Clay Tests

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PILE SPACING EFFECTS ON LATERAL PILE GROUP BEHAVIOR: LOAD TESTS

Kyle M. Rollins1, Ryan J. Olsen

2, Jeffery J. Egbert

3,

Derek H. Jensen4, Kimball G. Olsen

5and Brian H. Garrett

6

ABSTRACT

To investigate group interaction effects as a function of pile spacing, full-scale cyclic lateral load

tests were performed on pile groups spaced at 3.3, 4.4 and 5.65 pile diameters in the direction of

loading with as many as five rows of piles. Group interaction effects decreased considerably as

pile spacing increased from 3.3 to 5.65 D. Lateral resistance was a function of row location in

the group, rather than location within a row. For a given deflection, the leading (first) row piles

carried the greatest load, while the second and third row piles carried successively smaller loads.

Fourth and fifth row piles carried about the same load as the third row piles. For a given load,

the maximum bending moments in the trailing row piles were greater than those in the lead row,

but these effects decreased as spacing increased. Cyclic loading reduced the peak load by about

15% after 15 cycles; however, distribution of load within the pile group was essentially the same

at the peak load. Gaps significantly reduced resistance for small deflections.

INTRODUCTION

The lateral load resistance of pile foundations is critically important in the design of

structures which may be subjected to earthquakes, high winds, wave action, and ship impacts.

Because of the high cost and logistical difficulty of conducting lateral load tests on pile groups,

relatively few full-scale load test results are available that show the distribution of load within a

1Prof.,Civ. & Env. Engrg. Dept., Brigham Young Univ., 368 CB, Provo, UT 84602, [email protected] Engr., Kleinfelder, Inc., 2825 East Myrtle St., Stockton, CA 95205, [email protected] Engr., Earthtec Testing and Engrg. Inc., 115 N. 1330 W., Orem, UT 84057,[email protected] Engr., U.S. Engrg. Laboratories, Inc., 814 Parkway Blvd., Broomall, PA, 19008, [email protected]

5Staff Engr., GeoEngineers, Inc., 8410 154th Ave NE, Redmond, WA 98052, [email protected]

6Staff Engr., PDA Engrg., 7644 S. State, Midvale, UT 84047, [email protected]

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pile group (Brown et al, 1987; Brown et al, 1988; Meimon et al, 1986; Rollins et al, 1998; Ruesta

and Townsend, 1997). Nevertheless, the results from these tests indicate that the average load

for a pile in a closely spaced group (3D spacing) will be substantially less than that for a single

isolated pile at the same deflection and that leading row piles in the group will carry significantly

higher loads than trailing row piles at the same deflection.

The piles in trailing rows are thought to exhibit less lateral resistance because of

interference (shadowing) with the failure surface of the row of piles in front of them. This

shadowing or group interaction effect is expected to become less significant as the spacing

between piles increases because there is less overlap between adjacent failure planes.

Unfortunately, there is currently significant variation in the recommendations from various

agencies and researchers as to the appropriate adjustment factors to account for this reduction in

resistance with variation in pile spacing (AASHTO, 2000; US Army, 1993; Reese and Van Impe,

2001)

In addition, there is uncertainty about whether the reduction factors for group interaction

developed from tests on pile groups with three or less rows will be appropriate for subsequent

rows in a large pile group or whether the reduction factors will continue to gradually decrease

with each additional trailing row as observed for the second and third rows. Recent centrifuge

test results in sands (McVay et al, 1998) suggest that reduction factors may stabilize for greater

numbers of rows; however, no test results are yet available for clays.

Finally, previous full-scale and centrifuge pile group tests have not always evaluated the

effect of cyclic loading on group reduction effects or on bending moment-load or deflection-load

relationships. These effects are important to understand for events such as earthquakes which

produce multiple cycles of lateral load.

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To improve our understanding of pile group behavior as a function of pile spacing, a

series of full-scale cyclic lateral load tests were performed on three pile groups at spacings of

3.3, 4.4 and 5.65 pile diameters (D) and with up to five rows of piles. Similar cyclic lateral load

tests were also performed on companion single piles for comparison purposes. A drawing of the

layout of the single pile and pile groups used in the test program at the South Temple overpass

under Interstate 15 in Salt Lake City, Utah is shown in Fig. 1. The test program and test results

are described in this paper, while detailed analysis and development of p-multipliers are

described in a companion paper (Rollins et al, 2004). A complete summary of static and

dynamic testing of all foundations at the site is provided by Rollins et al (2003).

