Tunnelling in squeezing rocks: Case histories

Summary Squeezing rock conditions have posed and continue to pose a major obstacle to the construction of tunnels through mountains, as experience dating back more than a century shows. The paper deals with the study of past experiences in the light of present geotechnical engineering knowledge. Many of the transalpine tunnels were constructed before geotechnical engineering had been developed, and the principles underlying squeezing were not yet understood. Also construction techniques have changed with time. By studying past experience in the light of our present knowledge in geotechnical engineering (rock and soil mechanics), one may gain more insight into the nature and causes of squeezing ground behaviour. Here, a number of older and newer case histories are summarised, providing substantial insight into the phenomenon of squeezing rock. Squeezing rock behaviour is influenced by rock type and structure. Usually, in squeezing zones the rock is strongly jointed and fractured and has low strength. Overburden has also a significant effect and squeezing behaviour may occur abruptly in a tunnel once a limiting overburden has been exceeded. Water pressures in strongly jointed and often crushed rock are important and so are the adopted construction procedures and sequences. A support of substantial structural strength may be necessary to prevent long-term deformations and to withstand increased loading on the tunnel liner from the rock mass surrounding the tunnel.     

Migration of large earthquakes along the great fault zones in Middle Asia

An analysis of the distribution (both spatial and temporal) of large earthquakes (M 6.5) along the Gissar—Kokshaal and the Hindu-Kush—Darvaz—Karakul fault zones in Middle Asia has revealed the linear character of migration from the ends to the centre of the Pamir arcs at a rate of 1–2 km/year to 3–6 km/year. Migration of large earthquakes at a similar rate has also been found in some of the other great fault zones. An attempt has been made to evaluate the duration of a migration cycle.The regularity found, although it needs further confirmation, has been used to tentatively predict the possible sites of future large earthquakes likely to occur in the present century.     

Analysis of potential cave-in from fault zones in hard rock subsea tunnels

Summary As a part of a research program on the rock engineering aspects of hard rock subsea tunnelling, analyses of potential cave-in from fault zones have been carried out at the Norwegian Institute of Technology. This is a topic of great importance for the planning of future subsea tunnels, and particularly for the selection of the minimum rock cover of such projects. The paper is divided into three main parts: a) review of cases of instability in Norwegian subsea tunnels, b) evaluation of theoretical maximum sliding, and c) discussion of cases of cave-in in tunnels under land. In theory, a cave-in during subsea tunnelling may propagate far higher than the normal minimum rock cover. Taking into consideration the comprehensive geo-investigations that are always carried out for subsea tunnel projects today, it would, however, be unrealistic to base the dimensioning of rock cover for future projects on worst-case scenarios. Consequently, the main result of this study is to emphasize the importance of comprehensive geo-investigations, detailed tunnel mapping, a high degree of readiness during tunnelling and a thorough quality control.     

Application and evaluation of ground surface pre-grouting reinforcement for 800-m-deep underground opening through large fault zones

Faults are complex geological conditions that are commonly encountered during underground excavation. Many support schemes, such as using a single pilot heading method and 30-m-long borehole pre-grouting, have been implemented during the pilot excavation of an 800-m-deep underground opening that passes through large fault zones in East China. However, various geo-hazards, including groundwater inrush, debris flow, and roof collapse, are still occurring, which seriously threaten tunneling safety. To eliminate the geo-hazards and ensure tunneling safety, ground surface pre-grouting (GSPG) was proposed and implemented for the first time to reinforce the regional engineering rock mass for this proposed 800-m-deep underground opening passing through large fault zones. The minimum grouting pressure of GSPG at a depth of 800 m below the surface is put forward based on hydraulic fracturing theory, providing valuable guidance for GSPG engineering practice. Engineering practice demonstrates that GSPG eliminates geo-hazards, improves the objective rock mass stability, and ensures tunneling safety. Field measurements indicate that the displacement velocity of the surrounding rock shows an obvious fluctuation response under the influence of GSPG, and the impact of GSPG on the stability of the 800-m-deep underground opening that has been excavated dramatically decreases as the distance from the grouting borehole increases. Moreover, there is a strong negative exponential correlation between the maximum velocity of deformations and the distance from the grouting borehole. In addition, the safe distance underground during GSPG is greater than 137 m.     

