Search for: All records

Drilled Displacement Piles (DDP) provide an ideal foundation solution that combines the benefits of ground improvement with traditional advantages of piling systems. This paper offers insights gathered from 55 construction projects in which nearly 130 DDPs were installed and tested axially. High quality site exploration data (e.g., Cone Penetration Test (CPT) and Standard Penetration Test (SPT)) were evaluated to derive geotechnical analysis parameters. The test sites consisted of mostly mixed soil types with strongly stratified layers of sand, silt, and clay. Pile diameters ranged between 35 and 61 cm (14 to 24 inches). Prior to analyzing the axial performance of DDPs, a variety of failure interpretation methods were assessed to confidently extrapolate failure loads when field testing was terminated prior to pile failure. Results of this study suggested the Van der Veen’s (1953) method to most closely estimate the load that triggers pile plunging behavior specific to DDPs, followed by the Butler & Hoy (1977) and L1-L2 methods (Hirany and Kulhawy, 1989). Hereafter, in-situ axial load test results were compared with a wide range of analytical methods, including those developed specifically for DDPs. Predictive accuracy was assessed in terms of total pile capacity and pile settlement and separated based on pile diameter, stiffness, and soil type. Most examined analytical methods underpredict the in-situ pile capacities for both, CPT and SPT -based analysis. It was also found that the difference between the experimentally determined and predicted capacities is related to the level of improvement in the surrounding soil following pile installation. A general comparison between predictive axial accuracy and the observed level of ground improvement is also discussed for sandy and mixed type of soils. 

Embedded sensors within infrastructure elements are powerful catalysts for new designs and construction methods, enabling advanced data collection and informed decision making. This paper presents the development, validation, and implementation of a prototype instrumentation tool utilized in large-scale lateral load tests of rock-socketed pile foundations, with the objective to measure shear stresses near the rock-soil boundary. The proposed instrumentation is novel in that it will be the first attempt to determine experimentally the 3D strain field through embedded sensors with immediate application to a broad array of pile foundation engineering problems. Data obtained from the prototype instrumentation is used to clarify whether shear force amplifications in piles crossing soils with strong stiffness contrasts are real, or an artifact of analytical, Winkler-based design methodologies. Three reinforced concrete pile specimens with a diameter of 0.46 m and a length of 4.9 m were subjected to reverse cyclic lateral loading up to complete structural failure. The sensors’ development, design, and construction, as well as their performance in measuring shear stresses will be discussed by comparing experimental data with predictions from conventional software tools. Ultimately, this study aims to improve the design and construction of more practical, resilient, and economical infrastructure. 

Piles socketed into rock are frequently utilized to carry large loads from long-span bridges and high-rise buildings into solid ground. The pile design is derived from internal shear and moment magnitudes following code recommendation and numerical predictions. Little experimental data exist to validate code prescriptions and design assumptions for piles embedded in rock. To help alleviate the lack of large-scale test data, the lateral response behavior of three 18-in. diameter, 16 ft long, reinforced concrete piles was evaluated. The pile specimens were embedded in a layer of loose sand and fixed in “rock-sockets,” simulated through high strength concrete. The construction sequence simulated soil-pile interface stress conditions of drilled shafts. The pile reinforcement varied to satisfy the internal reaction forces per (1) code requirements, (2) analytical SSI predictions, and (3) structural demands only. The pile specimens were tested to complete structural failure and excavated thereafter. Internal instrumentation along with crack patterns suggested a combined shear-flexural failure, but do not support the theoretically predicted amplification and de-amplification of shear and moment forces at the boundary, respectively. 

The determination of internal pile reactions is critical to designing and assessing the structural performance of deep foundations. Internal shear and moment profiles strongly depend on lateral pile-soil interaction, which in turn depends on pile and soil stiffnesses as well as the stiffness contrast between soft and stiff strata, such as occurs at a soil/rock interface. At zones of strong geomaterial stiffness contrast, Winkler-spring-type analyses predict abrupt changes in the internal pile reactions for laterally-loaded foundation elements. In particular, the sudden deamplification of internal moments when transitioning from a soft to stiff layer is accompanied by amplification of pile shear. This “shear spike” can result in bulky transverse reinforcement designs for drilled shaft rock sockets that pose constructability challenges due to reinforcement congestion, increasing the risk of defective concrete on the outside of the cage. This paper presents an experimental research program of three large-scale, instrumented drilled shafts with simulated rock sockets constructed from concrete. Each shaft had a different transverse reinforcement design intended to bound the amplitude of the predicted amplified shear demand, with a particular emphasis on performance of shafts with shear resistance less than the predicted demand and below the code minimum. Test results suggested that the shafts experienced a flexure-dominated failure irrespective of the transverse reinforcement detailing.