Fig.1: Rock Mass for Foundation Construction
Investigation of Rock Mass Properties for Foundation Construction
1. Rock Mass Weathering
Weathering is the degradation and disintegration of the intact rocks and changing to soft rock and consequently reduces the ultimate strength of the intact rock. There are several factors that cause weathering of rock mass for instance environmental influences, leaching of ground water, and chemical attacks. The material that degraded and disintegrated due to weathering is commonly removed from its original place due to erosion and consequently cavities and fissures will be formed in the rock in addition to the formation of joints in rocks. As a result, the compressibility and permeability of the rock mass would increase. This is not desirable and could lead to different problems in the future when such rocks are used in the construction of the foundation structure. The intensity of factors that cause weathering varies from one location to another and hence the extent of the weathering would change as well. It is claimed that the influence of weathering can reach up to 100m. There are cases in which weathered mass rock shows false indication regarding the bearing strength of soil for example when small cores are drilled in partially weathered sandstone and generates loose debris which gives wrong indication about bearing capacity of the soil. It should be known that the effect of weathering on mass rock is influenced by the type of the rock and the mineral present in the composition of the rock. For example, limestone except chalk has a greater resistance against weathering compare with other types of rocks such as mudstone, siltstone, and shale. Another factor that leads to the degradation of mass rocks is the fluctuation of ground water. When ground water declines, chemical and bacterial reaction will occur in mass rocks that contain pyrite such as shale and mudstone. Nonetheless, it should be bore in mind that certain chemical solutions would create sizable fissures and caverns in limestone. Such phenomenon sometimes manifests itself by the formation of depressions on the ground surface unless younger soil layer does not lay over it. When limestone covered with younger rock layer, it would be considerably difficult to investigate the severity of the caverns and fissures. Nonetheless, geophysics method can be used to explore such features and could obtain satisfactory result to a certain extent. It is possible that cavities in limestone rock mass are commonly filled with loose material which makes this type of rock mass to have low bearing capacity and hence unsuitable for both spread and pile foundation. If the placement of such foundation cannot be prevented, then it should be designed to bridge across cavities.2. Faulting of Rock Mass
Faulting is another feature of rock mass that need to be investigated specifically in areas prone to earthquakes. One might argue that, movements along fault planes may have been finished with time and hence it should be considered during the design of foundation structure. However, it is necessary to conduct investigation in seismic regions and areas prone to mining subsidence.
Fig.2: Faulting of Rock Mass
3. Jointing in Rock Mass
Joints are fractures in rock mass that may or may not lead to small relative movement of rocks on both sides of the joint plane. By and large, the compressibility of rock mass which is loaded under the foundation is influence by not only joint spacing but also the width of the joint. Moreover, if the joints are inclined toward an excavation and filled with loose material such as weak clay, then the stability of the excavation would be in jeopardy and it probably leads to failure of foundation bearing on an irregular rock surface. Finally, it is required to conduct examination and exploration for the rock core so as to determine joint spacing, width, and inclination.4. Strength of Intact Rock
The strength of intact rock may be related to the bearing strength and compressibility of the jointed rocks mass. Added to that, values obtained from splitting test and uniaxial compression test can be used to specify whether the foundation can be excavated using explosive or not, or whether the excavated material can be used as a construction material or not.References
- MJ Tomlinson , 1999. Foundation Design and Construction. 6th ed. Edinburgh : Pearson Education Limited.
- Qiang Xu , Jianyun Chen, Jing Li, Chunfeng Zhao, Chenyang Yuan, 2015. Study on the Constitutive Model for Jointed Rock Mass. PLoS ONE, 10(4).