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88 Shaft Group, Load 5 (E-W) 2.5 1000 2 800 1.5 600 Displacement / Acceleration 1 400 Load, tons inches / g's Statnamic Acceleration 0.5 200 Lateral Translation, Top West Load tons 0 0 Lateral Translation, Top East -0.5 -200 Lateral Translation, Top Center -1 -400 -1.5 -600 -2 -800 0.4 0.6 0.8 1 1.2 seconds Time (a) Lateral Load versus Translation Static & Derived Statnamic - Shaft Group 700 140 600 120 500 100 Static % Damping 400 80 Load Derived Statnamic tons 300 60 % Damping Total Resistance (Static + Damping) 200 40 100 20 0 0 0 0.5 1 1.5 2 2.5 3 inches Tlti (b) FIGURE 75 Results of lateral STN test: (a) measured dynamic response; (b) derived static response. CONSTRUCTABILITY, INSPECTION, AND sociation of Foundation Drilling (ADSC). The FHWA QUALITY ASSURANCE Drilled Shaft Manual (O'Neill and Reese 1999) addresses constructability and its role in drilled shaft design. The man- These topics are considered together because they encom- ual also forms the basis of a National Highway Institute (NHI) pass activities having a single objective: construction of course on drilled shafts that is available through FHWA. A a high-quality, rock-socketed drilled shaft foundation that separate NHI course that certifies inspectors for drilled shaft performs in accordance with the design assumptions. As illustrated in the flow chart diagram of Figure 3, chapter one, construction (Williams et al. 2002) also has a strong empha- the final design is based on input from three general sources: sis on constructability and was developed with significant (1) site characterization, (2) geotechnical analysis, and contractor input. ADSC provides short courses, workshops, (3) structural analysis and modeling. Plans and specifications and a library of publications focused on construction-related are developed that reflect generally accepted practices based issues for drilled shafts. ADSC also provides "constructabil- on the collective experience of the construction and engi- ity reviews" of individual projects in which independent con- neering communities. Examples of model specifications tractors review the project plans and specifications and offer include those given in Chapter 15 of the FHWA Drilled Shaft advice on its constructability. This step could be incorporated Manual (O'Neill and Reese 1999), ACI Standard Specifica- into the overall process depicted in Figure 3, as denoted in the tion for the Construction of Drilled Piers, ACI 336.1-98 flow chart by "constructability review." (1998), and specifications developed by state and federal transportation agencies with extensive experience in drilled Integrating constructability into a drilled shaft project shaft use. In addition, effective specifications will address involves taking a common sense approach to design that issues that are unique to the specific conditions that determine accounts for the methods, tools, and equipment used by con- the final design, including constructability issues which, ide- tractors to build the shafts. No attempt will be made here to ally, are accounted for in all three of the input categories iden- identify all of these issues, but items identified by the survey tified previously. In the following paragraphs, these topics are and that relate specifically to rock sockets are discussed. discussed individually, but in practice they must be integrated into the design concepts discussed in this synthesis. Schmertmann et al. (1998) and Brown (2004) present guidelines for ensuring quality in drilled shaft construction Constructability and some recent advances in materials that have applications in both soil and rock. The key elements to be considered to Much emphasis has been placed on constructability of drilled avoid the most commonly observed construction problems shafts by FHWA and through efforts of the International As- are:

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89 Workability of concrete for the duration of the pour; almost certainly reduce side resistance compared with a shaft Compatibility of congested rebar and concrete; drilled and poured under dry conditions, in either soil or rock. Control of stability of the hole during excavation and However, if sound practices are followed by an experienced concrete placement, especially with casing; contractor and there is proper inspection, slurry drilling for Proper consideration and control of hydrostatic balance rock sockets can be an effective construction method, assum- and seepage; ing the slurry is handled in a manner that avoids contamina- Bottom cleaning techniques and inspection; and tion of the interface bond or excessive suspended sediment. Drilling fluid that avoids contamination of the bond between concrete and bearing material or excessive In certain rock types, there is evidence that use of polymer suspended sediment. slurry may be beneficial to rock-socket side resistance. The Kentucky DOT requires polymer slurry for drilling in rock New developments in concrete mix design, in particular that exhibits low values of slake durability index. Typically, mixes described as self-consolidating concrete (SCC), can this is the case in certain shale formations in Kentucky. Slak- provide benefits for drilled shaft construction. The charac- ing occurs when the shale is exposed to water, and can cause teristic of SCC that is most beneficial is very high slump formation of a smear zone, reducing side resistance consid- flow. Reinforcement cages with a high density of steel bars, erably, as demonstrated by Hassan and O'Neill (1997). often necessary especially for seismic design, make it diffi- Apparently, the polymer slurry prevents softening and the cult to provide the necessary clear spacing between bars that resulting smear zone, although there have not been load tests will ensure flow of concrete to the outside of the