Slopes - Geotechnical Investigation and Design Tables - page 181

Table 14.21 Seismic slope stability.

Consideration Long term seismic Short term seismic

Reasons for The soil has reached its long-term Seismic load, therefore soils (except

strength parameters, when the for some coarse gravels and cobbles)

seismic event is likely to occur. will not drain properly during seismic

Short-term (undrained) parameters shaking. The event is short term

are appropriate only during
construction

Method

•

Use effective stress parameters.

•

Use undrained shear strength, that has

Softened (Constant volume) reached its equilibrium, i.e. due to

values swelling/consolidation

Apply a horizontal seismic Apply a shear strength reduction • •

coefficient factor of 0.8

•

Apply a horizontal seismic coefficient

Factor of 1.15 (OBE) 1.0 (OBE)

> >

safety

>

1.0 (MCE)

Liquefiable Use c = 0, φ = 0 for a layer that is

zone liquefiable, i.e. no strength

Comments Due to the rapid rate of loading (period of

1 sec), conventional strength tests (with time
to failure of 10 minutes) may not be
appropriate. Typically this rate of loading
effect can increase the soil strength by
15% to 20% (Duncan and Wright, 2005). This
offsets the above strength reduction factor

•

Peak Ground acceleration (a max ) is derived from the Operational Basis Earthquake
(OBE) or Maximum Credible Event (MCE).

•

OBE derived from probability of occurrence, and usually provided in local codes.
However those codes may be 1 in 50 year occurrence and for buildings, which
may not be appropriate for some structures e.g. dams.

•

MCE derived from consideration of all available fault lengths, near sites, and
attenuated acceleration to the site.

14.22 Stable topsoil slopes
•

This is a surficial failure common during construction and following rainfall
events, when the vegetation has not been established to stabilise the slopes.

Table 14.22 Topsoil placement considerations.

Consideration Slope requirements Comments

Placing by machine Slopes 1 in 5 (19 degrees) required

>

Adhering to slope Slopes

>

1 in 3.5 (27 degrees) required

Grassing and Slopes 1 V in 2H Lesser slopes has increasing >

Planting difficulty to plant and

adherence of topsoil

Thickness Slopes 1V in 2H: Use 200 mm maximum Greater thickness may be used <

Slopes 1V in 2H to 1V in 3H: Use 300 mm with geocell or geo mats.

maximum
Slopes

>

1V in 3H: Use 400 mm maximum