Research on groundwater seepage through fault zones in coal mines

Hydrogeology Journal - Water inrush in coal mines is commonly linked to fault zones. Excavation of the coal seam can lead to new fractures in the associated fault zone. Many water inrush disasters...     

Experimental investigation of the hydraulic properties of large-scale irregular fractured rock masses in granite fault zones

Hydrogeology Journal - Permeability is one of the critical parameters for evaluating the hydraulic properties of water-conducting media. Variations in the hydraulic properties generally lead to...     

Influence of host lithofacies on fault rock variation in carbonate fault zones: A case study from the Island of Malta

Relatively few studies have examined fault rock microstructures in carbonates. Understanding fault core production helps predict the hydraulic behaviour of faults and the potential for reservoir compartmentalisation. Normal faults on Malta, ranging from <1 m to 90 m displacement, cut two carbonate lithofacies, micrite-dominated and grain-dominated carbonates, allowing the investigation of fault rock evolution with increasing displacement in differing lithofacies. Lithological heterogeneity leads to a variety of deformation mechanisms. Nine different fault rock types have been identified, with a range of deformation microstructures along an individual slip surface. The deformation style, and hence type of fault rock produced, is a function of host rock texture, specifically grain size and sorting, porosity and uniaxial compressive strength. Homogeneously fine-grained micrtie-dominated carbonates are characterised by dispersed deformation with large fracture networks that develop into breccias. Alternatively, this lithofacies is commonly recrystallised. In contrast, in the coarse-grained, heterogeneous grain-dominated carbonates the development of faulting is characterised by localised deformation, creating protocataclasite and cataclasite fault rocks. Cementation also occurs within some grain-dominated carbonates close to and on slip surfaces. Fault rock variation is a function of displacement as well as juxtaposed lithofacies. An increase in fault rock variability is observed at higher displacements, potentially creating a more transmissible fault, which opposes what may be expected in siliciclastic and crystalline faults. Significant heterogeneity in the fault rock types formed is likely to create variable permeability along fault-strike, potentially allowing across-fault fluid flow. However, areas with homogeneous fault rocks may generate barriers to fluid flow.     

Effect of anchor plate on the mechanical behavior of prestressed rock bolt used in squeezing large deformation tunnel

Anchor plate is a necessary component of the prestressed rock bolt, which plays a decisive role in actively resisting the rock mass deformation and improving the self-bearing capacity of the surrounding rock. In this paper, in order to investigate the effect of anchor plate on the mechanical behavior of prestressed rock bolt used in squeezing large deformation tunnel, a series of numerical, field and laboratory tests were performed. The mechanical characteristics of anchor plate with different shapes and sizes were systematically analyzed. Based on the Muzhailing Tunnel in Gansu Province, China, which is a typical deep-buried soft rock tunnel and suffered from serious squeezing large deformation disaster, the field verification was carried out. Meanwhile, the influences of spherical washer on the mechanical behavior of bolt shaft and anchor plate were also investigated. The result shows that the bowl-shaped plate has much better performance than the flat one, while the rounded bowl plate performs slightly better than the squared bowl plate. With the increase of plate thickness or plane size, the anchor plate has the better bearing capacity, while the stress transfer effect will be worse. Furthermore, the non-uniform deformation will gradually increase with the decrease of plate thickness or increase of plane size. Therefore, it is advised to increase the plate thickness rather than the plane size under the condition of better bearing capacity. The bending-induced stress of the anchor plate increases rapidly with the increase of the inclination angle between bolt shaft and anchor plate. Whereas the installation angle is unavoidable in practice, the spherical washer which has great effects on reducing the bending-induced stress could be a necessary component of the prestressed rock bolt system.

