Peak Strength Parameters Research Articles

Study on the Strength Characteristics of Columnar Jointed Basalt with a True Triaxial Apparatus at the Baihetan Hydropower Station

Columnar jointed basalt is widely distributed in the dam foundation and underground cavern of the Baihetan hydropower station, in which columnar jointing and microfractures are developed, the integrity of the rock mass is poor, and the material easily relaxes after excavation. Therefore, it is important to obtain the real mechanical parameters of columnar jointed basalt in rock mechanics problems. In the test cave at the dam site, six in situ samples with a size of 50 cm × 50 cm × 100 cm were successfully made, and in situ true triaxial test were carried out. This paper presents a method for obtaining the strength parameters of the Mohr–Coulomb criterion and Hoek–Brown criterion by true triaxial testing. The peak strength parameters under three states and yield strength parameters under one state were obtained by setting various stress paths for the undisturbed samples and failure samples. The strength characteristics of columnar joint basalt and the crack characteristics of the specimens are analyzed, and the results of the triaxial test and direct shear test are compared. By means of acoustic emission (AE) techniques and surface crack descriptions, the failure process and failure pattern of the samples were obtained. The abundant test results provide strong support for the selection of engineering rock mechanics parameters.

Analysis of Tunnel Support Requirements Using Deterministic and Probabilistic Approaches in Average Quality Rock Mass

The support design for rock tunnels involves a complex problem because of the various types of uncertainties present in rock mass properties; therefore, probabilistic approaches are used to consider these uncertainties systematically in the presented analysis. In earlier studies, uncertainties in peak strength parameters and deformation modulus were considered while uncertainty in residual strength parameters was neglected. Another important factor is the assumption regarding the postpeak behavior of the rock mass, which depends on the rock mass quality in the field. An average quality rock mass generally shows strain-softening behavior in a range from peak to residual strength, which generally has been neglected because of the assumption that the rock mass is elastic-perfectly plastic. However, it was observed that the yield zone depth and displacements around the tunnel are highly sensitive to residual strength parameters, and residual strength parameters can considerably influence the estimation of the support requirements of a tunnel. In this paper, a new computational approach, based on the geological strength index (GSI) and the use of deterministic and probabilistic methods, is described for the reinforcement design of a tunnel in an average quality rock mass. This approach considers the variability in residual strength parameters along with peak strength parameters and the deformation modulus. Moreover, the strength drop in the stress-strain behavior of the rock mass is also considered in the analysis. An underground powerhouse cavern from the Himalayan region of India is taken as the case study to show the methodology used in the research. The study brings out the advantages of the probabilistic over the deterministic approach for the support design of a tunnel. Another important conclusion from the study is that the support requirements for the tunnel are greatly influenced by residual strength parameters, and therefore, uncertainty and strength drop in the residual strength parameters should be properly considered when designing support for tunnels.

SEISMIC RISK EVALUATION OF IRRIGATION TANKS: A CASE STUDY IN IBIGAWA-CHO, GIFU PREFECTURE, JAPAN

Landslides are one of earth’s most serious types of natural disasters and they are often inducedby earthquakes. Predicting the approximate landslide displacements caused by earthquakes is helpful forassessing earthquake hazards and for designing slopes to withstand future seismic shaking. In this research, theauthors firstly focus on obtaining the residual strength parameters and the peak strength parameters bymultistage procedure with the Bromhead ring shear apparatus, which are used in the slope stability analysis.The maximum vertical settlement (MVS) of a slope is determined by the Newmark method using representativedata from three earthquakes, i.e., Great Hanshin Earthquake, Chuetsu Earthquake and Iwate-Miyagi NairikuEarthquake. It is experimentally demonstrated that the MVS is very large in both Tanigumi Pond and SugoPond. Once a great earthquake occurs, the progressive failure phenomenon appears and people living in thearea will be in danger. The results presented in this paper can be applied to future landslide hazard assessmentsand used to promote reconstruction efforts by the government in the affected areas and for future disasterprevention and mitigation.

