Multi-Level Report

EXECUTIVE SUMMARY

A 48-inch (1,219-mm) dedicated test shaft was tested by LOADTEST. The shaft was constructed under bentonite slurry to a depth of 49.5-foot (45.57-meter). Sub-surface conditions at the test shaft location consist primarily of alternating layers of sand and limestone.

The maximum bi-directional load applied to the shaft at the lower O-cell elevation during Stage 1 was 3,579 kips (15.92 MN). At the maximum load, the displacement below the lower O-cell assembly was 4.522 inches (114.86 mm). The maximum bi-directional load applied to the shaft at the upper O-cell elevation during Stage 2 was 3,216 kips (14.30 MN). At the maximum load, the displacements above and below the upper O-cell assembly were 0.718 inches (18.23 mm) and 0.473 inches (12.01 mm), respectively. Unit shear data calculated from strain gages indicated an average net unit side shear of 3.2 ksf (154 kPa) between the upper O-cell assembly and the top of concrete We also calculated a maximum applied end bearing pressure of 53.6 ksf (2,567 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 1,557 kips (6.9 MN), the adjusted test data indicate this shaft would settle approximately 0.25 inches (6.4 mm) of which 0.20 inches (5.1 mm) 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. For this test we calculated a buoyant weight of shaft of 137 kips (0.61 MN) above the lower O-cells and 86 kips (0.38 MN) above the upper O-cells.

Side Shear Resistance: The maximum upward applied net load to the upper side shear during stage 2 was 3,130 kips (13.92 MN) which occurred at load interval 2L-8 (Appendix A, Page 10, Figure 2). At this loading, the upward movement of the upper O-cell top was 0.718 inches (18.23 mm). The maximum downward applied load to the middle side shear during stage 2 was 3,216 kips (14.30 MN) which occurred at load interval 2L-8 (Appendix A, Page 10, Figure 2). At this loading, the downward movement of the upper O-cell base was 0.473 inches (12.01 mm).

In order to assess the side shear resistance of the test shaft, loads are calculated based on the strain gage data (Appendix A, Pages 11-18) and estimates of shaft stiffness (AE), which are presented in Appendix H. We estimate a weighted shaft stiffness (AE) of 6,000,000 kips (26,700 MN) using the tangent modulus analysis presented in Appendix H. Net unit shear curves are presented in Appendix G. Net unit shear values for loading increment 2L-8 follow in Table B:

Average Net Unit Side Shear Values

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 5) indicate that a creep limit of 2,050 kips (9.1 MN) was reached at a movement of 0.38 inches (9.8 mm) (Figure 6). The middle side shear creep data (Appendix A, Page 10) indicate that a creep limit of 2,500 kips (11.1 MN) was reached at a movement of 0.33 inches (8.4 mm) (Figure 7). The upper side shear creep data (Appendix A, Page 10) indicate that a creep limit of 2,125 kips (9.5 MN) was reached at a movement of 0.41 inches (10.5 mm) (Figure 8). A top-loaded shaft will not begin significant creep until all 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 6,700 kips (29.8 MN) by some unknown amount.

Equivalent Top Load: The test shaft was loaded to a combined side shear and end-bearing load of 9,932 kips (44.2 MN). For a top loading of 1,557 kips (6.9 MN), the adjusted test data indicate this shaft would settle approximately 0.25 inches (6.4 mm) of which 0.20 inches (5.1 mm) is estimated elastic compression. For a top loading of 3,296 kips (14.7 MN) the adjusted test data indicate this shaft would settle approximately 0.50 inches (12.7 mm) of which 0.42 inches (10.8 mm) is estimated elastic compression.

Shaft Compression Comparison: The measured maximum shaft compression at the end of Stage 1, taken as the sum of the averages of two ECTs, two telltales, and negative upper O-cell expansion, is 0.237 inches (6.01 mm) at 1L-10 (Appendix A, Page 5). Using an average shaft stiffness of 6,000,000 kips (26,700 MN) and the load distribution in Figure 3 at 1L-10, we calculated an elastic compression of 0.223 inches (5.66 mm) over the length of the shaft above the lower O-cells. We believe this good agreement provides evidence that the values of the estimated shaft stiffness are reasonable and that the lower O-cells loaded the shaft in accord with their calibrations.

The measured upper shaft compression at the end of stage 2, averaged from two telltales, is 0.170 inches (4.31 mm) at 1L-10 (Appendix A, Page 10). Using an average shaft stiffness of 6,000,000 kips (26,700 MN) and the load distribution in Figure 4 at 2L-8, we calculated an elastic compression of 0.188 inches (4.76 mm) over the length of the traditional compression telltales. We believe this good agreement provides evidence that the values of the estimated shaft stiffness are reasonable and that the upper O-cells loaded the shaft in accord with their calibrations.

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