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29 values. A low-strain modulus derived from downhole seis- Bieniawski (1978) mic measurements was used as a reasonable upper-bound Serafim and Pereira (1983) check on the rock mass modulus. The modulus of intact rock Ironton-Russell from laboratory uniaxial compression tests on core samples Regression is consistent with the observation of Heuze (1980) that field rock mass modulus values range from 20% to 60% of intact rock modulus and serve as an additional upper-bound check. INTERMEDIATE GEOMATERIALS A persistent challenge to the geotechnical engineer, and one that pertains directly to design and construction of drilled shafts, is defining the boundary between soil and rock. Dif- ferent approaches to site characterization and evaluation of geomaterial properties and different design methods are used when the geomaterial involved is clearly defined as soil or as rock. However, many geomaterials encountered in practice FIGURE 16 Ratio of rock mass modulus to modulus of intact exhibit properties that make it difficult to define them clearly rock versus Geological Strength Index (Yang 2006). as being soil or rock within the context of standardized clas- sification systems. Geologic processes provide us with a con- tinuum of geomaterial properties and characteristics, some of which defy simplified categorization. A case history described by LaFronz et al. (2003) illustrates the use of multiple methods for establishing design values of The term intermediate geomaterial (IGM) has been ap- rock mass modulus. Site characterization for the Colorado plied recently to earth materials with properties that are at River Bridge (Hoover Dam Bypass Project) included borehole the boundary between soil and rock (O'Neill et al. 1996). jack, downhole seismic (compression wave velocity), and lab- The criteria are based on (1) whether the material is cohe- oratory uniaxial compression tests. The major rock unit for the sionless or cohesive and (2) some index of material strength. abutment foundations on the Arizona side of the bridge is Cohesionless IGMs are defined by O'Neill et al. (1996) as Hoover Dam tuff (welded volcanic ash). Results of field and very dense granular geomaterials, such as residual, completely laboratory tests used to establish rock mass modulus in the tuff decomposed rock and glacial till, with SPT N60-values are summarized in Table 14. Values given for the correlation between 50 and 100. Cohesive IGMs are defined as materials with GSI reflect two values of GSI for the tuff, one corre- that exhibit unconfined compressive strengths in the range sponding to fracture conditions of width = 1 to 5 mm with soft of 0.5 MPa qu 5 MPa. Specific materials identified filling (GSI = 45) and the other corresponding to fracture width by O'Neill et al. (1996) as being cohesive IGMs include of 0.1 to 1 mm and no filling (GSI = 52). Modulus values based (1) argillaceous geomaterials, such as heavily overconsoli- on downhole p-wave velocities were calculated using equa- dated clays, clay shales, saprolites, and mudstones that are tions given by Viskne (1976), described by LaFronz et al. prone to smearing when drilled; and (2) calcareous rocks such (2003) as valid at the rock mass scale. as limestone and limerock and argillaceous geomaterials that are not prone to smearing when drilled. The term IGM as used Results were applied as follows. Borehole jack measured by O'Neill et al. (1996) and subsequently adopted in O'Neill values at stress ranges representative of expected footing and Reese (1999) and in the draft 2006 Interim AASHTO bearing pressures were taken as reasonable values for de- LRFD Bridge Design Specifications has been limited specif- veloping foundation load-deflection curves. Deformation ically to design of drilled shafts and has not been adopted in modulus predicted by the correlation to GSI (Table 12, Hoek the general geotechnical literature. For example, the term and Brown 1997) provided a cross-check on the borehole IGM is not used in the FHWA Manual on Subsurface Inves- jack measured values. The mean value of modulus from tigations (Mayne et al. 2001) or in "Evaluation of Soil and the borehole jack tests is in the range of the GSI-predicted Rock Properties," Geotechnical Engineering Circular No. 5 (Sabatini et al. 2002). Responses to Question 8 of the survey TABLE 14 show that most responding states (23) define IGMs for drilled MODULUS VALUES, HOOVER DAM TUFF (LaFronz et al. 2005) shaft design according to the criteria of O'Neill et al. (1996). Method Mean Modulus (GPa) However, six states responded that geomaterials are classified Borehole jack 2.83 as either soil or rock and IGM is not used. Correlation to GSI 2.34, 3.52 Downhole seismic 3.31 According to O'Neill and Reese (1999) cohesionless Uniaxial compression 13.79 IGMs may be treated, for practical purposes, in the same
