Slopes - Geotechnical Investigation and Design Tables - page 180

Table 14.19 Typical slopes under water (ICE, 1995).

Type of material Description Slopes in still water Slopes in active water

Rock Nearly vertical Nearly vertical

Clay Stiff 45 ◦ 1V: 1H 45 ◦ 1V: 1H

Firm 35 ◦ 1V: 1.4H 30 ◦ 1V: 1.7H

Sandy 25 ◦ 1V: 2.1H 15 ◦ 1V: 3.7 H

Sand Coarse 20 ◦ 1V: 2.7H 10 ◦ 1V: 5.7H

Fine 15 ◦ 1V: 3.7H 5 ◦ 1V: 11.4H

Silt Mud 10-1 ◦ 1V: 5.7H to 57H 5 ◦ 1V: 11.4 H or less <

14.20 Side slopes for canals in different materials
•

The side slopes in canals depends on the type of natural materials, and the canal
depth.

•

A canal that is 1.0m in depth may have material that can have a 1V: 1.0H slopes,
while at 2.0m depth a slope of 1V: 2.0H may be required.

•

The flow velocity in the canal may require revetment protection, and that may
govern the slope.

Table 14.20 Typical slopes for earthen canals in different soil materials.

Group symbol Material type Minimum side slope Comments

Rock 1V: 0.25 H Extent of weathering and joints

may affect slope design

Boulders, cobbles 1V:1.5H Good erosion resistance

Seepage loss

GW, GP Gravels, well or poorly graded 1V: 2.5H Good erosion resistance

SW, SP Sands, well or poorly graded Seepage loss

SC Clayey sands 1V: 2.5H Fine sands have poor erosion

SM Silty sands resistance

GM Silty gravels 1V: 1.5H Medium erosion resistance

GC Clayey gravels Medium seepage loss

ML Inorganic low plasticity silts 1V: 1.5H Poor erosion resistance for low

CL Inorganic low plasticity clays Plasticity index

OH Organic low plasticity clays Low seepage loss

MH Inorganic high plasticity silts 1V: 3.0H Low seepage loss

CH Inorganic high plasticity clays

OH Organic high plasticity clays

14.21 Seismic slope stability
•

Pseudo-static analysis is performed by applying an acceleration coefficient in the
analysis.

•

The long term parameters are considered appropriate, however both types of
analysis are presented in the table below. There seems to be a divided opinion in
the literature in using long term or short-term analysis.

•

Horizontal seismic coefficient (k h ) = a max /g.