Visualizing the effect of excavation rate on rock deformation and fracturing of tunnels using a transparent soft rock surrogate

One of the most important problems during tunneling in soft rock is deformation and fracturing of the rock during tunneling. The problem was successfully explored by using a transparent rock surrogate to simulate the behavior of soft rock, which permitted visualizing conditions within the rock. Synthetic soft rock was made using consolidated fused quartz saturated with a blend of two mineral oils that have the same refractive index as the quartz. The tunnel was simulated using a smooth aluminum tube and two tunneling methods representing machining and blasting were considered. Two observation planes made of seeding particles were pre-placed within the model and used to track soil movements and crack propagation. Images of both planes were captured simultaneously using two orthogonal cameras. Rock deformations were concentrated in the vicinity of the tunnel face, and deformation rates behind the tunnel face were significantly greater than those ahead of the face. However, deformation rates and patterns varied considerably depending on the excavation method/rate. Fracturing mechanisms exhibited similar differences, for machining deformations occurred higher above the crown and propagated toward the tunnel face. Conversely for blasting deformations sprang from the crown upwards. These observations can assist with numerical simulations and in planning tunnel support systems.

     

A transparent aqueous-saturated sand surrogate for use in physical modeling

This paper presents the geotechnical properties of a new family of synthetic transparent soils made of fused quartz, saturated with a matched refractive index water-based sucrose solution, suitable for modeling the behavior of sand in small-scale model tests. The dry density ranged between 1,134 and 1,358 kg/m³. The peak angle of friction was found to range from 46° to 57°. The average hydraulic conductivity was 1.7 × 10^?5 cm/s. The compressibility index (C _c) ranged from 0.34 to 0.57. The main advantage of fused quartz over available sand surrogates made of silica gel is that its solid structure better models the behavior of natural sand. The matching pore fluids are inert and non-toxic, which facilitates their use in educational settings. The availability of a safe and easy-to-use transparent sand permits measurement of three-dimensional deformation patterns and flow characteristics in controlled research experiments. The introduction of an aqueous solution permits the use of two immiscible pore fluids, one made of mineral oil and the other made of a sucrose solution, for modeling multiphase flow problems, as well as coupled flow-deformation problems.     

Simplified analytical solution for point load acting behind a cantilever wall

The analysis of earth pressure resulting from an applied point load behind a retaining wall is to be carried out in three dimensions as the earth pressure distribution varies both in vertical and horizontal directions. In this research, the semi‐empirical expressions developed by Terzaghi [Trans. ASCE 1954; 119 ] for earth pressure due to point load are integrated and a correction factor is introduced to develop equivalent equations for two‐dimensional analyses. A new parameter referred to as the design width, which is the horizontal distance retained by the individual vertical retaining element is introduced. The used procedure and the resulted equations are tested and verified by adopting different design width values covering the practical range. The resulted equations together with the semi‐empirical expressions of earth pressure due to line load developed by Terzaghi [Trans. ASCE 1954; 119 ] can be easily used for two‐dimensional analysis of the effect of point load. Copyright © 2011 John Wiley & Sons, Ltd.     

Use of machine learning for classification of sand particles

Acta Geotechnica - Particle classification is essential for geotechnical engineering practice since particle shapes correlate with the mechanical and hydraulic properties of sand layers....     

Effect of Face Losses and Cover-to-Diameter Ratio on Tunneling Induced Settlements in Soft Clay,Using Transparent Soil Models

Geotechnical and Geological Engineering - Reliable prediction of surface and subsurface settlements induced by shallow tunnels is important to minimize the adverse effects which may take...     

Relationship Between Temperature and Earth Pressure for a Rigidly Framed Earth Retaining Structure

The relationship between temperature and earth pressure acting on a rigidly framed earth-retaining structure (RFERS) subject to wide temperature variation was explored. A distressed RFERS open concrete garage that retains 11 m (36 ft) of soil was instrumented. After some repairs, movement of the building was monitored and recorded hourly for a period of four and a half years. The monitoring revealed complex temperature-dependent soil–structure interactions. The measured displacements were used to calculate the earth pressure coefficient using closed form equations that were developed by treating the structure as an equivalent cantilever beam, and calibrating the expression using a total of 42,000 FEM models. The data indicated that the coefficient of earth pressure behind the monitored RFERS had a strong linear correlation with temperature. During the cold season the building contracted, and the retained soil followed. During the hot season, the building was unable to overcome the earth pressure, thus it expanded away from the soil, resulting in a cumulative annual displacement. The coefficient of lateral earth pressure changed by approximately 0.005/°C, varying in the range of 1.25–1.5, depending on the season. The study also reveals that thermal cycles, rather than lateral earth pressure, caused some of the structural elements to fail.     