SITE CHARACTERIZATION

The subsurface profile was characterized using a variety of methods to provide basic

geotechnical data for use in subsequent computer analyses of the test results. Based on the

results of the field and laboratory testing, the soil profile shown in Fig. 2 was developed. The

soil profile generally consists of stiff clays with some sand layers to a depth of 5 m. The sand

layers were in a medium compact density state (Dr 60%). These soils were underlain by a soft

sensitive clay layer which was in turn underlain by interbedded layers of silty clay and sand.

Cone penetration test (CPT) soundings were performed at each group location to define the

stratigraphy and the variation across the site. These tests confirmed that the profiles were very

similar at test location. Logs of the average CPT cone tip resistance and friction ratio at the site

are presented in Fig. 2. Additional in-situ testing included borehole shear tests, vane shear tests,

standard penetration tests, cone pressuremeter tests, and shear wave velocity tests as described

more fully by Rollins et al (2003). Undisturbed and disturbed samples were also obtained for

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laboratory strength, consolidation and index testing. Unified Soil Classification symbols based

on the index testing are also shown in Fig. 2

The vane shear test was the primary means for evaluating the undrained shear strength of

the clay and the results from these tests are also shown in Fig. 2. In addition, undrained shear

strength was obtained from unconfined compression tests on undisturbed samples and from

correlations with the CPT cone resistance. In general, the agreement between the strength

evaluation methods was very good. The undrained strength values used subsequently in the

analysis are also identified in Fig. 2 and are in good agreement with the measured strength. The

pre-consolidation pressures obtained from the consolidation testing are also shown in Fig. 2

relative to the vertical effective stress. These results indicate that the clay is overconsolidated

near the ground surface but is close to normally consolidated below a depth of 5 m. Based on the

boreholes shear tests, the friction angles for the sand layers were typically between 36 and 38.

The water table was typically located 1.07 m below the ground surface during the testing.

CYCLIC LATERAL LOAD TESTING OF SINGLE PILES

Lateral load tests were performed on two isolated single piles to provide comparisons to

the pile group tests. The first single pile test was a virgin load test, while the second test was

performed on a pile that had previously been loaded in the opposite direction. The re-load test

was necessary to provide a comparison with one of the pile groups that was loaded statically in

one direction after it was loaded dynamically in the opposite direction using the statnamic

method.

The test piles were 324 mm OD steel pipes (9 mm wall thickness) and were driven

closed-ended to depths of approximately 11.9 m below the ground surface. The steel conformed

to ASTM A252 Grade 3 specifications and had an average yield strength of 404.6 MN/m2

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(58,700 psi) based on the 0.2% offset criteria with a standard deviation of 15,168 kN/m2

(2200

psi). The moment of inertia of the piles was 1.16 x 108

mm4

(279 in4). Strain gages were placed

on opposite faces of the pile at 10 depth levels (0.61, 1.22, 1.52, 2.74, 3.66, 4.57, 6.10, 7.62, 9.15

and 10.7 m below ground level) to determine bending moment profiles versus depth. Two angle

irons were attached to protect the strain gages, which increased the moment of inertia to 1.43 x

108mm

4(344 in

4).

The load tests were performed using a displacement control approach with approximately

nine displacement increments. The load applied by the hydraulic jack was measured with a load

cell while pile head deflection and rotation were measured with LVDTs. For each deflection

increment, 15 load cycles were typically applied to simulate the cyclic loading typical of a M7.5

earthquake (Seed et al, 1975) and to evaluate the change in lateral resistance due to cyclic

loading.

The peak load-deflection curves for the 1st

and 15th

cycles during the load test on the

single pile are presented in Fig. 3. For a given deflection, the drop in peak load from the 1st

to

the 15th cycle is about 15%. Most of this drop occurs in the first few cycles. Although the

difference in the peak load-deflection curves for the 1st

and 15th

cycles is relatively small, these

curves are somewhat deceptive because they do not show the full load-deflection curve before

the peak load. A more accurate indication of the reload behavior is provided by the complete

load-deflection curves for each fifteenth cycle, which are also included in Fig. 3. At deflections

short of the previous peak deflection, the load during the 15th

cycle is significantly below that for

the 1st

cycle. The curves for the 15th

cycle appear to be composed of two parts. The lower part

of the curve is relatively linear. The slope of the upper part of the curve increases rapidly and the

curve becomes parabolic with a concave upward shape.