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30 manner as coarse-grained (cohesionless) soils. They are as- There is no simple answer to the problem of classifying sumed to respond to loading by rapid dissipation of excess cohesive materials at the soilrock boundary. Various classi- pore water pressure (fully drained response) and are analyzed fications that distinguish geomaterials on the basis of com- within the context of effective stress. For strength analysis, pressive strength of unweathered rock material are summa- cohesionless IGMs are characterized in terms of the effective rized in Figure 18, which includes a proposed classification stress angle of friction '. It should be noted that some by Kulhawy et al. (1991) in which rock strength is defined empirical correlations that apply to cohesionless soils, such relative to that of concrete used in construction, which is as- as friction angle estimated from SPT N-values, may not be sumed to range from 20 kN/m2 (3 ksi) to 100 kN/m2 (15 ksi). applicable to cohesionless IGMs. Specific approaches for Rock at the high end of the strength scale (>100 kN/m2) is estimating design parameters of shafts in cohesionless IGM classified as strong and in most cases would be expected to are covered in chapter three. be an excellent founding material, except that it would be ex- pensive to excavate. Rock with compressive strength falling The definition of cohesive IGMs given earlier is based within the range of concrete strength is classified as medium on a single index, the unconfined compressive strength. Al- and the rock mass could be either weaker or stronger than though this categorization may be useful to identify mate- concrete, depending on weathering and structural features. rials falling into a defined range of intact strength, it does For rock classified as weak (<20 kN/m2) foundation capacity not necessarily provide the distinction between soil and is expected to be governed by the strength of the rock mass. rock most relevant to behavior of drilled shafts. To illus- Materials defined as cohesive IGMs by O'Neill et al. (1996) trate, consider Figure 17 from Kulhawy and Phoon (1993). fall into this strength range. To account properly for the be- This figure shows the relationship between unit side resis- havior of weak rock in engineered construction, the follow- tance determined from field load tests on drilled shafts and ing additional factors must be considered carefully: geologic one-half of the unconfined compressive strength. Both pa- origin, in situ weathering profile, state of stress, ground- rameters are normalized by atmospheric pressure pa. Two water, and construction practices. categories of load tests were defined; those conducted on shafts in fine-grained soils (clay) and those in rock. Kul- A defining characteristic of geomaterials at the soilrock hawy and Phoon relied on the judgment of the original boundary may be whether or not the in situ material was at one authors and the database compilers to establish whether the time rock (geologic origin). This is probably the distinguish- material was soil or rock. For convenience, the range of ing feature between clay and rock in Figure 17. The next geo- normalized strength that defines cohesive IGM is superim- logic consideration is the in situ weathering profile. Igneous, posed on Figure 17. It can be seen that the soil and rock data sedimentary, or metamorphic rocks subjected to in-place constitute apparently different populations, including over weathering result in geologic profiles that may exhibit the full the range of strength that defines cohesive IGM. For pur- range of characteristics, for example, as described in Figure 11 poses of drilled shaft side resistance, therefore, the classifi- (Key), Sheet 2, under "Rock Weathering--Alteration." The cation of IGM does not provide a smooth transition from descriptive terms are based on recommendations for describ- soil to rock. It may be more meaningful to define the mate- ing degree of weathering and alteration by the ISRM. One of rial as being one or the other on the basis of additional the criteria for distinguishing between residual soil and com- geologic information. pletely weathered or altered rock is whether the original rock IGM FIGURE 17 Side resistance versus geomaterial strength FIGURE 18 Classification for unweathered rock material (Kulhawy and Phoon 1993). strength (Kulhawy et al. 1991).