Flexural Performance of Drilled Shafts with Minor Flaws in Stiff Clay

Lateral loads are often the primary forces that act on drilled shafts when they support retaining walls, bridge piers, or building foundations. The construction of drilled shafts often inadvertently introduces flaws that are not always detectable with well-performed nondestructive evaluation (NDE) techniques. The effect of such undetectable minor flaws on the lateral-load performance of drilled shafts needs to be assessed and subsequently considered in the design. This paper summarizes a field study that consisted of NDE of six, full-scale drilled shafts with preinstalled voids and lateral-load tests that were performed on the six test shafts. Results from the field study indicated that undetectable (by NDE) void flaws occupying areas of up to 15% of the cross-sectional area of the drilled shaft could reduce free-head shear capacity up to 16%. A subsequent numerical analysis was performed to filter out all variables, other than void flaws, that could affect the lateral-load deformation of drilled shafts. Numerical analysis results validated the field tests measurements. A parametric study of variables affecting the load-deformation behavior of drilled shafts suggests that a reduction in moment capacity of up to 27% is possible with undetected voids present in the shafts that were tested.     

Constant Force Shaking of a Group of Four Drilled Shafts

A foundation system comprising a group of four drilled shafts, each 750 mm in diameter by 5.5 m long, constructed in silt/sand/gravel with a rigid pile cap, was subjected to constant force shaking by a servo-hydraulic actuator reacting against a heavy anchor block. Forces of up to 150 kN peak to peak were achieved with resulting displacements of up to 2 mm and accelerations of up to 0.5g. The displacement response of the pile cap was measured over a range of frequencies from 2 to 12 Hz and the resonant frequency and damping ratio were determined for three different levels of excitation. The foundation behaved much like a simple harmonic oscillator with a natural frequency of approximately 10 Hz. This resonant frequency was found to decrease slightly with increasing excitation while the damping ratio decreased slightly. Values for generalized stiffness, mass, and damping for an equivalent simple harmonic oscillator were back-calculated from the test data. The results of this study suggest that it may be possible to predict values for generalized mass and damping a priori by using simple empirical methods.     

Effect of Construction on Axial Capacity of Drilled Foundations in Piedmont Soils

A program of field loading tests was conducted to measure the axial response of drilled foundations constructed using a variety of different drilling techniques. The research was performed at the Auburn University National Geotechnical Experimentation Site at Spring Villa, Ala. in Piedmont geology composed of silty soils formed by weathering of parent metamorphic rocks. A total of ten drilled shafts (0.9 m diameter by 11 m deep) were constructed using techniques including dry construction with casing advanced ahead of the hole and with drilling slurry composed of polymer fluids and mineral (bentonite) fluids. The results demonstrate the great potential influence that differing construction techniques may have on the load transfer in side shear of drilled foundations. The mineral slurry resulted in significantly lower side shear relative to the other techniques.     

Experimental Evaluation of Lateral-Load Resistance of Pile Caps

Although pile caps have considerable ability to resist lateral loads, this resistance is often neglected in design. Published cases involving a variety of pile and cap sizes, soil conditions, and loading conditions indicate that the lateral-load resistance of pile caps can be significant, but it is difficult to generalize on the basis of these results because of the variations in conditions involved in the tests. To develop a more systematic basis for evaluating cap resistance, a field test facility was constructed to perform full-scale lateral-load tests on single piles and pile groups, with the pile caps embedded in the stiff natural soil at the site and with the pile caps backfilled with granular soil. Thirty-one tests were conducted to evaluate the lateral-load resistance of pile caps by comparing the response of pile groups with caps fully embedded and with soil removed from around the caps. The results of the tests show that pile caps provide significant resistance to lateral load. This resistance depends primarily on the stiffness and strength of the soil in front of the cap and the depth of cap embedment.     

