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There are many factors that influence the side resistance prediction of drilled shafts in poor quality rock. As part of a bridge replacement project in Oregon, we evaluated the performance of various design methodologies for predicting the side resistance in poor quality rock. In particular, we looked at the side resistance predicted in poor quality rock from the rock mass strength developed using Rock Quality Designation (RQD) as well as the Geological Strength Index (GSI) developed by Hoek and Brown. We predicted the capacities of drilled shafts at several bents along the bridge alignment and compared the estimated bedrock socket required to resist the structural loading. This paper summarizes the determination of rock mass quality using the RQD and GSI methods, describes the prediction methods used for determining the side resistance in poor quality rock, presents the comparison of RQD versus GSI predicted design values, and evaluates the performance of the predictive methods and discusses the difference in cost associated with the drilled shaft lengths predicted by each method. Based on the results of this study, using the RQD methods without any specific laboratory strength or deformation test results may under predict the ultimate shaft resistance within poor quality rock, i.e., RQD < 60 percent.
Ground modification has been used on many construction sites to densify granular material and reduce potential settlement or the susceptibility to liquefaction. This paper evaluates the benefits derived through several ground improvement methods, deep dynamic compaction, vibro-compaction, and vibroreplacement. These ground modification procedures were performed on several study sites with the effectiveness of the methods evaluated by comparing the tip resistance of pre-and post-construction cone penetrometer test (CPT) soundings. The results provided by the treatment methods were also evaluated with respect to the amount of fines observed in the subsurface profile. The results of our evaluation are intended to expand the availability of information on the effectiveness of these ground modification methods.
There are many factors that influence the skin friction capacity of steel Hand closed ended pipe (CEP) piles driven into sands. As part of a bridge replacement program near Texas gulf coast, we evaluated the performance of various design methodologies for predicting driven pile shaft resistance in sands. We predicted the shaft resistance of steel Hand CEP piles at one bridge location using the SPT Method (Meyerhof, 1979), FHWA DRIVEN computer program, and WEAP-SPT Method. The actual pile shaft resistances were then determined during end-of-drive (EOD) and at beginning of restrike (BOR) using dynamic testing and CAPWAP procedures. The dynamic testing shaft resistance distribution results were compared to our predictive methods to evaluate which predictive method would be best suited for estimating the pile capacity at the remaining bridge locations. This paper describes the installation of the driven steel Hand CEP piles, presents the dynamic load test results as compared with predicted design values and evaluates the performance of the predictive methods.
As part of a large expansion project in Las Vegas, Nevada, we evaluated performance of various analytical design methodologies for predicting drilled shaft side resistance in a soil profile of hard and soft clay with interlayered sand and cemented soils (caliche). We predicted side resistance capacities using the Federal Highway Adminstration (FHWA) methods and other developed relationships based on in situ testing. Two instrumented preconstruction Osterberg load tests were performed to refine capacity estimates. Further evaluation of drilled shaft installation was recommended by the design team to quantify the influence of groundwater on side resistance of the drilled shaft. The contractor elected to perform dynamic testing using the APPLE system. Results of the Osterberg and dynamic load testing were compared to our predictive methods to determine which of the methods used would be best suited for estimating side resistance of drilled shafts in heterogeneous soil conditions typical of the central Las Vegas Valley. This paper describes the installation of the drilled shafts and presents the Osterberg and dynamic load test results as compared to predicted design values determined using FHWA and other design methods. In addition, we compared the cost of the testing program with savings in foundation costs due to design efficiencies.
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