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This change in the slope of the load-deflection curve is readily explained by presence of

the gap that develops around the pile during virgin loading. During the first cycle, the applied

load is resisted by both the pile and the soil near the ground surface. During subsequent

loadings, a gap develops between the cohesive soil and the pile due to the previous loading. For

deflections less than the width of that gap, the primary resistance to loading is due to the pile

stiffness. This explains the approximately linear relationship between load and deflection when

the pile is pushed through the gapped region. As the deflection approaches the previously

achieved maximum deflection, the load-deflection relationship becomes non-linear with a

concave upward shape. This increase in the slope of the upper part of the curve is due to the pile

engaging the soil and receiving progressively more lateral soil resistance.

The peak load-deflection curves for the virgin pile test and the pile re-loaded in the

opposite direction from the original loading are presented in Fig. 4. The re-load curve is softer

and more linear than the virgin curve, particularly at smaller deflections, because gaps formed

around the entire perimeter of the pile during the virgin loading. Because the gaps reduce the

soil resistance, the pile itself provides a greater portion of the total lateral resistance and the re-

load curve becomes more linear than the virgin curve.

CYCLIC LATERAL LOAD TESTING OF PILE GROUPS

Cyclic lateral load tests were performed on three separate pile groups at different

longitudinal spacings. The piles in the group were also driven with a closed-end and had the

same properties as the single piles. The middle pile in each row was driven first and then the

outer piles were driven. During driving, ground heaving of 75 to 125 mm was observed within

the group. One group consisted of piles in a 3x3 arrangement with a longitudinal (direction of

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loading) spacing of 5.65 pile diameters on centers. A second group consisted of piles in a 3x4

arrangement with a longitudinal spacing of 4.4 pile diameters and the third group consisted of

piles in a 3x5 arrangement with a longitudinal spacing of 3.3 pile diameters. The transverse

spacing in all cases was 3.3 pile diameters. The load was applied to a load frame using two 1300

kN hydraulic jacks and measured with load cells.

The load frame was designed to provide the same displacement at each pile location and

be essentially rigid in comparison with the stiffness of the piles. Each pile was attached to the

load frame by a tie-rod with a pinned connection. Strain gages attached to each tie-rod provided

a continuous readout of the load carried by each individual pile during the test. Pile head

deflection and rotation were measured using LVDTs attached to an independent reference frame.

Strain gages were placed on opposite faces of the middle pile in each row at approximately the

same 10 depth levels described for the single pile. A portable data acquisition system was used

to record the information from up to 120 channels of data at one second intervals.

The load tests were performed in an incremental manner with a displacement control

method as described previously for the single pile tests. At each displacement increment a total

of 15 cycles were typically applied, prior to increasing the displacement for the next increment.

Load versus Deflection Relationships

The peak total load vs. deflection curves for the 1st

and 15th

cycles from the tests on the

three pile groups are provided in Fig. 5. In each case, the reduction in total resistance from the

1st

to the 15th

cycle is typically about 16 2% which is similar to that observed for the single pile

test. To better quantify the decrease in resistance as a function of depth, the stiffness at each

cycle was normalized by the stiffness for the 1st

cycle at a given deflection to obtain a stiffness

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ratio. The average stiffness ratio is plotted as a function of the number of cycles for each group

test in Fig. 6. A similar curve is also provided for the single pile test for comparison. The curves

for each pile group are very similar to each other and to that for the single pile test which

indicates that this reduction is not dependent on pile spacing. Typically, about half of the

decrease in stiffness (7%) occurred after the first cycle. Afterwards, the decrease became

progressively smaller as the number of cycles increased, producing a decrease of about 16% after

15 cycles.