Drilled Shaft Defects: Detection, and Effects on Capacity in Varved Clay

This paper presents the results of nondestructive integrity tests (NDTs) and axial static load tests on drilled shafts constructed in varved clay at the National Geotechnical Experimentation Site in Amherst, Mass. The shafts were constructed with built-in defects to study: (1) the effectiveness of conventional NDT methods in detecting construction defects and (2) the effect of defects on the capacity of drilled shafts. Defects included voids and soil inclusions occupying 5–45% of the cross section as well as a soft bottom. Nine organizations participated in a blind defect prediction symposium, using a variety of NDT techniques. Most participants located defects that were larger than 10% of the cross sectional area. However, false positives and inability to locate smaller defects and multiple defects in the same shaft were encountered. Static load tests indicated that (1) minor defects had little or no effect on skin friction; (2) a soft bottom resulted in a 33% reduction in end bearing relative to a sound bottom; and (3) reloading resulted in a 20–30% reduction in the geotechnical capacity.     

Strain Wedge Model Capability of Analyzing Behavior of Laterally Loaded Isolated Piles, Drilled Shafts, and Pile Groups

This paper demonstrates the application of the strain wedge (SW) model to assess the response of laterally loaded isolated long piles, drilled shafts, and pile groups in layered soil (sand and/or clay) and rock deposits. The basic goal of this paper is to illustrate the capabilities of the SW model versus other procedures and approaches. The SW model has been validated and verified through several comparison studies with model- and full-scale lateral load tests. Several factors and features related to the problem of a laterally loaded isolated pile and pile group are covered by the SW model. For example, the nonlinear behavior of both soil and pile material, the soil-pile interaction (i.e., the assessment of the p-y curves rather than the adoption of empirical ones), the potential of soil to liquefy, the interference among neighboring piles in a pile group, and the pile cap contribution are considered in SW model analysis. The SW model analyzes the response of laterally loaded piles based on pile properties (pile stiffness, cross-sectional shape, pile-head conditions, etc.) as well as soil properties. The SW model has the capability of assessing the response of a laterally loaded pile group in layered soil based on more realistic assumptions of pile interference as compared to techniques and procedures currently employed or proposed.     

Case History: Capacity of a Drilled Shaft in the Atlantic Coastal Plain

This paper presents a single case history of a drilled shaft constructed in the Atlantic Coastal Plain deposits for a bridge foundation that was subjected to axial loading. The predicted nominal axial capacity is estimated based on state of practice empirically derived methods specified in the current AASHTO LRFD Bridge Design Specifications. Predictions are compared to observed soil resistance derived from a static load test conducted on a full-size instrumented test shaft using the Osterberg Cell method. The results suggest that the AASHTO specified prediction methods should be applied cautiously for drilled shafts in the Atlantic Coastal Plain, incorporating an appropriate in situ testing program for evaluating soil design parameters, considering variations from the specific geologic environment and construction methodology used to develop the specified prediction methods, accounting for the load-deformation behavior of the shaft, and providing for instrumented static load testing to measure the actual behavior of the drilled shafts.     

Data Reduction of Horizontal Load Full-Scale Tests on Bored Concrete Piles and Pile Groups

This paper proposes a new approach for data reduction of horizontal load full-scale tests on piles and pile groups. This approach has been developed on results from tests run on bored concrete piles embedded in homogeneous and nonhomogeneous ground. Due to nonlinear response of pile material and also to nonhomogeneous embedding ground, the problem of fitting reliable curves for representing strains along shafts is increased. It is suggested that B-splines fixed by a weighted least-squares algorithm should be used to overcome that problem. Taking advantage of the mathematical properties of B-splines, an algorithm for computing the internal force distribution amongst pile heads direct from test results is also proposed for pile groups. It is shown that the integration of the curvatures to compute pile movements should be done using natural boundary conditions instead of pile head measurements whenever possible. Despite the concrete crack, the distribution of bending moments can be computed from curvatures provided a reliable reinforced concrete model is used. Finally, it is proposed to compute the soil reactions by the integration of bending moments, solving an integral equation by again using B-spline functions.     

Numerical Study of an Integral Abutment Bridge Supported on Drilled Shafts

The majority of integral abutment bridges (IABs) in the United States are supported on steel H-piles to provide the flexibility necessary to minimize the attraction of large lateral loads to the foundation and abutment. In Hawaii, steel H-piles have to be imported, corrosion tends to be severe in the middle of the Pacific Ocean, and the low buckling capacity of steel H-piles in scour-susceptible soils has led to a preference for the use of concrete deep foundations. A drilled shaft-supported IAB was instrumented to study its behavior during and after construction over a 45-month period. This same IAB was studied using the finite-element method (FEM) in both two- (2D) and three dimensional (3D). The 3D FEM yields larger overall pile curvature and moments than 2D because in 3D, the high plasticity soil is able to displace in between the drilled shafts thereby “dragging” the shafts to a more highly curved profile while soil flow is restricted by plane strain beam elements in 2D. Measured drilled shaft axial loads were higher than the FEM values mainly due to differences between the assumed and actual axial stiffness and to a lesser extent on concrete creep in the drilled shafts and uneven distribution of loads among drilled shafts. Numerical simulations of thermal and stream loadings were also performed on this IAB.     

