Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities

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13.4.3.1 Pre-Drilling and Surface Casing
Pre-drilling, either with or without removal of material, is sometimes used to facilitate the installation of temporary or permanent casing at the beginning of drilled shaft installation. If the auger diameter is larger than the diameter of the casing, pre-drilling may result in loosened material outside the completed shaft. This is not a concern if the depth of pre-drilling is limited to the shaft cutoff level or to the design scour depth. In other cases, the diameter of the auger used for predrilling should generally be not more than the diameter of the completed shaft. To further reduce the risk of soil disturbance outside the completed shaft, predrilling in such cases should be limited to just loosening the soil rather than removing it. If an oversized pre-drilled hole extends below the drilled shaft cutoff level, the contractor should be required to fill any voids around the casing with tamped granular backfill, or with grout, before continuing with shaft drilling operations. The use of an oversized surface casing to stabilize and support the soil near the top of the shaft can result in loosened material in the annular zone between the completed shaft and the surface casing. To mitigate this condition, the contractor can fully remove all soil within the surface casing prior to extending the shaft below the bottom of the surface casing, and then allow the shaft concrete to flow into the annular void as the temporary casing is removed with this approach, the contractor must prevent drilling spoil from falling into the annular void prior to concrete placement. If permanent casing is installed within the surface casing, the annular void should be backfilled with tamped granular backfill or with grout after the permanent casing is installed.
13.4.3.2 Structural Integrity
A major concern associated with construction of drilled shafts is the structural integrity of the concrete in the completed shaft. Poor concrete placement procedures or inappropriate concrete mixes can result in structural defects in the completed drilled shaft that can reduce its stiffness and lead to greater displacement during lateral loading. Such defects can also greatly reduce the structural integrity of the shaft for axial loading. Common defects may include necking of the shaft, honeycombing, soil intrusion, segregation of concrete aggregates, bleed-water channels, low strength concrete, concrete laitance, and cold joints. Nondestructive testing (NDT) methods, such as Crosshole Sonic Logging (CSL) per ASTM D and Sonic Echo / Impulse Response (SE/IR) methods per ASTM Dare typically specified to verify the structural integrity of the shaft concrete. Other, less commonly used methods include Gamma-
Gamma Logging (GGL) per California Department of Transportation Test No. 233, and Thermal Integrity Profiling (TIP) per ASTM D. These methods have advantages and disadvantages, as discussed in Brown et al. (2010); however, they offer a practical approach to identifying anomalies (i.e., potential defects) within the shaft that may warrant further investigation by concrete coring or, when anomalies are detected near the top of the shaft, by visual inspection of the exposed top and sides of the shaft. When such investigations confirm the presence of a defect in the shaft that will significantly impact the performance of the shaft, remediation measures, as discussed in Brown et al. (2010), can be performed to correct the defector one or more additional shafts can be installed to replace or supplement the defective shaft.

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13.4.3.3 Rock Sockets
Drilled shafts and other drilled-in foundation types, such as micropiles, can develop high lateral resistance as a result of fixity within a rock socket. However, to achieve this high resistance the designer must verify that the assumed top of rock elevation and rock quality is consistent with design assumptions. Complicating this issue is the fact that, for some geologic settings and in some rock formations, the rock surface elevation and the quality of the rock can vary dramatically over short distances. When bedrock conditions are encountered that differ appreciably from the conditions that could reasonably be expected based upon the construction documents, the work could be delayed as the engineer-of-record evaluates the impact of these changed conditions on the performance of the foundations. To reduce the risk of delays and costly design changes during construction, an appropriate subsurface investigation should be performed during the design phase of the project, as discussed in Chapter 3, to define the rock conditions at the site. In addition, projects requiring highly loaded drilled shafts or non-redundant drilled shaft foundations should require a rock core boring at each shaft location to define rock depth and rock quality in advance of drilling operations to allow time for the designer to confirm or adjust the rock socket depth and length, and time for the contractor to fabricate the reinforcement cage needed to suit the site-specific shaft and socket lengths. These additional core borings are typically included in the construction contract and performed prior to initiating shaft installation.

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