Weathered Rock Mass Research Articles

Site characterization through combined analysis of seismic and electrical resistivity data at a site of Dhanbad, Jharkhand, India

We present the seismic site characterization study using joint modelling of Horizontal-to-Vertical Spectral-Ratio (HVSR) and Rayleigh wave-phase velocity-dispersion curves obtained from Multi-channel Simulation with One Receiver (MSOR) in a part of Dhanbad, Jharkhand, India. The joint analysis of these two different but complementary datasets puts stronger constraints on the model parameter search space than one dataset and may help us in finding more unique shear-wave velocity model. The microtremor data from 12 observation points were utilized to iteratively search 1D shear-wave velocity profiles in a predefined model search space. These 1D shear-wave velocity models were interpolated to generate a 2D shear-wave velocity profile of the site using the cubic spline method. Our results show that the high peak amplitude value of HVSR is associated with low peak-period values of HVSR at a distance of ~ 60 m from the southern end of the profile; which may indicate the presence of the Basin Edge Effect. We identified four layers based on significant changes in the shear wave velocities to a depth of ~ 60 m. The major impedance contrasts are located at average depths of ~ 13 m, ~ 40 m and ~ 55 m, respectively. These layers from the surface may indicate the presence of soil, highly weathered rock mass, moderately weathered rock and bedrock, respectively. The depth of engineering solid bedrock (Vs > 600 m/s) is found at the depth of 55 m in the south which gradually decreases to a depth of 40 m in the northern end of the profile. The shear-wave velocity (Vs 30) for this area varies between 293 and 357 m/s; which can be classified as “D-type site”. For validation and comparison of our results, the Electrical Resistivity Tomography (ERT) data were also recorded along the same traverse using Wenner and Schlumberger configurations. Our results show a significant amount of correlation between the 2D shear-wave velocity and resistivity profiles obtained from joint analysis of tremor and ERT data.

ANALYSIS OF SLIDING DEFORMATION MECHANISM OF A TYPICAL HIGH CUTTING SLOPE IN THE MOUNTAIN OF FUJIAN PROVINCE

The K85 slope is a typical high cutting slope in the mountain of Fujian province, which is located in an expressway of Fujian province. During the slope treatment, there appeared cracking deformation and partially collapse, and formed a landslide disaster. The landslide affected the smooth opening of the line. So, for the similar slopes, it is great significance to study the deformation mechanism of this slope. In this paper, by analyzing the geological environment of the slope and the deformation of the slope excavation, the deformation and failure mechanism is described. It indicates that the deformation and failure mode is that the overlying weathered rock mass along the underlying rock surface caused the sliding deformation and damage. It uses finite element numerical simulation to analyze the steady-state of design schemes and actual construction conditions. And combining with on-site monitoring results, it confirms the deformation mechanism of these slopes. Meanwhile, it analyzes the risk prevention and control nodes about the treatment process in this kind of slope. Finally, the slope prevention and reinforcement scheme is optimized for the instability landslide. And, it is reasonable and feasible to verify the scheme by numerical simulation and new monitoring results.

Influence of weathering-induced iron precipitation on properties of sandstone in a tropical environment

Weathering-induced iron dissolution and precipitation in rock masses lead to reduction in shear strength of discontinuities, amongst others, while precipitation leads to an increase, and precipitated iron may reduce the susceptibility to future weathering. This effect is investigated in fieldwork campaigns with detailed rock mass characterization and chemical analyses in Kota Kinabalu, Malaysia. Many cut slopes in this area consist of different zones in which iron in various forms is either dissolved or precipitated. Iron-coated and -cemented discontinuities have a marked higher shear friction, while staining has little influence on friction. Discontinuities with iron-coating and -cementing reduce the susceptibility to weathering of a rock mass as weathered material is less easily eroded and underlying material is protected from further weathering. On the other hand, dissolution zones, which are mostly found below the precipitation zones, have considerably lower rock mass quality. In engineering works the latter are invisible before excavation. Just assuming the continuation downward of the surface rock mass with precipitated iron, or, worse than this, assuming a less weathered rock mass with better characteristics, may be very erroneous. This study emphasizes that forecasting the susceptibility to weathering should include the influence of morphology, environment and climate.