Probabilistic analysis of tunnels considering uncertainty in peak and post-peak strength parameters

Stability analysis of rock tunnels is a complex problem because of various types of uncertainties present in the rock mass properties and hence probabilistic approaches are used to systematically consider these uncertainties in the analysis. While the uncertainty in deformation modulus and peak strength parameters has been considered previously it has been observed that the uncertainty in post-peak strength parameters is generally neglected in earlier studies. Post-peak strength parameters are among the most important factors which significantly influence the plastic zone development and displacements around the tunnel and hence neglecting uncertainty in post-peak strength parameters is not appropriate. In the current study, a quantitative approach based on the Geological Strength Index (GSI) has been used to estimate the uncertainty in peak strength, residual strength and deformation parameters. Then the uncertainty in yield zone and displacement around the tunnel is estimated using Hong’s point estimate method coupled with finite element method. The approach was used to estimate the displacements around tunnels of different shapes in three case studies in average quality rock mass and the predicted displacements were compared with the in-situ measurements. A comparison is provided with the generally adopted conventional probabilistic approach in which uncertainty in peak strength parameters and deformation modulus is considered and rock mass is assumed as elastic-perfectly plastic. It was observed that predicted displacements were matching well with the measured displacements for all the tunnels by considering uncertainty in residual strength parameters while displacements were underestimated when the conventional approach was used. A parametric study was conducted to estimate the influence of Coefficient of Variation (COV) of different parameters on the plastic zone development and displacements of rock tunnel by selecting one of the case studies. It was observed that COV of yield zone depth and displacement was varying with the COV of the intact rock properties and thus influencing the probability of failure.

Impact of highly weathered geology on pipe-jacking forces

For the Kuching Wastewater Management System Phase 1 project in Kuching, Malaysia, 7·7 km of trunk sewer lines were constructed in the highly fractured, highly weathered Tuang Formation using a pipe-jacking method. The pipelines were founded at depths of up to 35 m below Kuching City, where the majority of the pipe-jacking activities would traverse the Tuang Formation. Jacking forces in highly fractured geology are not well understood as most jacking force models were derived for drives traversing soils. Therefore, a novel method was developed, whereby equivalent rock strength characteristics were interpreted from direct shear testing on reconstituted tunnelling rock spoils. Tangential peak strength parameters, [Formula: see text] and [Formula: see text], were developed from the nonlinear behaviour of the reconstituted spoils and applied to a well-established jacking model to assess arching and development of jacking forces from four documented drives. The back-analysed parameters μ avg and σ EV were used to demonstrate that geology had significantly affected the development of jacking forces. The back-analysis of the studied drives was successfully validated through finite-element modelling. This research shows that the developed parameters [Formula: see text] and [Formula: see text] and the back-analysed parameters μ avg and σ EV can be reliably used to predict jacking forces in highly fractured, highly weathered geology.

Negative aging of granular materials

Background: Use of abundantly available soft rocks for embankment construction is continuously increasing due to economic reasons. A detailed knowledge on time-dependent deterioration (negative aging) of such weak geomaterial becomes essential for better serviceability of geotechnical structures. Methodology: To explore possible effects of deterioration of soil grains on mechanical properties, a series of consolidated drained torsional shear tests were performed under saturated and dry conditions on crushed soft rocks from Japan and Pakistan. Results: Test results indicated that soils under dry conditions with intact grains exhibited stiffer stress-strain behavior, higher peak stress ratios, and strain softening response after peak shear stress. On the contrary, soils undergoing water-induced deterioration of grain showed enormous loss of strength upon submergence. Mohr-Coulomb failure envelopes elucidated large effects of saturation-induced disintegration of grains on effective angle of internal friction. In addition, peak strength parameters seemed to be relatively more vulnerable to saturated conditions as compared to residual strengths. Conclusion: It is concluded that the observed soil behavior can be crucial to embankments and structural foundations constructed on granular materials obtained from crushed soft rocks.

Study of Post-Peak Stress-Strain Relationship of Rock Mass with Joint

The post-peak stress strain relationship expression of intact rock cannot well reflect post peak properties of rock masses with joint, in order to obtain post peak stress-strain relationship suitable for rock masses with joint, based on the test data, based on the Kulun strength criterion, analysis evolution of post peak strength parameters of rock masses with joint. According to rock masses with joint under different confining pressure and fracture dip at different post peak behavior, the post peak stress-strain relationship is simplified as new kind type, regarding maximum principal strain as strain softening parameter, on the assumption cohesion and internal friction angle are the piecewise linear functions of maximum principal strain, the method of solving express of new post-peak stress-strain relationship of rock masses with joint is obtained.

