Question

For an opened joint with infill materials (i.e. filled joints), its shear strength is affected by the type and thickness of the infill material. State two (2) types of infill material and explain how each type affects the strength of the filled joint.

Answer 1

Ans : These infill material would reduce the shear strength of an open joint as compared to an open or a cleaned joint which is due to the fact that they won`t allow the walls of the rock joint from coming into contact during when shear is acting .

The two types of infill material are :

• Gypsum Cement ( CaSO₄.H₂O )
• Silty Clay ( 25% Fine Sand and 75% kaolinite )

These are the infill materials that are used to fill up the open spaces . These material can be easily available and are very inexpensive as compared other injection that are used to fill up empty space . They can be moulded into any sort of shape when mixed with water . The strength gained by these infill material is not dependent upon the time its dependent on the completion of chemical hydration . In case of gypsum its properties depends upon the water cement ratio(w/c ratio) . It gains compressive strength which is nearly about 60-70MPa after 1-2 weeks of curing . Natural joints where formed in this case with Gypsum cement .These silty clay have varying infill thickness to asperity height ratio according to which the results might vary . This type of infill material becomes slurry like when it approached saturation. After this they were wrapped up into a thin imprevious membrane . This is done to maintain the constant water content condition inside the joint . So that this infill material could provide strength and forms bond with the neighbouring particles . But the strength and bond formed is much better in case of gpysum cement as compared to silty clay .

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