Cyclic load effects on model pile behavior in frozen sand

Small cyclic loads, 3 to 5 percent of the long-term sustained load, superimposed on a static load will increase settlement (displacement) rates of friction piles embedded in frozen ground. Determination of the basic mechanisms responsible for this increase in pile settlement rates and a suitable theory for their prediction were the objectives of this research.; A series of model steel piles with controlled surface geometries (lug size, shape, and spacing) were frozen in sand and loaded while carefully controlling test variables. The data collected provided information on the displacement mechanisms operating at the pile/frozen soil interface. Pile settlement rates due to static and incremental static loading were analyzed and compared to rate increases resulting from superimposed dynamic loads. Variables controlled during these tests included cyclic load amplitude, frequency of load application, static load magnitude, and temperature. Sand density and ice volume fractions were controlled during sample preparation.; Experimental results showed that cyclic loads significantly increased model pile displacement rates over those observed for the sustained load, but were smaller than rates inferred from published results. Typically, superimposing a small cyclic load (with amplitude X) produced a displacement rate increase approximately equivalent to the rate increase produced by adding a small static load (magnitude close to 0.6 times X) to a similar pile. The measured displacement (creep) rates appeared to be independent of frequency in the range of 0.1 Hz to 10 Hz for small cyclic loads superimposed on a sustained load. Data corresponding to frequencies which induced resonant response in parts of the loading system were excluded from this study.; Larger lugs (more than 2 times maximum particle size) compressed frozen sand at their leading edge and densified the adjacent frozen soil. Particle crushing along with formation of a zone of material flow and/or slippage occurred in front of and around the lugs. Ice melting at particle contact points and water movement to adjacent locations with lower stress accompanied this slip process. The onset of increasing pile displacement rates (tertiary creep) was shown to be a function of lug height and spacing.