Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 5
5
CHAPTER 2
Available Ground Improvement
Case Histories and Approaches
Publications on ground improvement methods and their arrangement with a 0.75-m center-to-center (3D) spacing in
applications to foundation design are extensive. However, engi- both directions. The original clay profile consisted of stiff, over-
neering practice typically has not used soil improvement tech- consolidated clay with an undrained shear strength of about
niques in combination with deep foundations to increase the 1150 psf (57 kPa) at the ground surface, which increased to
lateral resistance of bridge foundations. If soil improvement about 3000 psf (150 kPa) at a depth of 18 ft (5.5 m) below
has been undertaken, it has generally been with the goal of pre- the ground. Approximately 9 ft of the clay was excavated and
venting liquefaction or increasing soil resistance so that deep replaced with a relatively uniform clean sand compacted to a
foundations were not required. Three excellent publications dry unit weight (d) of 98 pcf (15.4 kN/m3), which is a relative
that summarize the state of the art in soil improvement are density of about 50%. A direct shear test indicated a friction
provided by Mitchell (1981), Terashi and Juran (2000), and angle of 38.5°, but back-calculated friction angles using LPILE
ASCE (1997). Based on a review of methods documented in (Reese, Wang, Isenhower et al., 2004a) suggest a friction angle
these and other publications, specific improvement methods of around 50°. A plot showing the total load versus deflection
or technologies appropriate for applications to improve the lat- curves for the pile group in both clay and sand is presented in
eral resistance of soils (stiffness and strength) associated with Figure 2-1. At deflections less than about 20 mm, the lateral
pile foundations are summarized in Tables 2-1 and 2-2. resistance of the pile group in clay is about the same as that in
A review of the literature indicates that there are also a sand. However, at greater deflections, the lateral resistance of
number of case histories that provide some insight regarding the pile group in sand eventually exceeds that for the pile
the degree of increased lateral pile resistance that could be group in clay by over 28%, despite the fact that the clay was
obtained by employing soil improvement in concert with deep relatively stiff.
foundations. In connection with research studies, field load The pile group tested by Rollins et al. (2005) and Rollins,
tests have been performed on two pile groups where the native Snyder et al. (2010) consisted of 15 12.75-in. diameter steel
clay soil was excavated and replaced with compacted granu- pipe piles driven closed-ended to a depth of about 40 ft. The
lar soil. These studies were primarily undertaken to evaluate piles were driven in a 3 × 5 grouping with a center-to-center
group interaction factors under lateral loading. Brown et al. spacing of 4.17 ft (3.92D) in the direction of loading and 3.5 ft
(1987) conducted lateral load tests on a nine-pile group in sat- (3.29D) transverse to loading. The upper 2.5 m of clay in the
urated stiff clay. Later, Brown et al. (1988) excavated the clay, original soil profile had an undrained shear strength of about
compacted sand around the pile group, and repeated the lat- 900 psf. The pile group reacted against two 4-ft diameter drilled
eral load test. Rollins et al. (2005) performed cyclic lateral load shafts. Prior to the second set of tests, the upper 1 m of clay was
tests on a 15-pile group in medium-consistency clay. Rollins, excavated and replaced with compacted clean sand. In addi-
Snyder et al. (2010) excavated the clay, replaced it with clean tion, an extra 1.5 m of sand was compacted above the original
sand, and performed additional lateral pile group load tests. ground elevation so that the upper 2.5 of the profile consisted
Although these tests were not designed to evaluate the effect of clean sand compacted to 93% of the modified Proctor max-
of excavation and replacement on lateral pile group resistance, imum density. The load versus deflection curves for the pile
the test results can be compared to provide this information. group in clay and sand are compared in Figure 2-2.
The pile group tested by Brown et al. (1987, 1988) was a Because the clay strength was relatively soft, the lateral resist-
nine-pile group consisting of 0.25-m (10-in.) diameter steel ance of the pile group in sand was considerably higher than
pipe piles filled with grout. The piles were driven in a 3 × 3 that for the pile group in clay. Analyses using the computer
OCR for page 6
6
Table 2-1. Summary of soil improvement methods in loose cohesionless soils.
Method Advantages Disadvantages Strength Cost
Gain
Excavate
and Replace
Compacted ·Simple, widely used method ·May require excavation support or Low to mod. Low to mod.
