Lance A. Roberts

Probabilistic Axial Load Displacement Relationships for Drilled Shafts

The utility of reliability based design (RBD) methodology, such as the load and resistance factor design (LRFD) approach, is being increasingly recognized for the design of drilled shafts. The LRFD approach of drilled shaft design has the following advantages: (1) foundation design is easier and more efficient when the structure is designed using the LRFD method, as the load combinations need not be redefined, and (2) reliability may be rationally incorporat ed into the design process. In this paper, we develop a probabilistic mathematical model for drilled shaft load-displacement behavior. The soil-drilled shaft interaction is considered explicitly in the model development along the lines of the “t -z” approach. However, to ensure model simplicity, we consider the shaft -soil interface to be homogeneous and ideally elasto-plastic. Consequently, closed form expressions are derived for drilled shaft load -displacement behavior. These expressions are given in terms of the shear modulus of shaft-soil interface sub-grade reaction K,

Cyclic Loading Effects on the Creep and Dilation of Salt Rock

The Solution Mining Research Institute (SMRI) has embarked on inquiries into the effect cyclic loading might have on salt. This interest stems from the concept of using salt caverns as a storage medium for renewable energy projects such as compressed air energy storage where daily pressure cycles in the cavern are conceivable as opposed to the seasonal cycles that are typical for natural gas storage projects. RESPEC and the Institut für Aufbereitung und Deponietechnik at Clausthal University of Technology jointly executed a rock mechanics laboratory study using both facilities for performing triaxial cyclic loading creep tests on rock salt recovered from the Avery Island Mine in Louisiana, USA. The cyclic triaxial creep tests were performed under various load paths including compression, extension, and compression/extension. The tests were performed under both dilative and nondilative stress regimes. The cyclic compression creep data were compared to static creep tests performed under similar conditions to assess the effect of cycling of the applied stress. Furthermore, the cyclic compression tests were compared to a numerically simulated static creep test at the same stress and temperature conditions to determine if the creep behavior was similar under cyclic loading.

Design Methodology for Axially Loaded Auger Cast-in-Place and Drilled Displacement Piles

AbstractWith the increasing use of augered cast-in-place and drilled displacement piles in new construction, it is important that proper design parameters be incorporated when evaluating pile performance using reliability-based design methods. Although augered piles can be distinguished from bored piles, including drilled shafts, and driven piles by the magnitude of the effective stress changes they produce in the vicinity of the pile during construction, the current design methods for augered piles generally use the same design methods as those used for bored or driven piles. To enhance the efficiency of the augered piles, a unique design method must be developed. This paper focuses on developing a design methodology for axially loaded augered piles installed in predominately sandy soils using the t-z method. To develop the design parameters for augered piles, back-calculation of the t-z parameters was conducted using static load–test data. The data from 17 static pile load tests conducted on augered pil...

Performance-Based Design of Deep Foundation Systems in Load and Resistance Factor Design Framework

Performance-based design methods are increasingly being used for the design and analysis of structures. For performance-based design of deep foundation systems, limiting tolerable movements are one of the design criteria that can be used in lieu of the usual ultimate load capacity. The limiting tolerable movement is often selected to correspond to a movement that will cause damage to a structure but will not produce a collapsible failure of the structure. Performance-based design principles can be extended to include the axial design of deep foundation systems under strength and service loading conditions. In this extended approach, limiting tolerable settlements can be selected to correspond to a movement that will either cause excessive stresses in the structure or render a structure functionally inoperable because of serviceability limits. A performance-based design approach for axial design of deep foundations under the strength and serviceability limit states was developed. The design approach is integrated with the load and resistance factor design framework to develop an efficient and consistent methodology for satisfying both limit state criteria.

Performance-Based Design of Drilled Shaft Bridge Foundations

The design of deep foundations for bridge structures should ideally consider the interaction between the soil and the foundation element to properly model the side and tip resistance components. A soil-structure interaction model, such as the t-z model method, can be utilized to develop a load-displacement curve for a deep foundation subjected to axial loads. The load-displacement curve represents the behavior of the deep foundation over a range of applied axial loads. More importantly, however, the load-displacement curve can be analyzed by using performance-based design criteria, such as a tolerable displacement corresponding to the service limit state. The tolerable displacement of an individual deep foundation element can be selected to correspond to a movement that would cause adverse performance, excessive maintenance issues, or functionality problems with the bridge structure. In this paper, a performance-based soil-structure interaction design approach for the strength and service limit state axia...

