The notion of "poor subsoil" or " bad building ground " is relative. One is usually inclined to only reckon clay or peaty ground as coming within this category, hut other kinds of soil, such as fine sand, may also be qualified as unsuitable for supporting certain structures. For light buildings, such as houses, even clay or peaty soil often prove to be sufficiently good to build upon, provised that one at least takes care that settling takes place evenly, by founding them, for instance, on a continuous concrete slab. On the contrary, ground composed of fine sand may be insufficient to bear the weight of heavy edifices, in cases where unequal settling is not admissible. Lock heads of big ocean locks come within this category; they might, it is true, be founded on a continuous slab forming the floor of the lock, hut the vast extent of such slabs and the very unequal loads which would require them to be very thick might raise difficulties. One would then be obliged to divide the structure, by means of joints, into separate parts, some of which are light (such as the floors), whilst others are heavy (the walls). These separate parts would settle unevenly, which might be harmful to the working of the lock gates, especially in the case of rolling or sliding gates. There are instances of structures that have been damaged by unequal settling and in which the working of the gates has suffered in consequence: in one foreign country the rolling gate of a dry dock could not be used as such because the floor had settled less than the walls and the gate chamber. Settling is caused above all by the rising of suberranean water in the foundation-excavation that has been dried out, and this water makes the sand into quicksand in the upper layer on the floor of the excavation. Thus, it was found that the inland navigation lock recently constructed at Zutphen (Eefde) , at the entrance of the new Twenthe canal, the walls of which are founded on a continuous slab, has sunk, according to information supplied by W. Eggink, the engineer of the Waterstaat, over 5 cm, owing to the rise of suberranean water in the working sump after pumping stopped, although the average load on the subsoil had not yet reached 1 kg/cm". As a settling of this extent had been foreseen when drawing up the plans, they took the necessary measures to avoid the concrete resting on the screen of sheet piling (by keeping an interstice between these screens and the bottom of the concrete) or adhering to them (by greasing the parts of the sheet piling against which the concrete rested ) . If the sand on the spot had been less fine, the settling would probably not have taken place.