Reference Manual Chapter 9 Design of Pile Groups Reference Manual Chapter 9

Design of Pile Groups Group Capacity (compression loads) Group Settlement Group Capacity (uplift loads) Group Capacity (lateral loads) 9-116

Design of Pile Groups Piles for highway structures are almost always installed in pile groups. The design of a pile group must consider the group’s axial compression capacity, settlement, uplift resistance, and lateral load capacity.

Design of Pile Groups The efficiency of a pile group is the ratio of the ultimate capacity of the group to the sum of the candidates of the individual piles. Where: g = Pile group efficiency. Qug = Ultimate capacity of the pile group. n = Number of piles in the pile group. Qu = Ultimate capacity of each pile in the pile group. 9-116

Design of Pile Groups The group efficiency may be less than 1 for a pile group driven into a compressible cohesive soil, or into a dense cohesionless soil underlain by a weak cohesive deposit. The group efficiency in cohesionless soils is generally greater than 1. Vibratory densification Densification from displacement 9-118

Design of Pile Groups The settlement of a pile group is likely to be many times greater than that of a single pile carrying the same load as each pile in the pile group. 9-116

Stress Zones from a Single Pile and Pile Group 9-117

Overlap of Stress Zones 9-119

Group Capacity in Cohesionless Soils The ultimate axial compression capacity of a pile group driven in a cohesionless soil may be taken as the sum of the individual capacities, unless underlain by a weak deposit, jetted, or predrilled. If underlain by a weak deposit, the ultimate group capacity is the lesser of the 1) sum of the individual pile capacities, or 2) the group capacity against block failure. A minimum center-to-center pile spacing of 3 diameters is recommended. 9-118

Group Capacity in Cohesive Soils For pile groups in clays with undrained shear strengths less than 95 kPa (2 ksf), and the cap not in firm contact with the ground, use a group efficiency ranging from 0.7 for c-t-c spacings of 3 diameters, to 1.0 for c-t-c spacings of 6 diameters (interpolate in between). 9-120

Group Capacity in Cohesive Soils For pile groups in clays with undrained shear strengths less than 95 kPa (2 ksf), and the cap in firm contact with the ground, a group efficiency of 1.0 may be used. For pile groups in clays with undrained shear strengths greater than 95 kPa (2 ksf), regardless of pile cap/ground contact, use a group efficiency of 1.0. 9-120

Group Capacity in Cohesive Soils 4. Calculate the ultimate pile group capacity against block failure, and use the lesser capacity. 5. A center-to-center spacing less than 3 diameters should not be used. 9-120

Group Capacity in Cohesive Soils Short-term group efficiencies in cohesive soils 1 to 2 months after installation may be as low as 0.4 - 0.8 due to high driving-induced excess porewater pressures (results in decreased effective stress). Pile groups in clays which are loaded shortly after pile installation should consider the reduced short-term group capacity. In critical cases, piezometers should be installed to monitor porewater pressure dissipation with time. 9-120

Dissipation of Excess Pore Pressures + x - single pile - 9 pile group  - 13 pile group - 25 pile group M - 116 pile group - 230 pile group 9-121

Block Failure of Pile Groups Block failure of pile groups is generally only a design consideration for pile groups in soft cohesive soils or in cohesionless soils underlain by a weak cohesive layer. 9-122

Three Dimensional Pile Group 9-123

Block Failure of Pile Groups Qug = 2D (B + Z) cu1 + B Z cu2 Nc Qug = Ultimate group capacity against block failure. D = Embedded length of piles. B = Width of pile group. Z = Length of pile group. cu1 = Weighted average of the undrained shear strength over the depth of pile embedment for the cohesive soils along the pile group perimeter. cu2 = Average undrained shear strength of the cohesive soils at the base of the pile group to a depth of 2B below pile toe level. Nc = Bearing capacity factor. 9-122

Block Failure of Pile Groups The bearing capacity factor, Nc, for a rectangular pile group is generally 9. However, Nc should be calculated for pile groups with small pile embedment depths and/or large widths Nc = 5 [ 1+D/5B ] [ 1+B/5Z ] ≤ 9 9-122

