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Development and Field Testing of Multiple Deployment Model Pile (MDMP)

CHAPTER 8. SUMMARY CONCLUSIONS AND RECOMMENDATIONS

8.1 Summary

8.1.1 The MDMP Configuration and Specifications

The Multiple Deployment Model Pile (MDMP) is an in situ soil testing device composed of a series of modular sensors that can be assembled in various desired configurations. The MDMP can be either pushed or driven to the required testing depth, typically beyond the bottom of a cased borehole. The model pile is capable of measuring axial loads, pore water pressure, total radial stresses, local displacement, and pile acceleration. A typical configuration of the modular MDMP is shown in Figure 127. The MDMP instrumentation includes three load cells, three accelerometers, a displacement transducer, a pore pressure transducer, and a total pressure cell. The friction sleeve between the top and middle load cells is 83.5 cm (32.9 in) long and constitutes a surface area of 2000 cm² (310 in²). Table 47 summarizes the ranges of the MDMP instrumentation.

8.1.2 The Newbury Site Testing

The first field deployment of theMDMP was at a site located in Newbury, Massachusetts during March 1996. The test location was chosen because it contained a 9- to 12-m- (30- to 40-ft-) thick clay deposit close to the ground surface, allowing to assess the pile capacity gain and pore pressure dissipation with time. Additional full-scale instrumented pile testing was also carried out at the same location.

Table 47. Summary of the MDMP Instrumentation Ranges.

On March 6, 1996, the first of two model pile tests was conducted at the Newbury Site. The pile was driven about 9.33 m (30.6 ft) below ground surface, with the pressure cell and friction sleeve at a depth of 7.39 m (24.25 ft). An initial load test was performed in compression, followed by 11 load tests with time performed in tension. A final static-cyclic load test in compression was performed about 138 h after pile installation.

Figure 127. Typical Configuration of the Modular MDMP.

On March 13, 1996, the second of the two model pile tests was conducted at the Newbury Site. The pile was driven about 12.31 m (40.4 ft) below ground surface, with the pressure cell and friction sleeve at a depth of 10.46 m (34.33 ft). An initial load test was performed in compression, followed by nine load tests with time performed in tension. A final static-cyclic load test in compression was performed about 120 h after pile installation.

In both tests, the MDMP was monitored with the Pile Driving Analyzer (PDA) during installation and a restrike following the static load tests. When the pile was extracted (after the completion of each testing series), it was surrounded by a soil cake conforming to the 101.6-mm (4-in) casing, indicating that the shear in the soil had taken place some distance away from the pile wall.

8.1.3 Test Results

(1) Pore Pressure Dissipation.

The hydrostatic pressure was established to be 57.02 kPa and 87.43 kPa for the NB2 and NB3 testing depths, respectively. The initial (peak) and final measured pore water pressures during test NB2 were 217.3 kPa and 51.02 kPa, respectively. The initial and final measured pore water pressures during test NB3 were 224.0 kPa and 92.46 kPa, respectively. Using the methodology presented by Paikowsky et al. (1995), the rate of pore pressure dissipation, H_ut, was found to be 0.6047 and 0.6011 for NB2 and NB3, respectively. The time at 50% dissipation, t₅₀, for NB2 was 9.854 h (35476 s) and for NB3, it was 7.849 h (28256 s). When adjusted to the PLS Cell radius (19.177 mm), t_50(pls) was 2.493 h (8975 s) and 1.986 h (7149 s) for NB2 and NB3, respectively.

(2) Capacity Gain.

A compression load test was carried out 25 min after the installation of MDMP test NB2 in which a skin resistance of 0.16 kN (35 lb) was measured along the frictional sleeve (equivalent to a shear stress of 0.78 kPa (0.11 psi)). The final pull-out test (11^th in the sequence) was conducted 118.6 h after the end of installation in which a skin resistance of 5.54 kN (1246.5 lb) was measured along the frictional sleeve (equivalent to a shear stress of 27.72 kPa (4.02 psi)).

