Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Considerations for Subsurface Explorations when Lateral Loads are Significant
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| 3.3.3 Considerations for Subsurface Explorations when Lateral Loads are Significant When lateral loading on deep foundations is expected to be a significant aspect of the structure design, consideration should be given to performing certain in-situ testing to obtain data that can specifically be used for lateral pile analyses. The additional cost of these methods maybe offset by a more economical design for lateral loading. The PMT can be used to estimate the stiffness (E m ) and the mass strength of weak rock and fractured rock for development of p-y curves for lateral pile/shaft analysis (p-y curves will be discussed in more detail in Chapter 6). This technique has been increasingly used in the US. and elsewhere to estimate the elastic modulus of a soft rock mass. The PMT involves inflating a cylindrical probe against the sidewalls of a boring drilled in soil or rock. The most common technique is to insert the inflatable probe in a pre-bored hole before expansion takes place, although self-boring PMTs are also used. Pressuremeter testing for rock uses a device similar to that used in soil but with a stiffer membrane and higher pressure range. The term rock dilatometer is sometimes used to describe a pressuremeter used for testing rock this is not the same device as a flat- plate dilatometer for testing soil. According to ASTM Done of the most important aspects of the PMT is assuring that the sidewalls of the borehole are smooth, consistent, and of uniform diameter. In general, the PMT offers advantages in providing estimates of rock modulus over laboratory methods, particularly in weak rocks, because PMT provides a direct estimation of the modulus of the rock mass with little disturbance, does not require sampling, and it automatically considers the softening effects of fractures, joints, weathering on the lateral deformability of rock. The method also allows the indirect estimation of the rock mass strength. The PMT (and the SPT) can produce estimates of strength and modulus estimates in very weak and weak rocks (Abu-Hejleh et al. 2005). These categories can be defined as rock having unconfined compressive strengths between 20 ksf and 100 ksf, and between 100 ksf and 500 ksf, respectively however, the definition of weak or very weak rock may vary by local practice area or formation. In general, sampling of soil-like rock for subsequent laboratory testing can be particularly challenging. Some local agencies have developed region-specific guidelines or correlations for investigations and estimating parameters for weak and very weak rock. For example, the Colorado DOT has produced correlations between PMT and SPT and unconfined compression test results for weak rock (Abu-Hejleh et al., 2005). The PMT can be used with unusual soil types that may not correlate well with other tests, such as dense gravelly soils, for developing engineering parameters for laterally loaded pile/shaft analysis. PMT tests in soil formations can be used to develop load-deflection response curves that can be used in computerized lateral pile analyses, without being associated with a particular soil classification or without having to develop estimates of other engineering parameters. In addition to the above procedures, the Goodman Jack or borehole jack testis used occasionally Goodman et al. 1972) in rock formations to estimate their strength and deformation properties. A borehole jack device consists of exerting a unidirectional pressure on the walls of a borehole using two opposed curved steel plates. This apparatus is designed for use in inch diameter holes. The advantage of this system is that it allows higher pressures than the PMT; therefore, the response of stronger rock can be investigated. The borehole jack results must be corrected to account for the stiffness of the steel plates. 37 The rock pressuremeter and borehole jack devices require proper calibration and operation, and these may vary depending on the type or manufacturer of the particular device. The system should be calibrated prior to and after testing. Tests should include multiple loading and unloading cycles with pressure readings taken during both loading and unloading cycles (Brown et al. 2010). The DMT may also be used for developing correlations for laterally loaded pile/shaft design. The DMT is primarily intended for soils with particle sizes smaller than fine gravel and is not well-suited for soils with large gravel, cobbles, boulders, concretions, cementations, large shells, or rock layers. The DMT can be used with correlations to estimate the soil type, at-rest earth pressure, overconsolidation ratio, effective friction angle of sands, undrained shear strength of cohesive soils, and the dilatometer modulus. The use of PMT, DMT, and borehole jack tests and the calibration of the results of such tests is part of an evolving state-of-the-practice with regard to laterally loaded deep foundations, especially as additional research and testing is performed. For example, a study on rock socketed drilled shafts in Ohio included in-situ pressuremeter and dilatometer testing and full scale lateral load testing. The results of pressuremeter and dilatometer tests were used to develop load-displacement curves (p-y curves) for analysis of the laterally loaded shafts. The p-y curves were compared with and adjusted to match the full- scale load test results. One of the findings of the study was that the dilatometer test results could provide reasonable predictions of p-y curves for drilled shafts in rock by using a method developed by Briaud et al. (1983) with the modification of reducing the p-values by 50 percent (Nusairat et al. 2006). The in-situ tests in rock have several limitations. Most notably is that they test only a relatively small area of the rock mass. As a result, depending on the joint spacing, test results mayor may not be representative of the overall rock mass behavior. A similar consideration would apply with interbedded rock types which may have different properties, such as sandstone and shale. The testing should be performed at sufficient intervals within each rock type to prevent bias in the results for evaluating the overall rock mass. The rock pressuremeter is generally limited to soft to weak rocks. Download 6.03 Mb. Share with your friends:
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