Page 148 - Civil Engineering Project Management, Fourth Edition

Site surveys, investigations and layout
instance, it will have to be started between 3 and 3.5m square, because the
supports to it will have to be ‘brought in’ twice, and the reduced area at the
bottom of the pit must be large enough for the men to work in, with a crane
skip present and also possibly a pump. 133
Before starting a trial pit it is necessary to decide the depth and information
to be sought and whether other means, such as augering or a borehole, would
produce the information quicker and at less cost. If the requirement is simply
to find rock level, or to ascertain whether soft material lies below hard (such
as a boulder), a boring may be a better option. If one is looking for clay, silt or
soft material, a most important matter is whether the pit is to find the full
depth of such material or just penetrate into it. The former can be much more
difficult and expensive than the latter and may prove impossible without
groundwater lowering.
If undisturbed samples are to be taken it is necessary to know whether they
are to be taken horizontally into the sides of the pit, or vertically from the
bottom. Pushing a 100mm diameter sampling tube horizontally into the side
wall of a trial pit often involves the use of jacks, and digging the tube out is no
easy matter.

12.8 Exploratory holes

Exploratory holes can generally be classified into three kinds:
• rotary core drilling by diamond drill to obtain samples of soil and rock;
• cable percussion driven lined holes in soft ground, sunk by clay cutter or
shell;
• uncored holes drilled by rotary percussion drill in hard ground.
It is, of course, necessary to have an idea what sort of ground must be pene-
trated before the right type of investigation can be chosen; also it is necessary
to know the kind of information required. It is not always possible to know
the nature of the ground beforehand; soft ground can contain large boulders,
and hard ground bands of soft or loose material. Mixtures of this type will
cause delay.

Rotary core drilling

A rotary core drill uses a circular, diamond-embedded drill bit which cuts out
a core of rock. The standard sizes in use are given in BS 5930. The most usual
starting size adopted is ‘H’ (nominal hole diameter 99mm) to give good sized
cores of 76mm diameter which are less liable to fracture during the cutting
process and which permit size reduction to deepen a borehole. It is import-
ant that cores are inspected immediately upon withdrawal in order to note
whether fractures are fresh and caused by drilling, or whether they are natural