cage. The in which a direct comparison has been made. This issue flow properties of SCC have been shown to reduce potential deserves further research. defects associated with incomplete cover or voids caused by inadequate flow of concrete. One state DOT identified the following as a problematic construction issue: "various methods used to force a dry Prompt placement of concrete is another construction pour," indicating that some measures taken to avoid placing practice that promotes quality in the as-built shaft. Delay in concrete under water or slurry are more detrimental than concrete placement increases the potential for slump loss allowing a wet pour. Both Schmertmann et al. (1998) and and, in some cases, has been identified as a cause of reduced Brown (2004) describe a case that seems to contradict some side resistance (Schmertmann et al. 1998). commonly held ideas about casing versus wet hole construc- tion of rock sockets. A drilled shaft installed through 12 m Several states identified problematic construction issues of soil and socketed into rock was constructed using a full- when the slurry method of construction is used in rock sock- length casing (to provide downhole visual inspection). A ets. One issue is whether slurry has a detrimental effect on load test using the Osterberg load cell indicated a mobilized side resistance of rock sockets. Thirteen states indicated that side resistance in the socket of 0.5 MN, much less than they restrict the use of slurry in rock sockets and one state expected. A second shaft was constructed, but using a wet expressed "concerns with use of drilling fluids instead of cas- hole method with tremie placement of concrete and without ing." In many situations, if casing is used to support the hole, casing into the rock. Load testing of this shaft indicated more the need to use slurry is eliminated. Typically, casing need than 10 MN of side resistance in the socket. The difference only extend to the top of rock if the rock-socket portion of is attributed to a decrease in concrete workability during the the hole will remain open without caving. If there is water time required to remove the casing after concrete placement, in the overburden, the casing can be sealed into the rock, preventing formation of a good bond along the socket inter- dewatered, and the socket can then be excavated without sup- face. Trapping of debris between the casing and rock could port. However, there are situations where a contractor may also have occurred and may have smeared cuttings along the deem it necessary to introduce slurry. For example, when sidewalls. The lesson of this case is that the construction rock is highly fractured it may not be possible to seal the cas- method should be selected to provide the best product for the ing sufficiently to prevent water inflow, and a contractor may given conditions, and that in many situations a wet hole elect to use slurry. In this case, slurry may be used to balance method is the most effective and will not adversely affect the hydraulic head to prevent seepage into the hole that can shaft behavior if done properly. Forcing a dry pour may disturb the material at the base of the shaft, an issue related cause more problems than it solves. directly to design decisions on whether to include base resis- tance in the design. For reverse circulation drilling, slurry Another good reason to review the ground conditions care- may be used as the circulating fluid (e.g., the RichmondSan fully before allowing "dry hole" construction is identified Rafael Bridge shown in Figure 56). by Schmertmann et al. (1998). If the groundwater elevation is above the base of the hole, dry conditions inside the socket There are few data showing the effects of properly mixed result in a hydraulic gradient causing inward seepage as and handled slurry on rock-socket side or base resistances. illustrated in Figure 76. They describe several cases where Slurry that does not possess the appropriate viscosity, density, seepage degraded side resistance and base resistance. Main- and sediment content, or that is allowed to remain in the hole taining a slurry or water level inside the hole sufficient to bal- (and not agitated) long enough to form a thick filter cake, will ance the groundwater pressure eliminates the inward gradient

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90 casing tional information could improve their ability to perform the work. Another contractor interviewed for this study stated that the rock classification system of the ISRM is useful to determine what type of tool (rock auger, core barrel, or downhole hammer) will be most effective. The ISRM system places rock into one of seven categories (R0 through R6) based on strength, as described in chapter two (see Rock Material Descriptors). Zone of disturbed material at base caused by inward seepage An issue identified by several states is the discrepancy that sometimes occurs between the elevation corresponding to top of rock as shown in boring logs and as encountered during construction. The Washington State DOT uses lan- guage in their special provision for rock sockets that report- edly works well and is summarized as follows. For shafts with a specified minimum penetration into the bearing layer and no specified base elevation, the contractor furnishes each FIGURE 76 Development of disturbed base caused by high reinforcing cage 20% longer than specified in the plans. The seepage gradient toward bottom of a cased hole. increased length is added to the bottom of the cage. The con- tractor then trims the reinforcing cage to the proper length and prevents base and side disturbance. The authors cite before placement. The DOT assumes the cost of the excess several cases in which comparisons of Osterberg load cell test steel, but believes that cost is offset by avoiding construction results on shafts