     

On the internal structure and mechanics of large strike-slip fault zones: field observations of the Carboneras fault in southeastern Spain

Deciphering the internal structure of large fault zones is fundamental if a proper understanding is to be gained of their mechanical, hydrological and seismological properties. To this end, new detailed mapping and microstructural observations of the excellently exposed Carboneras fault zone in southeastern Spain have been used to elucidate both the internal arrangement of fault products and their likely mechanical properties. The fault is a 40 km offset strike-slip fault, which constitutes part of the Africa–Iberia plate boundary. The zone of faulting is 1 km in width not including the associated damage zone surrounding the fault. It is composed of continuous strands of phyllosilicate-rich fault gouge that bound lenses of variably broken-up protolith. This arrangement provides a number of fluid flow and fluid sealing possibilities within the fault zone. The gouge strands exhibit distributed deformation and are inferred to have strain hardening and/or velocity hardening characteristics. Also included in the fault zone are blocks of dolomite that contain thin (<1 cm thick) fault planes inferred to have been produced by strain weakening/velocity weakening behaviour. These fault planes have a predominantly R₁ Riedel shear orientation and are arranged in an en echelon pattern. A conceptual model of this type of wide fault zone is proposed which contrasts with previous narrow fault zone models. The observed structural and inferred mechanical characteristics of the Carboneras fault zone are compared to seismological observations of the San Andreas fault around Parkfield, CA. Similarities suggest that the Carboneras fault structure may be a useful analogue for this portion of the San Andreas fault at depth.     

Numerical modeling of fracking fluid migration through fault zones and fractures in the North German Basin

Gas production from shale formations by hydraulic fracturing has raised concerns about the effects on the quality of fresh groundwater. The migration of injected fracking fluids towards the surface was investigated in the North German Basin, based on the known standard lithology. This included cases with natural preferential pathways such as permeable fault zones and fracture networks. Conservative assumptions were applied in the simulation of flow and mass transport triggered by a high pressure boundary of up to 50 MPa excess pressure. The results show no significant fluid migration for a case with undisturbed cap rocks and a maximum of 41 m vertical transport within a permeable fault zone during the pressurization. Open fractures, if present, strongly control the flow field and migration; here vertical transport of fracking fluids reaches up to 200 m during hydraulic fracturing simulation. Long-term transport of the injected water was simulated for 300 years. The fracking fluid rises vertically within the fault zone up to 485 m due to buoyancy. Progressively, it is transported horizontally into sandstone layers, following the natural groundwater flow direction. In the long-term, the injected fluids are diluted to minor concentrations. Despite the presence of permeable pathways, the injected fracking fluids in the reported model did not reach near-surface aquifers, either during the hydraulic fracturing or in the long term. Therefore, the probability of impacts on shallow groundwater by the rise of fracking fluids from a deep shale-gas formation through the geological underground to the surface is small.     

Mechanism of frictional fusion in fault zones

Spherulitic pseudotachylytes from the Arunta Block formed by frictional fusion of mylonitic parent rocks during high-level reactivation of a previously ductile fault zone. Fusion occurred preferentially in mica-rich domains due to release of water through disruption of the mica lattice by frictional sliding. This generated selective localised melting of mica during frictional heating, with the production of initial pseudotachylyte melts enriched in water and ferromagnesian components. Subsequent fusion of adjacent salic phases, promoted by the high water content of the existing melt, would tend to shift the trend of later melts towards a total melt composition. Therefore, under conditions of frictional sliding, fusion appears to be favoured in crystalline quartzofeldspathic rocks possessing both a high shear strength, and a significant water content locked up in the lattices of hydrous minerals, principally biotite.The presence or pre-existence of glass in many pseudotachylytes demands that they cooled in a near-surface environment, i.e. at depths of less than about 5 km. Thus glassy pseudotachylytes must postdate associated mylonite series rocks, generally forming subsequent to exhumation of the mylonites to a higher level in the crust. Some non-glassy pseudotachylytes, however, may possibly form towards the end of movements in a ductile regime, as strain hardening sets in.     