Investigation on the characteristics of overlying strata caving in the Chengchao Iron Mine, China

In this study, a comprehensive assessment of the overlying strata caving characteristics of mental mines is conducted for the non-pillar sublevel caving method, especially considering the geological features and rock mass characteristics. In general, the overlying strata caving process can be divided into four stages: the intermittent roof caving stage, the crack extension and surface subsidence stage, the karst collapse stage, and the mining-induced collapse stage. Roof caving is an intermittent process, with higher caving occurring in stronger rock masses, and occurs more easily in weaker rock masses. Due to gaps generated in the overlying rock mass, the bulking coefficient of the overlying strata is much greater than the laboratory-measured value; it also varies with the mining advance lines. During the crack extension and surface subsidence stage, the surface deformation is less extensive when the rock strength is greater, and the gradient of surface deformation is lower when the rock mass integrity is lower. Thus, higher mining height and lower rock mass quality result in greater surface deformation. Furthermore, weak planes, such as faults and joints, can help to generate fissures in weathered rock masses; these fissures create passageways for groundwater flow, which accelerates crack extension and connection in reverse. In addition, surface collapses can be divided into karst collapses and mining-induced collapses. Because a large number of fissures and a high karst rate create favorable conditions for karst collapse, significant variations in groundwater levels can induce karst collapse at the surface, with high negative pressure forming in the karst caves. Mining-induced collapses are mainly related to the mining depth, mining height, and the properties of the overlying rock mass. Underground mining is a major factor in surface collapse, and heavy rainfall can induce a delayed but sudden collapse.

Landslides and dam damage resulting from the Jiuzhaigou earthquake (8 August 2017), Sichuan, China.

At 21.19 on 8 August 2017, an Ms 7.0 earthquake struck the Jiuzhaigou scenic spot in northwestern Sichuan Province, China. The Jiuzhaigou earthquake is a strike-slip earthquake with a focal depth of 20 km at 33.20° N and 103.82° E, and was caused by two concealed faults. According to emergency investigations and remote sensing interpretations, the Jiuzhaigou earthquake triggered 1780 landslides, damaged one dam (Nuorilang Waterfall) and broke one dam (Huohua Lake). The landslides mainly occurred in the Rize Valley and Shuzheng Valley and in Jiuzhai Paradise. The landslides involved hanging wall and back-slope effects, and the slope angle, slope aspect, seismic faults and valley trend were obviously related to the occurrence of the landslides. Specifically, most of the landslides were shallow landslides, rockfalls and rock avalanches and were small in scale. The failure modes of landslides mainly include wedge rock mass failure, residual deposit failure, relaxed rock mass failure and weathered rock mass failure. The initial low stability of the dam coupled with the topographic effect, back-slope effect and excess pore water pressure led to damage to the Nuorilang Waterfall dam.

2D and 3D stability analysis of tunnel roof collapse in stratified rock: A kinematic approach

This paper aims to predict the potential roof collapse of deep tunnels subjected to water table level changes and deterioration of rock overlying the tunnel crown. The effect of partly weathered rock masses is represented with lower mechanical parameters. Meanwhile, the pore water pressure is taken as external loads acting on the rock bank below the water table without consideration of capillary effect. Under the combined effects of the above two conditions, a novel and comprehensive failure mechanism is postulated for analyses of roof stability of tunnels. Both two-dimensional (2D) and three-dimensional (3D) collapse mechanisms formed by multi-surfaces are proposed in kinematic analyses where upper bound solutions of the failure mechanism are derived based on variational principle and stepwise optimization technique. Validation of the proposed approach is carried out which showed consistencies of failure mechanisms with analytical and numerical results presented in published literatures. Parametric studies with rectangular and circular tunnel cross-sections are also conducted in 2D and 3D analyses so as to depict the effect of weathered parameters and pore water on incipient falling blocks. The output from these parametric studies could potentially provide useful reference for engineers in the design and construction of deep-buried tunnels.