Lateral Capacity of Pile Groups

LATERAL CAPACITY OF PILE GROUPS The lateral deflection of a pile group is typically 2 to 3 times larger than the deflection of a single pile. Piles in trailing rows of pile groups have significantly less lateral load resistance than piles in the lead row. Laterally loaded pile groups have a group efficiency less than 1. 9-150

LATERAL CAPACITY OF PILE GROUPS The lateral capacity of an individual pile in a group is a function of its position (row) in the group, and the c-t-c pile spacing. A p-multiplier, is used to modify p-y curve Laterally loaded pile groups have a group efficiency less than 1. 9-150

LATERAL CAPACITY OF PILE GROUPS The lateral capacity of an individual pile in a group is a function of its position (row) in the group, and the c-t-c pile spacing. A p-multiplier: 0.8, 0.4, & 0.3 (thereafter) 9-150

TABLE 9-19 LATERALLY LOADED PILE GROUPS STUDIES Soil Type Test Center to Center Pile Spacing Calculated p-Multipliers, Pm For Rows 1, 2, & 3+ Deflection in mm (in) Reference Stiff Clay Field Study 3b .70, .50, .40 51 (2) Brown et al, (1987) .70, .60, .50, 30 (1.2) Medium Clay Scale Model- Cyclic Load .60, .45, .40 600 at 50 cycles (2.4) Moss (1997) Clayey Silt .60, .40, .40 25-60 (1.0 - 2.4) Rollins et al, (1998) V. Dense Sand .80, .40, .30 25 (1) (1988) M. Dense Centrifuge Model 76 (3) McVay et al, (1995) 5b 1.0, .85, .70 Loose M. Sand .65, .45, .35 F. Sand .80, .70, .30 25-75 (1-3) Ruesta et al, (1997)

Third & Subsequent Rows Lateral Load Lateral Load Third & Subsequent Rows Second Row Front Row ps Pm ps 9-151 Single Pile Model p-y Curves for Group

STEP BY STEP DESIGN PROCEDURE FOR LATERALLY LOADED PILE GROUPS STEP 1 : Obtain Lateral Loads. STEP 2 : Develop p-y curves for single pile. a. Obtain site specific single pile p-y curves from instrumented lateral pile load test at site. b. Use p-y curves based on published correlations with soil properties. c. Develop site specific p-y curves based on in-situ test data. 9-154

STEP 3 : Perform LPILE Analyses. Perform LPILE analyses using the Pm value for each row position to develop load-deflection and load-moment data. Based on current data, it is suggested that Pm values of 0.8 be used for the lead row, 0.4 for the second row, and 0.3 for the third and subsequent rows. These recommendations are considered reasonable for center to center pile spacing of 3b and pile deflections at the ground surface of .10 to .15b. For larger c-t-c spacings or smaller deflections, these Pm values should be conservative. Determine shear load versus deflection behavior for piles in each row. Plot load versus pile head deflection results similar to as shown in Figure 9.69(a).

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STEP 4: Estimate group deflection under lateral load. a. Average the load for a given deflection from all piles in the group (i.e., each of the four rows) to determine the average group response to a lateral load as shown in Figure 9.69(a). b. Divide the lateral load to be resisted by the pile group by the number of piles in the group to determine the average lateral load resisted per pile. c. Enter load-deflection graph similar to Figure 9.69(a) with the average load per pile to estimate group deflection using the group average load deflection curve.

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STEP 5: Evaluate pile structural acceptability. a. Plot the maximum bending moment determined from LPILE analyses versus deflection for each row of piles as illustrated in Figure 9.69(b). b. Check the pile structural adequacy for each row of piles. Use the estimated group deflection under the lateral load per pile to determine the maximum bending moment for an individual pile in each row. c. Determine maximum pile stress from LPILE output associated with the maximum bending moment. d. Compare maximum pile stress with pile yield stress.

STEP 6:. Perform refined pile group evaluation that STEP 6: Perform refined pile group evaluation that considers superstructure substructure interaction.

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