Weak Rock Research Articles

Small‐diameter tunneling in difficult ground – Analysis of TBM performance in hard rock

AbstractSmall‐diameter tunneling in hard rock is increasingly widespread due to the need for new and longer utility tunnels comprising sewer, stormwater, freshwater, or hydropower as well as cable tunnels and casings for pipelines transporting gas or hydrogen. Utility tunnels have to deal with a wide range of geological settings, like small overburden, weathered rock, rock–soil transitions, as well as fractured or intact hard rock with high strength and abrasivity. A database has been created including 35 hard rock projects with diameters between 1 and 5 m as well as more than 70,000 m of tunnel alignments, with a median drive length of less than 500 m. Challenges in creating it and some early interpretations based on the contents of the database are presented. Details about an exemplary pipe jacking project in basement rocks in Brittany, France, are given. The large variety in this small‐diameter range in hard rock includes different TBM types, cutterhead designs, cutter types, and geotechnical conditions. Potential pitfalls in small‐diameter TBM data analysis are shown and general drive parameter trends and penetration prediction approaches are presented and set in relation to the geotechnical conditions. Our analysis shows that difficult ground conditions do not only incorporate rocks with very high strength, but also generally weak rocks like schist or limestone could be responsible for low penetration rates and high thrust forces.

Numerical modelling of laterally loaded piles driven in low-to-medium density fractured chalk

Chalk’s sensitive, variable nature poses difficulties for foundation designers. It can present as weak rock and yet be de-structured to very weak putty by dynamic or high-pressure loading. The development of multiple offshore wind farms at north European chalk sites led to the recent ALPACA Joint Industry Programme, which undertook intensive material characterisation and large-scale field testing at St Nicholas at Wade (SNW), Kent, UK to capture and better understand the behaviour of piles driven in fractured low-to-medium density chalk. Noting that lateral loading response is a vital design concern for monopile and jacket supported structures, this paper focuses on 3D Finite Element (FE) modelling of ALPACA’s monotonic lateral loading field tests on open-ended driven tubular steel piles. The brittle chalk is modelled with a strain-softening Mohr-Coulomb model combined with nonlocal regularisation, calibrated meticulously against the ALPACA characterisation dataset. A second, simpler modelling approach, adopting a perfectly-plastic Mohr-Coulomb model, is also explored as a simplified practical alternative. Both approaches can match field lateral capacity and bending moment distributions, after taking due account of pile installation and chalk fracturing effects. The analyses indicate how robust, accurate and cost-effective lateral loading design may be approached for low-to-medium density fractured chalks.

The largest rock avalanches in Patagonia: Timing and relation to Patagonian Ice Sheet retreat

One of the largest concentrations of giant landslides (≥108 m3) in Patagonia is in the eastern part of Lago Cochrane/Pueyrredón (LP) valley in Argentina. In addition to minor earthflows and rock slides, this landslide cluster is dominated by rock and debris avalanches that affect the northern slope of Meseta Belgrano, the largest of which have volumes >1 km3 and a runout of >10 km. To determine the chronology of these large landslides and their relationship to the geological setting and the glacial history related to the Last Glacial Maximum (LGM) ∼20–18 ka ago, we combined geomorphological mapping with absolute dating (luminescence and radiocarbon dating) and numerical modelling of slope stability. Dating and cross-cutting relationships with glaciolacustrine deposits suggest that some of the largest rock avalanches collapsed directly into a glacial lake between ∼17 and ∼12 ka, soon after deglaciation, but some were pre-glacial and landslide activity continued until today, posing a potential hazard to the area. In agreement with these data, numerical modelling suggests that slope stability was only marginally affected by ice retreat and glacial lake drainage, and landslides were most likely favoured by relatively low rock strength, related glacially-conditioned topography, and, possibly, seismic activity. A newly identified active fault at the base of the Meseta Belgrano, whose activity was likely enhanced by postglacial rebound, was probably the key factor that concentrated postglacial rock avalanches into the LP valley. We conclude that exceptionally large (km-scale) landslides can occur on slopes made of relatively weak rocks in a glacially-conditioned topographic setting even without a strong direct triggering effect of deglaciation, while fatigue due to long-term seismicity may promote collapse.