The lateral resistance of the piles in the group was found to be a function of row

location within the group, rather than location within a row. Contrary to expectations based on

the elastic theory, the piles located on the edges of a row did not consistently carry more load

than the center piles for a given deflection. This result is consistent with observations from other

full-scale lateral pile group tests in clay (Brown et al, 1987; Meimon et al, 1986; Rollins et al,

1998), but conflicts with full-scale (Ruesta and Townsend, 1997) and centrifuge group tests in

sands (McVay et al, 1998). Plots of average row load versus deflection for each pile group are

presented in Fig. 7 for the 1st cycle of load. The curves are grouped by row with row 1 being the

lead or front row in the group. The load vs. deflection curve for the appropriate single pile test is

also shown in each plot for comparison.

The front row piles in each group carried the greatest load, while the second and third

row piles carried successively smaller loads for a given displacement. However, the fourth and

fifth row piles, when present, carried about the same load as the third row piles. In fact, the back

row piles often carried a slightly higher load than the piles in the preceding row. This finding is

consistent with test results reported by Rollins et al (1998) for full-scale tests in clay and McVay

et al (1998) for centrifuge tests in sand.

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Average lateral load resistance was also found to be a function of pile spacing. Relative

to the lateral resistance of the single pile, relatively little decrease in lateral resistance was

observed for the first two rows of the pile group spaced at 5.65 pile diameters although some

decrease did occur for the third row. However, the lateral resistance became progressively

smaller than that for the single pile as pile groups spacing decreased to 4.4, and then 3.3 pile

diameters on centers. Group interaction effects typically increased as the load and deflections

increased up to a given deflection but then remained relatively constant beyond this deflection.

The deflection necessary to fully develop the group interaction effects increased as the pile

spacing increased. This increase in required deflection is likely related to the increased

movement necessary to cause interaction between failure zones.

Approximately the same distribution of load was observed within the group for the 15th

cycle of loading as for the 1st

cycle of loading when the deflection level approached the peak

previous deflection. However, as illustrated in Fig. 8, at much smaller deflections the

distribution of load among rows with a pile group was much more uniform. At these smaller

deflections when the piles were still within the gapped region, the lateral resistance was

primarily provided by the pile stiffness itself, which is not influenced by group interaction

effects. When the piles engage the soil, at deflections somewhat less than the previous

maximum deflection due to rebound of the soil, group interaction effects again manifest

themselves and produce differences in load distribution within the group similar to that exhibited

during virgin loading.

Bending Moment versus Load

Maximum bending moment versus load curves developed from the 1st

load cycle are

provided for each of the three pile groups in Fig. 9. The maximum moment is the largest

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moment along the length of the pile and the load is the average load carried by piles in the row.

For each pile group, curves are plotted for each row in the group along with a curve from the

single pile test for comparison purposes. The curves for the group at the largest spacing (5.65D)

are relatively close to that for the single pile; however, the plots for the pile groups at closer

spacings (4.4D and 3.3D) indicate the effects of pile-soil-pile or group interaction. For the

closely spaced pile groups, the curves for the front row piles (row 1) are typically close to that

for the single pile, but the curves for rows 2 and 3 tend to develop progressively higher bending

moment for a given pile head load. The curves for rows 4 and 5, when present, are similar to

that for row 3. The tendency for higher moment for a given load is a result of group interaction,

which has the effect of softening the soil resistance and decreasing the rotational restraint in the

trailing rows.

Maximum moment versus load curves for the 15th

load cycle are also provided for the

3x4 pile group in Fig. 10 for comparison purposes. For a given load, the maximum moment

ranged from 14 to 30% higher for the 15th

cycle curve than for the 1st

cycle curves. On average,

the percent increase in moment was smallest for the single and 1st row piles and greatest for the

4th

row piles. The increase in the maximum moment for the 15th

cycle curves can be attributed to

the softening of the soil resistance produced by applying multiple cycles at a given deflection.

Reduced resistance provides less lateral restraint, which leads to greater bending at the same

load.

