Case studies to demonstrate challenges of driven piles on rock

Abstract

2In Wyoming, pile foundations for bridges are often driven on rock materials because of the state's shallow bedrock stratigraphy. Unfortunately, no static analysis methods are currently available for estimating the resistance of these driven piles. In this paper, two recently completed bridge projects (the Owl Creek and Woods Wardell sites) on steel H-piles in Wyoming are explicitly presented, and data from three past projects are included to highlight the limited knowledge and challenges pertaining to the present design and construction practices. Static analysis methods were used to estimate the geotechnical resistances of these piles. The wave equation analysis program and the case pile wave analysis program were used to verify their performances during construction. Structural capacities of these piles were also calculated. The results of the studies show that the static analysis methods and structural analyses yield inconsistent pile resistance estimations. Recommendations in terms of pile bracing and embedded pile length are proposed to predict better the resistances of piles on soft rock. NotationA g cross-sectional area of a pile F y minimum yield strength of a steel pile K effective length factor L unbraced pile length P e elastic critical buckling resistance P n nominal structural capacity of a pile P o equivalent nominal yield resistance Q applied load q u uniaxial compressive strength R nominal pile resistance R p nominal end bearing R s nominal side resistance r s radius of gyration about the axis normal to the place of buckling g load factor f resistance factor y slender element reduction factor IntroductionThe shallow bedrock stratigraphy in Wyoming, USA results in steel H-piles with a high driving durability on rock often being used as the foundation system to support bridges in the state. Driven steel H-piles are preferred over drilled shafts because of the local availability of steel pile materials, driving equipment and experienced contractors. Also, for a moderate structure load of a typical bridge in Wyoming, the driven steel pile system is more cost-effective than the drilled shaft foundation system. The total axial resistance of these piles consists of a combination of side resistance and end bearing. To attain the required capacity, particularly in a soft overburden soil, the pile would have to rely on its resistance on a stiff rock or intermediate geomaterials (IGMs) Aashto's LRFD bridge design specificationsRecommendations for piles driven on rock suggested by the AASHTO LRFD Bridge Design Specifications (Aashto, 2014) are described as follows.(a) The resistance of a pile driven on soft rock shall be determined in the same manner as soil (article 10.7.3.2.2). The resistance factors of the static analysis methods and dynamic methods for driven piles are summarised in article 10.5.5.2.3. (b) Piles driven in hard rock shall be governed by the structural limit state described in article 6.9.4.1. The structural compressive resistance of a pile shall be considered as the smallest value bas...

Evaluation of Static Load Test Systems for Driven Piles in Intermediate GeoMaterials

“…Pile resistances estimated using the signal-matching method from the CAse Pile Wave Analysis Program (CAPWAP) vary significantly with the resistances predicted using existing SA methods for driven piles in IGMs ( 3 ). Additionally, it has been observed that the pile resistances predicted for IGMs using current SA approaches vary from those found by dynamic load testing (DLT) ( 11 – 13 ). The prediction of pile resistances in soft rocks using twenty static pile load trials did not yield any applicable prediction equations, either ( 3 ).…”

“…If IGM is treated as soil, construction concerns, such as early pile refusal and pile breakage, might occur because of a lack of suitable design approaches and static pile load test data ( 11 ). The results of an investigation by Ng and Sullivan on fifteen steel piles driven into IGMs in Wyoming revealed that the current SA techniques designed for soils produced inconsistent and conservative predictions of geotechnical resistances ( 12 ). Pile resistances estimated using the signal-matching method from the CAse Pile Wave Analysis Program (CAPWAP) vary significantly with the resistances predicted using existing SA methods for driven piles in IGMs ( 3 ).…”

Editorial

“…In the next paper, Ng and Sullivan (2017) studied the common practice in Wyoming, USA where steel H-piles are driven to penetrate rock materials. Current static analysis methods, originally developed for soil, provided inconsistent and potentially conservative geotechnical resistance estimations for a driven pile on soft rocks.…”