Analytical Method for Load-Transfer Characteristics of Rock-Socketed Drilled Shafts

The load-settlement behavior of rock-socketed drilled shafts under axial loading is investigated by a load-transfer approach. Special attention is given to the shear load-transfer function and an analytical method for estimating load-transfer characteristics of rock-socketed drilled shafts. A nonlinear triple curve is employed to determine the shear load-transfer function of rock-socketed drilled shafts based on the constant normal stiffness direct shear tests and the Hoek-Brown failure criterion. An analytical method that takes into account the soil coupling effect was developed using a modified Mindlin’s point load solution. Through comparisons with field case studies, it is found that the proposed methodology in the present study is in good agreement with the general trend observed by in situ measurements and, thus, represents a significant improvement in the prediction of drilled shaft shear behavior.     

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.     

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.     

New Failure Load Criterion for Large Diameter Bored Piles in Weathered Geomaterials

In the literature, various “failure criteria” or methods of estimating the failure load in pile loading tests have been proposed. The criteria, based on varying assumptions, were intended for different methods of pile testing and were verified on tests of a variety of pile types and sizes. Most of the criteria were not developed for slow maintained loading tests of large-diameter (greater than 0.6 m) and long bored piles. Piles of this kind have considerable resistance, and it is often impractical to reach failure load as defined by the various criteria. In this paper, a total of 38 large-diameter bored piles (drilled shafts) that were tested, ranging from 0.6 to 1.8 m in diameter, varying from 12 to 66 m in depth, and founded in weathered geomaterials (rocks and saprolites), are critically reviewed and studied. Among them, a selection of seven pile load tests is examined in detail by using different existing failure criteria and specifications. The tests were chosen for their high degree of mobilization of pile capacity and the availability of reliable load-movement relationships. Specific aspects of pile behavior, such as the mobilization of toe resistance and shaft shortening, are also investigated using 31 loading tests to develop a new failure load criterion. The writers were heavily involved with the construction, testing, and analysis of 15 of the 38 piles. From the results of the study, a new nonsubjective, semiempirical method is proposed for estimating the approximate interpreted failure loads for piles founded in weathered geomaterials. The method is based on a moderately conservative estimation of the movement required to mobilize toe resistance and incorporates observations of shaft shortening from pile loading tests. Generally, the new method may allow more effective and consistent designs for large-diameter bored piles in weathered geomaterials.     

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.     

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.     

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.     

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.     

Capacity of Drilled Shafts in Burlington Limestone

Field load tests of three drilled shafts socketed in Burlington limestone were conducted using the Osterberg load cell. The objective of the testing was to compare the load capacities obtained from the field load tests with load capacities predicted using empirical methods. Based on the results of this study, the following conclusions can be drawn. The observed values of unit side resistance exceeded predicted empirical values for two of the three shafts tested (2,343 and 2,278 kPa observed versus 1,550 and 1,252 kPa predicted). However, for one of the shafts, the observed value of unit side resistance was only about ? of the more conservative predicted empirical value (916 kPa observed versus 1,252 predicted). Bearing capacity failure did not occur for two of the three shafts. Bearing capacity failure may have occurred for one of the shafts. Using a factor of safety of 3 applied to the lowest observed value of end bearing pressure implies that the allowable bearing capacity for the Burlington limestone at this site (3 MPa, or ?500 psi) exceeds the typical presumptive bearing capacity for sound limestone in mid-Missouri (1914 kPa or 277 psi).     

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.     

p-y Criterion for Rock Mass

Drilled shafts socketed in rock mass have been used frequently as a foundation system to support both vertical and lateral loads. Traditionally, the lateral interaction between the drilled shaft and the surrounding rock medium has been characterized by means of nonlinear p-y curves; however, there is a lack of well verified p-y criterion for rock mass. In this paper, a hyperbolic p-y criterion is developed based on both theoretical derivations and numerical (finite element) parametric analysis results. The methods for determining pertinent rock parameters needed for constructing the proposed p-y curves are presented in the paper. Two full-scale lateral load tests on large diameter, fully instrumented drilled shafts socketed in rock conducted by the writers, together with additional four load test results reported by Gabr et al. were used to validate the applicability of the proposed hyperbolic p-y curves for rock mass. The comparisons between the computed shaft responses (both deflections and bending moments) and the actual measured responses are considered acceptable.     