poured both wet and dry show this effect. delays, disputes, and claims that may occur otherwise. The most common factor cited in construction claims asso- Other specific issues identified by states in the questionnaire ciated with rock-socketed shafts is "differing site conditions," pertain to inadequate cleanout buckets, improper placement that is, the subsurface conditions actually encountered during of concrete with pump trucks, and a case in which temporary construction are claimed to be materially different from those casing to support the overburden with the same diameter as the shown in boring logs. Responses to the questionnaire did not rock socket resulted in the casing being dragged down into indicate that claims were a major obstacle to the use of drilled the socket, requiring additional socket drilling. There is a shafts for most states. However, one state DOT gave the fol- constructability lesson in each of these cases. lowing response when asked to comment on issues "pertain- ing to the use of rock-socketed drilled shafts by your agency" Certain geologic conditions are associated with more (Question 6): "Most result in claims due to the requirement to challenging construction and may require more detailed in- include `Differing Site Conditions' on all contracts." vestigation and flexibility in the approach to construction. Some of the more notorious of these include: (1) karstic con- The same agency responded as follows to Question 36 ditions associated with limestone and other rocks susceptible pertaining to perceptions of construction problems: to solution, (2) rock with steeply dipping discontinuities, (3) well-developed residual soil deposits grading into partially We design for low bidding contractors to get the contract and the weathered rock and then unweathered bedrock, (4) alternat- construction problems that will result. Rock may be harder than the contractor thought when bidding and planning the job. Thus ing hard and soft layers of rock, and (5) glacial till. Each of the drilling equipment brought out is often unable to drill or very these conditions presents its own unique set of construction slow to drill the rock. This results in costly contractor claims. challenges and different approaches are required to address them successfully. A question that often arises in some of Claims for differing site conditions are part of the geo- these environments is "what is rock?," or perhaps more im- technical construction field, but measures can be taken to portantly, "what is not rock?" On some projects, certain geo- minimize them. For example, one contractor interviewed for materials may be rock for pay purposes, but not for design. this study noted that geotechnical reports often place strong If these issues can be addressed before construction and there emphasis on rock of the lowest strength, because these lay- is good communication between owners and contractors, a ers may control side or base resistances for design. However, reasonable approach that results in a successful project can for estimating drilling costs, contractors need information on usually be developed. When the difficulties are not antici- rock layers of the highest strength, because that will dictate pated but are encountered during construction, the likelihood the type of drilling and tools needed to bid the job accurately of claims and disputes is much higher. Drilling of a trial in- and to carry out the construction properly. Transportation stallation shaft (also referred to as a "method" or "technique" agencies might consider surveying contractors to find out shaft) before bid letting can identify many of the problems exactly what information contained in their boring logs is that will be encountered during production drilling and most helpful for bidding on rock-socket jobs, and what addi- should be considered whenever there are major questions

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91 about the subsurface conditions and what is required to con- include the geotechnical report, boring logs, and communi- struct rock sockets successfully. cation with the design engineer. For rock sockets, inspectors should be trained to understand the information presented in boring logs pertaining to rock. This includes being familiar Inspection and Quality Assurance with the site and geomaterial characterization methods de- scribed in chapter two. Inspectors require basic training in Inspection is the primary method for assuring quality in the rock identification, testing, and classification, and should be construction of drilled shafts. The philosophy and methods familiar with rock coring procedures, the meaning of RQD, of drilled shaft inspection are covered in Chapter 16 of the compressive strength of intact rock, and terminology for FHWA Drilled Shaft Manual (O'Neill and Reese 1999) and describing characteristics of discontinuities, degree of weath- are the subject of a video and a Drilled Shaft Inspector's ering, etc. Inspectors should be aware of design issues such Manual (Baker 1988) available from the ADSC. A certifica- as whether the shaft is designed for side resistance, base re- tion course for drilled shaft inspectors is offered by the NHI sistance, or lateral resistance, and in which rock layers the of FHWA, and a Participants Manual was developed as part various components of resistance are derived. of the course (Williams et al. 2002). Table 21 is a partial list- ing of inspection issues pertaining specifically to rock-socket Before construction, inspectors should know how the con- construction. tractor plans to construct the shafts. This requires knowledge of the tools and methods used for construction in rock. A Special emphasis is required in making a strong connec- valuable aid is the Drilled Shaft Installation Plan, a document tion between drilled shaft design and inspection. Practically, describing