Static load test interpretation using the t-z model and LRFD resistance factors for auger cast-in-place (AC IP) and drilled displacement (DD) piles

Abstract Static load test data for augered cast-in-place (ACIP) and drilled displacement (DD) piles were interpreted using the t-z model method. Eighteen load tests consisting of load-settlement and load transfer data were obtained from two different construction sites. A series of back-calculations were performed using the t-z model method to enhance the static load test data, which were then analyzed to obtain resistance bias factors for the currently used ACIP and DD pile design methods. The t-z model method enhanced load test data provides the ability to interpret field data for loading conditions beyond those measured during test. The resistance bias for the various design methods were then used to obtain the resistance factors based upon the traditional calibration approach. Furthermore, resistance factors were also obtained using probabilistic analysis based upon the t-z model method. The comparison of these two sets of resistance factor indicates that those obtained from the traditional calibration approach contain a wide range of magnitudes, while the resistance factors from the t-z model method design result in a consistent level of reliability.

LRFD of shallow foundations at the service limit state

The design of shallow foundations typically involves two-steps: (1) the calculation of ultimate bearing capacity based on general bearing capacity theories and (2) the calculation of maximum contact pressure to produce an allowable total settlement. The allowable bearing capacity for the shallow foundation system is the lesser of the ultimate bearing capacity, divided by a factor of safety, and the computed maximum allowable contact pressure. Currently, these calculations utilise nominal values of soil strength and stiffness parameters and do not explicitly account for their uncertainty. This paper develops a method for the calibration of resistance factors for load and resistance factor design (LRFD) of shallow foundations at the service limit state. The bearing soil strength and stiffness parameters are assumed to be random variables and the standard elastic settlement equations are combined with the Monte Carlo simulation technique to develop a series of probabilistic pressure–settlement curves. Based on an allowable magnitude of total settlement, the pressure–settlement curves are analysed to compute the pertinent statistics for an allowable bearing resistance that are then utilised to develop the resistance factors. The computed resistance factors are observed to be highly variable and are dependent on design uncertainty and shallow foundation size.

Characterization of t-z Parameters and Their Variability for Auger Pressure Grouted Piles Using Field Load Test Data

With the increasing use of auger pressure grouted (APG) piles in new construction, it is important that proper design parameters be incorporated when evaluating pile capacity and performance using reliability-based design methods. This paper focuses upon developing t-z model parameters and their variability from analysis of static axial compression tests on APG piles. Data from ten static pile load tests conducted on APG piles from construction sites and literature were obtained. Load-settlement curve fittings between the measured data and the theoretical data were conducted using the t-z method to back-calculate a set of soil-pile interfacial and tip parameters. The t-z model parameters are compared to standard field investigation data, including standard penetration test (SPT), cone penetration test (CPT) and effective stress of the ground to develop correlations suitable for the design of APG piles. Subsequently, the t-z model for APG pile design was developed and the model efficiency was evaluated by comparing the predicted and measured load-settlement and load transfer curves.

Design of Bridge Foundations using a Performance-Based Soil-Structure Interaction Approach

The design of deep foundations must consider soil-structure interaction in order to properly model the side and tip resistance components. A soil-structure interaction model such as the t-z model method allows for the settlement analysis of a deep foundation over a range of applied vertical loads. The developed load-settlement curve can be analyzed using performance-based design criteria, such as a limiting tolerable settlement and a serviceability settlement. The limiting tolerable settlement can be selected to correspond to a movement that will either cause excessive stresses in the structure or render a structure inoperable while a serviceability settlement would correspond to a movement that would cause adverse performance or excessive maintenance issues with the structure. In this paper, a performance-based soilstructure interaction design approach for axial design of deep foundations under the AASHTO Strength and Service Limit States is presented. The design approach can be integrated within the Load and Resistance Factor Design (LRFD) framework to develop an efficient methodology for satisfying these limit state criteria.

Evaluation of Traditional Lateral Pile Capacity Method Using an LRFD Approach

In this paper, the lateral pile capacity obtained from a traditional design method is compared to that obtained from an LRFD approach. A novel methodology for determining the resistance factors for use in the LRFD approach is developed using a prescribed lateral load-displacement model for piles, field measured load-displacement curves, and probabilistic analysis. The p-y method is used for modeling the lateral load-displacement behavior. Model parameters are back-calculated from the field load-displacement curves and their variability is assumed. Probabilistic analysis is performed to obtain load capacity distributions and resistance factors as a function of site variability and reliability index. These resistance factors are used to obtain the design resistance, which is compared to the design capacity obtained through the traditional approach. The design capacity from the traditional approach is found to be satisfactory only for low variability sites that in turn allow for low reliability indices.