Geotechnical Engineering

DHARM

752 GEOTECHNICAL ENGINEERING

information about index properties, compressibility, shear strength, permeability, etc., should
also be provided.
The data obtained from a series of borings is presented in the form of a subsurface
profile. A sub-surface profile is a vertical section through the ground along the line of explora-
tion. It indicates the boundaries of different strata, along with their classification. The condi-
tions between bore holes are estimated by interpolation which may not be correct/true. Obvi-
ously, the larger the number of bore holes, the more accurate the sub-surface profile is.
The report should contain the discussion of the results which forms the heart of the
investigation report. The reporter should try to discuss the problem clearly and concisely with-
out ‘if ’s’ and ‘but’s’. The reporter should come straight to the point. For readability, this sec-
tion of the report should be divided into a number of sub-headings. In writing this section of
the report, care should be taken to avoid wishful thinking based on preconceived ideas on the
foundation design. The problem should be studied without prejudice. The recommendations
for foundation design must be based on the facts stated in the report, i.e., on the bore hold
records and test data. They must not be based on conjecture.

If the report is lengthy, it may be convenient to summarise the main findings in item-
ized form. This would be of help to the busy field engineer who may not have time to read
through pages of discussion. Alternatively, the report may commence with a brief summary of
the investigation procedure and the main conclusions which have been drawn from it.

The last stages are the final typing and checking of the report, printing the drawings,
and assembling and binding the whole. A neatly printed/type-written and bound report with
good, clear drawings, free of typing and draughtsman’s errors reflect the care with which the
whole preparation of the report has been made. Slipshod writing and careless typing and
drawing may lead the client to think that the whole investigation perhaps has been carried out
in a similar manner.

18.8 ILLUSTRATIVE EXAMPLES

Example 18.1: One sampler has an area ratio of 8% while another has 16%; which of these
samplers do you prefer and why? (S.V.U.—B.Tech., (Part-time)—Sept., 1983)

The sampler with area ratio of 8% is to be preferred since the sample disturbance is
inversely proportional to it. It is considered desirable that the area ratio be less than 10% for
undisturbed sampling.

Example 18.2: Compute the area ratio of a thin walled tube samples having an external
diameter of 6 cm and a wall thickness of 2.25 mm. Do you recommend the sampler for obtain-
ing undisturbed soil samples? Why? (S.V.U.—Four-year B.Tech.—Dec., 1984)

External diameter, De = 6 cm = 60 mm

Wall thickness = 2.25 mm

∴ Internal diameter, Di = (60 – 2 × 2.25) mm = 55.5 mm

Area ratio, Ar =

[60 (. ) ]

(.)

22

2

55 5

55 5

− = 16.88%

Since the area ratio is more than 10%, the sampler is not recommended for obtaining
undisturbed samples. The sample disturbance will not be insignificant.