The ratio of the sum of the horizontal forces to the sum of the vertical forces is referred to as the sliding factor (Fss). This is usually about 0.75 but must not exceed 0.90 under extreme loading. These figures represent the range of the coefficient of static friction normally encountered at the site of a gravity dam.
At or in the foundations, the horizontal loading will be resisted by cohesion and friction. The ratio of the total resistance by cohesion and friction to the horizontal load is termed the shear friction factor (Fsf). Most countries accept 4 as a minimum value. In practice the foundation is usually prepared in steps or is sloped upward in a downstream direction to provide resistance to failure far in excess of the above figure.
| Location of plane of shearing/sliding | Cohesion (c) | Friction tan ø |
|---|---|---|
| mass concrete intact | 1.5-3.5 | 1.0-1.5 |
| mass concrete horizontal construction joint | 0.8-2.5 | 1.0-1.5 |
| conrete/rock interface | 1.0-3.0 | 0.8-1.8 |
| rock mass sound | 1.0-3.0 | 1.0-1.8 |
| rock mass inferior | <1.0 | <1.0 |
Recommended shear friction factors, Fsf (USBR 1987)
| Load Combination | |||
|---|---|---|---|
| Location of sliding plane | Normal | Unusual | Extreme |
| dam concrete, base interface | 3.0 | 2.0 | <1.0 |
| foundation rock | 4.0 | 2.7 | 1.3 |