Failure responses of rock tunnel faces during excavation through the fault-fracture zone

It is essential to cast light on the construction risks in tunnel excavations through the fault-fracture zone (FFZ). This study adopts the material point method (MPM) to simulate the failure responses of a rock tunnel face during excavation through the FFZ. A numerical study was conducted to compare a physical model test and validate the feasibility of using the MPM in simulating tunnel face failure. One hundred ninety numerical simulation cases were constructed to represent a rock tunnel excavation project with different site configurations. The simulation results suggest that the cohesion and the friction angle significantly influence failure responses. The tunnel cover depth can magnify the failure responses, and the FFZ thickness significantly affects the mobilized rock mass volume when the FFZ consists of a weak rock mass. The numerical simulation results suggest three deformation patterns: face bulge, partial failure, and slide collapse. The failure responses can be characterized by stress arch, slip surface, angle of reposing, and influence range. The insights suggested by the face failure responses during excavation through the FFZ can aid field engineers in determining the scope of possible damage, and in establishing emergency measures to minimize losses if such failure occurs.

Friction to Unconfined Compression Strength Correlation for Evaluating Capacity of Weak Rock Foundation Sockets

Historically, deep foundations in weak rock have been designed as friction elements using frictional resistance (fs), calculated from the unconfined compressive strength (UCS) of rock. Most of the published correlations of fs to UCS were developed based on load tests on low-capacity piles in specific geological conditions, using UCS values not necessarily representative over the test depth. There is a large variation in foundation design depths calculated using these correlations. This paper presents a correlation between fs and UCS of weak rock, developed using data from 44 bidirectional load tests from high-capacity deep foundations in weak rocks. The dataset used in this study, is one of the largest used for weak rocks, with high test loads in the range of 100–320MN and the depth of foundations mostly in the range of 20–87 m below ground level. Bi-directional load test data from La Maison tower site is then simulated in Plaxis, and ultimate skin friction developed is compared against the skin friction calculated using the new corelation. The actual ground profile and foundation layout of La Maison tower is then modelled in Plaxis with the required foundation depth derived using the recommended corelation, to check serviceability limits. The resulting maximum settlements are found to be well within the acceptable limits. The correlation factor of 0.5 between fs and UCS is thus recommended for estimating rock socket friction for design of deep foundations in weak carbonate rock formations.

Effect of soaking time on the geotechnical behavior for different types of sedimentary rocks in Egypt

ABSTRACT Weathering problems of weak sedimentary rocks, especially when subjected to water immersion, are considered one of the serious geotechnical engineering issues. Examples of rock water submersion include foundations subject to seepage, regular irrigation, residence daily activities, service water, frequent plant watering, an increase in groundwater level and heavy rain, in addition to numerous urban and rural water use consequences; hence, it is necessary to quantify the effect of water on the physical and mechanical characteristics of sedimentary rocks. The objective of this study is to assess, evaluate and estimate the strength and stiffness of deterioration of sedimentary rocks in Egypt subject to freshwater soaking in three states (natural/presoaking state, after 48 h and after 30 days of soaking). An extensive laboratory testing program including slake durability, uniaxial compression and point load tests was conducted in the three states on sandstone and limestone specimens collected from Galala, Attaka-Suez Governorate, Egypt. The influence of water immersion on the physical and mechanical characteristics such as rounding, stress–strain envelope, strength and stiffness was studied. The results showed a reduction of up to 87% and 93% in the uniaxial compressive strength and elastic modulus, respectively, post 30 days of freshwater soaking. The limestone showed more resistance to deterioration compared to the sandstone. The stiffness and strength of sedimentary rocks decrease with long contact with water. This paper results provide guidance when assessing, planning and preliminarily evaluating similar rock formation/s considered as a candidate for engineering projects.

