State of the Practice for Design of Groups of Laterally Loaded Drilled Shafts

Many state departments of transportation (DOTs) are beginning to use a larger percentage of drilled shafts for their deep foundations. Therefore, a clear understanding of current design methodology and specifications is important. The three most common design methods in use today are the group reduction factor, the p-multiplier method, and the modulus of subgrade reaction multiplier. A brief discussion of each method is presented. Telephone interviews were conducted with state DOTs to determine their current design procedure for groups of laterally loaded drilled shafts. Responses were obtained from all 50 states. Additional interviews were conducted with several well-known researchers in the field of deep foundations to determine what procedure they recommend. From these interviews it was clear that there is no uniformity in design methods or even in interpretation of the AASHTO recommendations for laterally loaded drilled shaft groups. The different interpretations of the AASHTO recommendations may be due to wording used in the AASHTO specifications, namely, the “ratio of lateral resistance of shaft in group to single shaft.” It is somewhat unclear as to whether this wording refers to the single-pile capacity or to the soil resistance. To understand the intended method of application of these factors, a historical search of the literature was conducted. It is clear that the source cited in the AASHTO specifications recommended using the reduction factors to reduce the modulus of subgrade reaction.