UC Irvine

Constructing DDPs involves a drilling process which combines the application of an axial force with the generation of a simultaneous torque to create a borehole in which the soil is first loosened (drilled) and then displaced towards the outside of the borehole using a drilling rod with integrated displacement body. This installation method has the potential to induce soil densification around the borehole vicinity, the magnitude of which depends on the soil type, pile installation procedure, and pile geometry/spacing. This thesis offers insights gathered from 55 construction projects in which more than 100 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. 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. The use of SPT based predictions is recommended against.

An evaluation of pre- and post-installation soil properties surrounding newly installed DDPs have been carried out by interrogating a subset of data from 10 construction sites where Cone Penetration Tests (CPT) were performed before (CPTPRE) and after (CPTPOST) pile construction. CPT key measurements, i.e., tip resistance (q_t) and sleeve friction (f_s), were corrected for overburden stress to obtain the normalized cone resistance (Q_t) and normalized friction ratio (F_r) and compared with pre-installation in-situ data to quantify the soil improvement ratio. Comparisons of pre and post installation measurements suggests the change in CPT-based soil resistances to be mostly associated with soil behavior type (Ic) and soil fines content (FC). Cohesive soils suggested substantially lower improvement potential than cohesionless soils. The spatial improvement was assessed by comparing pre-and post-installation soil measurements within 2-3 times of the pile diameter surrounding the DDP. Results show that there is no significant improvement is yielded at distances larger than 2D. Analytical comparisons between axial pile load capacities estimated with CPTPRE and CPTPOST data does not differ considerably when using CPTPOST parameters for cohesive soils but showed a notable improvement (1.3-1.6 times) for mixed and cohesionless soils. However, when compared to in-situ failure loads observed from axial load tests, the use of post-installation CPT-based soil parameters for design was deemed too ambiguous.