Single Level Report - Rock Socket

EXECUTIVE SUMMARY

LOADTEST tested a 48-inch (1,219-mm) dedicated test shaft constructed to a depth of 46-feet (14 meters). Sub-surface conditions at the test shaft location consist primarily of silty clay underlain by shale and limestone.

The maximum bi-directional load applied to the shaft was 9,122 kips (40.57 MN). At the maximum load, the displacements above and below the O-cell were 0.117 inches (2.96 mm) and 0.118 inches (3.01 mm), respectively. Unit shear data calculated from Strain Gage Level 3 indicated an average net unit side shear of 24.5 ksf (1,172 kPa) between the O-cell and the top of the weathered shale. We calculate a maximum applied end bearing pressure of 708.4 ksf (33,916 kPa).

Using the procedures described in the report text and in Appendix C, we constructed an equivalent top load curve for the test shaft. For a top loading of 5,540 kips (24.6 MN), the adjusted test data indicate this shaft would settle approximately 0.25 inches (6.4 mm), all of which is estimated elastic compression.

TEST RESULTS AND ANALYSES

General: The loads applied by the O-cell act in two opposing directions, resisted by the capacity of the shaft above and below. Theoretically, the O-cell does not impose an additional upward load until its expansion force exceeds the buoyant weight of the shaft above the O-cell. Therefore, net load, which is defined as gross O-cell load minus the buoyant weight of the shaft above, is used to determine side shear resistance above the O-cell and to construct the equivalent top-loaded load-settlement curve. For this test we calculated a buoyant weight of shaft of 68 kips (0.30 MN) above the O-cell.

Side Shear Resistance: The maximum upward applied net load to the upper side shear was 9,054 kips (40.27 MN) which occurred at load interval 1L-15 (Appendix A, Page 3, Figure 1). At this loading, the upward movement of the O-cell top was 0.117 inches (2.96 mm).

In order to assess the side shear resistance of the test shaft, loads are calculated based on the strain gage data (Appendix A, Page 4) and estimates of shaft stiffness (AE), which are presented below. We used the ACI formula (Ec=57000f´c) to calculate an elastic modulus for the concrete, where f’c was reported to be 6,130 psi (42.27 MPa) on the day of the test. This, combined with the area of reinforcing steel and nominal shaft diameter, provided an average shaft stiffness (AE) of 13,100,000 kips (58,100 MN) in the 60-inch diameter shaft section, 10,700,000 kips (47,400 MN) in the 54-inch section and 8,500,000 kips (37,900 MN) in the 48-inch section. Net unit shear curves are presented in Appendix G. Net unit shear values for loading increment 1L-15 follow in Table A:

TABLE A: Average Net Unit Side Shear Values

Load Transfer Zone Load Direction Net Unit Side Shear
Top of Pile to Strain Gage Level 3 up 0.3 ksf (16 kPa)
Strain Gage Level 3 to Strain Gage Level 2 up 0.8 ksf (36 kPa)
Strain Gage Level 2 to Strain Gage Level 1 up 21.6 ksf (1,035 kPa)
Strain Gage Level 1 to O-cell up 58.4 ksf (2,798 kPa)


Strain Gage Load Distribution Curves

For upward-loaded shear, the buoyant weight of shaft in each zone has been subtracted from the load shed in the respective zone above the O-cell.

NOTE: Net unit shear values derived from the strain gages above the O-cell assembly may not be ultimate values. See Figure G-1 for net unit shear vs. upward shear zone displacement plots.

Combined End Bearing And Lower Side Shear Resistance: The maximum O-cell load applied to the combined end bearing and lower side shear was 9,122 kips (40.57 MN) which occurred at load interval 1L-15 (Appendix A, Page 3, Figure 1). At this loading, the average downward movement of the O-cell base was 0.118 inches (3.01 mm). The load taken in shear by the 0.3 feet (0.09 meters) shaft section below the O-cell is calculated to be 220 kips (0.98 MN) assuming an estimated unit side shear value of 58.4 ksf (2,798 kPa) and a nominal 48-inch (1,219-mm) shaft diameter. The applied load to end bearing is then 8,902 kips (39.59 MN) and the unit end bearing at the base of the shaft is calculated to be 708.4 ksf (33,916 kPa) at the above noted displacement. A unit end bearing curve is presented in Appendix G.
Creep Limit: See Appendix D for our O-cell method for determining creep limit. The combined end bearing and lower side shear creep data (Appendix A, Page 3) indicate that no apparent creep limit was reached at a movement of 0.12 inches (3.0 mm) (Figure 4). The upper side shear creep data (Appendix A, Page 3) also indicate that no apparent creep limit was reached at a movement of 0.12 inches (3.0 mm) (Figure 5). A top-loaded shaft will not begin significant creep until both components begin creep movement. This will occur at the maximum of the movements required to reach the creep limit for each component. We believe that significant creep for this shaft will not begin until a top loading exceeds 17,756 kips (79.0 MN) by some unknown amount.

Equivalent Top Load: Figure 2 presents the equivalent top-loaded load-settlement curves. The lighter curve, described in Procedure Part I of Appendix C, was generated by using the measured upward top of O-cell and downward base of O-cell data. Because it is often an important component of the settlements involved, the equivalent top load curve requires an adjustment for the additional elastic compression that would occur in a top-load test. The darker curve as described in Procedure Part II of Appendix C includes this adjustment.

The test shaft was loaded to a combined side shear and end-bearing load of 18,176 kips (80.8 MN). For a top loading of 5,540 kips (24.6 MN), the adjusted test data indicate this shaft would settle approximately 0.25 inches (6.4 mm) all of which is estimated elastic compression. For a top loading of 10,691 kips (47.6 MN) the adjusted test data indicate this shaft would settle approximately 0.50 inches (12.7 mm) all of which is estimated elastic compression.

Note that, as explained previously, the equivalent top load curve applies to incremental loading durations of eight minutes. Creep effects will reduce the ultimate resistance of both components and increase shaft top movement for a given loading over longer times. The Engineer can estimate such additional creep effects by suitable extrapolation of time effects using the creep data presented herein. However, our experience suggests that such corrections are small and perhaps negligible for top loadings below the creep limit indicated in Figure 2.

Shaft Compression Comparison: The measured maximum shaft compression, averaged from two telltales, is 0.068 inches (1.74 mm) at 1L-15 (Appendix A, Page 1). Using a weighted shaft stiffness of 9,850,000 kips (43,800 MN) and the load distribution in Figure 3 at 1L-15, we calculated an elastic compression of 0.071 inches (1.80 mm) over the length of the compression telltales. We believe this excellent agreement provides good evidence that the values of the estimated shaft stiffness are reasonable and that the O-cell loaded the shaft in accord with its calibration.

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