A compression load test was carried out 21.5 min after the installation of MDMP test NB3 in which a skin resistance of 0.23 kN (51 lb) was measured along the frictional sleeve (equivalent to a shear stress of 1.13 kPa (0.16 psi)). The final pull-out test (9^th in the sequence) was conducted 94.9 h after the end of installation in which a skin resistance of 4.67 kN (1,050 lb) was measured along the frictional sleeve (equivalent to a shear stress of 23.35 kPa (3.39 psi)).

Using the methodology presented by Paikowsky et al. (1995), the rate of capacity gain (C_gt) for both peak values and residual values measured during NB2 was 0.589. MDMP test NB3 resulted in a rate of capacity gain (C_gt) of 0.599 for peak values and 0.631 for residual values. The time to 75% gain of capacity, t₇₅, for NB2 was 67.7 h for peak values and 70.7 h for residual values, and for NB3, t₇₅ was 43.2 h for peak values and 38.2 h for residual values. Following the standard normalization used by Paikowsky et al. (1995), t₇₅ was adjusted to a 152.4-mm (6-in) radius pile. The t_{75(152.4 mm)} was 1083.2 h for peak values and 1131.2 h for residual values, and 691.2 h for peak values and 610.9 h for residual values for NB2 and NB3, respectively.

(3) Radial Consolidation.

The coefficients of horizontal consolidation, c_h, were 0.0135 cm²/s and 0.0170 cm²/s for NB2 and NB3, respectively.

(4) Radial Stresses.

The total radial stresses were measured to be similar to the magnitude of the pore pressures at the end of the MDMP installation (200 kPa for NB2). The total radial stresses then gradually decreased at a rate somewhat lower than the pore pressure dissipation, which resulted in a slow increase in the effective radial stresses. This increase was at a rate of 1.52 kPa/h beginning about 1 h after the end of the driving, and reached about 36 kPa 37 h later. At that point, the total radial stress was about equal to the total vertical stress. At the same time, the consolidation process was at about 90% and a sharp increase was observed in the radial total and effective stresses. The radial stresses seemed to stabilize about 67 h after the end of the MDMP NB2 installation and remained about constant thereafter, affected only by the load testing.

(5) Static - Cyclic Load Tests.

A test consisting of a large displacement (about 50 mm) downward, following by a static-cyclic full-mobilization load test completed the final testing of MDMP tests NB2 and NB3. The frictional behavior presented degradation with the displacement and repetitive behavior in the loading-unloading cycle of the testing. The peak forces along the friction sleeve in the push tests were 2.61 kN (588 lb) and 1.17 kN (263 lb).

(6) Dynamic Analysis.

Multiple dynamic measurements were carried out at the top of the drilling rods and inside the MDMP. The analysis of the measurements, while allowing insight for the behavior, encountered some difficulties due to the complexity of the pile geometry.

8.2 Conclusions

8.2.1 General Conclusions

1. The MDMP was successfully analyzed, specified, constructed, calibrated, and tested.
2. The dynamic monitoring during installation and subsequently following static loading provided the capability of examining the relationship between the dynamic resistance and the static components during driving and with time.
3. The ability to monitor the pile capacity gain with time, along with the controlling mechanisms (pore pressure and radial stress variations), provided significant insight into pile analysis, design, and testing.
4. The MDMP provided direct measurement of soil/structure interaction and served, therefore, as an ideal in situ tool that enabled direct correlation between the measured strength/capacity values and design parameters. Extrapolation of the measurements allowed for full-scale pile capacity time-dependent evaluation.