Discussion of “Modification to Translational Failure Analysis of Landfills Incorporating Seismicity” by Xuede Qian and Robert M. Koerner

Discussion of “Modification to Translational Failure Analysis of Landfills Incorporating Seismicity” by Xuede Qian and Robert M. Koerner

Different Approaches for Simulating Brittle Failure in Two Hard Rock Mass Cases: A Parametric Study

One of the aims of rock mechanics analysis is to predict fallouts in underground excavations. The objective of this paper was to study the relative importance of different strength parameters and their significance on the simulation of brittle failure and fallouts. This work was conducted as a parametric study, using numerical modelling and a number of approaches. The results were compared with observed fallouts. More obvious and distinct shear bands could be observed with decreased element sizes close to, and at, the boundary. The maximum shear strain was the most reliable indicator for fallout prediction. The results of the (instantaneous) cohesion softening friction softening models were sensitive to changes of the peak strength parameters and less sensitive to variations in residual parameters. The result from the cohesion-softening friction-hardening (CSFH) model, when using a peak cohesion equal to the intact rock strength, best captured the observed rock behaviour.

Determination of residual strength parameters of jointed rock masses using the GSI system

The Geological Strength Index (GSI) system, proposed in 1995, is now widely used for the estimation of the rock mass strength and the rock mass deformation parameters. The GSI system concentrates on the description of two factors, rock structure and block surface conditions. The guidelines given by the GSI system are for the estimation of the peak strength parameters of jointed rock masses. There are no guidelines given by the GSI, or by any other system, for the estimation of the rock mass’ residual strength that yield consistent results. In this paper, a method is proposed to extend the GSI system for the estimation of a rock mass's residual strength. It is proposed to adjust the peak GSI to the residual GSI r value based on the two major controlling factors in the GSI system—the residual block volume V b r and the residual joint condition factor J c r . Methods to estimate the residual block volume and joint condition factor are presented. The proposed method for the estimation of rock mass's residual strength is validated using in-situ block shear test data from three large-scale cavern construction sites and data from a back-analysis of rock slopes. The estimated residual strengths, calculated using the reduced residual GSI r value, are found to be in good agreement with field test or back-analyzed data.

Mechanical Tests on Pervasively Jointed Rock Material: Insight into Rock Mass Behaviour

In the exploration phase for the design of an underground cavern in a limestone formation a large number of triaxial compression tests were carried out on laboratory specimens which were characterized by a variable degree of fracturing. The data were analyzed to investigate the influence of fracture intensity and of confining stress on the mechanical parameters. In particular, investigations focused on the relationships between the parameters of the strength criteria, respectively in residual and peak conditions, on the decay of the Young modulus with stress level in the prepeak phase of the test, and on the brittleness of the rock in the postpeak phase. The tested rock can be considered as a small-scale model of a jointed rock mass, and the laboratory data therefore provide useful insight into the mechanical behaviour of rock masses, especially the relationships between residual and peak strength parameters, which are required in many analytical models and in numerical codes for the analysis of underground excavations.

Solifluction shears at Carsington, Derbyshire

Abstract A clayey solifluction deposit of Devensian age on the lower slopes of a broad valley in Namurian mudstone is about 1.2 m thick and near its base are shear surfaces, gently undulating and occupying on average 40% of the length (56 m) examined in trial pits. Within a shear zone up to 2 mm in thickness the clay mineral aggregates are aligned sub-parallel to the shear surface; otherwise the aggregates are in random orientation. Tests show that strength on the shear surface falls to the residual value (c′ = 0, φ r ′ = 12°) after small displacements. The resultant peak strength parameters of the deposit, allowing for the undulating and discontinuous nature of the shears, are c′ = 6 kPa and φ′ = 18°; as compared to c′ = 10 kPa and φ′ = 20° for the intact clay. Average index properties are w = 40, LL = 75, PL = 32 and clay fraction = 62%.

Bearing Capacity of Footings on Calcareous Sands

ABSTRACT The bearing capacity and compressibility of a calcareous sand were examined by drained triaxial compression tests and laboratory model tests on strip footings 51mm by 305mm. The tests were carried out on two sands—a calcareous sand having a carbonate content of 18% and a clean sand obtained by washing the calcareous sand with Hydrochloric Acid (HC1) and water. All tests were performed at a relative density of 66%. The results indicate increased compressibility and no change in the peak strength parameters due to the presence of carbonates. The bearing capacity of the calcareous sand was smaller than that of the clean silica sand. Various methods of analysis were discussed to account for the effect of compressibility on bearing capacity.

