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: Borehole blasting tests using 15- and 40-lb charges were conducted in silt at Fort Wainwright near Eielson Air Force Base. Tests were conducted with the silt unfrozen and with two thicknesses of frozen surface layers (2.0 and 3.7 feet). The standoff distance (the height of the charge above the soil surface) was varied to determine its effect on borehole dimensions. The 15-lb shaped charge was fired over unfrozen silt with a moisture content between 15 and 35% and at standoff distances of 2.0, 2.5, 3.0 and 3.5 ft produced a borehole of sufficient size to accommodate the 40-lb cratering charge having a 7-in. diameter and 2.0-ft length. The optimum standoff distance was 2.5 ft to produce a near-optimum depth for placing the cratering charge. The 15-lb shaped charge fired over the same material with the 2- and 3.7-ft-thick frozen surface layer did not produce a borehole of sufficient size to accommodate the 40-lb cratering charge. The 40-lb shaped charge fired at standoff distances of 3.5, 4.0 and 4.5 feet produced boreholes for the unfrozen and the frozen surface layer conditions that are larger than required to accommodate the 40-lb cratering charge; therefore, it is wasteful and more of a problem logistically. Firing a second 15-lb shaped charge in the same locations as the first over the 3.7-ft-thick frozen layer increased the hole cross section by about one-third; however, the hole was still not adequate. The second firing of the 15-lb charge increased the depth by nearly two times; however, with the 40-lb charge, little change in depth was observed.

Testing Shaped Charges in Unfrozen and Frozen Silt in Alaska