Frictional Characteristics Of Calcareous Sands From Offshore Florida

ABSTRACT A comprehensive laboratory testing program was recently undertaken on calcareous sands and silts from an offshore location in Florida. These tests provided a better understanding of the parameters that contribute to the low frictiona1 resistance known to be mobilized along a pile in such sediments. This paper describes some of the findings from this study. It is shown that the magnitude of reduction in the lateral stresses on the pile due to development of an arch in the soi1 formation can be determined from laboratory strength tests and incorporated in the conventional pile capacity formulation for granular material. Results also suggest that determination of maximum unit soil-pile friction from consideration of soil compressibility could provide realistic values. INTRODUCTION Calcerous sediments are often encountered in the relatively shallow water environment of the tropics. These biologically derived gravel, sand, silt and clay sized carbonate sediments have undergone post depositional alteration such as cementation and chemical replacement. A number of field pile load tests (1, 2, 6, 7, 8, 14) and laboratory model pi 1e tests (11, 13) have shown that the axial capacity of piles in calcareous sands is significantly less than in comparable silica sand formations. Such reductions in capacity are attributed to crushability of the calcareous sand grains and presence of cementation in the formation. For driven pipe piles in calcareous sands and silts, volume reduction due to grain crushing under pile driving stresses could equal or even exceed the volume displaced by the pile with very little radial displacement of the soil into the adjacent formation (7). This reduces the tendency of the soil to dilate, and the fines created by grain crushing could provide a 1ubricant at the pile-soil interface which reduces frictional resistance being mobilized against the pile wall. In the undisturbed zone adjacent to the pile, particle cementation contributes to the formation of an arch which reduces the 1ateral stresses against the pile wall. Results of 1aboratory tests (3, 5, 6, 7, 12, 13) show high friction angles, in the range of 40 to 55 degrees, for reconstituted calcareous sand and silt samples. Grain crushing at high confining pressures seems to have little effect on the friction angle; a reduction of only a few degrees (12). These reported friction angles are usually higher than those reported for siliceous sands of similar gradation and density (5, 12). This could be due to greater mineral frictional resistance between calcareous soil grains (9) and interlocking of irregular shaped cemented lumps of individual particles (10, 12). Soil steel friction test data indicates that the angle of friction between the soil and steel,?, is about the same for both calcareous andsi1iceous sands (12). These results simply that frictional characteristics of calcareous sands are not significantly influenced by grain crushing effects at high stress levels and are therefore not responsible for the low frictional load capacity of piles. Mobilization of very low lateral stresses against the pile appears to be the cause for the lower capacity.