Bending Moment versus Depth

Bending moment versus depth curves are shown for the three pile groups in Figs. 11, 12

and 13. Curves are shown for each row in the group at three to four deflection levels along with

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a curve for the single pile at the same deflection level for comparison. For the pile group with

the largest spacing (5.65 D), the maximum moments for the first two rows are quite close to one

another and to the single pile curve for a given deflection (see Fig. 11). As the pile spacing

decreases, the lead row (row 1) develops the greatest maximum bending moment while the

trailing row piles (rows 2 through 5) develop somewhat less moment at the same deflection, but

the difference is relatively small (see Figs. 12 and 13). This pattern develops as a result of two

counteracting effects. First, the trailing row piles receive somewhat less soil resistance due to

group interaction effects and, as a result, carry lower loads at the same deflection level than the

lead row piles. Because the loads in the trailing row piles are lower, the maximum bending

moments are also lower. Second, despite the fact that the loads are smaller for the trailing row

piles, trailing row piles develop higher moments for a given load than lead row piles, as

described previously. These two effects combine so that the variation in maximum bending

moment for the various rows is relatively small at any given deflection.

The depth to the maximum moment does not vary appreciably among the group piles

relative to the single pile. Nevertheless, as the spacing decreases the moment in the group piles

becomes progressively greater than that in the single pile at depths below the maximum. In this

zone, the moments are generally highest in the trailing row piles and lowest in the lead row piles.

The difference between the moments in the group piles and the single pile in this zone also

increases as the deflection level increases.

The patterns observed in the bending moment versus depth curves for the 15th

cycle of

loading are similar to those for the 1st

cycle; however, the maximum moments decrease relative

to the 1st

cycle moments and the depth to the maximum moment typically increases slightly due

to the softening of the soil caused by repeated loading.

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CONCLUSIONS

1. Average lateral load resistance was a function of pile spacing. Group interaction effectsbecame progressively more important in reducing lateral resistance as pile spacing

decreased from 5.65, to 4.4 to 3.3 pile diameters on centers.

2. The first row (leading row) piles in the groups carried the greatest load, while the secondand third row piles carried successively smaller loads for a given displacement.

However, the fourth and fifth row piles, when present, carried about the same load as the

third row piles. The back row piles often carried a slightly higher load than the piles in

the preceding row.

3. For these pile groups driven in clay, the lateral resistance was a function of row locationwithin the group, rather than location within a row. This behavior has been observed in

other full-scale tests in clay, but is contrary to expectations based on the elastic theory

which predicts that piles located on the edges of a row did not consistently carry more

load than those located within the group.

4. For a given load, the maximum bending moments in the trailing row piles were greaterthan those in the lead row due to group interaction effects, which essentially softened the

lateral soil resistance against the trailing row piles relative to the leading row piles.

5. For a given deflection, the bending moments in the trailing row piles were somewhatlower than those in the lead row because group interaction effects led to smaller load

carrying capacity and thus smaller moments, but the differences were generally small.

6. Cyclic loading reduced the peak load at the same deflection by about 15% after 15 cyclesand about half of this reduction occurred after only one cycle. However, at deflections

less than the peak, the reduction in lateral resistance was considerably greater.

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7. After 15 cycles of loading, the distribution of load within the group was essentially thesame as for the virgin loading once the pile moved through the gap and again engaged the

surrounding soil. For deflections less than the gap width load distribution was relatively

uniform within the pile group.

8. Cyclic loading also led to increases of 14 to 30% in the maximum bending moment for agiven load with the smallest increases in the single pile and lead row piles and the

greatest increases in the trailing row piles.

REFERENCES

AASHTO (2000). Bridge Design Specifications, American Association of State Highway andTransportation Officials, Washington, D.C.

Brown, D.A., Morrison, C., and Resse, L.C. (1988). Lateral load behavior of a pile group insand,J. of Geotech. Engrg., ASCE, 114(11), 1261-1276.

Brown, D.A., Resse, L.C., and ONeill, M.W. (1987). Cyclic lateral loading of a large-scale pilegroup,J. of Geotech. Engrg. ASCE, 113(11), 1326-1343.

McVay, M., Zhang, L., Molnit, T., and Lai, P. (1998). Centrifuge testing of large laterally

loaded pile groups in sands, J. of Geotech. and Geoenviron. Engrg., ASCE, 124(10), 1016-

1026.

Meimon, Y., Baguelin, F., and Jezequel, J.F. (1986). Pile group behavior under long term lateral

monotonic and cyclic loading, Proc., Third Intl Conf. on Numerical Methods in Offshore

Piling, Inst. Francais Du Petrole, Nantes, pp. 286-302.