Metasomatism and fluid flow in ductile fault zones

Observed major element metasomatism in 5 amphibolite facies ductile fault zones can be explained as the inevitable consequence of aqueous fluid flow along normal temperature gradients under conditions of local chemical equilibrium. The metasomatism does not require the infiltration of chemically exotic fluids. Calculations suggest that metasomatized ductile fault zones are typically infiltrated by 10⁵ moles H₂O/cm², fluid flow is in the direction of decreasing temperature, and fluids contain about 1.0 molal total chloride. Where available, stable isotopic alteration data confirm both flow direction and fluid fluxes calculated from major element metasomatism. The fluid fluxes inferred from metasomatism do not require large-scale fluid recirculation or mantle sources if significant lateral fluid flow occurs in the deep crust. Time-integrated fluid fluxes are combined with estimates of flow duration to constrain average flow rates and average permeabilities. Rocks in ductile fault zones are probably much more permeable during metasomatism (average permeabilities of 10^-17 to 10^-15 m²) than rocks normally are during regional metamorphism (10^-21 to 10^-18 m²). Estimated average fluid flow rates (3.5×10^-3 to 0.35 m/yr) are insufficient, however, to significantly elevate ambient temperatures within ductile faults. Fluid flow in the direction of decreasing temperature may increase the ductility of silicate rocks by adding K to the rocks and thereby driving mica-forming reactions.     

Reverse fault slip through soft rock and sand strata by centrifuge modeling tests

The devastating damage after the 1999 Chi-Chi and 1999 Izmit earthquakes has greatly motivated soil–reverse fault interaction studies. However, most centrifuge modeling studies have employed a single homogeneous soil layer during testing, which does not represent in situ conditions. Indeed, while geological conditions vary spatially, engineering soils are often underlain by soft rocks. Therefore, four centrifuge models were developed to evaluate the effect of soft rock layers on the ground surface and subsurface deformation. Sand–cement mixtures of varying thicknesses with a uniaxial compressive strength of 0.975 MPa, simulating extremely soft rock, were overlain by pluviated sandy soil. The model thickness was 100 mm, corresponding to 8 m in the prototype scale when spun at 80 g. Every model was subjected to a vertical offset of 50 mm/4 m (0.5 H; H: total sedimentary deposit thickness) along a reverse fault with a 60° dip. The results indicate that the presence of a soft rock stratum results in the creation of a horst profile at the ground surface. Additionally, the thinner the soil layer on top of the soft rock stratum is, the longer and higher the horst created at the ground surface. Consequently, the fault deformation zone lengthens proportionally with the increasing thickness ratio of the soft rock. Furthermore, the presence of soft rock as an intermediary stratum between bedrock and soil causes the deformation zone boundary on the hanging wall side to move in the direction of fault movement.

     

凝灰岩岩基风化带精细划分技术

正岩体风化带的划分是水利工程勘察重要的研究内容,提高岩基垂向上的精细识别对建基面的开挖、坝基防渗设计、坝基的稳定性和变形控制等方面具有重要意义(王旺盛等,2009;韩刚等,2011)。传统岩体风化程度的划分主要依据钻探过程中的人为感觉、现场岩样的直观判断以及室内的岩石物理力学试验等,在总结大量工程实践的前提下,可以说采用定性或半定量的判别手段在岩体划分中     

Two case histories of tunnels through squeezing rocks

Summary The expression squeezing rock is a concept too vague to be used by practicing engineers. In this paper, it is assumed to mean large convergences of the tunnel walls.Two case histories are briefly presented. The Frejus tunnel was driven with a large overburden; on the contrary, the Sidi Mezghiche tunnel was shallow. Different techniques were used to control the convergences. In both cases, the tunnels were located in complex formations; the complexity stems from the heterogeneity and anisotropy of the rock masses, and no efficient technique is available to determine the geotechnical characteristics and the natural state of stress in the formations.     

