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

A number of rock strength criteria are available for use in civil engineering designs. Although considerable uncertainty in geological conditions and variability in rock properties is involved in estimating rock mass strength, it may be necessary to divide the criteria into two categories based on rock failure behavior: linear and non-linear. In this research, both types of criteria are applied to estimate the strength of weak rock masses at five different shallow tunnel sites. The results show varying strength values. The variation in weak rock properties affects the variability of rock strength, depending on the frictional properties for the linear criterion, and on the geological strength index (GSI) for the non-linear criterion. Confinement may also influence both criteria, but the estimated strength of the non-linear criterion is still low for weak rock when the GSI is low. Accordingly, the implication of these variations and uncertainties in rock properties is that the linear criterion may be practically suitable for tunnelling at shallow depths where instability is mostly due to gravity loads. The criterion tends to provide moderate, conservative rock mass strength estimations for this type of tunnel, since shear mechanisms may dominate rock mass failures around it.

Shallow tunnel; Shear failure; Strength criterion; Uncertainty; Weak rock

Five tunnel cases were investigated: one case of the Athens Metro tunnel in Greece adopted from Kavvadas et al. (1996), and four cases in Indonesia. The required geological surveys and drilling were conducted at one site on Lombok Island, two sites on Sumbawa Island and one site in Sumatra, followed by laboratory tests to obtain rock material properties. The methods suggested by the International Society for Rock Mechanics (1981) were adopted in the laboratory tests.

The ISRM (1981) provides a definition of weak rock as rock material that has a uniaxial compressive strength (s_ci) of less than 20 MPa. The s_ci of weak rock could fall far below this value, and could be as low as 1.64 MPa (Agustawijaya, 2007). Difficulties may arise in the laboratory testing of such rock, mostly due to laboratory treatment and the nature of the rock (Agustawijaya, 2007; Prakoso & Kulhawy, 2011). Bieniawski (1989) suggests that rock materials that have a s_ci value of below 1 MPa should be treated as soil.

However, since in situ testing of the uniaxial compressive strength of rock masses is difficult to conduct in practice (Hoek & Brown, 1994), and many uncertainties arise in obtaining real values (Prakoso & Kulhawy, 2011), the strength of rock masses is ascertained from modelling based on their s_ci and geological properties (Equations 9 and 10). The GSI, scaled in 10s up to 100, as suggested by Marinos et al. (2005), has considerable potential for use in rock engineering because it permits the manifold aspects of rock to be quantified, enhancing geological logic and reducing engineering uncertainty, particularly for tunnelling in weak rock (Marinos et al., 2006). 

Many uncertainties and a high level of variability in rock properties are involved in estimating rock mass strength for underground design. Since in situ estimation of rock mass strength is very difficult to make, modelling consequently relies on intact rock material properties and geological rock mass structures. The estimation of rock mass strength for weak rock using linear and non-linear equations results a wide range of values. The non-linear model is particularly influenced by the GSI, and application in the field requires certain engineering judgment to describe the competency of weak rock masses; otherwise, it may affect the design. For a shallow tunnel design in weak rock, however, the stability of the tunnel may be greatly dependent on the frictional characteristics of the rock. The linear criterion seems to be more suitable from a practical point of view, as it could provide moderate, conservative rock mass strength estimations, and may be less sensitive to subjective indexes.

The author acknowledges the support from and access provided by the tunnel project contractors at Pandan Duri (PT. Waskita Karya) and Rababaka Komplek (PT. Nindya Karya). The author also acknowledges the data provided by Dr. Joko Susanto on the Ketaun tunnel.

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