Rock Mechanical Effects of Cement Grouting in Hard Rock

Doctoral thesis, 2001

A pre-grouted rock mass is subjected to stress re-distribution as the tunnel excavation progresses. The strength and deformation characteristics of grouted rock joints are therefore of interest in any effort to predict the rock mechanical behavior as well as the failure of sealing in a grouted rock mass. The objective of this thesis was to investigate the basic behavior of grouted rock joints and to study its rock mechanical interaction. Direct shear tests were performed on both grouted and ungrouted, identical high-strength rock joint replicas while injecting water in the joint plane. The tests were done under normal stresses between 2 and 4 MPa with cement grouts having water-cement ratios between 0.7 and 0.9. The obtained results were then fitted and used for numerical modeling of representative tunneling situations. The grouted cement affects the joint shear strength in two ways; a cohesive part is added and the friction is reduced. Grout sealing failure of a joint subjected to shear displacement occurs as a consequence of dilation. Test results indicated that the shear strength of grouted joints were reduced up to 25 % and seepage commenced when the range of elastic, relative shear displacement was exceeded. The results implied that for most applications, that the peak shear strength of grouted joints can be regarded as a function of the rock contact friction and grout bond strength, using a Mohr-Coulomb failure criterion. The grout sealing failure occurs concurrently with the shear failure. It was concluded from the numerical modeling that, shear displacement in a single, adversely located joint represents a possible seepage path. The most common adverse joint location was found to be parallel with the principal shear stress. It was also concluded that the secondary compressive stress field after tunnel excavation increased the joint shear strength and suppresses the dilation that causes grout sealing failure.