Fred H. Kulhawy

Axial Compression of Footings in Cohesionless Soils. I: Load-Settlement Behavior

The results of 167 full-scale field load tests were used to examine several issues related to the load-displacement behavior of footings in cohesionless soils under axial compression loading, including (1) method to interpret the “failure load” from the load-settlement curves; (2) correlations among interpreted loads and settlements; and (3) generalized load-settlement behavior. The L1 - L2 method was found to be more appropriate than the “tangent intersection” and “10% of the footing width” methods for interpreting the failure load. The interpreted loads and displacements indicate that footing load-settlement behavior is less elastic and more nonlinear than that of drilled foundations. The results show that the footing behavior will be beyond the elastic limit for designs where a traditional factor of safety between 2 and 3 is used. A normalized curve was developed by approximating the load-settlement curve for each load test in the database by hyperbolic fitting, and the uncertainty in this curve was qu...

Reliability-based design for transmission line structure foundations

Abstract This paper presents one of the geotechnical initiatives in reliability-based code development that has been sponsored by the Electric Power Research Institute for transmission line structure foundations. The framework for the development of a practical reliability-based design approach is illustrated using the design of drilled shafts (bored piles) for uplift under undrained loading. A target reliability index of 3.2 is selected based on the reliability indices implied by existing working stress designs. Two simple design formats (load and resistance factor design and multiple resistance factor design) are rigorously calibrated using the first-order reliability method to produce designs that achieve a known level of reliability consistently.

Drilled Shaft Foundations

A drilled shaft, also known as drilled pier, drilled caisson, caisson, bored pile, etc., is a versatile foundation system that is used extensively on a worldwide basis. In its simplest form, a drilled shaft is constructed by making a cylindrical excavation, placing a reinforcing cage (when necessary), and then concreting the excavation. With available drilling equipment, shaft diameters up to 20 ft (6 m) and depths exceeding 250 ft (76 m) are possible. However, for most normal applications, diameters in the range of 3 to 10 ft (1 to 3 m) are typical. This size versatility allows a single drilled shaft to be used in place of a driven pile group and eliminates the need for a pile cap. In addition, normal construction practices for drilled shafts effectively eliminate the noise and strong ground vibrations that develop during pile driving operations. For these and other secondary reasons, drilled shafts have become both the technical and economic foundation of choice for many design applications. In fact, they have become the dominant foundation type in many geologic settings around the world.

The impact of wave loads and pore-water pressure generation on initiation of sediment transport

The build-up of pore-water pressure by waves can lead to sediment liquefaction and subsequent transport by traction currents. This process was investigated by measuring pore-water pressures both in a field experiment and laboratory wave tank tests. Liquefaction was observed in the wave tank tests. The results suggest that sand is less susceptible than silts to wave-induced liquefaction because of the tendency to partially dissipate pore-water pressures. However, previous studies have determined that pore-water pressures must approach liquefaction before current velocities necessary to initiate transport are reduced. Once liquefaction has occurred more sediment can be transported.

Axial Compression of Footings in Cohesionless Soils. II: Bearing Capacity

An extensive database of full-scale field load tests was used to examine the bearing capacity for footings in cohesionless soils. Each load test curve was evaluated consistently to determine the interpreted failure load (i.e., bearing capacity) using the L1 - L2 method. This test value then was compared with the theoretical bearing capacity, computed primarily using the basic Vesic model. The comparisons show that, for footing widths B>1 m , the field results agree very well with the Vesic predictions. However, for B<1 m , the results indicated a relationship between B and the predicted-to-measured bearing capacity ratio. Accordingly, a simple modification was made to the bearing capacity equation, and the resulting predictions are very good.

Reliability-Based Design Approach for Differential Settlement of Footings on Cohesionless Soils

A probabilistic method is presented to estimate the differential settlements of footings on cohesionless soils, considering the uncertainties in both the load and capacity sides of the design equation. A random field approach is employed to characterize the inherent soil variability. This method is first compared to typical limit values from the literature to denote critical combinations of design parameters that can lead to exceedance of tolerable differential settlements. Then, reliability-based design equations are developed for the serviceability limit state (SLS) design of footings on cohesionless soils. The key parameters controlling the SLS are the allowable angular distortion, site variability, and footing spacing. The results are given in a straightforward design format and indicate that currently suggested deformation factors (resistance factors for SLS) equal to 1.0 are likely to be unconservative for most design situations.

On the Axial Behavior of Drilled Foundations

In this chapter, four representative types of drilled foundations (drilled shafts, augered cast-in-place piles, pressure-injected footings, and micropiles) are examined from the standpoint of axial capacity evaluation, generalized behavior, and normalized load-displacement response. The author shows how the capacities can be computed rationally, taking into account the particular characteristics of each foundation type. The author notes that the results agree well with the interpreted failure load from field load tests. The interpreted failure load also is linked consistently and simply to the elastic limit and the conventional slope tangent methods for interpreting compression and uplift load test results. The displacements exhibit consistent patterns of behavior in compression and uplift, at both the interpreted failure load and elastic limit. The author concludes that these results will be useful for better understanding axial foundation behavior over a range of drilled foundation types.

Practical Reliability-Based Design Approach for Foundation Engineering

A research study was completed recently that was directed toward the development of practical, reliability-based design (RBD) equations specifically for foundation engineering. Some of the key RBD principles used in the study are presented. The important considerations involved in the development of practical and robust RBD criteria are emphasized. In particular, the selection of an appropriate reliability assessment technique and the careful characterization and compilation of geotechnical variabilities are important because of their central role in the calculation of the probability of failure and the assessment of the target reliability level. An overview of a simplified RBD approach is given, and an application of this approach to the ultimate limit state design of drilled shafts under undrained uplift loading is discussed.

Evaluation of Drained Axial Capacity for Drilled Shafts

The drained side and tip resistances of drilled shafts in cohesionless soils were evaluated from field load test case histories with axial uplift and compression loading. The side resistance was evaluated using the effective stress or beta (β) method, and the tip resistance in compression was evaluated using bearing capacity theory. For the side resistance, the results show that β values can reach 6.5 at shallow depth, but they decrease with depth. The β values in uplift and compression are essentially the same and generally vary by less than 4%. Comparing the measured and predicted (traditional) values of β, the predicted β values are low and overly conservative. For the tip resistance, the average ratio of the measured to predicted values is about 0.3 at a displacement/diameter of 4%. This ratio shows some scatter at shallow depths, but the scatter decreases with increasing depth. To mobilize the tip resistance fully, a displacement/diameter of about 10% is required. Based on these data and evaluations, design recommendations are proposed.

Geo-RBD for Foundations - Let's Do It Right!

The transition to reliability-based design (RBD) of foundations has not been smooth. Design engineers do not necessarily object to RBD. Instead, they are concerned that the new design process restricts or eliminates their flexibility to mod- ify the process depending on site- or project-specific conditions. The broad issues that relate to these concerns are discussed, and an effort is made to clarify some key points that illustrate the strengths and weaknesses of RBD implementations. ABSTRACT: The transition to reliability-based design (RBD) of foundations has not been smooth. Design engineers do not necessarily object to RBD. Instead, they are concerned that the new design process restricts or eliminates their flexibility to mod- ify the process depending on site- or project-specific conditions. The broad issues that relate to these concerns are discussed, and an effort is made to clarify some key points that illustrate the strengths and weaknesses of RBD implementations.