Side Resistance In Piles and Drilled Shafts

More than 20 years have passed since a Terzaghi Lecture focused on the topic of deep foundations. However, considerable research has been performed, and experience gained, in this subject area in the intervening period. The objective of this paper is to update the earlier references on deep foundations by summarizing results of important recent research on a few aspects of the topic of side resistance, most notably (1) driven piles in saturated clay, (2) driven piles in siliceous sand loaded in compression and uplift, (3) drilled shafts in clay, and (4) drilled shafts in soft rock. It is concluded that, while simple design relations are available for topic (1), much is still to be learned. Under topic (2), the case is made that loading the pile in compression and uplift produces different values of unit side-shearing resistance. Regarding topics (3) and (4), the effects of details related to construction—such as stress relief, moisture migration from the concrete to the geomaterial, borehole roughness, and borehole smear—are shown to be significant. The final point made is that the design of deep foundations is a complex matter that should be addressed in a design context by engineers who are experienced in the observation of pile behavior, theoretical modeling, and the appropriate use of design methods.     

Nonlinear Efficiency of Bored Pile Group under Lateral Loading

A 3×3 bored pile group consisting of nine cast-in-drilled-hole reinforced concrete shafts and a comparable single-shaft were subjected to reversed cyclic, lateral head loading to investigate group interaction effects across a wide range of lateral displacements. The piles had the same diameter of d = 0.61?m and similar soil conditions; however, various equipment constraints led to two differences: (1) a fixed head (zero rotation) boundary condition for the single pile versus minor pile cap rotation in the vertical plane for the group and (2) shaft longitudinal reinforcement ratios of 1.8% for the single pile and 1% for the group piles. To enable comparisons between the test results, a calibrated model of the single pile (1.8% reinforcement) was developed and used to simulate the response of a single shaft with 1% reinforcement. Additional simulations of the pile group were performed to evaluate the effects of cap rotation on group response. By comparing the simulated responses for common conditions, i.e., 1% reinforcing ratio and zero head rotation, group efficiencies were found to range from unity at lateral displacements <0.004×d to 0.8 at small displacements ～ 0.01–0.02×d and up to 0.9 at failure (displacements >0.04×d). Hence, we find that group efficiency depends on the level of nonlinearity in the foundation system. The general group efficiency, although not its displacement-dependence, is captured by p-multipliers in the literature for reinforced concrete, fixed-head piles.     

Liquefaction-Induced Settlement of Pile Groups in Liquefiable and Laterally Spreading Soils

The results of a series of dynamic centrifuge tests on model pile groups in (level) liquefied and laterally spreading soil profiles are presented. The piles are axially loaded at typical working loads, which has enabled liquefaction-induced settlements of the foundations to be studied. The development of excess pore pressures within the bearing layer (dense sand) was found to lead to a reduction in pile capacity and potentially damagingly large coseismic settlements. As the excess pore pressure increased, these settlements were observed to exceed postshaking downdrag-induced settlements, which occur due to the reconsolidation of liquefied sand around the pile shaft. In resisting settlement, the pile cap was found to play an important role by compensating for the capacity lost by the piles. This was shown to be achieved by the development of dilative excess pore pressures beneath the pile cap within the underlying loose liquefied sand which provide increasing bearing capacity with settlement. The centrifuge test data show good qualitative and quantitative agreement with the limited amount of model and full-scale data currently available in the literature. The implications of settlement for the design of piled foundations to serviceability conditions in both level and sloping ground are discussed, with settlement becoming an increasingly important consideration for laterally stiffer piles. Finally, empirical relationships have been derived from the test data to relate suitable static safety factors to given increases in excess pore pressure in the bearing layer within a performance-based design framework (i.e., based on limiting displacements).     