A comparison between theoretical and measured creep profiles of landslides

Abstract Continuous creep contributes to the movements of a landslide in varved clays. The creep models developed in this paper are based on the Bingham rheological behaviour of the soil. In this Bingham model the yield strength value and the viscosity coefficient can be defined in different ways, as was done by Ter Stepanian (1963) and Yen (1969). The yield strength can be defined as a residual strength value for overconsolidated clays (Yen) or as a fixed fraction of the peak strength parameters Cp′ and φp′ (Ter Stepanian), and the viscosity coefficient can be considered as a constant (Yen) or as a flow factor which depends on the effective normal stress (Ter Stepanian). Creep tests, (reversal) direct shear tests and triaxial tests were carried out on varved clay samples in order to determine yield strength values and the viscosity parameter according to different definitions. Four creep models have been evaluated; these are based on four possible combinations of the different definitions of yield strength and the viscosity parameter. Yen's definition of the yield strength in combination with a flow factor which depends on the effective stress (Ter Stepanian) is found to give the best prediction of the creep velocity data that were measured in the field.

沖縄，北丘ハイツ地内泥岩すべりにおける地質構造規制とすべり面強度パラメータの算定

In order to increase the accuracy of stability analysis of the first time slide in the bedrock, it is important to clearly perceive the characteristics of geological structure, the residual strength acting on the slickenside surface of discontinuities, and the peak strength of the fractured mudstone in the slip surface zone.Concerning the landslide of the Neogene mudstone discussed in the present report, it was found that the slip surface has been formed by being strongly controlled by the strikes of bedding and fault planes, cutting through the fractured mudstone. The residual and peak strength parameters obtained from the ring shear test and the triaxial compression test were as follows: cr=7kPa and φr=16° for the slickenside surfaces; cf′=75kPa and φf′=36° for the specimens of mudstones having opening cracks; cf′=95kPa and φf=42° for the mudstones having internal cracks and microcracks. For the stability analysis a new method was introduced to assess the parameters of average shear strength along the slip surface, and applying the strength parameters mentioned above, c′=11kPa and φ′=17.5° were obtained.

Relict joints in completely decomposed volcanics in Hong Kong : Can Geotech J, V19, N2, May 1982, P117–123

In a major cutting in the completely decomposed volcanics in Hong Kong, closely spaced relict joints were found throughout the residual soil. The persistence of the exposed joint surfaces varies from several centimetres to more than 10 m. The joint surfaces, which are generally smooth and sometimes slickensided, are usually coated with a thin black-brown deposit. The origin of this deposit is believed to be the precipitation of iron-manganese decomposition products that have filled the joints in the parent rock during the course of weathering. Chemical analyses carried out on typical specimens have confirmed a high concentration of iron and manganese oxides. Results of laboratory shear strength testing show that the peak strength parameters along joint surfaces and of the intact material are: c′ = 0, and c′ = 6 kPa, respectively. Analyses of joint survey data indicate that relict joint orientations occur in systematic major sets and that joint spacings roughly follow a Poisson distribution.

Relict joints in completely decomposed volcanics in Hong Kong

In a major cutting in the completely decomposed volcanics in Hong Kong, closely spaced relict joints were found throughout the residual soil. The persistence of the exposed joint surfaces varies from several centimetres to more than 10 m. The joint surfaces, which are generally smooth and sometimes slickensided, are usually coated with a thin black-brown deposit. The origin of this deposit is believed to be the precipitation of iron-manganese decomposition products that have filled the joints in the parent rock during the course of weathering. Chemical analyses carried out on typical specimens have confirmed a high concentration of iron and manganese oxides. Results of laboratory shear strength testing show that the peak strength parameters along joint surfaces and of the intact material are: c′ = 0, [Formula: see text] and c′ = 6 kPa, [Formula: see text] respectively. Analyses of joint survey data indicate that relict joint orientations occur in systematic major sets and that joint spacings roughly follow a Poisson distribution.

The Analysis of Two Slope Failures in Cemented Champlain Clays

Detailed investigations have been made on the sites of two landslides which occurred in the sensitive Champlain clay deposits of the Saint-Laurent Lowlands. Laboratory tests made on specimens cut from block samples have led to a better understanding and evaluation of the brittle behavior of these cemented clays. The results have shown that very little strain is required for the strength to decrease from a well defined peak value to a residual value; this brittle behavior is evident only when tests are made at sufficiently low stress level such that the cementation bonds are preserved during consolidation.The analyses of the slides have shown that the use of the peak strength parameters led to a gross overestimate of the factors of safety; the residual strength parameters, however, give a fairly accurate assessment of the stability, the calculated factors of safety being close to unity. That behavior may be explained by the fact that the cemented clay is extremely brittle and, like any other strain softening material, is subjected to the mechanism of progressive failure. This study leads to a very efficient practical approach to the analysis of the stability of natural slopes.