A 5 m deep deposit of sand and silt containing organic layers is to be compacted using explosives placed in the boreholes located 8 m apart on a square grid. It is intended to create pore pressures that are at least equal to 80% of the effective overburden pressure. Estimate the "safe" distance from an underground explosion of 10 kg of TNT, beyond which little or no soil liquefaction is likely to occur.