Click for next page ( 31


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 30
31 fabric is still apparent. The highest degree of weathering ap- soil and rock properties. Wider use of RMR or GSI classifi- plies to materials derived from rock but for which the rock cation of rock mass is one way that state DOT agencies can fabric is not apparent. In this case, the material behavior is use the most up-to-date methods for characterizing RMS and controlled by soil fabric and the material should be classified deformation properties. as residual soil, even though it may contain fragments of weathered rock. Materials in which the original minerals have In situ testing methods that provide information on rock been completely decomposed to secondary minerals but mass modulus include PMT and borehole jack. Five states where the original fabric is intact may exhibit rock material reported using these tests to obtain modulus values for rock- behavior governed by rock mass features, including both rock socket design. To use the best available analytical models for material and discontinuities. The material should be consid- axial and lateral loading, as well as for effective interpretation ered to be rock mass, even though it may be highly weathered of load test results, rock mass modulus is a required parameter. or altered and exhibit low compressive strength. Judgment Currently, it is noted that there is no definitive in situ method is always required in assessing whether material behavior is or empirical equation for rock mass modulus that has been cal- governed by soil fabric or by rock mass fabric; however, this ibrated specifically for application to design of rock sockets. is a key factor to be assessed in a design approach. Whether a A case history example is presented in this chapter illustrating geomaterial is assigned the term "IGM" or "weak rock" is not the beneficial use of both in situ testing (borehole jack) and as important as understanding the geologic processes that give empirical correlations with GSI to establish representative val- the material its characteristics and engineering properties. ues of rock mass modulus for foundation design. Site and geomaterial characterization are interrelated with SUMMARY design, construction, and load testing of drilled shafts in rock. For design, Figure 3 shows that rock mass engineering In this chapter, site characterization methods used to define properties required for analysis of rock-socket capacity and subsurface conditions at bridge sites underlain by rock were load-deformation response are obtained through field and reviewed. The survey shows that eight states currently use geo- laboratory testing. Table 15 is a summary of rock mass char- physical methods to determine depth to bedrock and that acteristics used in design methods for axial and lateral load- seismic refraction is the method used. The literature review ing. A large X indicates the property is used directly in suggests that resistivity methods based on the use of multi- design equations that are currently applied widely in practice, ple arrays can provide detailed profiles that may be useful whereas a small x indicates that the characteristic is used for both design and construction. Karstic areas in limestone indirectly in the design or that it is required for a proposed or dolomite terranes with irregular, pinnacled rock surfaces design method that is not used widely. For example, intact rock or solution cavities are examples of sites where recent de- modulus ER is not used directly to analyze load-displacement velopments in geophysical methods could be applied. response of socketed shafts, but may be used to estimate the rock mass modulus EM, which is used directly in the analytical Every agency responding to the survey uses rock core equations. drilling as the primary method of subsurface investigation for rock sockets. Current practice for description and classifica- Information obtained through the site investigation tion of rock core is reviewed. The survey shows that most process will be used not only by design engineers but by states routinely determine the RQD of rock core and that contractors who will bid on the work and construct the foun- the uniaxial compressive strength of intact rock (qu) is also dations. As indicated in the flowchart shown in Figure 3, measured by one of the standardized methods. Also from the a goal of site characterization is to obtain information on survey, it was determined that five states currently classify constructability. O'Neill and Reese (1999) point out that con- all rock mass according to the Geomechanics Classification tractors will be most interested in knowing the difficulties System, in which rock mass is assigned a RMR. Twelve that might be encountered in drilling the rock. Specific in- states use RMR occasionally, whereas 14 states indicated formation that is useful in assessing the difficulty of drilling that RMR is never used. Some of the analytical methods de- in rock includes loss or gain of drill water; rock type with veloped in recent years and described in subsequent chapters lithological description; rock strength; characteristics of of this report require the rock mass classification in terms of weathering; and rock mass characteristics such as the pres- RMR. Specifically, RMR can be used to evaluate strength ence, attitude, and thickness of bedding planes, foliation, parameters according to the HoekBrown failure criterion, a joints, faults, stress cracks, cavities, shear planes, or other useful approach to quantifying strength of intact or highly discontinuities. Boring logs, containing most of the informa- fractured rock masses. RMR is used to establish GSI, which tion determined by the site investigation, are incorporated is required to use the most up-to-date version of the directly into the construction plans by most state DOTs. Any HoekBrown criterion. RMR and/or GSI are useful for esti- of the above information not given in the boring logs should mating rock mass modulus using the empirical correlations be made available to bidders to facilitate informed decisions. given in Table 12. The RMR is also recommended in current The same information will be used by the design engineer to FHWA manuals on site characterization and evaluation of forecast potential construction methods and construction

OCR for page 30
32 TABLE 15 ROCK MASS ENGINEERING PROPERTIES REQUIRED FOR ROCK-SOCKET DESIGN Design Applications Axial Loading Lateral Loading Load-Displacement Unit Side Unit Base Axial Load- Ultimate Continuum p-y Curve Rock Mass Characteristic Resistance Resistance Displacement Resistance Methods Parameter Compressive strength, intact rock, qu X X X x X Split tensile strength, intact rock, qt X Rock mass strength by Mohr Coulomb or Hoek and Brown X x X Shear strength of joint surfaces X x Elastic modulus, intact rock, ER x x x x Elastic modulus, rock mass, EM x X X X Rock quality designation (RQD) x X x x Rock Mass Rating (RMR) X x x Geological Strength Index (GSI) X x x X Notes: X = property is used directly in equations that are currently applied widely in practice. x = characteristic is used indirectly in the design or it is required for a proposed design method not widely used. problems to develop specifications for the project and to load-deformation response. Results of field load testing make cost estimates. Rock cores should be photographed also provide the basis for many of the design methods dis- and, when practical, retained for examination by prospective cussed in the next two chapters. For correct interpretation bidders. of load test results, it is imperative that subsurface conditions and soilrock engineering properties be evaluated as care- Field load testing, shown in the flowchart of Figure 3 fully as possible. The properties required for design and and described in chapter five, provides direct verification of listed in Table 15 are also required for load test interpretation design assumptions regarding axial and lateral capacity and and for proper extrapolation of load test results to design.