Fill ·Equipment widely available dewatering
·Compaction can be measured
easily
Flowable Fill ·No special equipment or ·Must dispose of excavated material Mod. to high Mod. to high
personnel required
·No compaction necessary
In-Situ
Densification
Dynamic Densification to 25-ft depth from ·Decrease in improvement with depth Low Low
Compaction surface impact ·Produces vibration and noise
·Could produce downdrag in existing
piles
·Requires overhead clearance for
dropping weight
Vibro ·Relatively uniform improvement ·Could produce downdrag in existing Low Moderate
Compaction with depth piles
Stone ·Increased densification relative to ·Must install stone through entire layer to Low Moderate
Columns vibrocompaction treat loose sand at depth
·Increased shear resistance of ·Could produce downdrag in existing
reinforced soil mass piles
·Drainage provided by columns in
the event of liquefaction concerns
Stone ·Improved effectiveness with high ·Increased cost and logistical effort of Low Moderate
Columns fines content soils installing wick drains prior to stone
with Wick ·Does not produce spoils column treatment
Drains
Compaction ·Can be used for retrofit below a ·Less effective at shallow depths where Moderate Moderate
Grouting pile cap pressure is restricted
·Can treat zones of interest ·More difficult to evaluate improvement
without treating all soil above the for retrofit conditions below pile
zone
·Does not produce spoils
Soil Mixing
Deep Soil ·Mixing can occur to 60- to 80-ft ·Can decrease the strength of sensitive Mod. to high Mod. to high
Mixing depths clays
·Significant strength gain can be ·Produces spoils
achieved
· Can produce columns (3-ft dia.)
or wall panels at desired depths
Grouting
Permeation ·Can be used for retrofit below a ·Limited to very coarse sands Moderate Moderate
Grouting pile cap ·More difficult to evaluate improvement
·Can treat zones of interest for retrofit conditions below pile than in
without treating all soil above the areas around periphery of pile group
zone ·Uniformity of treatment is often difficult
·Does not produce spoils
Jet Grouting ·Low noise and vibration ·Creates spoil material High High
·Can treat soil under pile cap after ·Requires mobilization of specialized
construction equipment and personnel
·Can transform pile group into
equivalent pier for scour resistance
OCR for page 7
7
Table 2-2. Summary of soil improvement techniques in soft cohesive soils.
Method Advantages Disadvantages Strength Gain Cost
Excavate
and
Replace
Compacted ·Simple, widely used method ·Must dispose of excavated material Low to mod. Low to mod.
Fill ·Equipment widely available ·May require excavation support or
·Compaction can be easily measured dewatering
Flowable ·No special equipment or personnel May need to dispose of excavated Mod. to high Mod. to high
Fill required material
·No compaction necessary
Cement ·Strength can be increased by 3 to 5 ·Requires special care in mixing Mod. to high Mod. to high
Treated times the in-situ strength operations
Excavated ·Significant laboratory testing
Clay necessary to develop treatment plan
·Field strength gain usually much
less than laboratory because of less
efficient mixing
Lime ·Strength can be increased 3 to 5 times ·Requires special care in mixing Mod. to High Mod. to High
Treated the in-situ strength operations
Compacted ·Requires less cement to achieve same ·Significant laboratory testing
Clay strength as using cement alone necessary to develop treatment plan
·Field strength gain usually much
less than laboratory because of less
efficient mixing
·Difficult to treat soils with high
sulfate content
Rammed ·Can be constructed above or below Creates relatively little increase in Low Low
Aggregate water table density of surrounding soil
Piers ·Creates extremely dense gravel ·Columns provide no flexural
columns with high-friction angle resistance.
·Increases lateral pressure in ·Creates extremely dense high-
surrounding soil friction angle columns
·Increases shear resistance of the
reinforced soil mass
In-Situ Soil
Mixing
Vibro ·Increases shear resistance of the ·Can cause heave of surrounding Low Low
Replacement reinforced soil mass ground
Compaction ·Can be used for retrofit below a pile ·Less effective at shallow depths Moderate Low
Grouting cap where pressure is restricted
·Can treat zones of interest without ·Can decrease the strength of
treating all soil above the zone sensitive clays
·Does not produce spoils ·More difficult to evaluate
improvement for retrofit conditions
below pile
Deep Soil ·Mixing can occur to 60- to 80-ft depths ·Can decrease the strength of Mod. to high Mod. to high
Mixing ·Significant strength gain can be sensitive clays
achieved ·Produces spoils
·Can produce columns (3 ft dia.) or
wall panels at desired depths
Mass ·Mixing can occur in-situ to 10- to 15-ft ·Significant laboratory testing Mod. to high Mod. to high
Mixing depths necessary to develop treatment plan
·No need for excavation or ·Field strength gain usually much
recompaction less than laboratory because of less
·Strength increase of 3 to 5 times efficient mixing
original shear strength
Grouting
Jet Grouting ·In-situ treatment with flexibility to ·Creates spoil material High High
produce variety of geometries (columns ·Requires mobilization of
and panels) specialized equipment and personnel
·Flexibility to treat only zones of
interest
·Can treat soil under pile cap after
construction
·Can transform pile group into
equivalent pier for resistance during
scour events
OCR for page 8
8
900
800
700
600
Total Load (kN)
500
400
300
Sand
200 Clay
100
0
0 10 20 30 40 50 60 70
Displacement (mm)
Figure 2-1. Load vs deflection curves for nine-pile groups
in stiff clay and dense sand based on Brown et al.