Reese, L.C. and Van Impe, W.F. (2001). Single piles and pile groups under lateral loading, A.A.

Balkema, Rotterdam, Netherlands.

Rollins, K. M., Peterson, K. T., and Weaver, T. J. (1998). "Lateral load behavior of full-scale

pile group in clay," J. of Geotech. and Geoenviron. Engrg., ASCE, 124(6), 468478.

Rollins, K.M., Olsen, R.J., Egbert, J.J., Olsen, K.G., Jensen, D.H., Garrett, B.H. (2003).Response, analysis, and design of pile groups subjected to static and dynamic lateral loads.Report No. UT-03.03, Research Div., Utah Dept of Transportation, Salt Lake City, Utah, 523 p.

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Rollins, K.M.,Olsen, K.G., Jensen, D.H., Garrett, B.H., Olsen, R.J., Egbert, J.J. (2004). Pilespacing effects on lateral pile group behavior: Analysis, Submitted for possible publication inJ.of Geotech. and Geoenviron. Engrg., ASCE.

Ruesta, P.F. and Townsend, F.C. (1997). Evaluation of laterally loaded pile group at Roosevelt

Bridge,J. of Geotech. and Geoenviron. Engrg., ASCE, 123(12) 1153-1161.

Seed, H.B., Idriss, I.M., Makdisi, F., and Banerjee, J. (1975). Representation of Irregular Stress

Time Histories by Equivalent Uniform Stress Series in Liquefaction Analysis, Report No.EERC 75-29, Earthquake Engineering Research Ctr., Univ. of Calif., Berkeley, CA.

US Army (1993). Design of Pile Foundations,Technical Engineering and Design Guides No. 1,U.S. Army Corps of Engineers, Washington, D.C.

ACKNOWLEDGEMENTS

This project was supported by Departments of Transportation from the states of Arizona,

California, New York, Utah, and Washington through a pooled-fund arrangement. This support

is greatly appreciated. The Utah Dept. of Trans. served as the lead agency with Sam Musser and

Blaine Leonard as the Project Managers. Doug Alexander, Tom Shantz, Paul Bailey, and David

Sowers served on the Technical Advisory Panel. However, the views and recommendations

expressed in this paper do not necessarily reflect those of the sponsors. Build, Inc. provided

personnel and pile driving equipment at cost and this contribution is appreciated. Finally, we

thank David Anderson, the BYU Civil Engineering Dept. Technician, for his invaluable efforts

during the testing and for debugging the data acquisition system.

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Figure Captions

Fig. 1 Layout of single piles and pile groups at test site below South Temple overpass on I-15

corridor in Salt Lake City, Utah.

Fig. 2 Interpreted soil profile along with results from field and laboratory tests at the test site in

Salt Lake City, Utah.

Fig. 3 Load-deflection curves for the peak points on the 1st and 15th cycles along with the

complete load-deflection curve for each 15th cycle on the single pile test.

Fig. 4 Comparison of initial load versus deflection curve for 324 mm single pile with curve for

subsequent test in the opposite direction.

Fig. 5 Load-deflection curves for the peak points on the 1st

and 15th

cycles of the pile group tests

of the (a) 3x3 pile group at 5.65D, (b) 3x4 pile group at 4.4D, and (c) 3x5 pile group at 3.3D.

Fig. 6 Average ratio of lateral stiffness for a given cycle (K) to the stiffness for the 1st

cycle (Ki)for each pile group test and the single pile test.

Fig. 7 Average pile load-deflection curves for each row in the (a) 3x3 pile group at 5.65D, (b)3x4 pile group at 4.4D, and (c) 3x5 pile group at 3.3D in comparison with curve for appropriate

single pile test.

Fig. 8 Average pile load-deflection curves for each row in the 3x4 pile group at 4.4D spacing fora complete load cycle following application of 15 cycles at 25 mm deflection.

Fig. 9 Maximum bending moment versus average applied load for each row in the (a) 3x3 pilegroup at 5.65D, (b) 3x4 pile group at 4.4D, and (c) 3x5 pile group at 3.3D in comparison with

curve for appropriate single pile test.

Fig 10 Maximum bending moment versus average applied load for each row in the 3x4 pile

group for the 15th

cycle of loading.

