Murat Karakus

Effects of different tunnel face advance excavation on the settlement by FEM

The excavation process for a tunnel changes in terms of its service requirements, ground conditions and the stability of surface buildings in urban areas. When a tunnel is excavated, there will be settlement, which may cause damage to surface structures. To control and limit the settlement caused by tunnelling operations, there have been many tunnelling techniques proposed. Thus, in this paper, a number of Finite Element Method analyses were conducted to investigate the effects of different patterns for advancing the tunnel face on the settlement. The Heathrow Express Trial tunnel was constructed in accordance with the principles of the New Austrian Tunnelling Method (NATM). The settlement measurements taken during its construction were used to validate the results from the analyses undertaken. Three different face advance techniques were used during the construction of the Heathrow Express Trial tunnel viz. Twin sidewall excavation (TS1), single sidewall excavation (TS2), and Crown, Bench and Invert excavation (TS3). As the trial work proved that TS2 produced the minimum settlement above tunnel centreline, only TS2 was subjected to the FEM analysis in this research. In order to simulate TS2 correctly three types of excavation models were devised and the results compared to field measurements of TS2. For the FEM analysis the Hypothetical Modulus of Elasticity (HME) soft lining approach was used and a practical method to estimate HME is proposed for when it is used for different face advance sequences. Results proved that when the excavation pattern was changed, the HME value was also changed and settlement over the tunnel centreline changed in terms of the face advance pattern adopted.

Determination of Mohr-Coulomb shear strength parameters from generalized Hoek-Brown criterion for slope stability analysis

Rock slope stability is critical for many aspects of mining and civil engineering projects, such as open pit mining and large dam construction. One of the most popular approaches for estimating the factor of safety (FOS) of a given slope is the limit equilibrium method (LEM) where rock mass strength is usually expressed by the linear Mohr– Coulomb (MC) criterion. Currently, a widely used criterion to estimate rock mass strength is the non-linear Generalized Hoek–Brown (GHB) failure criterion since it is able to estimate the shear strength of various types of intact rock and rock masses (Priest 2005). If the GHB criterion is used in conjunction with LEM for analyzing the rock slope, methods are required to determine the equivalent MC shear strength parameters cohesion c and angle of friction / at the specified normal stress rn from the GHB criterion. The determination of reliable shear strength values is a critical step in slope design as small changes in shear strength parameters can result in significant changes in the value of the FOS (Wyllie and Mah 2004). In past decades, methods for the determination of shear strength from the Hoek– Brown criterion for slope stability analysis were proposed by Hoek (1983), Priest and Brown (1983), Londe (1988), Hoek (1990), Hoek and Brown (1997), Kumar (1998), Hoek et al. (2002), Carranza-Torres (2004), Priest (2005), Fu and Liao (2010), Yang and Yin (2010). Comprehensive review of the literature of estimating shear strength of the Hoek–Brown criterion can be found in the paper by Carranza-Torres (2004). However, as Brown (2008) has noted, deriving exact analytical solutions for estimating the shear strength of a rock mass modeled using the GHB criterion has proven to be a challenging task due to the complexities associated with mathematical derivation. In the special case of the Hoek–Brown parameter a = 0.5, an analytical solution proposed by Bray and reported by Hoek (1983) yields accurate results for intact rock with the geological strength index (GSI) equal to 100. However in the more general case of a = 0.5, no accurate analytical solution is available (Carranza-Torres 2004). In this paper, an approximate analytical solution for estimating the equivalent MC parameters for highly fractured rock masses governed by the GHB criterion is proposed. The proposed approximate analytical solution yields fairly good results when GSI\40 and provides great flexibility for the application of the GHB criterion in conjunction with LEM for highly fractured rock mass slope stability analysis.

Function identification for the intrinsic strength and elastic properties of granitic rocks via genetic programming (GP)

Abstract Symbolic Regression (SR) analysis, employing a genetic programming (GP) approach, was used to analyse laboratory strength and elasticity modulus data for some granitic rocks from selected regions in Turkey. Total porosity ( n ), sonic velocity ( vp ), point load index ( Is ) and Schmidt Hammer values ( SH ) for test specimens were used to develop relations between these index tests and uniaxial compressive strength ( σ c ), tensile strength ( σ t ) and elasticity modulus ( E ). Three GP models were developed. Each GP model was run more than 50 times to optimise the GP functions. Results from the GP functions were compared with the measured data set and it was found that simple functions may not be adequate in explaining strength relations with index properties. The results also indicated that GP is a potential tool for identifying the key and optimal variables (terminals) for building functions for predicting the elasticity modulus and the strength of granitic rocks.

A Simplified Failure Criterion for Intact Rocks Based on Rock Type and Uniaxial Compressive Strength

The uniaxial compressive strength (UCS) of intact rock, which can be estimated using relatively straightforward and cost-effective techniques, is one of the most practical rock properties used in rock engineering. Thus, constitutive laws to represent the strength and behavior of (intact) rock frequently use it, along with additional intrinsic rock properties. Although triaxial tests can be employed to obtain best-fit failure criterion parameters that provide best strength predictions, they are more expensive and require time-consuming procedures; as a consequence, they are often not readily available at early stages of a project. Based on the analysis of an extensive triaxial test database for intact rocks, we propose a simplified empirical failure criterion in which rock strength at failure is expressed in terms of confining stress and UCS, with a new parameter which can be directly estimated from the UCS for a specified rock type in the absence of triaxial test data. Performance of the proposed failure criterion is then tested for validation against experimental data for eight rock types. The results show that strengths of intact rock estimated by the proposed failure criterion are in good agreement with experimental test data, with small discrepancies between estimated and measurements strengths. Therefore, the proposed criterion can be useful for preliminary (triaxial) strength estimation of intact rocks when triaxial tests data are not available.

