of sand produced. A compari-
tance alone. son between the load-displacement curves for Tests 3 and 4 at
Figure 3-38 provides a comparison of the load-displacement the greatest displacements indicates that the ultimate passive
curves for Test 3 relative to Test 2. In contrast with Test 5 where force with the sand backfill was approximately 32 kips. This
the compacted fill stopped at the end of the pile cap, the com- passive force is actually less than the 50-kip passive force mea-
pacted fill extends 5 ft beyond the end of the cap for Test 3. For sured when the native clay was left in place adjacent to the pile
both Tests 2 and 3 the soil adjacent to the pile cap was exca- cap face in Test 1, as discussed previously. This decrease in pas-
vated so no passive resistance was present in either test. A com- sive force occurs because the native clay in the upper 2.5 ft of
parison of the two curves indicates that the compacted fill the profile is desiccated and relatively strong. However, if the
increased the lateral soil-pile resistance by about 40 kips. As clay in the upper 2.5 ft of the profile were softer, excavation and
expected, extending the compacted fill 5 ft beyond the cap replacement with compacted sand could have increased the
increased the lateral resistance; however, the increase was rela- passive force. For example, if the clay surface layer had an
tively small. The increased resistance represents an increase of undrained shear strength of only 500 psf, the passive force in
18% relative to a comparable pile group in untreated clay. This the clay would only have been about 25 kips.
increase in lateral resistance can only be attributed to increased
soil-pile resistance because there was no soil adjacent to the pile
Rammed Aggregate Pier Construction
cap. The increase of 18% is comparable to results reported by
Brown et al. (1986, 1987) when a stiff clay was replaced with Rammed aggregate piers (RAPs) are a shallow alternative
compacted sand at a relative density of 50%. to deep foundations. They create a dense gravel column that
350
300
250
Load (kips)
200
150
100
Untreated Clay T1 Cap 1
50 Compacted Sand Under Cap T5 Cap 4
0
-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Displacement (in)
Figure 3-37. Load displacement comparison of Test 1 with
Test 5 (shifted to the right 0.4 in.).
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350
300
250
Load (kips)
200
150
100
Compacted Sand Beyond Cap T3 Cap 4
50 Untreated Clay After Excavation T2 Cap 1
0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Displacement (in)
Figure 3-38. Comparison of load-displacement curves for Pile
Cap 4 with compacted sand extending 5 ft beyond the cap
and Pile Cap 1 in native clay without soil adjacent to cap.
reinforces the surrounding soil. In addition, they increase 13-pier configuration consisted of 4 piers next to the cap,
the normal stress in the surrounding soil and compact the 5 piers in the middle row, and 4 piers in the row farthest from
surrounding soil if it is cohesionless. When testing was com- the cap. The row farthest from the cap was installed first and
plete on the compacted fill, 30-in. diameter geopiers were the row closest to the cap was installed last. Each column
installed in a grid pattern south of Pile Cap 4. Plan and pro- extended to a depth of 12.5 ft below the top of the pile cap.
file drawings are shown in Figure 3-40. The RAPs were Dynamic cone penetration tests were performed on three of
spaced at 36 in. center to center (c-c) in the direction of load- the columns and penetration resistance exceeded 40 blows
ing and 40 in. c-c in the direction transverse to loading. The per 1.75-in.
350
300
250
Load (kips)
200
150
100 Sand Adjacent to Cap T4 Cap 4
No Sand Adjacent to Cap T3 Cap 4
50
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Displacement (in)
Figure 3-39. Comparison of load-displacement curves for Pile
Cap 4 with and without sand adjacent to the pile cap.
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Figure 3-40. Plan and profile view of Pile Cap 4 showing the locations of the rammed aggregate piers and location of excavated zone for
subsequent test.
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350
300
250
Load (kips)
200
150
RAP to Cap Top T6 Cap 4
100 Untreated Clay to Cap Top T5 Cap 4
Untreated Clay T1 Cap 1
50
0
-0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Displacement (in)
Figure 3-41. Comparison of load-displacement curves for Pile
Cap 4 with RAP extending to the top of the cap relative to
tests on Cap 4 without RAP columns and Cap 1 in untreated
native clay.
RAPs are a relatively inexpensive means of retrofitting a pile columns (Test 5). At a displacement of about 1.3 in., the addi-
cap, but they are not designed specifically to increase lateral tion of the RAP columns increased the total lateral resistance
resistance. Nevertheless, comparison tests were performed to by about 40 kips. This represents an increase of about 15% rel-
explore the potential for increasing lateral resistance using this ative to the cap without the RAP columns. Figure 3-42 plots
approach. the load-displacement curves for Cap 4 after treatment with
RAP columns before (Test 6) and after excavation (Test 7) of
the soil adjacent to the pile cap. Because of reloading effects, the
Test Results for Rammed Aggregate Piers
curves for Test 7 at small displacements are not particularly
Figure 3-41 provides a comparison of the lateral load- meaningful; however, at larger displacements they appear to be
displacement curves for Cap 4 after treatment with RAP reasonable based on comparison with similar tests. The differ-
columns (Test 6) in comparison with the same cap without the ence between the curves for Tests 6 and 7 would represent the
350
300
250
Load (kips)
200
150
100
RAP to Cap Top T6 Cap 4
50 RAP to Cap Base T7 Cap 4
Untreated Excavated to Cap Base T2 Cap 1
0
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Displacement (in)
Figure 3-42. Comparison of load-displacement curves for Pile
Cap 4 after RAP treatment with and without soil adjacent to the
pile cap.