Utilization of weathered rock mass as the China Three Gorges Project Dam foundation

The utilization of the weathered rock mass in the Three Gorges Dam Area is one of the key geotechnical issues for the China Three Gorges project (CTGP) construction. Engineering geological investigations and tests had been carried out to study the physical, chemical, mineral and mechanical properties and distribution characteristics in detail for more than 30 years. The classification of the weathered zone has been further divided. The utilization of the lower part of the weakly weathered zone as the Three Gorges dam foundation rock mass had been researched by consolidation grouting test. The research results show that: (1) The weathered rock mass in Three Gorges dam area has a great thickness of 40–70 m, with inhomogeneous behavior, physical and mechanical properties in different weathering zones. (2) It is feasible that the lower part of the weakly weathered zone could be used as the dam foundation after consolidation grouting treatment. This approach can save a large amount of construction investment. (3) The weathered rock mass can be used as different civil engineering foundation, and different weathered zone can satisfy different foundation bearing layer requirements. (4) The weathered rock mass can be utilized as construction material. The completely-weathered rock mass can be used as cofferdam filling sand material. The weakly and slightly weathered granite can be used as filling rock materials of cofferdams.

CHARACTERISTICS OF BOULDERS FORMED IN TROPICAL WEATHERED GRANITE: A REVIEW

Boulder is well known as one of the obstruction material and problematic to the underground excavations and constructions. Although the presence of boulder in weathered rock mass was revealed and reported by many researchers, but the occurrence and physical characteristics of boulders in weathering profile are still not clearly understood. This paper attempts to revise the issues and characteristics of boulder formed in weathered granite which include the formation, distribution, properties and location of boulders found in tropical weathering profile. As a weathering product, boulder is dominantly found in moderately to completely weathered zone of rock mass (Grade III to V). Boulder consists of corestone Grade I or II and surrounded by some concentric sheet of weathered rock Grade III to V or rindlets which formed due the reaction of spheroidal weathering. The rindlets with thickness ranges 0.2 mm to 2.0 m is the zone of decomposition and dissolution of biotite and feldspar that gradually transforming to saprolite. This information is useful to geotechnical engineers and researchers for engineering purposes and weathering zone classification especially in underground excavation and structure design.

Geological Investigation and Tunnel Excavation Aspects of the Weakness Zones of Xiang’an Subsea Tunnels in China

Tunneling through weakness zones under deep seawater involves significant risks and may have disastrous consequences. This paper presents a case study of geological investigation and excavation aspects of subsea tunnels in major weakness zones. The subject is the Xiang’an subsea tunnels, the first subsea tunnel project in mainland China. The Xiang’an subsea tunnels passed through four major weakness zones, mainly consisting of highly to completely weathered rock mass. The weakness zones were characterized by a combination of long-distance and short-distance, destructive and nondestructive methods, which supplement and verify information with each other. The weakness zones were treated by full-face curtain grouting, using both cementitious and chemical grouts. The interrelationship between the settlement at tunnel crown and ground surface, as well as the interrelationship between ground settlement and ground cracking, were explored based on instrumentation data recorded during tunneling on land. The pre-warning, warning, and limiting values of tunnel crown settlement during excavation of each heading of subsea sections were established to protect the seabed against cracking. Engineering means and methods were developed to control the ground deformation during excavation.

Characteristics of weathering zones of granitic rocks in Malaysia for geotechnical engineering design

Tropical climatic conditions cause severe weathering to rock masses resulting in thick weathering profiles consisting of different weathering zones with complex characteristics. Feasible geotechnical engineering design requires a detailed understanding of the characteristics of each weathering zone. A typical weathering profile is usually characterized by field-scale observation and geological study. This paper presents the characteristics of weathering zones of various granitic rocks in the south of Malaysia. This region has a tropical climatic condition. The study was carried out in four active quarries located in Johor province, Malaysia, and forty sections were investigated in the study areas. A typical granitic weathering profile was characterized based on the geological and structural parameters (i.e. joint characteristics, corestone occurrence, rock/soil ratio, mass homogeneity, color of rock, and discoloration at joints' surfaces). The profile includes common zones ranging from fresh to residual soil; however, two subzones are introduced based on corestone (boulder) occurrence in the highly weathered and completely weathered zones. This study, based on detailed field observations at the mentioned quarries followed by analysis, presents an amalgamated picture of weathering zones in granitic profiles of a tropical region like Malaysia. Moreover, it is believed that the characterization of the typical weathering profile allows further classification of generally complex weathered rock masses for a specific geotechnical engineering design.

