While the mining and the construction of dug-outs was being carried on as intensively as possible, an enormous amount of spoil (excavated earth and chalk) was accumulated. The disposal of this spoil became a very serious problem. Chalk cannot be scattered broadcast except at a distance from the shaft-heads, as its presence would disclose to the enemy that work was in progress and draw shell-fire. The accumulation of spoil was thus very difficult to dispose of, and frequently the trenches became almost impassable owing to the heaps of sandbags full of chalk stacked against their sides waiting to be emptied. The usual method of disposing of this spoil was to dump it into disused trenches and shell-holes. These, however, were soon filled, and it became evident that elaborate arrangements must be made beforehand for dealing with the spoil whenever operations entailing excavations on a large scale are planned, otherwise vast accumulations will be formed. In this sector it was therefore necessary to establish a carefully-organised system of spoiling parties to keep pace with the output of the tunnellers and pioneers. It was only thus that it could be ensured that the intensive mining and dug-out construction would not be checked owing to delays in dealing with the excavated earth. Large numbers of men had thus to be found daily for this work, with the result that there was little labour available for the ordinary duties of trench maintenance. As long as the weather remained fine, difficulty was not experienced on this account. Subsequently, however, when the weather changed, the whole Division was taxed to its utmost to keep the trenches in a condition which rendered them passable to troops. This was particularly difficult in the sector between La Boisselle and Thiepval. Here above the chalk was a deep overlay of clay. Thus, when towards the end of the year there was a heavy fall of rain and subsequently of snow, the country in this area became water-logged, and in consequence the sides of the trenches were continually collapsing. This was partly due to the fact that the trenches constructed by the French were cut with perpendicular sides. Trenches of this pattern stood well enough in fine weather or where cut in chalk, but when cut in clay could not withstand the additional pressure which the weight of absorbed rain-water brought into play. The result was that throughout this portion of the sector the sides of the trenches fell in, and they became merely shapeless ditches knee- or waist-deep in mud and water. Water and mud, too, flowed down the stairs of the dug-outs unless dams were carefully made, while the "Bairnsfather" type of shelter collapsed on to its occupants at least once with fatal results.