The bearing capacity of rock is usually very high as you would expect compared to soil. In most cases the bearing capacity of rock is so high that smaller foundations can be used when they are bearing directly onto rock. However there are a number of special rock types such as chalk, Keuper Marl and fractured, fissured or dissolvable rocks which can have much lower bearing capacities depending on the condition of the rock strata.
This article provides typical allowable bearing capacities for different types of rocks and also outlines the detailed BS 8004 procedure for assessing the condition of the rock. There are also detailed discussions of special rock types.
The CivilWeb Soil Bearing Capacity Calculation Excel Suite cannot be used to calculate the bearing capacity of rock though it does include a 74 page Ultimate Guide to rock and soil bearing capacities which details the BS 8004 procedure which the designer can use to determine the allowable bearing capacities of rock layers.
Typical Bearing Capacity of Rock
For preliminary design it is often not possible to calculate the bearing capacity of the soils because no site investigation has taken place. Many publications have included typical bearing capacities of different soils and rocks for use in preliminary design.
There are many examples of preliminary bearing capacities in the literature. For shallow foundations the bearing capacities of unweathered rock are so large as to become effectively infinite. When shallow foundations bear directly onto the bedrock bearing capacity is no longer relevant except for extremely heavy structures.
There are many examples of presumptive bearing capacities for various rock types, some are reproduced below. Where values are specified in national or local standards these should take precedence over any other values found in the literature.
Weathered Rock
BS 8004 contains a detailed procedure for the determination of bearing capacities in weathered rocks. These bearing capacities are calculated using the rock type and weathering in accordance with BS 5930. They are calculated assuming a settlement to foundation width ratio of 0.5% with rigid square foundations. Other shapes can be accommodated using shape factors as shown below.
Bearing Capacity of Rock - Special Rock Types
The bearing capacity of certain types of rock cannot be determined using the standard procedures detailed above. These rocks require special consideration. Below we have detailed the most commonly encountered special rock types in the UK.
Chalk
Chalk is a sound, soft white limestone rock found all over the South of England. Chalk is susceptible to softening when subject to percolating water. This can even cause sinkholes in some cases.
The below table from BS 8004 can be used for foundations in chalk.
Greywacke
Greywacke is a term used for badly sorted and muddy sedimentary rocks. They often include large deposits of coarse materials in deep troughs which can be thousands of metres deep.
The presumed bearing capacity for Greywacke rocks is around 550 – 1200kN/m2.
Bunter Sandstone
Bunter Sandstone is a soft rock consisting of cemented particles of sand. The presumed bearing capacity for Bunter Sandstone is around 450 – 900kN/m2.
Keuper Marl
Keuper Marl or Mercia Mudstone is a rock formation running through the pennines into Shropshire and the west midlands in the UK. This mudstone is very fissured which allows water ingress and severe softening and weathering. Its weathered state resembles a cohesive soil whereas the unweathered state behaves as a mudstone.
For this reason BS 8004 includes guidance on allowable bearing capacities on Keuper Marl based on the weathering of the rock. These are presented below. A map showing the extent of Keuper Marl in the UK is presented on the following page. Further information can be found in Ciria Report C570 – Engineering in Mercia Mudstone.
CivilWeb Soil Bearing Capacity Calculation Excel Suite
The CivilWeb Soil Bearing Capacity Calculation Excel Suite includes a 74 page Ultimate Design Guide which contains typical values and full details of the methods required for determining the bearing capacity of rock.
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