Pile Design for Downdrag: Examples and Supporting Materials (2024)

APPENDIX H

Design Example 6 — Liquefaction in Sand (Pipe Pile) Using ALLCPT and TZPILE

In Design Example 5, the amount of downdrag, drag load, pile settlement, and soil settlement for an H-pile within a liquefiable soil deposit were determined. In this design example (Design Example 6), the amount of amount of downdrag, drag load, pile settlement and soil settlement for a pipe pile within the same liquefiable soil deposit were examined. This pipe pile design case is presented because the load development can be different for a pipe pile than for a H-pile.

The Innovative Geotechnics (2023) ALLCPT program was used for this design example and the results from the program are presented herein. The results from the Innovative Geotechnics (2023) ALLCPT program are for a fully-mobilized condition like those required for using Method A suggested by the NCHRP-12-116A project team. The obtained unit side resistance and unit end bearing output from the ALLCPT program were used as input in the Ensoft (2021) TZPILE program to perform a partially-mobilized analysis (Method B suggested by the NCHRP 12-116A project team). Using the TZPILE program, the influence of soil settlement was considered during pile load and pile settlement calculations. The flowcharts and steps for Method A and Method B that were followed during this design example are included herein. The Method A approach is presented first, followed by the Method B approach.

Step 1: Establish soil data

The same CPT data and interpreted soil profile that were used in Design Example 5 were reused in Design Example 6. The CPT data are presented in Figure H1 and Table H1.

Step 2: Determine soil settlement

The soil settlement profiles for Events 1 and 2, with a peak ground acceleration of 0.1g and a moment magnitude of 6.5 (PGA =0.1g, M_w=6.5) for Event 1 and PGA=0.4g and M_w=7.7 for Event 2, are presented in Figure H2 and tabulated in Table H2. Moreover, the procedures for determining post-liquefaction reconsolidation settlement and the soil settlement profile that were used in Design Example 5 were reused for Design Example 6.

Table H1. Average CPT sounding record for the Blytheville, AR Test Site.

z	fs	qt	u2	Comments: z=Depth [ft], fs=Sleeve friction [tsf], qt=Tip resistance [tsf], u2=Pore pressure [psi] Values collected every z=0.164ft but reported herein every z=0.984ft (except for first and last values).
0.163	0.163	5.990	-0.080
1.148	0.343	6.644	-4.346
2.133	0.321	5.024	-3.507
3.117	0.232	30.509	-0.474
4.101	0.283	8.452	0.052
5.085	0.259	3.602	1.817
6.070	0.279	4.583	2.308
7.054	0.321	5.619	3.310
8.038	0.314	7.221	3.799
9.022	0.178	4.880	4.400
10.007	0.153	26.554	4.783
10.991	0.283	55.585	4.052
11.975	0.380	57.579	3.349
12.959	0.299	41.164	3.934
13.944	0.332	44.718	1.888
14.928	0.412	80.277	0.862
15.912	0.485	99.291	1.548
16.896	0.549	102.243	3.270
17.881	0.435	82.030	3.717
18.865	0.413	81.470	3.345
19.849	0.259	50.410	3.707
20.833	0.233	30.238	3.041
21.818	0.294	56.976	4.014
22.802	0.274	48.270	7.523
23.786	0.306	58.057	7.823
24.770	0.316	59.929	8.438
25.755	0.303	59.692	9.553
26.739	0.271	53.975	11.429
27.723	0.290	60.599	11.954
28.707	0.307	68.630	12.150
29.692	0.328	76.193	13.576
30.676	0.350	86.086	13.470
31.660	0.336	89.231	14.176
32.644	0.419	98.042	15.180
33.629	0.396	96.314	15.467
34.613	0.372	107.343	15.953
35.597	0.410	113.077	16.297
36.581	0.508	143.899	16.744
37.566	0.666	185.609	16.821
38.550	0.608	179.809	17.287
39.561	0.656	185.487	17.292
40.546	0.840	206.957	17.771
41.530	0.866	205.632	16.569
42.569	0.925	210.830	18.586
43.553	0.995	232.122	18.903
44.537	1.028	245.359	19.529
45.549	1.077	259.473	19.777
46.533	1.035	227.953	20.100
47.517	1.000	211.495	20.318
48.529	0.967	220.913	20.881
49.513	1.070	253.789	20.833
50.498	0.895	268.331	21.315
51.482	0.757	256.276	21.870
52.466	0.680	247.751	22.581

Table H2. Results from Yoshimine et at. (2006) and Idriss and Boulanger (2008) calculations.

