20 SEWER DESIGN 



determined by borings or test-pits, the latter being preferable. 

 Enough soundings should be taken to explore thoroughly the 

 ground through which the sewer is to pass, since the location 

 of the mains may depend on the character of the ground. If 

 there are two possible locations for a deep main, one through 

 rock and one through soil, the cheaper design will of course 

 locate it in the soil as determined by the borings, and even a 

 longer line may be cheaper to build on account of the character 

 of the trenching. Contracts are now rarely let at a lump sum 

 for the system, but rather at unit prices for the different kinds 

 of work, so that rock found in unexpected places has to be 

 paid for, and goes to make the work cost more than the engineer's 

 estimate. Where water and quicksand are encountered, there 

 has as yet been found no just way of paying for the extra work 

 involved, and it must be covered by the percentage added by the 

 contractor to cover such contingencies. It may be noted here 

 that a cheaper and fairer method would be to pay the contractor 

 directly, and just in proportion to the amount of this extra 

 work. In laying out the best lines, the designing engineer should 

 have the location of rock, quicksand, water-pockets, and soft 

 clay in mind, to avoid them if possible, and get the maximum 

 efficiency at the minimum cost. The proper attitude towards 

 the contractor, also, is to give him all the information possible 

 as to the nature of the work, in order to reduce the percentage 

 added for unknown difficulties and to secure closer bids. 



The examination for rock is most easily made by driving 

 a bar or pipe, i to i^ inches in diameter, to refusal, although 

 the method is open to the objection that a large boulder may be 

 mistaken for the solid rock. Such a rod, driven by mauls and 

 twisted by a wrench as it goes down, will easily penetrate 30 

 to 40 feet of soil or clay, and by the use of an open pipe a core 

 may be brought up. A f-inch pipe will drive better (in 8-foot 

 lengths the driving protected by a cap), but a 2-inch pipe will 

 bring up the better core. If it is decided to thoroughly explore 

 the ground, it is a simple and effective plan to rig up a small 

 hand pile-driver, using a block of wood for the weight. 