Roughness and Unit Side Resistances of Drilled Shafts Socketed in Clay Shale and Limestone

Rock socketed drilled shafts are being used increasingly to support heavily loaded structures. Rock sockets provide resistance to the load through a combination of side and base resistances. In this study, the effect of drilling tools such as an auger and a core barrel on the unit side resistance was investigated. A total of four field studies were performed on clay shale (compressive strength of 1–2?MPa) and limestone (compressive strength of 10?MPa). Borehole roughnesses produced by the different types of drilling tools in clay shale and limestone were measured using a laser borehole roughness profiler developed in this study to measure roughness to 0.5?mm in the boreholes. Based on the results of this study, it was observed that the drilling tools developed different socket roughnesses, which in turn affected the side resistances of the rock socketed drilled shafts.     

Load Transfer Curves along Bored Piles Considering Modulus Degradation

The load-transfer (or t-z) curve, which reflects the interaction between a pile and the surrounding soil, is important for evaluating the load-settlement response of a pile subjected to an axial load using the load-transfer method. Preferably, the nonlinear stress-strain behavior of the soil should be incorporated into the t-z curve. This paper presents a practical approach for the estimation of t-z curves along bored piles by considering the nonlinear elastic properties and modulus degradation characteristics of the soil. A method for evaluating the modulus degradation curve from the results of a pressuremeter test is proposed. The results of load tests on one instrumented bored pile in Piedmont residual soil in Atlanta and another in the residual soil of the Jurong Formation in Singapore provide verification of the validity of the proposed approach.     

Influence of Torque on Lateral Capacity of Drilled Shafts in Sands

As a result of recent changes in the requirements involving hurricane extreme events (e.g., wind velocities), the Florida Department of Transportation has moved away from cable-stayed signs, signals, and lights systems to mast arm/pole structures. Unfortunately, the newer systems develop significant lateral and torque loading on their foundations (e.g., drilled shafts). Current design practice for a mast arm/pole foundation is to treat lateral loading and torsion separately (i.e., uncoupled); however, recent field-testing suggests otherwise. This paper reports on the results of 91 centrifuge tests. 54 of the tests were conducted in dry sand and 37, in saturated sands. The tests varied the lateral load to torque ratios, shaft embedment depths, and soil strengths. The experiments revealed that even though the torsional resistances of the shafts were not influenced by lateral load, the shafts’ lateral resistance was significantly impacted by torsion. Reductions in lateral resistance of 50% were recorded for shafts under high torque to lateral load ratios. Using the free earth support assumption and the ultimate soil pressure the soil pressure distribution along the shaft was developed. Using force and moment equilibrium, as well as the applied torque, maximum shear, and moments were computed. The predicted values were found to be within 25% (10% on average, except for the tests in saturated dense sand with polymer slurry) of the experimental results.     

Curvature and Bending Moments from Inclinometer Data

Bending moments in excavation support walls and deep foundations are frequently estimated using curvature derived from inclinometer surveys. The basic idea involves fitting a curve or a series of curves to the inclinometer data. The curvature is then estimated as the second difference of the displacement profile. Although many methods are available to estimate curvature, there is a lack of a consistent standard in deducing curvature from inclinometer readings. Twelve methods for estimating curvature that lend themselves well to spreadsheet or programmable calculations are reviewed. They are then applied to 60 sets of inclinometer readings obtained from a variety of walls and drilled shafts. Included are six laterally load tested drilled shafts which were monitored using both inclinometers and strain gauges. A direct comparison of curvatures from inclinometers with those from strain gauges was performed. The comparisons show that a piecewise cubic polynomial curve fitting a moving window of five successive inclinometer data points provide curvature values that are in good agreement with those from strain gauges, and is proposed as a reasonable method for estimating bending moments from inclinometer data.     

