FOUNDATIONS, GASWORKS BUILDINGS, ETC. 41
the smaller holes will give ampie indication of the nature of the ground. The 4-inch boring is, however, not to be advised ; and for the maj ori ty of sites the 8-inch drill—as stipulated by the London County Council—is certainly to be recommended. Having once bored the hole, observations of the rise and fall of water must be made. These should extend over some period, and may usually be carried out merely by dipping at fairly frequent intervals, par-ticular attention being given to the effect of heavy rainstorms, or other causes likely to influence the water level. Trial boreholes must on all occasions be taken lower than the probable depth. of the foundations, so as to remove all doubt as to the character of the proposed bed ; wliilst strata of a gravelly nature should be regarded with suspicion, in that they may eventu-ally prove to be interspersed with seams of running sand.
The information obtained from borings about the nature of the ground is not always reliable. Certain classes of boring machinery, particularly in rock, yield ■cores whichgive information of great value, but often it is very diffi.cu.lt to form a just estimation of the ground whcnthe material has to be broken up by the drill before it is brouglit to the surface. All that is obtained. is, after all, a rough idea of the nature of the ground at the exact spot where the boring has been made, and often very different information is obtained if the boring is made 50 feet north, south, east, or west. Again, borings give very little information as regards the quantity of water which may be met with, and dealing with which may be a very important item both in cost and in time. It is also very difficult to determine by borings what effect the pressure of the ground may have on the works or what safe
Fro. 1.—Stbata Dkill in Operation, fitted with Boring Auger to recover Subsoil IN LEXCTHS OF 3 FEET 6 INCHES.