(1987, 1988).
program GROUP (Reese, Wang, and Arrellaga et al., 2004b) 1.9-m (6.3-ft) wide anchor block. The block was originally
were successful in matching the measured response of the pile poured flush against an excavation into partially saturated stiff
groups in clay. However, for the pile groups in sand, successful clay and a lateral load test was performed. The clay was then
agreement with measured response generally required the excavated and replaced with a compacted sandy gravel backfill
use of friction angles that are somewhat higher than would and the test was repeated. Rollins, Gerber, and Kwon (2010)
normally be used in engineering practice. The potential evaluated the passive force provided by various soils against a
for improved resistance using excavation and replacement pile cap that was 1.1 m (3.67 ft) deep and 5.2 m (17 ft) wide.
increases as the clay becomes softer and compacted granular Tests were conducted on a silty sand at two densities and on
soil becomes denser. Increases of 60% are possible (Rollins, loose silty sand with a 1- to 2-m wide zone of dense compacted
Snyder et al., 2010). gravel immediately adjacent to the pile cap.
Two field test studies have evaluated the passive force on a The native clay in the tests performed by Mokwa and Dun-
pile cap as a function of soil type and density. Mokwa and can (2001) was partially saturated. Triaxial shear tests on the
Duncan (2001) performed tests on a 1.1-m (3.5-ft) deep and clay at the natural moisture content indicate that the cohesion
2000
1800
1600
Total Group Load (kN)
1400
1200
1000
800
Sand
600
Clay
400
200
0
0 20 40 60 80 100
Average Group Deflection (mm)
Figure 2-2. Load vs deflection curves for 15-pile group in
medium stiff clay and dense sand based on Rollins et al.
(2005) and Rollins, Snyder et al. (2010).
OCR for page 9
9
700
600
500
Total Load (kN)
400
300
200 Clay
Gravel
100
0
0 10 20 30 40 50
Displacement (mm)
Figure 2-3. Comparison of passive force provided by stiff,
partially saturated clay and compacted sandy gravel
against 1.1-m deep 1.9-m wide cap block.
was 1 ksf and that the friction angle ranged from 32° to 38°. Tests also were performed using the loose silty sand backfill
The clay was excavated to the base of the cap block and along with a well-compacted zone of sandy gravel adjacent
replaced by a compacted, sandy gravel. The sandy gravel to the pile cap. The compacted zones were 0.9 m (3 ft) and
(GW-GM in the Unified Soil Classification System) was com- 1.83 m (6 ft) thick. These tests indicate that compacting rela-
pacted to a relative density of approximately 80%. Triaxial tively thin zones (3 to 6 ft wide) of sandy gravel around a pile
shear tests indicate that the friction angle could range from a cap can significantly increase the passive resistance as illus-
low of 48° to a high of 52°. A comparison between the passive trated in Figure 2-5. In this case, replacing a 0.9 m (3 ft) zone
force-deflection curves for the clay and gravel is provided in of loose silty sand around the pile cap with compacted gravel
Figure 2-3. In this case, the lateral resistance provided by the increased the lateral resistance on the pile cap from an initial
stiff, partially saturated clay was considerably higher than that value of 70 kips to more than 180 kips, which is an increase of
for the gravel at the shallow depths involved. As a result, the lat- over 200%. Crack patterns from the tests, shown in Figure 2-6
eral resistance actually decreased substantially when the com- indicate that the compacted gravel zones increase the effective
pacted gravel was used in place of the clay. Duncan and Mokwa
(2001) concluded that the log-spiral method provided the best 400
estimate of the ultimate capacity and that the passive force- Dense Silty Sand
350 (98%)
deflection relationship could be estimated reasonably using a
Loose Silty Sand
hyperbolic curve. This conclusion was supported by analyses 300 (88%)
Passive Force (kips)
of additional large-scale tests by Rollins and Cole (2006) and
250
Cole and Rollins (2006).