8.2.2 Major Conclusions

1. The capacity gain with time was found to be an extreme phenomenon in which the soil resistance changed from about zero (fluidized soil) to a significant portion of its natural shear strength. The capacity gain was related to the variation of skin friction with time. This, in turn, was found to depend on the radial effective stresses. In contrast to common knowledge and one-dimensional consolidation theories, as a result of a total radial stress decrease after installation, the radial effective stresses were found not to increase at the same rate as the pore pressure dissipation. These observations suggested redistribution of total stresses around the pile after driving, resulting in a delay in the capacity gain beyond the direct relationship to pore pressure dissipation.
2. The exact analysis of pore pressure dissipation rates and time called for accurate and reliable initial pore pressure measurements. These were difficult to achieve and required careful examination of the obtained data. However, their influence on the time required for the completion of the processes (pore pressure dissipation and capacity gain) was limited.
3. The load-displacement measurements provided direct soil/structure interaction monitoring with time, which allowed both design parameters and modeling relationships.
4. The use of dynamic measurements along with the MDMP installation provided insight into pile behavior and the dynamic measurement capabilities. The geometrical complications of the MDMP restricted the effective use of the dynamic methods of analysis and further development was required in this area.

8.2.3 Detailed Conclusions

1. The initial (peak) pore pressure measurements were found to be 20% to 47% lower than the u₃ measurements obtained in the CPT testing (Paikowsky and Chen, 1998). As the CPT measurements closely matched the expected values presented by Paikowsky et al. (1995), it is believed that a modification is required that addresses the combination of filter saturation and that data acquisition sampling rate. The MDMP installation method (driving) created challenges not common to steady-state penetration methods (e.g., CPT).
2. The rate of pore pressure dissipation in both tests was practically identical to an average H_ut of 0.603. This value falls within the range of recorded values for all normally consolidated soils (from 0.325 to 0.763), but indicates a faster dissipation than the mean value of Hut=0.466±0.089 (33 cases). The measured rate was also about 20% higher than the mean value found for BBC at the Saugus, MA site of Hut=0.492±0.072. The analyzed rates of pore pressure dissipation and time related to the process were affected by the initial pore pressure readings. The obtained results were very good, in particular, the final process predictions were affected only in a limited way by the initial pore pressure readings. These observations suggest that: (1) site-specific testing is recommended and (2) the above modification to the control of the initial pore pressure readings is important.
3. The radial consolidation values were about half of the ch values calculated from the CPT dissipation tests. The difference between the coefficients of consolidation obtained from the CPT and the MDMP was most likely the result of the variation in the initial pore pressure measurements that markedly affected the determined t50.
4. The rate of capacity gain denoted by the parameter, C_gt was found at the Newbury site to be 1.6 times higher than values determined from the data base. The t75 obtained at the Newbury site was also larger than the ones obtained at other locations, resulting in an overall longer capacity gain time even though the capacity gain was faster. A possible reason for this was that rarely (if at all) were any of the capacity gain rates in the database based on a complete monitoring of the skin friction from a very short time after installation to the end of consolidation.
5. A reasonable match was obtained between the predicted capacity at the end of driving based on the Energy Approach, the CAPWAP prediction at the time of restrike, and the final static capacity. The high dynamic resistance during driving and the low static resistance afterwards indicated the significance of the soil inertia and viscosity in resisting pile penetration.

8.3 Recommendations

1. Following the initial testing described in this research, the MDMP requires "fine-tuning," including: (1) an improved saturation procedure; (2) rebuilding and testing of the total soil pressure cell; (3) restoration and recalibration of strain-gauged load cells; (4) examination of calibration in light of temperature changes; (5) remachining of parts in the load test assembly, allowing easier frame mounting at the end of driving; (6) examination of lower load cell and accelerometer circuitry as a result of a reverse signal; and (7) building new drilling rods for deeper penetration testing.
2. Further investigation is required to determine the influence of the testing procedures and frequency on the final shear strength, e.g., conducting only one test at the end of the consolidation period versus multiple tests during the consolidation process.
3. Further investigation is required to determine the extent and the influence of the shear zone at the end of the consolidation process. The investigation of this zone can provide insight into the mechanism taking place around loaded piles and the accuracy of the assumed shear stresses.
4. Further investigation is required to be better understand the redistribution of the radial stresses around the pile as they control the effective stresses and the actual rate of capacity gain.