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8 CHAPTER TWO SITE AND GEOMATERIAL CHARACTERIZATION SCOPE SITE GEOLOGY This chapter describes site investigation methods, classifica- Understanding the geologic environment provides informa- tion systems for intact rock and rock masses, and field and tion used to plan the more detailed, subsequent phases of ex- laboratory tests used to determine rock engineering proper- ploration. Site geology refers to the physiography, surficial ties. The focus is limited to information relevant to the design geology, and bedrock geology of the site. The starting point and construction of rock-socketed drilled shafts. Several is a thorough survey of existing information. In many cases, references are available that provide guidance on strategies existing data will enable identification of geologic features and methods of site characterization and material property that will determine the feasibility of rock-socketed founda- evaluation for geotechnical practice, with a focus on trans- tions or will have a major impact on their design or con- portation facilities. These include the FHWA Manual on struction. The amount and quality of information gathered Subsurface Investigations (Mayne et al. 2001), "Evaluation can then be used to establish the type and extent of additional of Soil and Rock Properties," Geotechnical Engineering data that will be required. General knowledge of the site Circular No. 5 (Sabatini et al. 2002), and the AASHTO Man- geology is required in the first phase of the design process ual on Subsurface Investigations (1988). In addition, the U.S. outlined in chapter one, Conceptual Bridge Foundation Army Corps of Engineers has published several manuals rel- Design, to establish anticipated site conditions, feasibility of evant to this topic (Rock Testing Handbook 1993; Rock rock sockets, and conceptual evaluation of potential geo- Foundations 1994; "Geotechnical Investigations" 2001). technical hazards. The purpose of site characterization is to obtain the infor- Sources of existing data include: geologic and topographic mation required to develop a model of the site geology and to maps, publications, computer databases, aerial photographs, establish the required engineering properties of the geomateri- and consultation with other professionals. Many references als. The information obtained is used for two general purposes: are available that provide detailed information on sources and (1) analysis of capacity and load-deformation response, which applications of existing data to geotechnical site characteriza- determines the foundation overall design; and (2) construction tion (e.g., Mayne et al. 2001). A detailed treatment of the topic feasibility, costs, and planning. Once the site for a bridge or is beyond the scope of this report and only the general aspects other transportation structure has been established, all aspects of such data sources will be summarized. of the site and material characterization program are focused on the soil and rock conditions as they exist at that site. Geologic maps are used to transmit information about geo- Geologic conditions and rock mass characteristics can exhibit logic features at or near the earth's surface. Maps are pre- such a wide degree of variability that it is not possible to estab- pared at various scales and for a variety of purposes (Varnes lish a single standardized approach. The scope of the program 1974). A geologic map may be prepared to depict the general is determined by the level of complexity of the site geology, geology of a large region, for example bedrock geology of an foundation loading characteristics, size, configuration, and entire state, or it may cover a relatively small area and con- structural performance of the bridge, acceptable levels of risk, tain detailed information about specific geologic features, for experience of the agency, and other factors. Some of the infor- example engineering geology of a single quadrangle. A good mation needed to establish the scope of site characterization starting point is the geologic map of the state. These maps are may only be known following a preliminary study of the site. produced at a scale that makes it possible to identify the underlying bedrock formations in a general area. Often this Rock and IGM exhibit behaviors that are unique and is sufficient to know immediately whether a bridge is located require special techniques for application to engineering where bedrock conditions are favorable or unfavorable for problems. Two aspects of rock behavior that are paramount foundations in rock, or even whether bedrock exists at rea- are: (1) natural rock masses may exhibit a high degree of sonable depth. Most state DOT geotechnical engineers and variability and (2) properties of a rock mass are determined geologists with experience have familiarity with the geology by the combined properties of intact rock and naturally of their state and incorporate this step unconsciously. The occurring discontinuities, such as joints, bedding planes, next logical step is to determine if more detailed geologic faults, and other structural features. maps or reports are available for the particular area in which