Bulking factors and extents of caved zones in weathered overburden of shallow abandoned underground workings

A comparison between the bulking factor and height of the caved zone for different weathered subsurface rocks over shallow abandoned mines and for strong rocks at intermediate and large depths of longwall mining is performed. In this study, vertical boreholes drilled from the ground surface have been used to detect the height of caving in weathered (W) and very weathered (VW) rock mass overlying abandoned underground workings at a shallow depth, and then to calculate the bulking factor. It is found that key parameters that control the bulking factor (k) and the ratio between the heights of caving and mining (H/h) are different for W and VW rocks. In W rocks, such parameters are the rock strength of immediate roof and rock porosity, whereas those for VW rocks are rock quality designation (RQD, %) and standard linear fracture intensity (P10). It is argued that H/h in VW rocks increases according to linear-logarithmic law with an increasing ratio P10/RQD in the immediate roof and P10 in four rock layers over the immediate roof. Observations show that VW rocks exhibit smaller values of k=1.06–1.165 and larger H/h=6.07–15.6 compared to W rocks having 1.09<k<1.24 and 4.1 <H/h <11.25.

Performance evaluation of existing surface excavation assessment methods on weathered sedimentary rock

This paper is intended as a contribution to the discussion of the overview of existing excavatability assessments and their reliability in assessing the state of tropically weathered rocks, specifically in Nusajaya, Johor, Malaysia. For this purpose, case studies were conducted on sedimentary rock masses of the Jurong formation, located in Nusajaya, Johor. The existing available rock mass excavatability assessments were first critically reviewed. Three case studies were then conducted in order to assess the performance of surface excavatability in these sedimentary rock masses by practical excavation tests on site. Comparisons were then made between the assessments of those assessors and that obtained through the practical excavation test. The conclusions made in the paper about the results of the study show many divergent opinions among the assessors. It emphasizes the limitations of existing available assessments in predicting the excavatability of tropically weathered rock masses. Further complication may arise in sedimentary rock masses where different layers of rock are interbedding, producing different weathering profiles. In view of these, a simpler graphical method, which considers only two parameters, namely point load strength and spacing of surface discontinuities, was introduced to overcome the problems, thus might be misjudged during early excavation assessment. This subjectivity becomes more important as the heterogeneity of a rock mass increases with increasing interbedding and inherent anisotropy of the rock material, giving rise to quite different evaluations, even for a rock with the same state of weathering. Most of the existing excavatability assessment methods are found to be less reliable because nature, lithology and weathering state at various rock mass layers is not being taken into account. A more practical and appropriate excavatability assessment method is thus required to economically assess the tropically weathered sedimentary rock during the preliminary stage.

Mode of Slope Failure of Moderately to Completely Weathered Metasedimentary Rock at Bukit Panji, Chendering, Kuala Terengganu

Geology of Bukit Panji, Chendering, Kuala Teregganu consists of interbededmetasedimentary rocks (slate, phyllite and schists with minor quartzite) which haveexperience regional metamorphism. The age of this rock is Carboniferous. A development project which under construction in Bukit Panji, Kuala Terengganu hasenabled a landslide assessment to observe the modes of failure in moderately tocompletely weathered metasedimentary rock. Development on hillsides caused manyslope had to be cut to provide space for the infrastructure construction. From assessment analysis, a total of 21 cases of landslide failure occurred involving 17 cut slopes, and 4 cut-fill slopes. The most common type of failures is gully failures, with 9 cases represent 43% of all the observed slope failure. This was followed by 6 wedge failures, two planar and rock fall failures and one shallow sliding and toppling respectively. Cut slope failure involving moderate weathered rock mass (grade III) to the residual soil (grade VI). Relict structure was identified as the main factors controlling the failure, as well as water, natural slope-forming materials and the use of appropriate slope stabilization.

A Photogrammetric Approach for Stability Analysis of Weathered Rock Slopes

The geological strength index (GSI) system is dependent on the rock block volumes and the joint surface conditions. The weathering degree of rock slopes and their strength properties also depend on these characteristics. This study thus focuses on the use of the GSI system and the Hoek–Brown strength criterion to estimate the engineering parameters for weathered rock masses. Photogrammetric methods based on 3D surface models are used to obtain reliable data on the joint sets in rock slopes, instead of general site investigation using labor-intensive techniques. Photogrammetric surveys were conducted on weathered rock slopes in Gold Coast, Australia to obtain the joint spacing, orientation and roughness. The 3D models are then used to estimate the block sizes the joint roughness coefficients (JRC). The block volumes and JRC values were then used to estimate GSI values. Then parametric studies using the finite element method is conducted to investigate the stability of the slope using the GSI values.