Simplified method for estimating the Hoek-Brown constant for intact rocks

AbstractThe constant mi is a fundamental parameter required for the Hoek-Brown (HB) criterion to estimate the strength of rock materials. To calculate mi values, triaxial tests need to be carried out; however, triaxial tests are time-consuming and expensive. In this research, a simplified method is proposed that can estimate mi values using information on rock types and the uniaxial compressive strength (UCS) of intact rocks. To evaluate the reliability of the proposed method, mi values estimated from the proposed method are used in the HB criterion to predict intact rock strength. The predicted intact rock strength is then tested against experimental intact rock strength using existing triaxial tests and tests in the authors’ laboratory. Results from the comparison show that mi values calculated from the proposed method can be used reliably in the HB criterion for estimating intact rock strength when triaxial test data are not available.

Coal burst induced by rock wedge parting slip: a case study in Zhaolou coal mine

Abstract Rock wedge parting is an important factor that can induce coal burst. In this study, a physical model for the stick-slip model was developed to investigate coal burst induced by rock wedge parting. The Maxwell model was used to explain the stick-slip motion of the rock wedge parting. The evolution of microseismic source locations in the coal face was also obtained to correlate rock wedge parting with coal burst. It was found that superposition of the high static stress concentration and the dynamic stress from the fracture and caving of key stratum induced the coal burst.

Development of a Tool Condition Monitoring System for Impregnated Diamond Bits in Rock Drilling Applications

The great success and widespread use of impregnated diamond (ID) bits are due to their self-sharpening mechanism, which consists of a constant renewal of diamonds acting at the cutting face as the bit wears out. It is therefore important to keep this mechanism acting throughout the lifespan of the bit. Nonetheless, such a mechanism can be altered by the blunting of the bit that ultimately leads to a less than optimal drilling performance. For this reason, this paper aims at investigating the applicability of artificial intelligence-based techniques in order to monitor tool condition of ID bits, i.e. sharp or blunt, under laboratory conditions. Accordingly, topologically invariant tests are carried out with sharp and blunt bits conditions while recording acoustic emissions (AE) and measuring-while-drilling variables. The combined output of acoustic emission root-mean-square value (AErms), depth of cut (d), torque (tob) and weight-on-bit (wob) is then utilized to create two approaches in order to predict the wear state condition of the bits. One approach is based on the combination of the aforementioned variables and another on the specific energy of drilling. The two different approaches are assessed for classification performance with various pattern recognition algorithms, such as simple trees, support vector machines, k-nearest neighbour, boosted trees and artificial neural networks. In general, Acceptable pattern recognition rates were obtained, although the subset composed by AErms and tob excels due to the high classification performances rates and fewer input variables.

Estimation of Joint Roughness Coefficient from Three-Dimensional Discontinuity Surface

List of symbols k Roughness index Rp Roughness profile Z2 0 The modified root mean square value Rp Roughness profile index D Fractal dimension h The inclination of the individual line segments Dr1d 9 Kv A parameter of capturing the overall roughness characteristics of natural rock joints well by fractal techniques Rs Roughness coefficients B A parameter relate to JRC H The mean height of surface asperities in the mean line L The length of the 2D-profile M The divided number of discontinuity surface T The peak shear strength of rock joints rn The effective normal stress s The peak shear strength of the joint /b The total friction angle of the flat surface Ac The potential contact area A0 The maximum possible contact area h max The maximum apparent dip angle in the shear direction C A ‘‘roughness’’ parameter rc The compressive strength of the intact material obtained from a standard uniaxial test /r The residual friction angle a The angle between the schistosity plane and the normal to the joint Z 0 The position of the horizontal plane Lx Length in the x-direction of roughness surface sample Ly Length in the y-direction of roughness surface sample

The effect of freeze–thaw process on the physical and mechanical properties of tuff

In this study, we used a decay function to predict the long-term durability of tuffs against freeze–thaw (F–T) processes that are responsible of most of damages in natural building stones. The model proposed by Mutluturk et al. (Int J Rock Mech Min 41:237–244, 2004) provides meaningful parameters (including decay constant and half-life) for assessing the integrity loss of stones against freeze–thaw action. After their mineralogical characterization, six different types of tuffs were submitted to freeze–thaw cycles. Their deterioration was investigated by determining the physical (including effective porosity and P-wave velocity) and mechanical properties (including Brazilian tensile strength and uniaxial compressive strength) 10 cycles of F–T up to 70 cycles. Slake durability test was performed after 15, 30 and 60 cycles of F–T action. Results showed that slake durability index cannot be used for assessing the effect of F–T processes on the durability of the samples being studied. The decay model applied to the data indicated that, due to difference of petrographical characteristics of studied samples, decay constant and half-life were different for all types of tuffs. As with increase in lithic fragments and grain size of samples, their decay constant will increase (half-life will decrease) and, consequently, durability will decrease. Furthermore, based on achieved results from values of decay constant and half-life of the different tuff types, the destructive effect of F–T process on tensile strength was higher than on compressive strength. Brazilian tensile strength is a more relevant measurement to assess long-term durability of rocks to F–T.