Rollins, Gerber, and Kwon (2010) performed lateral load 200
tests on a pile cap supported by 12 0.324-m diameter pipe 150
piles. The piles provided sufficient vertical resistance so that
100
the full wall friction force could develop. Basic passive force-
deflection relationships were developed for two tests involv- 50
ing silty sand compacted at 88% and 98% of the modified
0
Proctor maximum unit weight as shown in Figure 2-4. The 0 0.5 1 1.5 2 2.5 3
increased compactive effort produced a considerable increase Deflection (inches)
in passive resistance. Preliminary analyses indicate that this Figure 2-4. Measured passive force vs deflection
behavior is predicted quite well using the Duncan and Mokwa relationships for two full-scale tests with silty
(2001) approach along with the soil properties measured in sand compacted to 88% and 98% of the modified
the field. Proctor maximum unit weight.
OCR for page 10
10
300 Direction of
Lateral Load Bf
Loose Silty Sand/6 ft
250
Gravel Backfill
Dense Sand
Pile Cap or
Loose Silty Sand/3 ft H
(D)
200 abutment
Gravel Backfill
Load (kips)
Loose Silty Sand
150 2 ft min.
100
Loose Sand (L)
50
0
0 1 2
Deflection (inches)
Figure 2-7. Layout of geometry for limited width
Figure 2-5. Passive force vs deflection curves for
dense backfill zone adjacent to pile cap or abutment.
loose silty sand against a 17-ft wide by 3.67-ft high
pilecap after excavation and replacement with 3-ft
and 6-ft zones of compacted gravel backfill against the friction angle of the looser surrounding sand (L ). Fig-
the cap. ure 2-7 shows the layout of the improved zone relative to the
pile cap or abutment and defines the basic parameters involved.
width of the pile cap and reduce the pressure on the loose silty To facilitate generalization, the width of the dense zone is nor-
sand behind it, thereby increasing passive resistance. Rollins malized by the height of the pile cap. In addition, the ultimate
and Nasr (2010) completed a follow-up study to develop a gen- passive resistance provided by the limited-width dense granu-
eralized equation to predict the increased passive resistance lar backfill (PLW) is normalized by the ultimate passive resist-
that could be obtained by constructing a narrow dense granu- ance for a homogeneous dense backfill (PFW) that fully encloses
lar zone adjacent to a pile cap or abutment surrounded by loose the passive failure surface. This ratio is defined as the passive
sand. This investigation followed the same pattern employed force ratio (PFR). PFR is plotted as a function of normalized
in this NCHRP study. The full-scale load tests reported by width (Bf /H) for a number of cases in Figure 2-8 where the
Rollins, Gerber et al. (2010) and Gerber et al. (2010) were used dense zone consisted of gravel. As the normalized width
to calibrate soil parameters within the finite element computer increases, the PFR increases. When Bf/H exceeds 1.0, the PFR
program PLAXIS. In addition, the computer model was veri- is normally greater than 60%, and the PFR increases as the fric-
fied against analytical solutions for computing passive force tion angle of the surrounding sand increases. Typically, the
(e.g., Duncan and Mokwa, 2001). Once the FEM analysis Bf/H ratio would need to be around 4.0 to fully enclose the fail-
model was calibrated, parametric studies were performed to ure surface; therefore, the narrow dense zone is relatively effec-
evaluate the influence of changes in wall height (H), the dense tive in mobilizing the majority of the total passive resistance.
zone width (Bf), the friction angle of the dense zone (D) and Based on the result from the FEM parametric study, Equation 1
2 ft x 2 ft Grid 2 ft x 2 ft Grid
10 ft x 17 ft x 3.67 ft Gravel Backfill 10 ft x 17 ft x 3.67 ft
Pile Cap Loose Silty Sand Pile Cap
(a) 3 ft sandy gravel zone plus loose silty sand backfill (b) 6 ft sandy gravel zone plus loose silty sand backfill
Figure 2-6. Plan view of crack patterns behind a pile cap after excavation and replacement of
loose silty sand with (a) a 3-ft and (b) a 6-ft zone of compacted sandy gravel behind the pile cap.