in detail the contractor's tools and methods of this involves providing inspection personnel with the knowl- construction. O'Neill and Reese (1999) describe the mini- edge and tools required to verify that drilled shafts are con- mum requirements of an installation plan and recommend structed and tested in accordance with the design intent. The that it be a required submittal by the contractor. starting point for inspection personnel is to have a thorough understanding of (1) subsurface conditions, (2) the intent of A fundamental design issue is the degree to which the the design, and (3) how items 1 and 2 are related. The in- rock mass over the depth of the socket coincides with the spector's sources of information for subsurface conditions conditions assumed for design. Therefore, some type of TABLE 21 INSPECTION ITEMS FOR ROCK SOCKETS Inspection Responsibility Primary Items to Be Addressed Required Skills or Tools Knowledge of site Rock types, depths, thicknesses, engineering Competency in rock identification and conditions properties (strength, RQD); groundwater classification; ability to read and interpret conditions core logs Knowledge of design Rock units providing side, base, and lateral Basic understanding of design philosophy issues resistances for drilled shafts under axial and lateral Design parameters: shaft locations, socket loading depths and diameters, reinforcement details Familiarity with standard specifications, plans, special provisions, shop drawings, and contractor submittals Knowledge of contractor 's Rock excavation tools (augers, coring, hammers, Review of Drilled Shaft Installation Plan plan for socket other) and methods (e.g., casing, slurry) construction Classification of rock for pay purposes Observations and record Identification and logging of excavated rock Competency in field identification of keeping during socket Tools used by contractor for each geomaterials; excavation geomaterial (tool description, diameter, rate Appropriate forms*, including: of excavation) Rock/Soil Excavation Log Occurrence of obstructions, removal method Rock Core Log Depth to top of rock Inspection Log Sidewall conditions (roughness, smearing) Construction and Pay Summary Roughening or grooving of sidewalls Use and handling of slurry and casing Inspection methods and devices (e.g., SID) Coring at the base Cleanout specs., verification method Sampling and testing Sampling of rock for lab tests; Proper sampling/testing equipment and Field tests on rock; e.g., point load, hardness; knowledge of procedures NDT/NDE *See Williams et al. (2002) for descriptions of inspection forms. Notes: RQD = rock quality designation; SID = shaft inspection device; NDT = nondestructive testing; NDE = nondestructive evaluation.

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92 downhole inspection is needed. Responses to Question 11 Most states include specifications for conditions at the of the survey reveal a wide variety of methods used for this bottom of the hole that must be satisfied before pouring purpose. Nine states reported that coring is required into rock concrete. Some distinguish between shafts designed for base below the bottom of the shaft after the excavation to base resistance and those designed under the assumption of zero elevation is complete. Typical required depths range from base resistance. A very typical specification (five states) 1.5 m to 10 m, three diameters, etc., although one state re- is "minimum 50% of the base area to have less than 12 mm quires coring 15 m below the bottom of the shaft. Coring be- (0.5 in.) and maximum depth not to exceed 38 mm (1.5 in)." low the base during construction allows a determination to Some states allow up to 300 mm (6 in.) of loose material be made of the adequacy of rock below the base to (1) pro- when base resistance is neglected. vide the base resistance assumed in the design; (2) ensure that the base is bearing on bedrock and not an isolated boul- When sockets are poured under dry conditions, common der ("floater"); and (3) detect the presence of seams, voids, inspection methods to verify bottom conditions are either or other features that would require changes in the base visual inspection or downhole cameras. For wet pours (under elevation or other remedial actions. slurry or water) the most common method is to lower a weighted tape (e.g., a piece of rebar on the end of a tape mea- Five states reported using a probing tool to inspect core sure) to the bottom of the hole and "feel" the bottom condi- holes at the bottom of the completed excavation (Figure 77). tions by bobbing the weight against the bottom. Although This method, which in most cases requires downhole entry somewhat subjective, an experienced inspector can differen- by the inspector, is most useful for detecting seams of soft tiate between clean water or slurry and contaminated condi- material in discontinuities. It is most applicable in limestone tions. Downhole cameras are available that permit viewing and dolomite where the bedrock surface is highly weathered, of conditions under water or slurry. A device used by the irregular, and filled with slots and seams of clayey soil. Florida DOT referred to as a shaft inspection device or SID has been used successfully in slurry shafts (Crapps 1986). Proper safety measures are paramount for downhole entry. The device, shown in Figure 78, has a color television cam- Five states reported using fiber optic cameras for inspection of core holes, which is safer and provides visual evidence of seams, cavities, and fractures, but does not provide the "feel" of probing that may be useful in karstic formations. Four of the states reporting use of probe rods are in the Southeast where karstic conditions are most common. Base of Socket Probe Rod FIGURE 77 Rock probing tool (after Brown 1990). FIGURE 78 Shaft inspection device or SID.