Hubert Law

Bridge foundation design has progressed towards larger diameter and longer drilled shafts in recent years, pushing the practical limits of construction. Load testing can lead to reliable foundation performance with shorter piles, helping to reduce cost and mitigate construction issues associated with unnecessarily long piles. A case study of an Osterberg load test program on a 10-ft (3-m) diameter 140-ft (43-m) long drilled shaft is presented. Results from the load test are used to determine final pile lengths for the project. A reduction of about 25 ft (7.6 m) or more is achieved per pile compared with lengths determined using conventional methods, providing cost savings, and improved constructability. Relatively simple numerical modeling tools are demonstrated to provide useful information during design and construction. Beam-spring models with side resistance (t-z) and end bearing (q-z) springs developed based on the load test illustrate the overall pile capacity within a displacement-based design framework. Axis-symmetric finite element (FE) models are calibrated using load test data and extended to address top-down loading and effects of potential anomalies on the pile capacity.

Soil-structure interaction modeling techniques are presented for bridges supported on large gravity caissons at deep-water sites. The fundamental mode of vibration of these massive caisson foundations tends to be in the short period range, in contrast with the longer period superstructure response. This results in an overall system with complex behavior during an earthquake, which needs to be carefully considered in the analysis and design. Modeling techniques with varying levels of complexity are demonstrated based on project experience from three bridges in highly seismic regions including: Second Tacoma-Narrows Bridge (WA), Strait of Messina Bridge (Italy) and South Park Bridge (WA). Use of numerical model pushover analyses to develop nonlinear distributed soil springs is shown to help overcome several of the limitations of the more basic lumped linear stiffness matrix approach. The relative importance of wave scattering effects and the evaluations of earth pressure distributions considering seismic loading scenarios for assessment of required reinforcements are also addressed.

This paper presents a case study of the evaluation of seismic soil-pile performance in a sloping river embankment containing soft, saturated low-plasticity fine-grained soil. Topics addressed include: i) liquefaction susceptibility of the site soils according to SPT- and CPT- based analysis procedures; ii) cyclic simple shear tests performed in order to assess the fine-grained soil response under earthquake conditions; iii) conducted non-linear numerical time history analyses of the embankment-foundation response to design ground motions; iv) comparison of 3D and 2D model results of ground-pile response in order to determine useful scaling factors for incorporating 3D effects (e.g., soil arching) in 2D models; and v) evaluation of driven timber and reinforced concrete piles for ground improvement as well as stiffening the drilled shaft foundations as strategies for helping the bridge meet the performance-based design criteria under lateral spreading conditions.

Mass mixing is one of main soil improvement methods by mixing the native soil and grout spoil to a relatively homogeneous mixture to increase the soil resistance. Numerical study using finite element method (FEM) of soil improvement effects by mass mixing on the lateral capacity of pile group in soft clay is presented. The clay is modeled by three-dimensional (3D) nonlinear pressure-independent elastoplastic solid elements, and the piles are modeled by one-dimensional (1D) linear elastic beam-column elements. The gapping between the soil and pile is taken into account using non-extension zero-length spring elements. The mass mixing block is also modeled as 3D linear elastic solid elements due to its relatively higher stiffness and strength than the surrounding virgin soft clay. The soil parameters are calibrated by simulating the lateral behavior of single pile in virgin weak clay, and of pile groups in virgin and improved soil. A series of parameter study including the depth and length effect of mass mix block, the effect of strength of mass mix on the lateral resistance of soil are preformed using the calibrated soil parameters and validated FEM model. Analyses show significant effect of mix depth and length and less significant effect of mix strength on the lateral capacity of pile group in soft clay.

This paper presents a brief summary of seismic analyses for Caldecott Tunnel Improvement Project. It includes seismic hazard analysis, wave scattering analyses to evaluate soil structure interaction (SSI) for the proposed 4^th bore, and development of seismic coefficient for slope stability design. Probabilistic Seismic Hazard Analysis (PSHA) was used to establish the rock motion criteria; 1,500-year and 300-year return periods were adopted for the Safety Evaluation Earthquake (SEE) and Functional Evaluation Earthquake (FEE), respectively. The wave scattering analyses were performed using the spectrum-compatible time histories to account for localized geologic features including site specific topography and soil properties specific to the current project. The analyses enable evaluation of the degree of ground distortion that contributes to racking and ovaling of the tunnel liner. Seismic coefficients for several critical slopes near portals were also derived considering the concept of wave propagation within the slope mass.