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9 the bridge is located. Sources of such maps and publications of shafts in the Eagle Ford Shale, a rock unit commonly include U.S. Geological Survey and state Geological Sur- encountered in north-central Texas, most notably in the veys, university libraries, and Soil Conservation Service. The Dallas area. Results of load tests on drilled shafts in mica use of Internet search engines has added a powerful tool for schist of the Wissahickon Formation, commonly encoun- locating such information and most governmental geologic tered in Philadelphia and other parts of eastern Pennsylvania, publications can now be identified and obtained on-line. are given in Koutsoftas (1981) and Yang et al. (2004). Turner Detailed geologic maps normally provide useful information et al. (1993) and Abu-Hejleh et al. (2003) consider side re- on characteristics of bedrock and, in some cases surficial, sistance from load tests on shafts socketed into Pierre and geology relevant to foundation engineering. These maps pro- Denver Formation shales. McVay et al. (1992) present a thor- vide descriptions of rocks in terms of lithology (rock type, ough study on the design of shafts in Florida limestone. mineralogy, and genesis), age, and structure (strike and dip Numerous other examples could be cited. Whenever such of sedimentary rocks). In addition, major structural features publications are available they should be used as a source of are identified, such as faults, folds, and contacts between background information during the planning phase of any rock units (formations or members). Geologic maps prepared project where the same rock units are present. Results of load specifically for engineering purposes may include data on tests at different locations, but in the same rock unit, cannot discontinuity patterns and characteristics, rock material be applied without judgment and site-specific considera- strength, Rock Mass Ratings (RMRs), groundwater condi- tions, but they do provide a framework for considering tions, and depth to bedrock (Radbruch-Hall et al. 1987). design issues and may provide insight on expected perfor- Many will identify geologic hazards such as swelling soils mance. Similarly, publications describing construction chal- or rock, landslides, corrosion potential, karst, abandoned lenges in certain geologic environments and strategies for mines, and other information of value. If engineering geo- addressing them can be useful. Schwartz (1987) described logic maps are available they are an essential tool that construction problems and recommended solutions for rock- should be used. socketed piers in Piedmont formations in the Atlanta area. Brown (1990) identified problems involved in construction The most practical aerial photographs for geotechnical of drilled shafts in the karstic limestone of northern Alabama purposes are black and white photographs taken with stereo and suggests methods and approaches that have been suc- cessful for dealing with such challenges. A literature review overlap and with panchromatic film, from heights of between often is all that is necessary to locate this type of useful 500 m and 3,000 m, at scales of about 1:10,000 to 1:30,000. information. The higher level photographs provide a resolution most use- ful for larger-scale features such as topography, geology, and landform analysis, whereas the lower-level photographs pro- Where bedrock is exposed in surface outcrops or exca- vide more detail on geologic structure. Landslides and debris vations, field mapping is an essential step to obtaining in- flows, major faults, bedding planes, continuous joint sets, formation about rock mass characteristics relevant to design rock outcrops, and surface water are some of the features and construction of foundations. A site visit is recom- that can be identified and are relevant to the siting of bridge mended for reconnaissance and field mapping following a structures. review of existing information. A competent engineering geologist or geotechnical engineer can make and record ob- A potentially valuable source of existing data may be servations and measurements on rock exposures that may consultation with other geoprofessionals with design or con- complement, or in some cases exceed, the information ob- struction experience in the same rock units. Geotechnical tained from borings and core sampling. Rock type, hard- engineers, geologists, groundwater hydrologists, contrac- ness, composition, degree of weathering, orientation and tors, mining company personnel, well drillers, etc., may be characteristics of discontinuities, and other features of a able to provide geotechnical engineering reports from rock mass may be readily assessed in outcrops or road cuts. nearby projects, photographic documentation of excavations Guidance on detailed geologic mapping of rock for en- or other construction works, and unpublished reports or test- gineering purposes is given in Murphy (1985), Rock ing data. In addition, such individuals are often willing to Slopes . . . (1989), and ASTM D4879 (Annual Book . . . share relevant experience. Bedian (2004) describes a case 2000). Photography of the rock mass can aid engineers and history in which experience at an adjacent site was used to contractors in evaluating potential problems associated with develop a value engineer proposal for the design of rock- a particular rock unit. The major limitation lies in whether socketed foundations for a high rise building. the surface exposure is representative of the rock mass at a depth corresponding to foundation support. When rock cor- The geotechnical literature contains many useful papers ing and surface mapping demonstrate that surface exposures describing design, construction, and/or load testing of rock- are representative, the surface exposures should be ex- socketed drilled shafts in which the focus is on a particular ploited for information. Figure 4 shows a bridge site where type of rock or a specific formation. For example, Hassan mapping of rock exposures could provide much of the rele- and O'Neill (1997) present correlations for side resistance vant data for design of foundations.