OCR for page 11
11
100%
90%
80%
70%
PFR = PLW/P FW Gravel
60%
(gravel)=39°
(sand)=36°
50%
(gravel)=39°
40%
(sand)=32°
30%
(gravel)=39°
(sand)=27.7°
20%
10%
0%
0 0.5 1 1.5 2 2.5
BF/H
Figure 2-8. Percentage of total passive resistance developed by a
narrow dense gravel zone adjacent to an abutment relative to that
for a homogenous dense gravel zone.
was developed to predict the PFR for a limited-width dense to treatment, correlations with SPT and CPT test results
granular backfill. (Kulhawy and Mayne, 1990) indicated that the sand had a
relative density of about 50%; however, after treatment, the
PFR = 3.1418 - 0.139 D - 0.033 D + 0.484 (B f H ) relative density was increased to between 85% and 100%.
- 0.043(B f H ) + 0.003 2 - 0.007 L (B f H )
2 L
(1) A comparison of the load versus deflection curves for the
CISS pile and the pile group before and after treatment but
At a test site on Treasure Island in San Francisco Bay, stone prior to blast liquefaction are shown in Figure 2-10. The lateral
column treatment was used to improve a 6-m-thick liquefi- resistance was increased by about 25% for the pile group and
able sand layer around two test foundations (Ashford, Rollins, about 33% for the CISS pile. Back-analyses using the computer
and Baez 2000a; Ashford, Rollins, Bradford et al., 2000b; and program LPILE indicate that the friction angle was increased
Weaver et al., 2005). One test foundation was a 0.6-m dia- from about 39° prior to treatment to about 48° after treatment.
meter cast-in-steel shell (CISS) shaft that reacted against a Prior to stone column treatment the sand layer liquefied
group of four 0.324-m diameter driven steel pipe piles. Lateral (excess pore pressure ratios of 100%) following blasting and
load tests were first performed on the test foundations prior to remained liquefied for at least 10 minutes. Settlement fol-
treatment, then comparable lateral load tests were performed lowing dissipation of excess pore pressures amounted to
after treatment for comparison. A high-speed hydraulic actu- about 12 in. A plot of the load-deflection curve for the CISS
ator was used to apply load. In addition to the conventional pile following blast-induced liquefaction is provided in Fig-
static lateral load tests, a pattern of small explosive charges was ure 2-11(a). The pre-blast load-deflection curve also is shown
detonated sequentially to produce a liquefied volume of soil for comparison. Following liquefaction, about 11 times more
within which the test foundations also could be laterally movement was required to develop the same lateral resist-
loaded to large displacement levels (9 in.). The layout of the ance as that prior to liquefaction. After treatment, high excess
test foundations is shown in Figure 2-9. The 0.9-m diameter pore pressure developed initially after the blasting but dissi-
stone columns were installed in a square pattern with a spac- pated within a few seconds, presumably due to the increased
ing of 2.5 m from center to center (see Figure 2-9) using the drainage provided by the stone columns. The load-deflection
dry, bottom feed method. The sand had a mean grain size of curve after blasting is presented in Figure 2-11(b) for compar-
about 0.2 mm with a fines content between 5% and 10% and ison and the stiffness of the curve is several hundred percent
was initially placed using hydraulic filling techniques. Prior higher in comparison to that prior to treatment.
OCR for page 12
12
Limit of Excavation
o o o
Limit of Densification
x x
o o
x x
x x
o o
x x
x x
o o
x x
x x
o o
x x
o o o
Scale: 1m
= 2 x 2 Pile Group x = Charges for first and second blast @ 3.5 m
o = Charges for second blast only @ 3.5 m
= 0.6 m CISS Pile = Stone Column, 0.9 m Diameter,
Installed to a depth of 6 m
Figure 2-9. Layout of test foundations, stone columns, and
explosive charges.
OCR for page 13
13
(a) Pile Group (b) CISS Pile
400 400
Post-Treatment Post-Treatment
300 300
Pre-Treatment Pre-Treatment
Load (kN)
Load (kN)
200 200
100 100
0 0
-100 -100
-20 -10 0 10 20 30 40 50 -20 -10 0 10 20 30 40 50
Displacement (mm) Displacement (mm)
Figure 2-10. Static load vs deflection curves before and after stone column treatment for (a) pile group and
(b) 0.6-m CISS pile.
700 700
(a) Before Stone Column Treatment (b) After Stone Column Treatment
600 600
Pre-Blast
500 Non-Liquefied 500 Post-Blast
Liquefied
400 400
Load (kN)
Load (kN)
300 300
200 200
100 100
0 0
-100 -100
-200 -200
-25 0 25 50 75 100 125 150 175 200 225 250 -25 0 25 50 75 100 125 150 175 200 225 250
Displacement (mm) Displacement (mm)
Figure 2-11. Pre- and post-blast cyclic load-deflection curves (a) before and (b) after stone column treatment for
a 0.6-m CISS pile.
