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93 era encased in a watertight bell and equipped with a light that may be of a smaller diameter than the shaft above the source and a water jet for clearing sediment to provide clear socket. pictures of the shaft sides and base. The SID was developed in Australia specifically for inspection of rock sockets under bentonite slurry. North Carolina also reported using a SID, EXAMPLES OF DIFFICULT GEOLOGIC CONDITIONS and several other states use downhole cameras to inspect sockets under water or slurry. Some of the most difficult conditions for drilled shaft con- struction and inspection are karstic limestone and residual The survey shows that some states neglect socket-base re- profiles that grade from soil to weathered rock to intact rock. sistance altogether if concrete is placed under slurry or water Experiences and approaches to these conditions identified by (Question 14). The rationale is that base conditions cannot be the literature review are summarized here. verified with sufficient reliability to be sure that a poor base, or "soft bottom," condition is avoided. This refers to a layer of disturbed soil, slurry, or contaminated concrete at the base, Shafts in Limestone which may allow excessively large downward movement be- fore the resistance of the underlying rock can be mobilized. Use of drilled shafts in karstic terrain is considered by Knott et These concerns may be justified under some conditions. al. (1993), Sowers (1994), and others. Brown (1990) describes However, as described in chapter three (see Table 16 and design and construction challenges of using drilled shafts in hard Figure 23), there are good reasons to account for base resis- pinnacled limestones and dolomites encountered in the Valley tance even for shafts constructed under wet-hole conditions. and Ridge and Cumberland Plateau physiographic provinces. Construction and inspection practices that can be taken to Subsurface conditions are highly irregular owing to extensive avoid poor base conditions include appropriate specifications weathering. Although intact rock strengths may be high (up to 70 MPa or 10,000 psi), numerous seams, slots, and cavities and quality control on properties of slurry at the bottom of the are typically filled with residual clayey soils (see Figure 79). hole prior to concrete placement, cleanout of slurry contam- Boulders and chert nodules are often embedded in the soils. inated with cuttings or suspended particles before concrete Drilling through soil is often performed in the dry soil and then placement, use of a weighted tape to "feel" the bottom of the a casing set when rock is encountered. Drilling in the rock is hole as an inspection tool, downhole viewing devices for in- difficult and can involve a combination of rock augers, drill spection of bottom conditions (e.g., SID), and proper use of and shoot methods, and core barrels. Sudden groundwater in- a pig or other device in the tremie pipe to prevent mixing flow is common upon encountering soil seams and slots. of concrete and slurry. Post-grouting of the shaft base is a measure that could be incorporated into design and con- In this environment of extreme variability the actual soil struction to provide quality base conditions in drilled shafts. and rock conditions for a specific drilled shaft cannot be determined with any degree of accuracy before construction. It is instructive to observe that most states that have Design, construction, and inspection have to be flexible incorporated field load testing of rock sockets into their foun- enough to adjust to conditions actually encountered. For dation programs, using a method that allows measurement example, where shafts can be shown to bear at least partially of base load-displacement, now include both side and base on sound rock, base resistance is assumed, but highly resistances in their design calculations. This is based on load test results that show, when proper quality control is applied, that base resistance is a significant component of shaft resis- tance at service loads. Nondestructive Testing and Evaluation Field tests to evaluate the integrity of as-built drilled shafts are now used widely in the industry as part of overall quality assurance. Nondestructive methods for testing (NDT) and evaluation (NDE) are covered in Chapter 17 of the Drilled Shaft Manual (O'Neill and Reese 1999) and in several other publications. The survey for this study included a question asking respondents to identify any issues pertaining to NDT and NDE that are unique or important specifically for rock- socketed drill shafts. No issues were identified, other than the need to consider locating NDT access tubes in the reinforcing cage so that the entire assembly is able to fit into a socket FIGURE 79 Features of karstic terrain (Knott et al. 1993).

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94 (a) (b) (c) FIGURE 80 Commonly encountered conditions for shafts in pinnacled limestone (Brown 1990). conservative values are used to account for the presence of by decomposed metamorphic rocks and a weathering profile seams at the base. This case is illustrated in Figure 80a, in characterized by unpredictable variability in the thickness and which a probe rod placed down one or more probe holes quality of the weathered materials. Drilled shafts are used ex- drilled into the base can be used to determine the extent and tensively for major structures in this region, primarily because nature of the seam. One criterion for acceptance is rock cov- it has been recognized that large axial loads can be supported if erage of 75% or more of the base area and vertical seams. a shaft is extended to either decomposed or intact rock. Figure 80b shows a nonvertical seam, which should be detectable by one of the probe holes and might necessitate Gardner (1987) identified three general weathering hori- additional drilling to preclude shear failure along the seam. zons in the Piedmont: (1) residual soil, representing advanced Alternatively, the seam could be excavated and grouted. This chemical alteration of the parent rock; (2) highly altered and technique would not be recommended if seepage is expected leached soil-like material (saprolite) retaining some of the into the excavated seam. Where shafts are bearing on a sec- structure of the parent rock; and (3) decomposed rock (locally tion of rock bounded by vertical seams or slots and the pos- referred to as partially weathered rock), which is less altered sibility of fracturing exists, rock anchors are sometimes used but can usually be abraded to sand- and silt-sized particles. to transfer load across potential fracture planes, as illustrated The underlying intact rock is typically fractured near its sur- in Figure 80c. Rock anchors or micropiles are also used to face but increases in quality with depth. The thickness and transfer load across horizontal seams filled with soft soil and characteristics of each zone vary considerably throughout the detected by probing beneath the base. region and may vary over short horizontal distances, and boundaries between the horizons may not be distinct. Figure To provide the flexibility needed for design, inspection, 81 shows a typical profile based on borings at one site. Factors and construction, creative contracting approaches are also that make drilled shafts challenging to design and build in the needed. Brown (1990) reported that contracting such work Piedmont are: on a unit cost basis provided the flexibility needed to deal with the unknown quantities of soil versus rock drilling, con- · Highly variable subsurface profiles, crete overpours, rock anchoring, drilling of probe holes, etc. · Presence of cobbles and boulders, The engineer estimates the unit quantities, but actual pay- · Steeply dipping bedrock surfaces, and ment is based on unit costs of material quantities actually · Difficulties in distinguishing between soil, partially used. This requires careful inspection and record keeping. weathered rock, and intact rock for pay purposes. The first of these makes it difficult to determine ahead of Drilled Shafts in the Piedmont time what the final base elevation will be for shafts required to reach intact rock. At least one boring at each drilled shaft The Piedmont Physiographic Province of the eastern United location can help to address this issue. Figure 82 illustrates States, extending from Alabama to New Jersey, is characterized two conditions that can cause "refusal" before the shaft