Step 3: Establish pile data

The input parameters for the Bustamante and Gianeselli (1982) LCPC analysis, performed within the ALLCPT program, are provided in Figures H3 and H4. These parameters are similar to the parameters used in the ALLCPT program for Design Example 5 but are used for a 21.64m long pipe pile instead of a 26.67m long H-pile. The input length, diameter, and wall thickness for the closed-ended pipe pile were 71ft, 18in, and 0.5in, respectively. Because the ALLCPT program only accepts metric units, the length, diameter, and wall thickness for the closed-ended pipe pile are 21.64m, 0.457m, and 0.013m, respectively. As with Design Example 5, the factors of safety were set to unity. The default values for dimensionless flexibility factor and effective length coefficient were also used. The soil stiffness at the pile base was changed to 106400kPa based upon the value reported in row 430 (pile toe) of the ALLCPT correlated soil parameters (106.438MPa). The concrete-filled steel shell pile elastic modulus was set to 20684280kPa (30000ksi).

Step 4: Compute Incremental Side Resistance

The output from the ALLCPT Pile Capacity Analysis Tool included program output data to determine the load and resistance in the pile (Figures H5 and H6; Table H3). The reported values of ∆Q in Table H3 are the incremental side resistance values.

Step 5: Develop a depth-dependent load profile

The loads in the pile (Q_wUTL), as a function of depth, are presented in Table H3. The cumulative load, as a function of depth, was obtained by adding the incremental side resistance from the top of the pile to the bottom of the pile. The unfactored top load was also added to all of the obtained depth-dependent load values.

Step 6: Calculate end bearing resistance; develop a depth depended resistance profile

The resistances in the pile (R), as a function of depth, are also presented in Table H3. The cumulative resistance, as a function of depth, was obtained by adding the incremental side resistance from the bottom of the pile to the top of the pile. The end bearing at the pile toe (Q_b=206.465tons at a depth of 71.063ft) was also added to all of the obtained depth-dependent resistance values.

Step 7: Develop the depth-dependent combined load profile

The load-resistance curve was developed by plotting the minimum load (Q_wUTL) or resistance (R) values at a given depth as a function of depth. The combined load-resistance curve is presented in Figure H7.

Table H3. Results from ALLCPT Pile Capacity Analysis and calculations (reported in imperial units).

Step 10: Calculate the toe settlement and elastic compression in the pile

A pile settlement curve (Figure H8) was developed. The pile settlement data were processed along with the Davisson (1972) technique to determine the pile head settlement (0.400in as shown in Figure H9). As shown previously in Table H3, the cumulative elastic compression was calculated using the load in the each pile segment that was provided by ALLCPT. The cumulative elastic compression was subtracted from the pile head settlement as a function of depth to obtain the pile settlement curve. As shown in Figure H10, the pile settlement curve is plotted along with the soil settlement data that were previously presented in Table H2 and previously shown in Figure H2.

Step 11: Calculate the geotechnical resistance of the pile

The geotechnical resistance of the pile is also presented in Figure H9. The geotechnical resistance was determined by identifying where the Davisson (1972) failure line crosses the ALLCPT-generated load-settlement curve. A value of 405 tons was obtained for the geotechnical resistance.

Step 12: Identify the location and settlement of the neutral plane (from the soil settlement-pilesettlement profile)

The obtained location of the neutral plane, as identified from the soil settlement-pile settlement curve is identified in Figure H10. The neutral plane was identified to occur at a depth of 29.7ft. The resulting settlement at the depth of the neutral plane (downdrag) was 0.293in.

