Permeability and its influence - Geotechnical Investigation and Design Tables - page 95

8.9 Dewatering methods applicable to various soils
•

The dewatering techniques applicable to various soils depend on its predominant
soil type.

•

Refer to Figure 8.1 for the drainage capabilities of soils.

Table 8.9 Dewatering techniques (here from Hausmann, 1990; Somerville, 1986).

Predominant
soil type

Clay Silt Sand Gravel Cobbles

Grain size
(mm)

0.002

0.06 2 60 >60

Subaqueous excavation or
grout curtain may be required.
Heavy yield. Sheet piling or
other cut off and pumping

Wells and/or
well points
with vacuum

Dewatering
method

Electro-
osmosis

Gravity
drainage

Gravity
drainage too
slow

Drainage
impractical ⇐

Sump
pumping

Range may be extended by using
large sumps with gravel filters

•

Well points in fine sands require good vacuum. Typical 150mm pump capacity:
60 L/s at 10m head.

8.10 Radius of influence for drawdown
•

The Drawdown at a point produces a cone of depression. This radius of influence
is calculated in the table.

•

There is an increase in effective pressure of ground within cone of depression.

•

Consolidation of clays if depression is for a long period.

•

In granular soils, settlement takes place almost immediately with drawdown.

Table 8.10 Radius of drawdown (Somerville, 1986).

Drawdown (m) Radius of influence (metres) for various soil types and permeability (m/s)

Very fine sands Clean sand and gravel mixtures Clean gravels

10 − 5 m/s 10 − 4 m/s 10 − 3 m/s

1 9 30 95

2 19 60 190

3 28 90 285

4 38 120 379

5 47 150 474

7 66 210 664

10 95 300 949

12 114 360 1138

15 142 450 1423