Field sampling and testing - Geotechnical Investigation and Design Tables - page 50

Table 4.16 Surface strength from site walk over.

Pressure from Typical undrained shear strength (kPa) Factor of safety

person support (bearing)

Light Medium Heavy

Typical pressure 20 kPa 30-40 kPa 50 kPa

No visible depressions 15 kPa 20-25 kPa 30 kPa 2.0

Some and visible depressions 10 kPa 15-20 kPa 25 kPa 1.5

Large depressions 5 kPa 10-15 kPa 15 kPa 1.0

•

Very Soft Clays ( < 12 kPa) will have some to large depressions even with a light

person pressure.

•

Soft Clays will have visible depressions except for a light person. Depressions for
all other persons.

•

Firm to stiff clay typically required for most (medium) pressure persons so as not
to leave visible depressions.

•

A heavy person pressure requires a stiff clay, so as not to leave visible depressions.

4.17 Surface strength from vehicle drive over
The likely minimum strength of the ground may also be assessed from the type of
vehicle used.

Table 4.17 Trafficability of common vehicles.

Vehicle type Minimum strength for

vehicle to operate

Passenger car 40 kPa

10 tonne (6

∗

4) truck 30 kPa

3 tonne (4 4) truck 25 kPa

∗

1 tonne 4 wheel drive vehicle 20 kPa

4.18 Operation of earth moving plant
•

Many earth moving equipment use large tyres or tracks to reduce the ground
pressure. The table provides the shear strength requirement for such equipment
to operate:
- Feasible - Deepest rut of 200mm after a single pass of machine.

- Efficient - Rut ≤ 50mm after a single pass.