Pioneer performed a comprehensive geotechnical investigation for a proposed 25-story building which included a 3-story parking structure. The investigation consisted of rotary mud drilling through a surficial layer of typical Chicago urban fill; followed by a layer of Chicago “blue clay”; followed by stiff to very tough silty clay; followed by “hardpan” which was present at a depth of approximately 75 feet below grade; and finally into the underlying dolomite bedrock.
During the field investigation, Pioneer performed field vane shear tests in the cohesive soft materials in order to assist in the caisson design and determination if downhole casing would be required during construction. Pioneer also performed pre-bored pressure meter tests in the “hardpan” materials overlying the bedrock in order to determine the allowable bearing pressure for these sediments. A Rock Quality Determination (RQD) was also completed for the underlying bedrock to determine the potential suitability for end-bearing caissons on the bedrock surface.
Based on an advanced analysis of the field and laboratory data, in conjunction with the anticipated high maximum column loads, Pioneer determined that belled-caissons bearing on the “hardpan” materials would provide for adequate support and prove to be more economical than deeper caissons bearing on or tied into the bedrock.