UCLA Civil and Environmental Engineering

When the direct shear test is performed in accordance with ASTM guidelines, the measured shear stresses at failure estimate drained strength parameters.  We investigate the possibility of estimating undrained strength using direct shear testing at variable shear displacement rates on specimens composed of various combinations of kaolinite and bentonite. Even at fast displacement rates, constant volume conditions are not achieved in the direct shear device because of changes in specimen height that are large relative to allowable ASTM thresholds for constant volume simple shear testing. However, undrained strengths established by constant volume simple shear testing at slow strain rates are well approximated by direct shear tests conducted at fast shear displacement rates (time to failure < t₅₀/8, where t₅₀=time to 50% consolidation in a conventional oedometer test). Because of the simplicity of direct shear testing, such estimates of undrained strength may be useful in engineering practice when access to a simple shear device is limited. Nevertheless, fast direct shear tests have shortcomings, including lack of control of rate effects, and constant volume testing is recommended for critical projects.

We investigate through laboratory testing the volume change characteristics of peaty organic soil from Sherman Island, California under static conditions (consolidation, secondary compression) and post-cyclic conditions. Incremental consolidation tests indicate the material to be highly compressible (C_c = 3.9, C_r = 0.4) and prone to substantial ageing from secondary compression (C_a/C_c = 0.05 following virgin compression). Strain-controlled cyclic triaxial testing of the peat finds the generation of cyclic pore pressures for cyclic shear strain levels beyond approximately 0.5-1.0%, with the largest residual pore pressure ratios r_ur (cyclic residual pore pressure normalized by pre-cyclic consolidation stress) being approximately 0.2-0.4. Post cyclic volume change occurs from pore pressure dissipation and secondary compression. The level of post-cyclic secondary compression increases with r_ur. Many of these phenomena have not been documented previously and suggest the potential for seismic freeboard loss in levees due to mechanisms other than shear failure.

This research involved analysis and field testing of several foundation support components for highway bridges. Two classes of components were tested - cast-in-drilled-hole (CIDH) reinforced concrete piles (drilled shafts) and an abutment backwall. The emphasis of this document (Part I of the full report) is CIDH shafts.

CIDH shafts are among the most common support structures in highway construction. Typically, drilled shafts have simple, prismatic geometries; yet, they display a complex, inelastic response under applied loading. The two major factors that affect their behavior are the interaction between the shaft and surrounding soil media, and the material inelasticity of the shaft itself. In this report we document the results of two single shaft tests and one shaft group test. All specimens are two-feet diameter reinforced concrete drilled shafts that extend approximately 24ft below ground line. The single shaft specimens include one in a flagpole configuration extending 13.3ft above ground line and the other capped at the surface in a fixed-head configuration. The group test specimen had 9 individual shafts in a 3 by 3 configuration anchored at the ground surface (with a moment connection) in a reinforced concrete cap. The test site consists primarily of low plasticity alluvial clay that is expected to exhibit an undrained response to the cyclic lateral loading. The quasi static loading was applied with a hydraulic control system in displacement-control mode, with the full suite of loading taking several days to complete for each test. The test data have been reduced to provide complete load-deflection backbone curves for loading in both directions, curvature profiles at pre-yield deflection levels, hysteresis curves documenting the cyclic behavior of the shaft soil system at pre-yield displacements, p-y curves for the single shaft specimens, and group interaction factors for the group specimen.

Pre-test response predictions of the CIDH specimens were obtained via (1) a three dimensional finite element model, (2) a macro-element model, developed at UCLA, and (3) the so-called strain wedge model adopted from the literature. Simulation results were compared with each other and with field measurements. It was observed that all of the three numerical approaches yielded reasonably accurate predictions for these small diameter shafts. We provide p-y curves in the API format calibrated to the test data and show that those curves improve the accuracy of predictions relative to generic p-y curves in commonly used design guidelines published by the American Petroleum Institute (API).

The p-y curves obtained from the experiments are shown to differ from what would be predicted using standard API models, with the data indicating a stronger and stiffer response at shallow depths where the shaft-soil interaction is most pronounced. We also compare results of various tests to evaluate head fixity effects on p-y curves and the adequacy of the diameter effect built into API p-y guidelines.

This research involved analysis and field testing of numerous foundation support components for highway bridges. Two classes of components were tested - cast-in-drilled-hole (CIDH) reinforced concrete piles (drilled shafts) and an abutment backwall. The emphasis of this document (Part II of the full report) is abutment backwall elements.

The backwall test specimen was backfilled to a height of 5.5 up from the base of the wall with well-compacted silty sand backfill material (SE 30). The wall is displaced perpendicular to its longitudinal axis. Wing walls are constructed with low-friction interfaces to simulate 2D conditions. The backfill extends below the base of the wall to ensure that the failure surface occurs entirely within the sand backfill soil, which was confirmed following testing. The specimen was constructed and tested under boundary conditions in which the wall was displaced laterally into the backfill and not allowed to displace vertically.

A maximum passive capacity of 497 kips was attained at a wall displacement of about 2.0 in, which corresponds to a passive earth pressure coefficient Kp of 16.3. Strain softening occurs following the peak resistance, and a residual resistance of approximately 460 kips is achieved for displacements > 3.0 inch. The equivalent residual passive earth pressure coefficient is Kp = 15.1 and the equivalent uniform passive pressure at residual is approximately 5.1 ksf, which nearly matches the value in the 2004 Seismic Design Criteria of 5.0 ksf. The average abutment stiffness K50 was defined as a secant stiffness through the origin and the point of 50% of the ultimate passive force. For an abutment wall with a backfill height H of 5.5 ft, this stiffness was found to be K50 = 50 kip/in per foot of wall. The load-deflection behavior of the wall-backfill system is reasonably well described by a hyperbolic curve.

The passive pressure resultant is under predicted using classical Rankine or Coulomb earth pressure theories. Good estimates of capacity are obtained using the log-spiral formulation and the method-of-slices. The method-of-slices approach is implemented with a log-spiral hyperbolic method of evaluating backbone curves that provides a good match to the data.