Relationship between Texas Cone Penetrometer Tests and Axial Resistances of Drilled Shafts Socketed in Clay Shale and Limestone

Modern methods for designing drilled shafts in soft rock require knowledge of the compressive strength and modulus of the rock. However, rock jointing at many sites prohibits the recovery of samples of sufficient length and integrity to test rock cores in either unconfined or triaxial compression tests. Since rational design procedures usually require values of compressive strength, surrogate methods must be employed to estimate the compressive strength of the rock. The surrogate methods considered in this study was Texas cone penetrometer tests, and performed at several sites in North Central Texas. In order to develop the relationships between Texas cone penetrations and side and base resistances of rock socketed drilled shafts, three field load tests were conducted. Based on the field study and literature reviews, a relationship between Texas cone penetration tests and axial resistances of rock socketed drilled shafts was proposed.     

Cyclic Lateral Load Behavior of a Pile Cap and Backfill

A series of static cyclic lateral load tests were performed on a full-scale 4×3 pile group driven into a cohesive soil profile. Twelve 324-mm steel pipe piles were attached to a concrete pile cap 5.18×3.05?m in plan and 1.12?m in height. Pile–soil–pile interaction and passive earth pressure provided lateral resistance. Seven lateral load tests were conducted in total; four tests with backfill compacted in front of the pile cap; two tests without backfill; and one test with a narrow trench between the pile cap and backfill soil. The formation of gaps around the piles at larger deflections reduced the pile–soil–pile interaction resulting in a degraded linear load versus deflection response that was very similar for the two tests without backfill and the trenched test. A typical nonlinear backbone curve was observed for the backfill tests. However, for deflections greater than 5 mm, the load-deflection behavior significantly changed from a concave down shape for the first cycle to a concave up shape for the second and subsequent cycles. The concave up shape continued to degrade with additional cycles past the second and typically became relatively constant after five to seven cycles. A gap formed between the backfill soil and the pile cap, which contributed to the load-deflection degradation. Crack patterns and sliding surfaces were consistent with that predicted by the log spiral theory. The results from this study indicate that passive resistance contributes considerably to the lateral resistance. However, with cyclic loading the passive force degrades significantly for deflections greater than 0.5% of the pile cap height.     

Reduction Factor for the Unloading Point Method at Clay Soil Sites

Full-scale testing can be an integral component of quality control/quality assurance for projects involving construction of deep foundations. Rapid load tests are being used in the deep foundation industry as a method for assessing the axial static behavior of deep foundations. Since rapid load tests involve dynamics, inertial and damping forces must be considered in analyzing measured pile response to estimate the static pile response. The unloading point method (UPM) is typically used for this purpose. Generally considered a consequence of load rate effects in clays, results from the UPM must be further modified by a reduction factor to obtain a reasonable estimate of the static pile response. A reduction factor of 0.65 applied to the UPM for clay soil sites has been recommended by others. However, a review and analysis of readily available literature reporting static and rapid pile load test results at sites predominantly consisting of clay soils indicate that an average reduction factor of 0.47 is more appropriate. Rapid load testing should be used judiciously. When using the UPM to estimate static pile capacity from rapid load tests in clay, static load tests should be performed to validate the reduction factor used to interpret rapid load tests.     

Lateral Load Capacity of Drilled Shafts in Jointed Rock

Large vertical (axial) and lateral loads often act on the heads of drilled shafts in jointed rock. In current design practice, the p-y curve method used in design of laterally loaded drilled shafts in soil is often also used for shafts in jointed rock. The p-y curve method treats the soil as a continuum, which is not appropriate in jointed rock, particularly when failure occurs due to sliding on joints. A new discontinuum model was developed to determine the lateral load capacity of drilled shafts or piers in a jointed rock mass with two and three joint sets. It consists two parts: a kinematic and a kinetic analysis. In the kinematic analysis, Goodman and Shi’s block theory is expanded to analyze the removability of a combination of blocks laterally loaded by a pier. Based on the expanded theory, a method was developed to select removable combinations of blocks using easily constructed two-dimensional diagrams. In the kinetic analysis, each kinematically selected removable combination of blocks is examined with the limit equilibrium approach to determine the ultimate lateral load capacity. Although the procedure is similar to slope stability analysis, it is more complicated with the addition of a lateral force and the vertical load exerted by the pier. Simple analytical relations were developed to solve for the ultimate lateral load capacity.     

