In general, the discontinuities are filled with the poor quality materials, cohesion less and coarse soils (sands, gravels etc.) or cohesive soils (silts and clays), which were carried by water flows, gravity or by both or result of the fracturing or weathering of the rocks material blocks. The presence of these infill materials produces the weakest planes in a rock mass mainly owing to its low frictional properties. (Brown, Oliveira, & Indraratna, 2010). However the mechanical characteristics of the filled material are expected to control the shear strength of the discontinuity in study of the infilled material. Factors which controlled the infilled behavior are:
1. Grain size of the infilled particle.
2. Atterberg limits
3. Overconsolidation ratio (for clay infill)
4. Water content and permeability
5. Wall roughness
6. Thickness of infill
7. Fracturing and crushing of wall rock.
There are three ways in which filling material may reduce the shear resistance of the discontinuity namely: Reduction of the micro-roughness due to space between the joint wall and coarse grains of the infill, change in frictional properties of the shear surface due to relative size of particles in infill and joint wall, lastly reduction of the “effective roughness” due to presence of the infill, the morphology of the shear surface will be changed.(Papaliangas, Hencher, Lumsden, & Manolopoulou, 1993)
Barton, (2013) suggests that shear strength of infilled rock joints is governed by ‘points in contacts’ with joint infill and rock joint. These contact points are of highly stressed asperities or opposing infill particles, these contact points may be close to their crushing strength. The shear strength of the infilled rock joints follow the below equations.
1. \(\frac{\tau }{{\sigma }_{n}}=\text{tan}[JRC.{\text{log}}_{10}\left(JCS/{\sigma }_{n}\right)+{\varnothing }_{r}]\) applies to rock joints.
2. \(\frac{\tau }{{\sigma }_{n}}=\text{tan}[R.{\text{log}}_{10}\left(S/{\sigma }_{n}\right)+{\varnothing }_{b}]\)applies to rock fill.
3. \(\frac{\tau }{{\sigma }_{n}}=\text{tan}[JRC.{\text{log}}_{10}\left(S/{\sigma }_{n}\right)+{\varnothing }_{r}]\) might apply to rock fill interface.
Whereas JRC is joint roughness coefficient.
R is infill surface roughness coefficient.
JCS is joint compressive strength
S is infill shear strength.
∅b is the internal friction angle of joint wall
∅b is the residual frictional angle
If the roughness amplitude and particle size exceeds about 7, then experimental results suggest that behavior will be infilled controlled (R-controlled) as shown in Fig. 1.