Application of Vertical Electrical Sounding for Subsurface Characterization to Determineslope Instability at Perizie, Nagaland

Vertical Electrical Sounding (VES) technique which employs the Schlumberger depth sounding method was used to investigate the subsurface conditions at the landslide in Perizie colony, Kohima. The interpretation of the VES curves shows differing lithology consisting of weathered shales with associated clay, which is considered an important factor for slope instability. The shear strength of the slope materials, including the country rocks and the soil cover, is also significantly reduced and easily detached due to the effects of weathering and erosion, causing the landslides. Three to six possible geo-electric layers are delineated from the variations in the resistivity values, with the lithologies consisting of topsoil followed by alternate layers of weathered and fractured rocks of varying thickness. The analysis also points to the presence of a weak zone at a depth of about 10 m, which is not apparent from the surface. The formation of groundwater aquifers in the fractured zones indicates high risk for slope failure as this accelerates the weak country rocks to weather. The results obtained by this study correspond well with the available borehole data of the area.

An operator view on trends in core acquisition

Abstract Core samples from the subsurface can provide unambiguous direct information to guide operator decisions. Core may be acquired with drilling equipment (full-bore core) or by post-drill wireline methods (sidewall core). Both approaches have distinct profiles of cost, risk, sample type and value, and an operator must select the most appropriate to progress business in an informed way. The option to selectively core after drilling and perceptions of lower cost and risk might indicate that sidewall coring will always be the best approach. Recent developments to increase the size and quality of rotary sidewall samples would only add weight to this view. It is not all good news for sidewall core, however. Individual sample size and total volume delivered per run are tiny; weak rock or high overbalance pressure may cause poor recovery and biased datasets; time between drilling and logging allows mud invasion and borehole relaxation, so samples are often broken and pore fluids contaminated. Sidewall sample sets therefore leave a higher degree of uncertainty when compared to full-bore core. It is this operator's view that both approaches have a role to play in reducing subsurface uncertainty, and cost, risk and value should be carefully considered when deciding which to apply.

Face stability analysis of circular tunnels in layered rock masses using the upper bound theorem

An analysis of tunnel face stability generally assumes a single homogeneous rock mass. However, most rock tunnel projects are excavated in stratified rock masses. This paper presents a two-dimensional (2D) analytical model for estimating the face stability of a rock tunnel in the presence of rock mass stratification. The model uses the kinematical limit analysis approach combined with the block calculation technique. A virtual support force is applied to the tunnel face, and then solved using an optimization method based on the upper limit theorem of limit analysis and the nonlinear Hoek–Brown yield criterion. Several design charts are provided to analyze the effects of rock layer thickness on tunnel face stability, tunnel diameter, the arrangement sequence of weak and strong rock layers, and the variation in rock layer parameters at different positions. The results indicate that the thickness of the rock layer, tunnel diameter, and arrangement sequence of weak and strong rock layers significantly affect the tunnel face stability. Variations in the parameters of the lower layer of the tunnel face have a greater effect on tunnel stability than those of the upper layer.

Spectral Deconvolution Analysis of Mafic Mineral in Irregular Mare Patches on the Moon

Irregular mare patches (IMPs), recently discovered on the Moon, are eruptions of magma on the lunar surface, and their origins are still in question. While prior studies on IMPs have mainly focused on optical image analysis, in this study, an analysis of the characteristics of minerals is performed exemplary for the first time. Modified Gaussian model (MGM) deconvolution was applied to the infrared spectrum to confirm the properties of the mafic mineral. Mafic minerals were analyzed for 6 olivine-rich (Ol-rich) IMPs out of 91 currently reported, and only 4 of them yielded results of significance. All four sites showed more abundance of Fe than Mg, and manifested a weak relationship with Mg-suite rock. However, a problem was discovered during the MGM application process due to pilot implementation. In order to solve this problem, it is required to adjust the MGM initial condition settings more precisely and to increase the signal to noise ratio of the observation data. Moreover, it is necessary to analyze the mineral properties for all IMPs considering minerals other than Ol and utilize them to deduce the origin of the IMPs.