풍화암반 저심도 터널 굴착 중 발생한 지표침하의 특성

The characteristics of subsidence above a shallow tunnel excavated in weathered rock mass is analyzed. The tunnel is excavated minimum about 11m beneath some buildings and the width of the tunnel is 11m, too. Subsidence pins are installed at 23 locations on surface along the tunnel, 180m long, adjacent to the buildings. Subsidences are measured for about 2 years and they are optimized to analyze three dimensional deformed ground surface, trough width parameter K and sectional volume loss of unit tunnel length Vs of the surface deformation profile.

Finite Element Analysis for the Stress Field and Seepage Field Interaction within Qingdao Submarine Tunnel Rockmass

The submarine geology characteristic for Qingdao submarine tunnel is fracture and fragmentation with smash rock mass and unevenly distributed rock mass strength. The submarine tunnel is excavated in a weathered rock mass containing water. Before the tunnel lining is constructed, it is important to assure the stability and safety for the rock tunnel and tunnel lining structure during construction and operational periods. As the submarine tunnel is under severe condition, the rock mass stabilization is to a large extent to be determined by the underwater seepage effect. It is fundamental to investigate how the seawater interacts with the tunnel rock mass stress field. To investigate the interaction between the rock mass seepage field and the secondary stress field, the nonlinear finite element software ABAQUS is used to analyze the interaction behavior between the rock mass seepage effect and the tunnel secondary stress field. The rock mass seepage field, stress field are analyzed in detail using the numerical simulation method. Also, the distribution of the tunnel rock mass plastic region is obtained. The numerical analysis results provide guidance for the submarine tunnel construction.

Spatial Variability Modelling of Geotechnical Parameters and Stability of Highly Weathered Rock Slope

In recent years, spatial variability modeling of geotechnical parameters using random field theory is gaining strength in the reliability based analysis of geotechnical problems. Slope stability analysis of highly weathered rock slope requires proper modeling of geotechnical parameters with due consideration of extent of weathering and overburden pressure. The present study demonstrates the approach for spatial variation modeling of geotechnical parameters and reliability analysis based stability assessment of highly weathered rock slope by considering a typical case of height = 5.0 m and slope angle of 30°. Commercially available software FLAC 5.0, via inbuilt FISH function, has been utilized for the numerical analysis purpose with the assumption that highly weathered rock mass can be analyzed within the framework of the concept of equivalent continuum model. A parametric study is performed to investigate the following: (i) influence of coefficients of variation of geotechnical parameters and (ii) auto-correlation distances on the reliability analysis results. For the reliability analysis, information on mean and variance of output parameter, i.e., factor of safety of the given rock slope is obtained from 2000 Monte Carlo simulations and results are utilized with first order reliability method to obtain reliability index values. The results of the analysis clearly demonstrated that numerical modeling of spatially varying geotechnical parameters gives more realistic treatment to the property variation of a natural material and stability assessment in reliability analysis framework is more comprehensive than conventional limit equilibrium based factor of safety approach.

The Influence of Geological Engineering and Geotechnical Conditions on Parameter Selection of the Primary Lining of a Road Tunnel in Laliki