The neutral plane locations from the combined load-resistance curve (58.8ft) and from the soil settlement-pile settlement curve (29.7ft) are not within the required 5 feet difference. Because the neutral plane locations are not within the required difference, Step 13 of the NCHRP12-116A Method A flowchart cannot be completed and a different pile geometry should be selected or Method B should be attempted. Because the difference is so large, modifications to the pile geometry are not expected to alter the difference in the locations of the neutral plane. Therefore, for this design example, it is recommended that the Method B flowchart be followed.

Method B: TZPILE design calculations with ALLCPT input

For ease of use, t-z and Q-w curves that were “Generated by the program” were used. The use of these curves allows for soil layer data to be input instead of t-z and Q-w curves. These data are presented in Step 1. Steps 2 and 3 for Method B are identical to those listed above for Method A. Therefore, these steps are not repeated in this section.

Step 1: Establish soil data

The design profile that was developed from correlations with the CPT data is presented in Figure H11. The developed design profile parameters, the ALLCPT output unit side resistance (fs), and the ALLCPT output unit end bearing (fb) from the ALLCPT Pile Capacity Analysis Tool were used as input in the TZPILE program. The input windows from TZPILE for this problem are presented as Figures H12 through H15. The pile properties are included in Figure H12. The soil properties are included in Figures H13 and the soil settlement is included in Figure H14 (Event 1) and Figure H15 (Event 2). As discussed previously in other design example problems, the TZPILE program uses units of inches and pounds, so the output from the ALLCPT program were converted prior to input into the TZPILE program. Also, the unit weight profile and undrained shear strength had to be converted from units of lb/ft³ and lb/ft² to lb/in³ and lb/in², respectively.

Steps 2 and 3 Identical to Method A presented previously but in TZPILE. The pile information is presented in Figure H12, and the soil settlement data resulting from the two design earthquake events are presented in Figures H13 and H14.

Step 4: Select t-z models and q-z model

As previously mentioned, the t-z and q-z curves that were selected were “Generated by the program”. Specifically, the API curves were selected. The inputs for these curves are shown in Figure H15.

Step 5: Iterate toe movement to obtain the unfactored top load

As shown in Figure H16, the toe movement was iterated by the changing the values in the TZPILE program to obtain and unfactored top load of 107tons. A toe movement of 0.0305in was used for Event 1 and a toe movement of 0.069in was used for Event 2. The program also uses the calculated elastic compression in the pile to determine the pile settlement as a function of depth (Figure H17). Therefore, the selected toe movement corresponded with a pile head movement of 0.251in for Event 1 and 0.323in for Event 2.

The results that were output from the TZPILE program included load as a function of depth, pile settlement as a function of depth, and soil settlement as a function of depth (Table H4 for Event 1 and Table H5 for Event 2). The soil settlement (show previously in Table H2) as a function of depth are output even though these values were directly input and no changes to the values occurred during the execution of the program.

Table H4. TZPILE output for Event 1.

Table H5. TZPILE output for Event 2.

Step 6: Develop the depth-dependent combined load is the pile

The depth-dependent load-resistance curves for the two events (Event 1 and Event 2) are presented as Figures H18 and H19. The maximum load in the pile from these two events was 153.3 tons and 176.5 tons. The increased soil settlement resulting from the larger earthquake event resulted in the larger maximum load in the pile occurring at a deeper depth.

Steps 7 and 8: Identify the location of the neutral plane and calculate the amount of drag load

As mentioned in Step 6, the increased soil settlement resulting from the larger earthquake event resulted in the maximum load in the pile occurring at a deeper depth (as shown in Figures H18 and H19 above). The location of the maximum load in the pile corresponded with the location of the neutral plane. The neutral plane locations for Events 1 and 2 were at depths of 32.5ft and 37.0ft, respectively. The drag loads calculated for Events 1 and 2 were 46.3tons and 69.5tons, respectively.