Behavior of Slender Piles Subject to Free-Field Lateral Soil Movement

Soil movements associated with slope instability induce shear forces and bending moments in stabilizing piles that vary with the buildup of passive pile resistance. For such free-field lateral soil movements, stress development along the pile element is a function of the relative displacement between the soil and the pile. To investigate the effects of relative soil-pile displacement on pile response, large-scale load tests were performed on relatively slender, drilled, composite pile elements (cementitious grout with centered steel reinforcing bar). The piles were installed through a shear box into stable soil and then loaded by lateral translation of the shear box. The load tests included two pile diameters (nominal 115 and 178?mm) and three cohesive soil types (loess, glacial till, and weathered shale). Instrumentation indicated the relative soil-pile displacements and the pile response to the loads that developed along the piles. Using the experimental results, an analysis approach was evaluated using soil p-y curves derived from laboratory undrained shear strength tests. The test piles and analyses helped characterize behavioral stages of the composite pile elements at loads up to pile section failure and also provided a unique dataset to evaluate the lateral response analysis method for its applicability to slender piles.     

Numerical Study of the Effect of Verification Core Hole on the Point Bearing Capacity of Drilled Shafts

This paper presents a numerical investigation of the effect of a verification core hole on the point bearing capacity of drilled shafts installed in clay shales. The verification core extracted at the shaft tip may reduce the point bearing capacity of drilled shafts as a result of degradation of clay shales and imperfect core hole infill. Finite-element analyses were conducted using the Mohr-Coulomb model with total stress material parameters estimated from laboratory tests. A series of load-displacement curves was calculated for 1 cycle of air drying and wetting; different drying durations and different core hole conditions were considered; and the point bearing capacity was determined at 3 and 5% shaft diameter displacements. The numerical analyses indicate that the point bearing capacity of drilled shafts with a verification core hole does not decrease for most cases, and the maximum reduction merely reaches 5%. Recommendations are made to reduce the effect of the verification core extracted at the shaft bottom during construction.     

Liquefaction-Induced Softening of Load Transfer between Pile Groups and Laterally Spreading Crusts

Laterally spreading nonliquefied crusts can exert large loads on pile foundations causing major damage to structures. While monotonic load tests of pile caps indicate that full passive resistance may be mobilized by displacements on the order of 1–7% of the pile cap height, dynamic centrifuge model tests show that much larger relative displacements may be required to mobilize the full passive load from a laterally spreading crust onto a pile group. The centrifuge models contained six-pile groups embedded in a gently sloping soil profile with a nonliquefied crust over liquefiable loose sand over dense sand. The nonliquefied crust layer spread downslope on top of the liquefied sand layer, and failed in the passive mode against the pile foundations. The dynamic trace of lateral load versus relative displacement between the “free-field” crust and pile cap is nonlinear and hysteretic, and depends on the cyclic mobility of the underlying liquefiable sand, ground motion characteristics, and cyclic degradation and cracking of the nonliquefied crust. Analytical models are derived to explain a mechanism by which liquefaction of the underlying sand layer causes the soil-to-pile-cap interaction stresses to be distributed through a larger zone of influence in the crust, thereby contributing to the softer load transfer behavior. The analytical models distinguish between structural loading and lateral spreading conditions. Load transfer relations obtained from the two analytical models reasonably envelope the responses observed in the centrifuge tests.     

Impulse Response Evaluation of Foundations using Multiple Geophones

A new method to nondestructively evaluate existing deep foundations is described for the situation where access to the top of a deep foundation is prevented by an intervening structure. The test consists of striking a surface of a structure and simultaneously recording velocities with vertical geophones at several locations on an impacted surface. Arranging the geophones at different distances from both the impact location and the source of surface wave reflections allows one to minimize the interfering effects of surface waves on recognizing the compression wave reflections from an underlying deep foundation. The processed data can be evaluated as either a conventional sonic echo or impulse response test. Results of field tests of the system are described for groups of shafts at the National Geotechnical Experimentation Site at Northwestern University. Results of these tests indicate that there are preferential locations for the geophones that enhance the interpretation of the condition of an underlying drilled shaft. A case study is presented that describes the application of the method to evaluate possible concrete deterioration of existing bridge piers.