The road tunnel in Laliki was excavated in highly heterogeneous, severely tectonically damaged and mainly very weak rocks of the Western Carpathians flysch. In particular, the conditions were characterized by a high percentage of very weak laminated shale and weathered rock mass, an unfavorable and very steep slope of the rock layers and unstable hydrological conditions with outflows of water in loosened tectonic zones. That structure and properties of the rock mass highly uncertain.This paper describes the influence of geological engineering and geotechnical conditions on the primary lining of a main road tunnel. The deformation of the primary lining was analyzed in terms of the percentage share of sandstones and shale, geomechanical classifications RMR (Bieniawski 1989) and QTS (Tesar 1979), types of the primary lining and the use of rock bolts and micropiles. The analysis was preceded by characterization of geological engineering conditions and technological characterization of applied primary linings.Displacements of the primary lining, greater than acceptable, occurred several times in a top heading during tunneling. The primary lining was reinforced by additional rock bolts and wire mesh, a thicker layer of shotcrete and micropiles if deformation reached the emergency state for some types of linings and they didn't indicate any tendency for stabilization. The reinforcement was used until the deformation stabilization was achieved. In the most difficult conditions, the lining was reinforced by a longer micropile umbrella. Parameters for the primary lining were selected on the basis of ongoing geological engineering and geotechnical measurements, in accordance with NATM's principles.The rock mass around the tunnel in Laliki is an example of weak carrying capacity. The observed displacements in the rock mass indicate that the disturbed zone around the tunnel was heavily developed. The primary lining used in such conditions must bear a relatively high load capacity from overlying loosened material and therefore the lack of interaction with the surrounding rock mass should be assumed.The data obtained indicate that the use of the primary lining in the highly variable conditions in the Carpathian flysch requires accurate geological engineering and geotechnical analysis during the day-to-day process of tunneling in order to verify the projected assumptions. The primary linings should be reinforced as needed based on the results of geotechnical measurements, monitoring the interaction between the rock mass and the system of lining.

Modelling the behaviour of a retaining wall in residual soils for a cut and cover construction of a deep station in Metro do Porto

This paper presents a re-appreciation of the ground characterisation and the criteria to select the most representative geomechanical parameters to consider in a numerical model to predict the behaviour of a retaining wall of a deep excavation in highly weathered granite rock masses and residual soils. This study was focused in the construction of a deep station of Metro do Porto, which involved a cut and cover solution, with unusual proportions (in plant and in depth), built in the typical Oporto's granite weathered profiles, being the excavation conducted with retaining walls consisting of multi-anchored concrete piles. Specific sections were carefully instrumented, due to the presence of historic buildings in the vicinity. The definition of representative model parameters was based on precise laboratory tests over high quality soil samples, including oedometer and high-precision triaxial tests. Geotechnical and geological characterisation of all the area for the original design, was initially based on in situ tests, such as SPT and rock masses classification, and on the local experience on this type of ground. After this construction, the assumptions of parameterisation, using a constitutive modelling based on new laboratory tests over high quality block samples, allowed a reanalysis of the assumptions on the design phase. A back-analysis of monitored displacements and forces during the construction was made, assuming the designed structural solutions, which were in fact implemented in construction, but considering the new approaches on the definition of the geomechanical parameters for the prevailing weathered rock masses, necessary for the numerical simulation based on the commercial software Plaxis®, using the Mohr-Coulomb and “Hardening-Soil” models. Some specific changes of the constructive sequence during the excavation and activation of supporting system were attained, by looking at the information found during the construction. The results of this parametrical re-approach and analysis of the singularities of highly weathered granite and corresponding residual soils masses for modelling of retaining walls of large excavations are discussed.

Weathering as a predisposing factor to slope movements: an introduction

Recognition of weathering as a primary process in the development of landforms dates back at least to the late 19th and early 20th century, as indicated by several contributions from various countries (e.g. Cortese 1895; Branner 1896; Falconer 1911; Jutson 1914; Walther 1915); nevertheless, it was only in the second half of the last century that systematic studies on weathering and related topics such as geomorphology, engineering geology and petrography were carried out. In a recent review paper Ehlen (2005) found through an on-line search more than 9000 hits where the term ‘weathering’ was used, dating from the mid-1950s to the beginning of the 21st century. Such a huge number of citations clearly demonstrates the attention that the scientific community has dedicated to the in-place breakdown of rocks by chemical, physical and biological processes. However, despite the high frequency of landslides and erosional phenomena in weathered materials, and the damage and casualties they repeatedly cause, not very much is known about the direct and indirect relationships between weathering and slope movements. The matter is further complicated by the high variability of landslide features (Working Party on World Landslide Inventory 1993). According to the local conditions, a variety of slope movements may take place in the weathered rock masses. Shallow soil slips evolving to rapid and catastrophic debris flows are probably the most common type in steep residual soil slopes, whereas thicker failures, also including deep-seated complex movements, can affect large volumes of weathered rocks. Following the first, simplified attempts to classify slides in natural slopes of residual soils (e.g. Morgenstern & de Matos 1975; Vargas & Pichler 1975), Durgin (1977) provided a comprehensive scheme of the relationships existing between landslides and weathering (Table 1). In general terms, studies on weathering-related landslides showed that in the …