Geotechnical Engineering

DHARM

SOIL EXPLORATION 737

Table 18.3 Requirements of Sampling Tubes (I.S: 2132-1986)

Inside diameter, mm 38 70 100 Area ratio in this case is

Outside diameter, mm 40 74 106

DD

D

ei

i

22

2

F −

HG

I

KJ

Minimum effective length 300 450 450 where

available for soil sample, mm De = External dia.

Area Ratio, Ar% 10.9 11.8 12.4 Di = Internal dia.

Note: Sampling tubes of intermediate or larger diameters may also be used.
After having extracted the sample in the same manner as in the case of split spoon type,
the tube is sealed with wax on both ends and transported to the laboratory.

18.4.6 Ground Water Level

Determination of the location of ground water is an essential part of every exploratory pro-
gramme. Ordinarily, it is measured in the exploratory borings; however, it may sometimes
become necessary to make borings purely for this purpose, when artesian or perched ground
water is expected, or the use of drilling mud obscures ground water.

A correct indication of the general ground water level is found by allowing the water in
the boring to reach an equilibrium level. In sandy soils, the level gets stabilised very quickly—
within a few hours at the most. In clayey soils it will take many days for this purpose. Hence,
standpipes or piezometers are used in clays and silt. A piezometer is an open-ended tube (may
be about 50 mm in diameter) perforated at its end. The tube is packed around with gravel and
sealed in position with puddle clay. Observations must be taken for several weeks until the
water level gets stabilised. The arrangement is shown in Fig. 18.8.

Puddle

clay seal

Gravel

Observation pipe

or piezometer

GL

Fig. 18.8 Piezometer for observation of GWL in a bore hole
In the case of impermeable clays, pressure measuring devices are used.
The elevation of ground water table affects the design of the foundation, since the bear-
ing capacity and a few other engineering properties of the soil strata depend upon it.