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3 CHAPTER ONE INTRODUCTION BACKGROUND PROBLEM DEFINITION Highway bridges represent a large investment in the U.S. The engineering problem addressed by this synthesis is transportation infrastructure, and structural foundations shown in Figure 1. A drilled shaft foundation is to be de- account for a significant percentage of total bridge costs. signed and constructed for support of a bridge structure. Sub- Current foundation engineering practice in the transporta- surface conditions may consist of soil underlain by rock. tion industry represents a dramatic advancement compared Upper portions of the rock may be partially to highly weath- with 25 years ago. Development of this topic is illustrated ered, giving these materials engineering properties that are by considering that NCHRP Synthesis 42: Design of Pile transitional between soil and rock, sometimes referred to as Foundations (Vesic 1977) does not mention rock-socketed intermediate geomaterials, or IGM. Loads to be considered drilled shafts. At the time of its publication, NCHRP Syn- for design typically are determined by AASHTO Bridge thesis 42 was the most comprehensive study extant on Design Specifications, with proper consideration of load the use of deep foundations for transportation structures. combinations and load factors. For foundation analysis, de- According to DiMaggio (2004), in 1980, driven piles sign loads may be resolved into vertical (P), horizontal (H ), accounted for more than 95% of transportation market and moment (M) components at the head of the shaft. A sub- share, based purely on repeating previous practice. Today, surface investigation is required to provide information on all the practice is oriented toward matching the foundation of the geomaterials through which the shaft must be con- type to project conditions. This has led to a wider variety structed and from which the foundation will derive its resis- of deep foundation types selected on the basis of subsur- tance to the design loads. The foundation designer then must face conditions, structural behavior, constructability, envi- determine the required dimensions (depth and diameter) ronmental constraints, and cost. A foundation type that has and structural properties of drilled shafts that will provide steadily increased in use over this time is the drilled shaft, adequate resistance and will limit vertical and horizontal a deep foundation constructed by placing fluid concrete in deformations to a level that provides adequate service per- a drilled hole. formance of the bridge. Trial designs are developed and evaluated with respect to: (1) cost, (2) performance, and A potentially effective way to use a drilled shaft is by (3) constructability. A major factor in all three criteria is bearing on, or extending into, rock. To achieve the perfor- whether the shaft needs to be extended into the rock or mance and economy potentials of rock-socketed shafts, de- IGM layers. Rock sockets will generally increase costs, signers must be aware of the many issues that affect both improve load-carrying and load-displacement perfor- cost and performance. Drilling and excavation in rock is mance, and make construction more challenging. generally more expensive and time consuming than in soil. Construction of a rock socket poses challenges and difficul- ties that are unique and may require specialized techniques, SCOPE AND OBJECTIVES equipment, and experience. The first issue confronted by a foundation designer is to determine whether a rock-socketed The overall objectives of this synthesis study are to foundation is necessary for bridge support. Factors to con- sider include the nature and magnitude of structural loads Collect and summarize information on current practices and factors related to rock mass characteristics, including pertaining to each step of the process described pre- depth to rock, rock type, rock mass engineering properties, viously, along with their limitations and sources of and constructability. The additional costs and effort of con- uncertainty; struction in rock must be offset by its benefits. The princi- Identify emerging and promising technologies in each pal benefits normally are higher load-carrying capacity and of these areas; the ability to limit deformations, compared with foundations Identify the principal challenges in advancing the state not founded in rock. To make the appropriate cost compar- of the practice; and isons, rock-socket design must be based on rational models Provide suggestions for future developments and im- of behavior that reliably predict the capacity and load- provements in the use and design of rock-socketed deformation behavior. shafts.