Page 151 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
136
As an alternative to rotary core drilling the same rig can be used for open
hole drilling where the drill bit cuts all material within the hole. Casing may
be needed in unstable ground. This can be a rapid means of reaching a
required depth to carry out a test or install instruments.
12.10 Judging the safe bearing value of a foundation

The safe bearing value for a given foundation material ought not to be
decided by the resident engineer but by the engineer or his specialist advisers.
However, the engineer will not thank the resident engineer for referring to
him questions about foundation materials which are obviously satisfactory,
such as gravel and rock, where the load thereon is well within the tradition-
ally accepted bearing strength. Standard field descriptions are given in BS
5930 for various materials and BS 8004, Table 1 shows allowable bearing
values for such materials for preliminary design purposes.
In clays or silts, or materials having clay bands or organic layers, and other
mixtures containing weak layers, special investigations, sampling techniques
and sophisticated analyses may be necessary before a safe bearing value can
be advised – dependent upon the type of structure the formation is to support.
These matters need to be considered by an experienced geotechnical engineer.
Site tests, such as the ‘standard penetration test’, vane shear tests and permea-
bility tests, may be used but these must be regarded as an adjunct only to
more sophisticated investigation techniques. Details of the standard penetra-
tion test are given in BS 1377 Part 9: 1990, para 3.3. Its widest use on site is to
reveal any weak spots in an otherwise consistent foundation material.

12.11 Testing apparatus for a site soils laboratory

The usual apparatus suitable for a small soils laboratory on site, to be run by
the resident engineer’s staff after proper instruction from a geotechnical engi-
neer, is set out below.

For moisture content determinations

1. Beam balance weighing by 0.01g divisions.
2. Drying oven, thermostatically controlled. (Not absolutely essential. For
rough measurement of moisture content the sample can be dried on a flat
tray over a stove.)
3. Six drying trays.