Investigation on progressive failure process of tunnel lining induced by creep effect of surrounding rock: A case study

Creep of weak rock is an important and challenging consideration during operation of tunnels, since it may lead to failure of tunnel lining with the increase of pressure and the additional deformation. In this paper, the progressive failure process of tunnel lining induced by surrounding rock creep effect was studied through time-dependent numerical simulation. In this numerical analysis, the Nishihara model considering D-P yield criterion compiled by UMAT was used to characterize the creep characteristics of surrounding rock, and the creep mechanical parameters of surrounding rock in the section with serious tunnel lining diseases were obtained by triaxial creep tests. The concrete damaged plasticity (CDP) model was used to represent the damage characteristics of lining. Numerical simulation was verified by comparing with the tunnel displacement monitoring results. It is shown that long-term stability and progressive failure mode of tunnel lining are affected by the creep effect of weak rock. The stress combination dominated by thrust and bending moment in the first 30 years makes the lining concrete unable to maintain stability. The failure modes of lining were divided into early compression damage of arch crown and late tension damage of arch shoulder.

Influence of climate-dependent variables on deformation and differential erosion of stratified sedimentary rocks

Rockfalls are major natural hazards for road users and infrastructures in northern Gaspésie (Eastern Canada) where nearly 25 km of road runs along imposing rockwalls. Since 1987, the Ministère des Transports du Québec (MTQ) has reported >17,500 rockfalls that have reached the roadway. While rockfalls are primarily gravitational phenomena, other factors contribute to their development. The long-term interaction of a series of processes leads to rock instabilities. They are never caused solely by the most recent visible change. This study aims to better understand the influence of climate-dependent variables on the mechanical deformations that leads to the erosion of stratified sedimentary rock sequences. To achieve this, a flysch rockwall was instrumented with crack meters and a network of hydrometeorological sensors. The major deformations affecting stratum of siltstone and sandstone were identified and correlated with climate-dependent variables. Results highlighted that irreversible deformations in the first decimeters of rock wall surface are mainly induced by rainfall and snowmelt, by temperature fluctuations around the freezing point and to a lesser extent, by large thermal variations. The water content fluctuations at the rock surface are more effective at generating irreversible deformations in the clayey siltstone strata than freeze-thaw cycles and high amplitude thermal variations. In the sandstone strata, irreversible deformations were primarily caused by freeze-thaw cycles and water content fluctuations at the rock surface. The long-term trends in the sandstone and siltstone strata recorded by crack meters, along with field observations, highlight mechanisms that lead to the development of superficial rock instabilities in this type of geology. Our results show that the retreat and settlement of the weak rock strata (clayey siltstone) causes the gradual overhang of the resistant rock strata (sandstone). As a result, sandstone blocks will eventually slide or topple on the eroding weak rock strata below. The results of this study underscore the importance of studying rock wall dynamics as a whole to understand the mechanisms that lead to the development of hazardous large-scale rock instabilities.

A Geotechnical Perspective on a Complex Geological Environment in a High-Speed Railway Tunnel Excavation (A Case Study from Türkiye)

The T26 tunnel was designed within the scope of the Ankara-Istanbul high-speed railway in accordance with the speed of 250 km/h. Some serious problems and excessive deformations were encountered during the excavation works. The deformations in the tunnel caused subsidence on the surface and the Tunnel Boring Machine (TBM) became stuck; therefore, tunnel excavation works were suspended. Design works for re-excavation in the T26 tunnel and extracting of the TBM were carried out and the tunnel was re-designed by the New Austrian Tunneling Method (NATM) system. The main purposes of the present study are to describe the problems encountered during the T26 tunnel and to discuss the sources of the problems. The advantages and disadvantages of TBM and NATM methods for the tunnel having difficult ground conditions were discussed. Critical points needing to be considered for the tunnels excavated with TBM through weak ground conditions and the effect of the TBM selection process were discussed. Considering the complex geological and geotechnical structure of the tunnel route, it is possible to say that the T26 case is an interesting case for tunnel engineering. Along the tunnel route, landslides, high seismic activity, groundwater conditions, and extremely weak rock mass features coexist. Therefore, the tunnel route is a very complex environment. However, due to the geometric limitations of the high-speed railways, relocation of the route is not possible. The experiences gained from tunnel excavations under difficult conditions are capable of bringing new horizons to future tunneling studies.

