written by: Jayant R Row
• edited by: Lamar Stonecypher
• updated: 2/18/2011
There are a number of techniques available for estimating the bearing capacity of rock foundations. These techniques include analytical methods, traditional bearing capacity equations, and field load tests. Of the various methods, field load tests are the least commonly used.
slide 1 of 4
Conditions That Are Encountered For Rock Foundations
To determine the rock mass bearing capacity in any rock it is necessary to ensure that the rock has continuity that is at least four to five times the width of the foundation. So if a foundation has dimensions of 100 feet by 200 feet, the rock mass should have a continuity of 400 feet by 800 feet. This means that in that area of 400feet by 800 feet there should be no joints in the rock, which can cause any weakness in the capacity of the rock to bear the foundation. Normally joints in rock formations are very widely spaced and may not be a criterion. The bearing capacity of rock masses in rocks will depend on the condition of the rock, orientation and as mentioned above, the continuity of the rock. While condition of the rock can be determined by taking core samples, the orientation would depend on the dip in the rock more a subject for geologists. Some rocks are formed in layers and their bearing capacity may depend on the layer on which the foundation is laid, and the presence of soft layers further down.
While estimating the rock mass bearing capacity engineers would be interested in two aspects of the bearing capacity. The first is the ultimate bearing capacity of the rock, which if exceeded would cause failure in the rock and hence the foundation. The second is the allowable bearing capacity of the rock mass, which is the maximum pressure that any foundation would exert on the rock and would generally be much lower than the ultimate bearing capacity and ensure a factor of safety.
slide 3 of 4
Testing Of Rock Mass Bearing Capacity
Load-bearing tests are either horizontal or vertical and require a test pit to be excavated. This pit has to be made with the least bit of disturbance to the foundation rock and any blasting must be restricted. Such tests are largely replacing the seismic tests that were earlier used to get the necessary information, and are more useful when the structures are heavily loaded. Another test that is used for estimating bearing capacity is a stamp test in which the penetration depth of a tungsten carbide button is used to fracture the rock that is being tested. There are also analytical methods to estimate ultimate rock mass bearing capacity which depend on constants for unit weight, the foundation location and other bearing capacity factors for the rock that are determined from laboratory tests.
Rock mass strength increases as foundations go deeper as the rock has greater strength to resist passive forces, and weathering action is minimal. Maximum loads that can be applied on rocks are limited by reducing the ultimate bearing capacity by a factor of safety that is quite often empirical and based on experience.