Fracture networks and fluid transport in active fault zones

Field measurements were made of 1717 mineral-filled veins in the damage zone of an active dextral strike-slip fault zone in Iceland. Most veins are composed of quartz, chalcedony and zeolites, strike roughly parallel or perpendicular to the fault zone, and are members of dense palaeo-fluid transporting networks. A common vein frequency in these networks is 10 veins per metre. Cross-cutting relationships indicate that 79% of the veins are extension (mode I) cracks and 21% are shear cracks. The apertures of most veins, measured as mineral-fill thicknesses, are from 0.1 to 85 mm, and the aperture frequency distribution is a power law. The outcrop trace lengths of 384 veins (of the 1717) could be measured accurately. These 384 veins are mostly small and range in length from 2.5 to 400 cm, in aperture from 0.01 to 0.9 cm, and have an average length/aperture ratio of about 400. Simple analytical models are derived and used to make rough estimates of the volumetric flow rates in hydrofractures of dimensions equal to those of typical veins. The results indicate that volumetric flow rates for a horizontal fracture and a vertical fracture in a rigid (non-deforming) host rock would be around 1.5×10⁻⁴ and 8.9×10⁻⁴ m³s⁻¹, respectively. The volumetric flow rate in a vertical fracture of equal size but in a deforming host rock, with buoyancy added to the pressure gradient, is around 1.3×10⁻³ m³s⁻¹. Thus, vertical fluid transport is favoured under these conditions.     

华北地区主要断裂带的现今运动特征

用中国地壳网络观测中心提供的华北地区最新GPS观测数据, 研究了华北地区主要断裂带的现今运动特征, 得到NE走向断裂带由北至南存在由挤压转变为拉张、右旋走滑速率增大的特征, NW走向断裂带以左旋挤压变形为主, 走滑速率NW向断裂带大于NE走向断裂带, 据此认为华北地区煤层气开发的有利区块位于南部地区。     

Exploring the seismic expression of fault zones in 3D seismic volumes

Mapping and understanding distributed deformation is a major challenge for the structural interpretation of seismic data. However, volumes of seismic signal disturbance with low signal/noise ratio are systematically observed within 3D seismic datasets around fault systems. These seismic disturbance zones (SDZ) are commonly characterized by complex perturbations of the signal and occur at the sub-seismic (10 s m) to seismic scale (100 s m). They may store important information on deformation distributed around those larger scale structures that may be readily interpreted in conventional amplitude displays of seismic data. We introduce a method to detect fault-related disturbance zones and to discriminate between this and other noise sources such as those associated with the seismic acquisition (footprint noise). Two case studies from the Taranaki basin and deep-water Niger delta are presented. These resolve SDZs using tensor and semblance attributes along with conventional seismic mapping. The tensor attribute is more efficient in tracking volumes containing structural displacements while structurally-oriented semblance coherency is commonly disturbed by small waveform variations around the fault throw. We propose a workflow to map and cross-plot seismic waveform signal properties extracted from the seismic disturbance zone as a tool to investigate the seismic signature and explore seismic facies of a SDZ.     

Dynamic differentiation and association of chemical elements in fault zones

This paper deals with compositional variations in fault zones from a dynamic point of view. In the fault zonen consisting of silicates, relative accumulation of Si and Fe is noticed in response to the leaching-out of K, Na, and to a lesser extent, Mg, Ca and Al. The ordee of petrogenetic elements from stable to mobile is tentatively suggested as follows: Si→Fe→Mg→ Ca→Al→K→Na. The difference in ionic radius for these chemical elements is thought to be the major factor controlling dynamic differentiation. In the fault zones arc silicates on one side and carbonates on the other, and new minerals are recognized in tectonites. On the silicate side Ca and Mg increase but Si and Al decrease; and the opposite is true on the carbonate side. This phenomenon indicates that migration of elements in the fault zones is accelerated by dyna mic effect.