Fig. 11 Bending moment versus depth curves for the three rows of the 3x3 pile group (5.65 pilediameter spacing) at four deflection levels along with the curve for the single pile at the same

deflections.

Fig. 12 Bending moment versus depth curves for the four rows of the 3x4 pile group (4.4 pile

diameter spacing) at four deflection levels along with the curve for the single pile at the same

deflections.

Fig. 13 Bending moment versus depth curves for the five rows of the 3x5 pile group (3.3 pile

diameter spacing) at four deflection levels along with the curve for the single pile at the samedeflections.

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N

3x5 Pile Group

(3.3D Spacing, 324 mm OD)

3x3 Pile Group

(3D Spacing, 610 mm OD)

3x4 Pile Group

(4.4D Spacing, 324 mm OD)

Geopier Reaction Pier

3x3 Pile Group

(5.65D Spacing, 324 mm OD Piles)

0 5 m 10 m 20 m

324 mm OD

Single Pile

Geopier Reaction Pier

Loading

Loading

Loading

Fig. 1 Layout of single piles and pile groups at test site below South Temple overpass on I-

15 corridor in Salt Lake City, Utah.

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0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

0 100 200

Undrained Shear Strength ,

Used in

Analysis

CPT Su

Vane Shea

Test

Unconfined

Compressio

Test

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0.00.51.0

Lean CLAY (CL)

with silt (ML) layers

Sensitive Fat

CLAY (CH)

Silty SAND (SM)

Fat CLAY (CH)

Lean CLAY (CL)

Silty SAND (SM)

Lean CLAY (CL)

with silt (ML) and

silty sand (SM)

layers

Idealized Soil Profile

Silty SAND (SM)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

0 2 4 6

Friction Ratio, %

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

0 5000 10000 15000

Tip Resistance (qc), kPa

Fig. 2 Interpreted soil profile along with results from field and laboratory tests at the test si

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0

50

100

150

200

250

0 20 40 60 80

Deflection (mm)

Load(kN)

1st Cycle Peaks

15th Cycle Peaks

Full 15th Cycle

100


and 15th

cycles at each

displacement increment along with the complete load-deflection curve for each 15th

cycle of

the single pile test.

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0

50

100

150

200

250

0 20 40 60 80 100

Deflection (mm)

Initial Load

Load in opposite direction

19

L

oa

d(kN)

Fig. 4 Comparison of initial load versus deflection curve for 324 mm single pile with curve

for subsequent test in the opposite direction


20/28

0

500

1000

1500

2000

2500

0 20 40 60 80 10

Avg. Group Deflection (mm)

TotalGroupLoad(kN)

1st Cycle Peak

15th Cycle Peak

3 x 5 Row Group

(3.3 Diameter Spacing)

0

200

400

600

800

1000

1200

1400

1600

0 10 20 30 40 5


TotalGroupLoad

(kN)

1st Cycle Peak

15th Cycle Peak

3 x 4 Row Group

(4.4 Diameter Spacing)

0

200

400

600

800

00

00

1400

1600

0 20 40 60 8


Tota

lGroupLoad(kN)

1st Cycle Peak

15th Cycle Peak

3 x 3 Row Group

(5.65 Diameter Spacing)12

10

0

0

0


and 15th

cycles of the pile group

tests of the (a) 3x3 pile group at 5.65D, (b) 3x4 pile group at 4.4D, and (c) 3x5 pile group at

3.3D.

20


21/28

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0 5 10 15

Number of Cycles

K/Ki

Single Pile

9 Pile Group

12 Pile Group15 Pile Group

Fig. 6 Average ratio of lateral stiffness for a given cycle (K) to the stiffness for the 1st

cycle

(Ki) for each pile group test and the single pile test.

21


22/28

0

50

100

150

200

250

0 20 40 60 8


Avg.

PileLoad(kN)

Single

Row 1

Row 2

Row 3

0

0

50

100

150

200

0 20 40 60 8


Avg.

PileLoad

(kN

Single

Row 1

Row 2

Row 3Row 4

0

0

50

100

150

200

250

0 20 40 60 80 100


Avg.