Step 9: Calculate the geotechnical resistance of the pile

The geotechnical resistance was determined in TZPILE by repeating Step 5 of the Method B flowchart. For these analyses, the soil settlement was neglected by turning off the Include Down-Drag (negative Skin Friction) toggle within TZPILE (Figure H20) and by also selecting the Load Method as User-Specified Tip Movements (Figure H21). Specifically, multiple toe movements (Figure H22) were evaluated to develop a load-settlement curve (Figure H23 and Table H6). This curve represents the pile head axial load and the pile head settlement. Two specific toe movements were included during the creation of the load-settlement curve; toe movements corresponding with a toe movement of 0.0118in, which was

calculated when the unfactored design load (214.3kips) was obtained at the top of the pile, and a toe movement of 0.05B (0.9in).

Table H6. TZPILE obtained load-settlement curve.

Step 10: Identify the location and settlement of the neutral plane (from the soil settlement-pilesettlement curve)

The amount of elastic compression within the pile was automatically calculated in the TZPILE software program. These automatically generated values simplify efforts compared to the hand calculation that was presented in Design Example 1. Just like Design Example 1, the TZPILE elastic compression calculations are based on the amount of calculated load within the pile at each incremental depth. The calculated pile settlement were tabulated and shown in Table H2 and Figure H2. From Figures H24 and H25, the neutral plane that was obtained from the soil settlement-pile settlement curves for Events 1 and 2 were 32ft and 37ft, respectively. The amount of settlement of the neutral plane (downdrag) for events 1 and 2 was 0.138in and 0.185in, respectively.

The differences between the neutral plane locations for Events 1 and 2, as obtained during Method B Step 7 and Step 10, were within the tolerable limits. For example, for Event 1, the neutral plane location calculated in Step 7 was 32.5ft and in Step 10 was 32ft. Likewise, for Event 2, the neutral plane location calculated in Step 7 was 37ft and in Step 10 was 37ft.

Step 11: Perform limit state checks

Limit state checks were performed to determine if the pile size was suitable for the design loads. For the structural strength limit state, the determined drag load associated with Event 1 (46.3tons) was multiplied by the drag load factor (γ_DR=1.1) to obtained a factored load of drag load 50.9tons. The unfactored top load (107tons) placed on the top of the pile was multiplied by the deadload factor (γ_D=1.25) to obtained a factored deadload of 134tons. The combined total factored load was 185tons. The concrete compressive strength for the concrete-filled steel pipe pile was assumed to be 5000psi resulting in a factored structural stress of 3750psi (0.75*5000psi) and a factored structural strength of 477tons when the stress was multiplied by the cross-sectional area of the pile (254.5in²). If a concrete compressive strength of 5000psi, was used for the concrete-filled steel pipe pile then the pile is adequately sized because the factored structural strength (477tons) was determined to be greater than the combined total factored load (185tons).

Conclusion:

The ALLCPT and TZPILE programs were used to identify the location of the neutral plane, the amount of drag load, and the amount of downdrag. The load and resistance curve that was developed using the ALLCPT program was for a fully-mobilized condition (full mobilization of side resistance and full mobilization of end bearing resistance). The influence of post-liquefaction reconsolidation settlement within the soil deposit was not accounted for when using the ALLCPT program. After accounting for the post-liquefaction reconsolidation settlement, using the TZPILE program, the neutral plane moved up

within the soil deposit. However, the neutral plane moved down within the soil deposit when the pile was subjected to additional downdrag from a larger earthquake event.

Specifically, the influence of mobilization was evident when considering the difference in responses between the Event 1 and Event 2. The additional soil settlement resulted in more mobilization of side and end bearing resistance and more load in the pile. The neutral plane lowered from a depth of 32 feet (Event 1) to 37 feet (Event 2) with the increased soil settlement from 2.465 inches at the ground surface (Event 1) to 12.23 inches at the ground surface (Event 2). The maximum load in the pile increased from 153.3 tons (Event 1) to 176.5 tons (Event 2). Therefore, the drag load increased from 46.3 tons (Event 1) to 69.5 tons (Event 2).

The piles were shown to be adequately designed for the geotechnical and structural strength limit states. The piles were also shown to be adequately designed for the geotechnical serviceability limit state. However, the large ground surface deformations in the soil surrounding the piles may led to a serviceability issues.

References