CHARTS TO FACILITATE COMPUTATION OF SKIN FRICTION ON DRIVEN NON-TAPERED PILES IN COHESIONLESS SOIL GEOTECHNICAL ENGINEERING MANUAL GEM-11 Revision #2 AUGUST 2015 GEOTECHNICAL ENGINEERING MANUAL: CHARTS TO FACILITATE COMPUTATION OF SKIN FRICTION ON DRIVEN NON-TAPERED PILES IN COHESIONLESS SOIL GEM-11 Revision #2 STATE OF NEW YORK DEPARTMENT OF TRANSPORTATION GEOTECHNICAL ENGINEERING BUREAU AUGUST 2015 EB 15-025 Page 1 of 19 TABLE OF CONTENTS ABSTRACT .....................................................................................................................................3 DESIGN PROCEDURE ..................................................................................................................4 EXAMPLE PROBLEM ...................................................................................................................5 DESIGN CHARTS ..........................................................................................................................9 1 φ as a Function of NNY and Po ................................................................................10 2 Pile Type: CIP 10 in. Diameter ..............................................................................11 3 Pile Type: CIP 12 in. Diameter ..............................................................................12 4 Pile Type: CIP 12 ¾ in. Diameter ..........................................................................13 5 Pile Type: CIP 14 in. Diameter ..............................................................................14 6 Pile Type: HP 8x36 (48 ton Capacity) ...................................................................15 7 Pile Type: HP 10x42 (56 ton Capacity) .................................................................16 8 Pile Type: HP 12x53 (70 ton Capacity) .................................................................17 9 Pile Type: HP 14x73 (96 ton Capacity) .................................................................18 APPENDIX ....................................................................................................................................19 A. Derivation of Design Charts ............................................................................... A-1 B. "Bearing Capacity of Piles in Cohesionless Soils" by R.L. Nordlund, 1963 ....... B-1 C. "Point Bearing and Shaft Friction of Piles in Sand" by R.L. Nordlund, 1979 .....C-1 EB 15-025 Page 2 of 19 ABSTRACT The Geotechnical Engineering Bureau currently uses the method developed by Nordlund (see Appendices B and C) to determine the static capacity of a pile in cohesionless soil. This method has been shown to accurately predict pile lengths and capacities for New York State soils, based on pile driving records and dynamic load test results. However, the drawbacks to using Nordlund’s method are that it is complex and time consuming. The purpose of these charts is to permit a more rapid determination of the sin friction on driven non-tapered piles in cohesionless soil. The charts in this manual were developed from Nordlund’s method and results obtained using either Nordlund’s method or this manual should correspond. The interested reader will find the development of these charts in Appendix A. EB 15-025 Page 3 of 19 DESIGN PROCEDURE 1. Draw the plot of overburden pressure versus depth at the proposed pile location. 2. Break the soil in layers according to properties. 3. Calculate the average sampler blow count (NNY) for each layer, from the boring logs. 4. Calculate the average overburden pressure (Po) for each layer. 5. Use the values of Po and NNY calculated in Steps 3 and 4 to find θ from Figure 1. 6. Use Po and φ to find qs (side resistance per linear foot (meter) of pile) using the appropriate chart from Figures 2 through 9 corresponding to the pile type being investigated. 7. Multiply the value of qs by the layer thickness to determine the side resistance for each layer. 8. Sum the values of side resistance for each layer to determine the total side resistance, QS. 9. Calculate end bearing resistance, QP, as described in Appendices B and C. 10. Sum QS and QP to obtain the total static pile resistance, QT. EB 15-025 Page 4 of 19 EXAMPLE PROBLEM The purpose of this example problem is to illustrate the use of the design charts. Problem: Find the length of a 12¾” diameter CIP pile required to support a design load of 35 tons. Given: The soil is a Gravelly SAND. The water table is 10 ft. below the existing ground surface, and the proposed bottom of footing elevation is 5 ft. above the water table elevation. The sampler blow counts are tabulated below: Depth Below Existing Ground Surface NNY (ft.) (bpf) 1.5 4 6.5 5 11.5 9 16.5 11 21.5 13 26.5 13 31.5 16 36.5 18 41.5 15 Solution: Follow design procedure. Draw the plot of overburden pressure (Po) versus