PileLoad(kN

Single

Row 1

Row 2

Row 3

Row 4

Row 5

Fig. 7 Average pile load-deflection curves for each row in the (a) 3x3 pile group at 5.65D, (b)

3x4 pile group at 4.4D, and (c) 3x5 pile group at 3.3D in comparison with curve for appropriate

single pile test.

22


23/28

0

25

50

75

100

125

0 5 10 15 20 25 30 35

Average Group Deflection (mm)

Previous cyclic deflection

Row 1

Row 2

Row 3

Row 4

Avg.

Loa

di

nR

o

w

(

kN

Fig. 8 Average pile load-deflection curves for each row in the 3x4 pile group at 4.4 pile

diameter spacing for a complete load cycle following application of 15 cycles at 25 mm

deflection.

23


24/28

0

50

100

150

200

250

300

0 50 100 150 200

Average Load on Piles in Row (kN)

Max.

Moment(kN-m)

Row 4

Row 3

Row 2

Row 1

Single

(b)

0

50

100

150

200

250

300

0 50 100 150 200


Max

.Moment(kN-m)

Row 3

Row 2

Row 1

Single

(a)

0

50

100

150

200

250

300

0 50 100 150 200


Max.

Moment(kN-m)

Row 5

Row 4

Row 3

Row 2

Row 1

Single

(c)

Fig. 9 Maximum bending moment versus average applied load for each row in the (a) 3x3

pile group at 5.65D, (b) 3x4 pile group at 4.4D, and (c) 3x5 pile group at 3.3D in

comparison with curve for appropriate single pile test.

24


25/28

0

50

100

150

200

250

300

0 50 100 150 200


Max.

Moment(kN-m)

Row 3

Row 4

Row 2

Row 1

Single

Fig. 10 Maximum bending moment versus average load on piles in each row for the 15th

cycle of the 3x4 pile group at 4.4D spacing.

25


26/28

Fig. 11 Bending moment versus depth curves for the three rows of the 3x3 pile group (5.65

pile diameter spacing) at four deflection levels along with the curve for the single pile at the

same deflections.

1st cycle 12.7 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-10 0 10 20 30 40 50 60 70 80

Bending Moment (kN-m)

DepthBelow

ExcavatedGround

(m)

Single

Row 1

Row 2

Row 3

1st cycle 25.4 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-25 0 25 50 75 100 125 150


DepthBelow

ExcavatedGround

(m)

Single

Row 1

Row 2

Row 3

1st cycle 38.10 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250 300


DepthBelowExcavatedGround(m

)

Single

Row 1

Row 2

Row 3

1st cycle 50.80 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250 300


DepthBelow

ExcavatedGround(m)

Single

Row 1

Row 2

Row 3

26


27/28

Fig. 12 Bending moment versus depth curves for the four rows of the 4x3 pile group (4.4

pile diameter spacing) at three deflection levels along with the curve fro the single pile at

the same deflections.

1st cycle 12.7 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-10 0 10 20 30 40 50 60 70 80 90


DepthBelow

ExcavatedGround(m)

Single

Row 1

Row 2Row 3

Row 4

1st cycle 25.4 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-25 0 25 50 75 100 125 150


DepthBelow

ExcavatedGround(m)

SingleRow 1Row 2Row 3

Row 4

1st cycle 38.10 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250 300


DepthBelow

ExcavatedGround(m)

Single

Row 1

Row 2

Row 3Row 4

27


28/28

Fig. 13 Bending moment versus depth curves for the five rows of the 3x5 pile group (3.3

pile diameter spacing) at four deflection levels along with the curve for the single pile at the

same deflections.

12.7 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-10 0 10 20 30 40 50 60 70 80


DepthBelowExcavatedGround(m

)

Single

Row 1

Row 2

Row 3Row 4

Row 5

25.4 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-25 0 25 50 75 100 125 150


DepthBelow

ExcavatedGround(m

)

Single

Row 1

Row 2

Row 3Row 4

Row 5

38.10 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250 300


DepthBelow

ExcavatedGround(m)

Single

Row 1

Row 2

Row 3Row 4

Row 5

50.80 mm

-1

0

1

2

3

4

5

6

7

8

9

10

-50 0 50 100 150 200 250 300


DepthBelow

ExcavatedGround(m)

Single

Row 1

Row 2

Row 3Row 4

Row 5

Date post:	04-Apr-2018
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