Simulated envelopes of non-isotropically scattered body waves as compared to observed ones: another manifestation of fractal heterogeneity

Envelopes of scalar waves are simulated at various distances from an instant point source embedded in a random uniformly scattering medium by means of direct Monte-Carlo modelling of wave-energy transport. Three types of scattering radiation pattern ('indicatrix') are studied, for media specified by (1) a Gaussian autocorrelation function of inhomogeneities, (2) a power-law ('fractal', k -^α) inhomogeneity spectrum and (3) the mix of case (1) and the isotropic indicatrix (very small + large inhomogeneities). We look for a model that can qualitatively reproduce the two most characteristic features of real S-wave envelopes of near earthquakes, namely (1) the broadening of the 'direct' wave group with distance and (2) the monotonously decaying shape of the coda envelope that does not deviate strongly from that expected in the isotropic scattering case. Both properties are observed for any band over a wide frequency range (1-40 Hz). The well-studied isotropic scattering model realistically predicts the appearance of codas but fails to predict pulse broadening. The model of large-scale inhomogeneity realistically predicts the mode of pulse broadening but fails to predict codas. We have found that, for a particular frequency band, within each class of inhomogeneity studied, both requirements can be qualitatively satisfied by a certain choice of parameters. In the Gaussian-ACF case, however, this match can be obtained only for a narrow frequency range. In contrast, the fractal case (with a value of exponent a of about 3.5-4) reproduces qualitatively the observed wide-band behaviour, and we consider it a reasonable representation of the gross properties of the earth medium.     

Soil–projectile interaction during penetration of a transparent clay simulant

Visualization of soil structure interaction during projectile penetration of clay is made possible by use of a surrogate composed of magnesium lithium phyllosilicate combined with high-speed photography and digital image correlation. A free-falling penetrator striking at 5.5 m/s simulated a projectile. Penetration resistance was constant within the resolution of the experiment; it was mainly due to the bearing resistance of the soil in contact with the nose, rather than skin friction. Bearing resistance in dynamic penetration for a hemispherical-nose rod was about 20% higher than quasi-static tests using a sphere. Bearing resistance was also about 20% higher for a hemispherical nose compared to a conical nose. Cavitation behind the nose is dependent on its shape with soils rebounding toward the projectile for conical noses but not hemispherical ones.

     

Approximate deflection of rigidly framed earth retaining structures due to an unknown earth pressure distribution

A simple and reliable method for predicting the relationship between lateral displacement and earth pressure for rigidly framed earth retaining structures (RFERS) was developed. Closed‐form equations were derived such that if one value of displacement or pressure is known (or assumed) the other can be computed for hydrostatic, seismic, uniform, and semi‐elliptical earth pressure distributions. Additionally, the general form of the equations can be used to predict the magnitude of the lateral force even if the shape of the earth pressure is unknown, with a reasonable degree of accuracy. The expressions for deflection were derived by treating the structure as an equivalent cantilever beam and calibrating the resulting expression using the finite element method (FEM). A parametric FEM analysis, of 42 000 different RFERS configurations, was performed to calibrate the expressions, using multivariate non‐linear regression between the derived expressions and FEM. A Weibull statistical analysis was performed for each equation and determined that the equations had better than 80% probability to yield deflections that are within 25% of the value computed using FEM. Furthermore, there is a 98% certainty that each equation will yield a deflection that is within 50% of that computed using FEM. Copyright © 2011 John Wiley & Sons, Ltd.     

Effects of shoreline and bedrock irregularities on the morphodynamics of the Alexandria coast littoral cell,Egypt

Beach-nearshore profiles combined with beach and surficial sediment samples were analyzed in conjunction with wave, current, littoral drift and sea-level data to determine the effect of bedrock on morphodynamic processes within the littoral zone of Alexandria on the Mediterranean coast of Egypt. This 14.5-km-long littoral cell is bounded by pronounced embayments and pocket beaches separated by headlands which prevent bypassing of beach sands, in effect making this cell a large, semi-closed basin. The compartmented nature of this cell acts together with the rough irregularity of the rocky seafloor to trap a thin veneer of sediment (<3 m thick), showing proportional mixing between two sedimentary provinces. A modern fine-grained sediment facies consisting of mixed carbonate/siliciclastic sand flanks most of the nearshore zone down to a depth of 8–10 m. Beyond this depth, considered to be the depth of closure, a relict late Pleistocene to mid-Holocene coarse-grained facies composed of biogenic carbonate sand is found. Along a short section of the coastline (km 3–6), the coarser sediment also occupies the nearshore zone. Over most of the study area the two sediment types are mixed in various proportions, largest mixing coinciding with poorest sorting. Profile analyses revealed seasonal changes in sediment volume along the coast which closely follow the cyclicity of seasonal changes in wave climate. The present shoreline orientation, headlands and rough, irregular rocky seabed are reflected in the erosion/accretion pattern, sediment characteristics, and the reversibility of longshore currents and littoral drift. Although there is a marked deficiency in the sediment balance, the sand budget for this cell, including artificial material (2.339*10⁶ m³) has increased slightly by 0.041*10⁶ m³ year^–1 as a result of engineering works carried out to widen the coastal road (Corniche). In addition to the physical properties of the bedrock (degree of induration), the accelerating sea-level rise during the Holocene and human influences, the modern morphology of the coast, the erosional seabed features in the nearshore zone, and the texture of seabed sediments are all controlled by the original geometry of the coast which consisted of an elevated subaerial ridge.