Capacity of Drilled Shafts in Burlington Limestone

Field load tests of three drilled shafts socketed in Burlington limestone were conducted using the Osterberg load cell. The objective of the testing was to compare the load capacities obtained from the field load tests with load capacities predicted using empirical methods. Based on the results of this study, the following conclusions can be drawn. The observed values of unit side resistance exceeded predicted empirical values for two of the three shafts tested (2,343 and 2,278 kPa observed versus 1,550 and 1,252 kPa predicted). However, for one of the shafts, the observed value of unit side resistance was only about $\frac{3}{4}$ of the more conservative predicted empirical value (916 kPa observed versus 1,252 predicted). Bearing capacity failure did not occur for two of the three shafts. Bearing capacity failure may have occurred for one of the shafts. Using a factor of safety of 3 applied to the lowest observed value of end bearing pressure implies that the allowable bearing capacity for the Burlington limestone at this site (3 MPa, or ≅500 psi) exceeds the typical presumptive bearing capacity for sound limestone in mid-Missouri (1914 kPa or 277 psi).