A Treatise On The Principles And Practice Of Dock Engineering

248 DOCK ENGINEERING. River Humber, but the fact that the sand between the two beds of clay was grey and loamy, whereas the water-borne sand was yellow, induced the engineers to make trial borings through the lower clay. This was found to be a stiff brown layer, 42 feet in thickness, and the borehole remained quite dry until the bottom was reached, when water charged with yellow sand flowed up the hole with considerable force, showing that the boils had their origin in the sand bed which immediately overlay the chalk. As the chalk wolds extend over a large area, attaining an elevation of 500 feet at no great distance from Hull, and giving rise to copious springs at their base, it was then considered probable that the influent was mainly due to land water accumulated in the chalk, though the fact that the stream was brackish indicated some connection with the sand beds of the River Humber. Sometimes the source of leakage, being nearer at hand, is more obvious. In the reconstruction of the Canada Lock at Liverpool, the site of which comprises an alluvial bed overlying two layers of boulder clay, intersected by a bed of sand and gravel of varying thickness, considerable difficulty was experienced at first owing to inbursts from the river through the sand. Excavations at the time were in progress, continuously within the lock chamber, under protection of the gates, and intermittently at the outer sill, at low water ofspring tides. The removal of the upper clay in both situa- tions was coincident with the flooding of the lock chamber at high tide, clearly under the head afforded by the water in the river. It was found impossible to keep down the water in the lock, and the interior work had to remain in abeyance until the outer sill was completed. The larger area involved in the construction of looks and entrances generally renders it difficult, and not always advisable, to adopt the method of treatment recomraended for infiltrations of water to wall foundations— viz., to lead them to some suitable spot where they can be provided with a vent. Discharge within the lock itself is inconvenient in the case of small streams and impracticable in the case of large ones. On the other hand, to convey a discharge outside the lock area would be a matter often attended by needless difficulty and expense. Furthermore, there is the risk that the effluent might carry with it material in suspension, unless it were entirely checked by a counteracting head. In view of the diversity of conditions under which constructive operations have to be carried on, it would be obviously impossible to lay down any general rules of procedure in case of leakage arising from boils or springs. All that is permissible is to briefly indicate a few of the methods which have been successfully adopted in actual cases, putting on one side altogether the question of their applicability elsewhere. 1. Where the discharge has been slight and of the nature of an infiltra- tion, it has been easily checked by the rapid deposit of a large bulk of concrete upon the spot, the concrete being mixed fairly dry, so as to allow for its admixture with the water in situ.