Field Behavior of an Integral Abutment Bridge Supported on Drilled Shafts

The abutments of integral bridges are traditionally supported on a single row of steel-H-piles that are flexible and that are able to accommodate lateral deflections well. 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 drilled shaft foundations. A drilled shaft-supported integral abutment bridge was monitored from foundation installation to in-service behavior. Strain gauge data indicate that drilled shaft foundations worked well for this integral bridge. After 45 months, the drilled shafts appear to remain uncracked. However, inclinometer readings provide a conflicting viewpoint. Full passive earth pressures never developed behind the abutments as a result of temperature loading because thermal movements were small and the long term movements were dominated by concrete creep and shrinkage of the superstructure that pulled the abutments towards the stream. In the stream, hydrodynamic loading during the wet season had a greater effect on the abutment movements than seasonal temperature cycling. After becoming integral, the upright members of the longitudinal bridge frame were not vertical because the excavation and backfilling process caused deep seated movements of the underlying clay resulting in the drilled shafts bellying out towards the stream. This indicates the importance and need for staged construction analysis in design of integral bridges in highly plastic clays. Also, the drilled shaft axial loads from strain gauges are larger than expected.     

Critical Evaluation of Compression Interpretation Criteria for Drilled Shafts

This paper is a critical evaluation of the interpretation criteria of drilled shafts under axial compression loading. A wide variety of load test data are used for analysis, and these data are divided into drained and undrained databases. The interpretation criteria are examined from these load test results to establish a consistent compression interpretation criterion. Among these criteria, the range of each interpretation method presents approximately the same trend for both drained and undrained conditions. The statistical results show that the smaller the compression displacement, the higher the coefficient of variation. Moreover, the undrained load test results reveal less variability than the drained results. The load-displacement curve of a drained loading also demonstrates more ductility than that for undrained loading. Based on these analyses, the relative merits and interrelationships of these criteria are established, and specific design recommendations for the interpretation of compression drilled shaft load test, in terms of both capacity and displacement, are given.     

Case History Evaluation of Laterally Loaded Piles

This paper examines seven case histories of load tests on piles or drilled shafts under lateral load. Since the current design software to estimate lateral load resistance of deep foundations requires p-y curves. The first approach used was correlative whereby soil parameters determined from in situ tests [standard penetration test (SPT) and cone penetration test (CPT)] were used as input values for standard p-y curves. In the second approach p-y curves were calculated directly from the stress deformation data measured in dilatometer (DMT) and cone pressuremeter tests. The correlative evaluation revealed that, on the average, predictions based upon the SPT were conservative for all loading levels, and using parameters from the CPT best predicted field behavior. Typically, predictions were conservative, except at the maximum load. Since traditionally SPT and CPT correlation-based p-y curves are for “sands” or “clays,” this study suggests that silts, silty sands, and clayey sands should use cohesive p-y curves. For the directly calculated curves, DMT derived p-y curves predict well at low lateral loads, but at higher load levels the predictions become unconservative. p-y curves derived from pressuremeter tests predicted well for both “sands” and “clays” where pore pressures are not anticipated.     

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.     

Numerical Solution for Laterally Loaded Piles in a Two-Layer Soil Profile

Piles are often embedded in a layered soil profile, such as sand or clay layer underlain by rock. Several existing solutions are available for laterally loaded piles in a layered soil system. However, these solutions are only applicable to constant soil stiffness for each layer. In this paper, a variational approach is employed to numerically solve the problem of laterally loaded piles in layered soils using beam on an elastic foundation model. The soil stiffness can be either constant with depth or linearly varying with depth. The numerical solution is validated against an existing solution for linearly varying soil stiffness in a single soil layer system and an existing solution for a two-layer soil system with constant soil stiffness. Case studies using the proposed solution for field lateral load tests on full size drilled shafts embedded in weak rock with an overlying sand layer are presented. The simplicity and the relative ease of using the solution make it a good alternative approach for estimating the deflection and moment responses of a laterally loaded pile in a two-layer soil profile.