=135. Excavation in Quicksand.=[85]—A sand or other granular material in which there is sufficient upward flow of ground water to lift it, is known as quicksand. Its most important property, from the viewpoint of sewer construction, is its inability to support any weight unless the sand is so confined as to prevent flowing of the sand, or unless the water is removed from the sand.
Excavation in quicksand is troublesome and expensive and is frequently dangerous. The material will flow sluggishly as a liquid, it cannot be pumped easily, and its excavation causes the sides of the trench to fall in or the bottom to rise. The foundations of nearby structures may be undermined, causing collapse and serious damage. These conditions may arise even after the backfilling has been placed unless proper care has been taken. The greatest safeguard against such dangers is not only to exercise care in the backfilling to see that it is compactly tamped and placed, but to leave all sheeting in position after the completion of the work.
The ordinary method of combating quicksand and in conducting work in wet trenches is to drive water-tight sheeting 2 or 3 feet below the bottom of the trench, and to dewater the sand by pumping. When dry it can be excavated relatively easily. A more primitive but equally successful method is to throw straw, brickbats, ashes, or other filling material into the trench in order to hold the excavation once made, or this may supplement the attempts at pumping, or the wet sand may be bailed out in buckets. Successful excavation in quicksand requires experience, resourcefulness. and a careful watch for unexpected developments. The well points described in Art. 142 are used for dewatering quicksand.
=136. Pumping and Drainage.=—Ground water is to be expected in nearly all sewer construction and provision should be made for its care. Where geological conditions are well known or where previous excavations have been made and it is known that no ground water exists it may be safe to make no provision for encountering ground water. Where ground water is to be expected the amount must remain uncertain within certain rather wide limits until actually encountered.
In order to avoid the necessity for pumping, or working in wet trenches it is sometimes possible to build the sewer from the low end upwards and to drain the trench into the new sewer. The wettest trenches are the most difficult to drain in this manner as the material is usually soft and the water so laden with sediment as to threaten the clogging of the sewer. It is undesirable to run water through the pipes until the cement in the joints has set. This necessitates damming up the trench for a period which may be so long as to flood the trench or delay the progress of the work. If it is not possible to drain the trench through the sewer already constructed the amount of water to be pumped can be reduced by the use of tight sheeting.
