Influence of Rock Properties in Estimating Rock Strength for Shallow Underground Structures in Weak Rocks

Abstract

Two popular rock strength criteria, the linear Coulomb and non-linear Hoek-Brown, are widely used in underground designs. These two criteria may be applied differently depending on rock conditions. Weak rocks may have different properties compared to hard rocks. Both criteria have been applied in a current research to practically determine the applicability of the criteria in estimating the strength of weak rock masses of five shallow underground structures. Results show that both criteria are able to model the strength of the five weak rock masses, but as expected the criteria provide quite different values for each type of rocks. The strength of rock masses around underground structures depends on uniaxial compressive strength and confinement; but the linear criterion very much depends on shear characteristics of rock materials. Whereas, the non-linear criterion relies on the geological strength index (GSI). Although the GSI may have served practical descriptions for rock masses, some difficulties were found when using the GSI for very weak pyroclastic rocks. The GSI seems to provide underestimated indexes for these rock types. Estimations show that the non-linear criterion may not really exhibit curved strength envelopes rather linear in some sense, for five weak rock masses. Thus in general, when an underground structure is reasonably shallow, has a lack of confinement, and where the shear behaviour dominates rock failures, the linear criterion is more preferable than the non-linear criterion in modelling the strength of weak rock masses.

Stability of the Meninting Diversion-Spillway Tunnel Constructed into Weak Volcanic Rock Masses Influenced by the Lombok Earthquake 2018

“…One suggested stability improvement of many suggestions [16] would be the application of grouting and rock bolting [17], which could increase the shear strength, and reduce deformation of rock masses [18,19]. The Meninting tunnel construction might be sufficient to stand earthquake stresses, but since it was excavated into a hill of various volcanic rock materials, the stability of rock masses would be a problem [20,21]. On the top of the tunnel, loose bouldery agglomerates and colluviums had low strength [22], particularly with a high coefficient of permeability; they could be easily to lose their shear strength under earthquake forces.…”

Practical Applications of Strength Criteria in Civil Engineering Designs for Shallow Tunnels in Weak Rock

“…However, characterization using the GSI for volcanic rock masses encounters difficulties (Agustawijaya, 2018), similar to those for weak ophiolite rocks (Marinos et al, 2006). Volcanic rock commonly has no significant structure, apart from irregular depositional bedding; boulder fragments are bonded with very weak volcanic mud, causing low strength.…”

“…Case I is the Athens Metro tunnel in Greece. The data available from Kavvadas et al (1996) were analyzed by Hoek and Brown (1997) and Agustawijaya (2011Agustawijaya ( , 2018. The tunnel, with a length of 18 km, was constructed in poor quality decomposed schist rock masses at a depth of 20 m.…”

“…Most uncertainties are caused by the nature of the material (soil and rock), testing conditions and design application (Kulhawy et al, 2001;Prakoso & Kulhawy, 2011;Serra & Miranda, 2013). In current tunnel projects, two rock strength criteria are applied, namely those of Coulomb and Hoek-Brown (Agustawijaya, 2018).…”

“…Thus, in such a situation, confinement 3 and ci should be important in Equation 9, which could represent the case for tunnelling in weak rock. In particular, for tunnelling at shallow depths, the confinement should be very low; consequently, the tunnel would depend highly on frictional characteristics, and the confinement may be calculated as follows (Agustawijaya, 2018):…”