depth at the proposed pile location. Assume: γT = 0.120 kcf EB 15-025 Page 5 of 19 EB 15-025 Page 6 of 19 EB 15-025 Page 7 of 19 Layer Average Po at qs Layer QS NNY per Midpoint φ (kips/ft.) Thickness (kips) Layer (ksf) (ft.) 1 5.0 0.90 29° 0.80 5 4 2 11.5 1.78 31° 2.00 20 40 3 17.0 2.60 33° 3.50 10 35 QS = 79 Check point resistance: QP AP xNq xPo 2 2 Ap = π (d /4) = 0.887 ft Po = 2.9 ksf use NNY = 15 φ = 32° From Appendix B, Fig. 1 Nq = 26 2 QP = (0.887 ft ) x (26) x (2.9 ksf) = 67 kips QT = QS + QP = 79 + 67 = 146 kips = 73 tons Since 70 ton of resistance are needed to provide a safety factor of two against soil bearing capacity failure (2 x 35 ton = 70 tons), a pile length of 35 ft. is sufficient. EB 15-025 Page 8 of 19 CHARTS TO FACILITATE COMPUTATION OF SKIN FRICTION ON DRIVEN NON-TAPERED PILES IN COHESIONLESS SOIL EB 15-025 Page 9 of 19 EB 15-025 Page 10 of 19 EB 15-025 Page 11 of 19 EB 15-025 Page 12 of 19 EB 15-025 Page 13 of 19 EB 15-025 Page 14 of 19 EB 15-025 Page 15 of 19 EB 15-025 Page 16 of 19 EB 15-025 Page 17 of 19 EB 15-025 Page 18 of 19 APPENDIX A EB 15-025 Page 19 of 19 APPENDIX A DERIVATION OF DESIGN CHARTS The design charts contained in this manual were derived form the method developed by Nordlund (see Appendices B and C). The development of these charts is shown in this Appendix. The equation for side resistance of a pile in cohesionless soil is as follows: Equation 1 Q P (K ' sin C )d S o d Kδ’ depends on the displacement volume of the pile Cd depends on the least pile perimeter δ/φ is assumed to be a constant for each pile type It follows, therefore, that since the factors Kδ’, sin δ, Cd depend on pile type, they may be combined into one factor for each pile type, for various values of φ. First, the value of Kδ (for δ=0) must be determined for each pile type. Kδ depends on w, the pile taper, φ, the soil friction angle, and V, the volume displaced by a 1 ft. length of pile. Since tapered piles are less frequently used, these charts were developed for non-tapered piles with w=0. A graph of Kδ vs. V (for δ=0) equal to 25°, 30°, 35°, and 40° may be plotted from the information in Fig. 4 through Fig. 7 in Appendix C. This graph of Kδ vs. V is shown as Figure 10. Using this figure, values of Kδ for each volume corresponding to a pile type may be found. These values of Kδ are given in Table 1. Using the values from Table 1, δ/φ and Cd, it is possible to determine sin δ and Kδ’ (for δ≠φ) for each pile type. The product of Kδ’, sin δ, and Cd is shown for each pile type in Table 2. The values of (Kδ’ sin δ Cd) are plotted versus θ in Figure 11 and Figure 12 so values for any friction angle between 25° and 40° can be determined. The values of (Kδ’ sin δ Cd) and θ are shown for CIP and H-Piles in Table 3. If we assume that d=1 ft. in Equation 1, than the value of qs, side resistance per linear foot of depth, can be defined as follows: Equation 2 q P (K ' sin C ) S o d EB 15-025 A-1 APPENDIX A The value of qs can be calculated for different values of Po, for each pile type. The relationship between qs and Po for various pile types are shown on Figure 2 through Figure 9. Note that the value of φ must be known to use Figures 2 through 9. Nordlund has correlated φ with Nc. Nc is the standard sampler blow count for the last foot of driving, corrected for overburden pressure. The value of Nc is defined as follows: Equation 3 Nc N STD xCN CN is the correction factor for overburden pressure; NSTD is the blow count on a sampler for the last foot of driving using a 140 lb. hammer falling 30 in. NSTD can be defined in terms of NNY as follows: Equation 4 NSTD 1.29 xN NY NNY is the blow count on a sampler for the last foot of driving using a 140 lb. hammer falling 30 in. Substituting Equation 4 into Equation 3 and solving for Nc yields: Equation 5 ' Nc 1.29 xCN xN NY 1.29 xNc N’c is the New York sampler blow count for the last foot of driving, corrected for overburden pressure. EB 15-025 A-2 APPENDIX A According to Peck, Hanson and Thornburn, the correction factor CN is defined as: Equation 6 20 CN 0.77 Log Po (tons) The relationship between φ and NNY is shown on Figure 13. Please note that Figure 13 has been developed by Nordlund for pile analysis only. This figure should not be used to correct blow counts for settlement analysis. If NNY is multiplied by CN, the product will equal the New York sampler blow count corrected for overburden pressure. Using this relationship and Figure 13, values of φ corresponding to NNY and Po can be determined. These are shown in Table 4, and plotted in Figure 1. EB 15-025 A-3 APPENDIX A EB 15-025 A-4 APPENDIX A Table 1 – Values of Kδ V δ = φ Pile Type Volume 25° 30° 35° 40° (ft3/ft) HP 8x36 0.074 0.68 0.81 1.07 1.53 HP 10x42 0.086 0.69 0.83 1.11 1.61 HP 12x53 0.108 0.71 0.86 1.17 1.75 HP 14x73 0.148 0.73 0.90 1.25 1.93 10 in.