Effect of Constructing a New Tunnel on the Adjacent Existed Tunnel in Weak Rock Mass: A Case Study

This study takes a new Shidao tunnel where the left line constructing in weak rock mass as a case study, and the effect of the new constructing tunnel on the existed tunnel are studied by the numerical analysis. High-precision field investigations are conducted to provide accurate parameters for the numerical model. The modified generalized Zhang-Zhu (GZZ) constitutive model is applied, and the numerical analysis results containing horizontal convergence displacement of side walls of the new constructing tunnel and a longitudinal crack in existed tunnel are validated by real-time monitoring. The vertical displacement of the vault, the horizontal displacement of the adjacent side wall, the plastic zone of the new constructing tunnel are studied. The effect of the new constructing tunnel on the existed tunnel is studied by analyzing the vertical displacement of the vault and the horizontal displacement of the adjacent side wall of the existed tunnel. During the constructing process of new tunnel, the maximum width variation of longitudinal crack is less than 0.3 mm. The maximum vertical displacement of the vault is less than 1 mm, and the maximum horizontal displacement of the adjacent side wall is less than 0.5 mm for the existed tunnel. Finally, the effects of tunnel spacing between new constructing and existed tunnels and geological condition represented by geological strength index (GSI) are investigated. The result shows that the maximum vertical displacements of the vault and the maximum horizontal displacement of adjacent side wall can reach −10.4 mm and −4.9 mm respectively when tunnel spacing is 0.5 d (d is actual spacing). When GSI is increased from 15 to 30, the maximum vertical displacement of the vault is reduced obviously.

New analytical approach to simulate the longitudinal fiberglass dowels performance installed at the face of a tunnel embedded in weak and weathered rock masses

A new theoretical method is proposed to simulate the fiberglass dowels performance in the confinement of the tunnel face in weak rock masses. In this method, to find the stress distribution ahead of the tunnel face, the spherical symmetry hypothesis is used. Thus, the tunnel's cylindrical shape is approximated by a hemispherical void with the same radius. Considering the relative shear displacements between the reinforcing elements and rock mass, the distribution of the axial forces and shear stresses along the reinforcements are obtained. Some examples are solved, and the results are compared with those derived from FLAC3D simulations. A good agreement is observed between the results obtained with both methods.

Prediction of the longitudinal ground pressure-acting roof of the shield during single-shield TBM excavation in weak rock masses

Prediction of the longitudinal ground pressure-acting roof of the shield during single-shield TBM excavation in weak rock masses

Water Projects in Ethiopia and their Implications for the Future of the Nile Water

Ethiopia is well known as the “East Africa water tower.” Water flows from it in all directions, north and west to the Nile River (Sudan and Egypt), And south to Lake Turkana (Kenya), Shabelle and Juba (Somalia). To the east, Lake Assal (Djibouti), Mountainous nature, and complex terrain dominate Ethiopia. Rainwater and its run-off mountain slopes and hills are the source of surface water for most Ethiopian lands. This paper discusses Ethiopia’s water situation, as the primary source country, which contributes about 85% of the Nile’s water, and discusses Ethiopia’s possibility to establish great dams. And the most important geological reasons behind the failure of many water projects in Ethiopia, whether on a large or small scale. Including the temporal and spatial diversity of rainfall, the high evaporation rate (87%), the different terrain, the severity of the slopes, the spread of weak basaltic volcanic rocks and fractured limestone, and a large number of faults and faults as a result of the seismic activity associated with the African trench, as well as the increase of the Ethiopian plateau. Alluvial sedimentation that negatively affects the storage capacity of dams.It also aims to assess the impact of existing or under-construction water projects on the water share of Egypt and Sudan and to identify future water projects in Ethiopia by analyzing previous data, reports, and scientific research, and finally finding ways to get out of the current crisis between countries The source on one side and the downstream countries (Egypt and Sudan) on the other side.

