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95 HORIZONTAL SCALE (FT) 0 100 200 300 400 500 930 B3 B1 B2 10 10 31 PARTIAL LEGEND 920 10 8 ZONE I 8 C.T. CORING TERMINATED 6 11 12 17 910 11 -10 PENETRATION RESISTANCE 18 18 NX-18% CORE RECOVERY 55 ZONE II 12 50/3" 16 900 RQD-82 ROCK QUALITY DESIGNATION 16 NX-18% 10 24 ZONE I FILL RQD-0 19 890 50/5" ZONE II RESIDUAL SOIL NX-95% 53 RQD-82 NX-15% ZONE III 50/3" ZONE III PARTIALLY WEATHERED ROCK 880 RQD-0 C.T. 50/4" ZONE IV ROCK NX-90% 50/5" GROUNDWATER, TIME OF BORING 870 RQD-79 C.T. NX-87% RQD-10 24-HR GROUNDWATER 860 ZONE IV NX-95% RQD-51 850 C.T. ELEVATION (FT) FIGURE 81 Typical Piedmont subsurface profile (after Schwartz 1987). is drilled to its design base elevation. When refusal is en- These examples illustrate the challenges that can be en- countered on a boulder that is "floating," questions may countered in the design and construction of rock-socketed arise concerning whether the boulder is an obstruction or drilled shafts as a result of certain geologic conditions, as constitutes drilling in rock. Similarly, when sloping bedrock well as approaches that others have found successful for is first encountered, the volume of material excavated to addressing such challenges. Every foundation site is unique reach base elevation may be disputed as to whether it is soil geologically, and successful design and construction ap- or rock, and drilling into sloping rock can be difficult. One proaches are those that are adapted to fit the ground condi- approach is to install casing until one edge of the casing hits tions. Mother Nature is quite unforgiving to those who behave rock, then drill a smaller diameter pilot hole into the rock otherwise. followed by drilling to the design diameter and advancement of the casing. SUMMARY Gardner (1987) reviews design methods for axial load- ing of drilled shafts in Piedmont profiles, including recom- Construction and issues related to constructability are inte- mendations for design side and base resistances in rock and gral parts of drilled shaft foundation engineering. A review methods used to determine relative load transfer between of rock drilling technologies is presented and shows that a side and base. Harris and Mayne (1994) describe load tests wide variety of equipment and tools is available to contrac- in Piedmont residual soils. O'Neill et al. (1996) used the tors for building drilled shafts in rock. The design, manufac- tests of Harris and Mayne to develop the recommendations turing, and implementation of rock drilling tools is a field for side resistance in cohesionless IGM from Standard Pen- unto itself and it is important for foundation designers to be etration Test results, as presented in chapter three. Both knowledgeable about the availability and capability of tools Gardner (1987) and Schwartz (1987) outline measures that and drilling machines. Constructability issues are interrelated can be taken to minimize construction delays and contract with all of the steps shown in the flowchart of Figure 3, de- disputes when building rock-socketed shafts in Piedmont picting the design and construction process for rock-socketed profiles. The principal requirements are: (1) thorough site shafts. Beginning with site characterization and continuing investigation, (2) design and construction provisions that through final inspection, constructability is taken into ac- can accommodate the unpredictable variations in subsur- count in foundation selection, in design methods through the face materials and final base elevations, and (3) construc- effects of construction on side resistance, in critical design tion specifications and contract documents that facilitate decisions such as whether base resistance will be included, field changes in construction methods and shaft lengths. in writing of specifications pertaining to use of slurry and Successful construction also depends on highly qualified bottom cleanout, and in matching inspection tools and pro- inspectors and clear communication between design engi- cedures to construction methods. The literature review iden- neers, contractors, and inspectors. tified many aspects of constructability pertaining to rock
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96 sockets and these are summarized. Practices that can improve constructability; for example, the use of SCC and installation of method shafts are identified. Field load testing of rock sockets has increased since the advent of innovative load testing methods, especially the O-cell and the STN. The basic mechanics of these and other tests are described, followed by a review of current applica- tions of each to testing of rock-socketed shafts. The survey shows that many states are using the O-cell to verify, and also to improve, design methods of rock sockets. A description of the KDOT experience with O-cell testing in rock is presented as an example. Load testing is also shown to be a factor in increased use of rock-socketed drilled shafts by transporta- tion agencies. Finally, load testing with the O-cell has been a useful tool for identifying and evaluating poor versus good construction practices. The report by Schmertmann et al. (1998), referenced several times in this chapter, is a particu- larly useful source for that information. Inspection and field quality control are recognized in the drilled shaft industry as the critical link between design and construction. Excellent sources of information on inspection are available and these are identified. The NHI inspector certification course is highly recommended for all inspec- tion personnel. Some of the tools identified by the survey and literature review that can be most effective for rock- socket inspection are the SID, coring of rock beneath the socket-base, use of probing tools, and downhole fiber optic cameras. Two geologic environments in which rock-socket con- struction poses special challenges, karstic limestone and Piedmont residual profiles, are presented to illustrate some of the practices that lead to successful projects. Matching of FIGURE 82 Typical drilling in Piedmont soils and rock design and construction strategies to ground conditions is the (Schwartz 1987). essence of constructability.