Heap leaching involves the stacking of gravel-size materials produced by grinding. Such materials can contain significant amounts of fines arising from the alteration or weathering of the parent rock. To enable transport, placing and pile operation, it is convenient to induce fines agglomeration with the gravel particles by mixing the material with moderate amounts of water. Moreover, the placing procedure in the heap is often conducted by pouring the material over the top of the pile from portable conveyor belts with no additional compaction. The combination of these procedures yields extraordinarily loose states. For instance, in the case studied in this paper, the placing dry unit weight measured in the field was as low as 16 KN/m3, while the laboratory determined minimum and maximum dry densities (ASTM D4254 & D4253) were 19 and 21 KN/m3, respectively. Despite the fact that the leach heaps are designed to operate in unsaturated condition, experience shows that it is not uncommon that some parts of the stack become saturated at some point of their operation or the closure phase. This may arise from just the sprinkling of leaching solution, or in combination with water inflow from rain or snowmelt. Moreover, segregation during placing, transit of equipment, fines migration and clogging may cause local variations of the materials permeability, producing the elevation of the phreatic level or even perched water tables. If such conditions are met in a significant volume within the pile, the undrained behaviour of the material is of paramount importance, considering its expected contractive tendency. Static and earthquake-triggered liquefaction have already been observed in some few cases around the world. In this paper, the undrained shear strength of a heap leach pile material is investigated by means of large-size triaxial tests. A series of consolidated-undrained tests were conducted on remolded specimens prepared at various initial void ratios. Emphasis was placed on careful measurement of initial void ratio and volume changes during the test, in order to obtain the best possible approximation to the critical state line.