Research on Construction Technology of Tunnel Face Slump in Soft Rock Tunnel of China-Laos Railway

<p><strong><em> </em></strong>In Taos barrow of jade grinding rail tunnel engineering ii, of rich water karst region slipped in weak rock tunnel collapse mechanism and treatment technology for research, slipped through the establishment of the tunnel surrounding rock collapse mechanics analysis model, the theoretical calculation and finite element calculation software simulation method of combining the, constraints of the tunnel to slip down position at the beginning of surrounding rock and the structural stability analysis, According to the analysis results, the corresponding treatment measures are put forward. The results show that the settlement value of surrounding rock vault is 84.7mm, and the horizontal convergence value of arch waist is 167.4mm. The deformation value of surrounding rock is large, and it is easy to collapse. The plastic zone of surrounding rock mainly occurs at the junction of upper and lower steps on the face of the face, and the maximum plastic strain is 6.63×10<sup>-2</sup>. The arch waist of the primary support structure is prone to stress concentration, which should be reinforced during construction to prevent instability failure. According to the actual situation of tunnel construction site, combined with the finite element calculation results, the treatment structure of concrete filling combined with small pipe reinforcement was used to treat the tunnel slide collapse section. The treatment effect of tunnel slide collapse section was better, which provided a reference for the treatment of the same situation in the subsequent construction section.</p>

Analysis and Modeling of Longitudinal Deformation Profiles Considering the Reinforcement Effect of Tunnel Face in Weak Rocks

Convergence-confinement method (CCM) has become a common method for tunnel design due to its simplicity and practicality. The ground reaction curve (GRC), the support characteristic curve (SCC) and the longitudinal deformation profile (LDP) are all considered in this method. One of these is the LDP which serves as a unique identifier for the location of the tunnel support. The installation point of the tunnel support is one of the most important outcomes of the method. LDP is more impacted by the reinforcement of the tunnel face. However, more existing LDP calculation methods do not consider the reinforcement of the tunnel face. A study on methods for calculating LDP considering the tunnel face reinforcement was carried out to address this constraint of CCM. Firstly, the transformation of tunnel face extrusion deformation and pre-convergence deformation at the tunnel face is realized in accordance with equivalent volume. Then a model of LDP with the tunnel face extrusion deformation as the variable was established considering the tunnel face reinforcement effect. The proposed model's accuracy is assessed in comparison with other typical models. We conclude with parametric analyses concerning the tunnel face reinforcement's effects on the point of support installation, as well as convergence deformation of the tunnel. By taking into account tunnel faces' reinforcement effects, tunnel support design can be more accurate, and the construction costs can be reduced.

New rock strength-based UCS-V[formula omitted] correlation equation for different rock types by statistical regression methods

Uniaxial compressive strength (UCS) is an essential parameter for the characterization of rock strength. P-wave velocity (Vp) measured using ultrasonic testing has been commonly used to predict UCS because this method has many advantages over conventional uniaxial compressive tests, such as its simple operation, low cost, and non-destructive nature. Vp can also be used to calculate dynamic elastic moduli. A Vp-based equation that can be used to evaluate the UCS of numerous types of rocks with high accuracy, however, remains a challenge. Effects such as porosity, weathering, and mineralization have been seldom considered although they significantly affect the UCS-Vp correlation, especially in weak rocks. This research proposes a rock strength-based UCS-Vp regression equation for engineering purposes, which can be used to predict the UCS of different rock types that have a Vp over a critical value and that have been classified by strength (high or low). The training data on which this equation is based yield a coefficient of determination of 0.96 and a standard deviation of 18.1 MPa, and the mean absolute error of the regression model is 14.7 MPa after testing. A critical Vp value of 3.5 km/s is used to evaluate how a rock is affected by porosity; if the rock Vp is greater than this critical value, a more in-depth strength analysis is needed. The critical rock strength judgment involves several hammer blows being applied to a rock sample to fracture or chip it. This determines whether the rock sample has a UCS above or below 100 MPa and low or high porosity, and thus the variable input values for the prediction equation. The UCS-Vp correlation is linear for rocks with greater than critical strength and logarithmic for rocks with lower than critical strength. The new model provides a quick and effective way to evaluate UCS for engineering projects and significantly extends the usage of the UCS-Vp correlation equation to different rock types while maintaining a high degree of accuracy.