Triaxial Test Specimens Research Articles

A Laboratory-Scale Evaluation of Smart Pebble Sensors Embedded in Geomaterials.

This paper introduces a novel approach to measure deformations in geomaterials using the recently developed 'Smart Pebble' sensors. Smart Pebbles were included in triaxial test specimens of unbound aggregates stabilized with geogrids. The sensors are equipped with an aggregate particle/position tracking algorithm that can manage uncertainty arising due to signal noise and random walk effects. Two Smart Pebbles were placed in each test specimen, one at specimen's mid-height, where a geogrid was installed in the mechanically stabilized specimen, and one towards the top of the specimen. Even with simple raw data processing, the trends on linear vertical acceleration indicated the ability of Smart Pebbles to assess the geomaterial configuration and applied stress states. Employing a Kalman filter-based algorithm, the Smart Pebble position coordinates were tracked during testing. The specimen's resilient deformations were simultaneously recorded. bender element shear wave transducer pairs were also installed on the specimens to further validate the Smart Pebble small-strain responses. The results indicate a close agreement between the BE sensors and Smart Pebbles estimates towards local stiffness enhancement quantification in the geogrid specimen. The study findings confirm the viability of using the Smart Pebbles in describing the resilient behavior of an aggregate material under repeated loading.

Triaxial Shear Test for Strength Behavior of Saturated Sand–Steel Interface Based on Preformed Failure Plane

Abstract An innovative test method for analyzing the strength characteristics of a saturated sand–steel interface by triaxial apparatus is presented. A two-part triaxial test specimen was utilized incorporating a preformed interface along which surface sliding took place. The triaxial apparatus is used to control the drainage state and stress path of the specimen to implement expected interface boundary condition. Four steel specimens with various surface roughness (R1 to R4, smooth to noticeably rough) were used to analyze the effect of surface roughness on the sand–steel interface shear behavior. Typical drained and undrained interface behavior are demonstrated, and the influence of boundary conditions, initial confining pressure, and roughness on drained sand-steel interface are discussed. Results show that the strength of specimen with preformed failure plane is less than that of pure sand specimen. The strain hardening occurs in R1, R2, and R3 roughness during the tests, but the deviator stress has the tendency to decrease after reaching the turning point in R4 roughness. In addition, the normalized efficiency parameter E and the interface friction angle φint increased approximately linearly with Ra in a semi-logarithmic scale. Comparison with direct interface shear tests indicates that friction angles obtained by triaxial tests are only slightly higher than those derived from direct shear tests, implying that the triaxial interface shear test can reasonably obtain the index of the saturated soil–steel interface shear strength.

Total Volume Change Measurement of Unsaturated Soil in Triaxial Tests: A New Approach

Abstract In the evaluation of unsaturated soil behavior in the laboratory, it is required to measure the total volume change of triaxial soil specimens. This, however, is not an easy task, with setups proposed in the literature being quite expensive or presenting variable setbacks. In this context, this article presents a new, reliable, cheap system to estimate the total volume change of the soil in unsaturated conditions in triaxial tests. To do so, a double-chamber triaxial cell was developed, with the inner cell connected to an air-tight container subjected to the same cell pressure and positioned on a precision analytical scale. The measure of the total volume change is given by the mass of water entering or leaving the air-tight container. After calibration, the system was tested under consolidation using saturated soil samples. Comparing the conventional change in water volume of the saturated specimen with the variation of the total volume obtained through the developed system, a maximum difference of 0.01 cm3 is obtained, which equals only 0.01 % of the specimen volume. The effects of temperature and of cell pressure variation, which might occur during the saturation of unsaturated specimens in conventional triaxial tests, are also presented and discussed.

Strength-weakening effect and shear-tension failure mode transformation mechanism of rockburst for fine-grained granite under triaxial unloading compression

The occurrence of rockburst is closely related to initial high stress state and excavation unloading effects. To investigate the effects of three-dimensional (3D) stress state and unloading rate on the rockburst mechanism, triaxial unloading compression tests were performed on cubic fine-grained granite specimens (50 mm × 50 mm × 50 mm). Two unloading rates (0.08 and 40 MPa/s) were employed in the tests with seven initial 3D confining pressures (5, 10, 20, 30, 40, 50, and 60 MPa). For comparison, triaxial and biaxial compression tests were conducted at the same initial confining pressures. The results show that, under the same confining pressures, the variations in peak strength of granite specimens are in the following order: triaxial compression test > biaxial compression test > triaxial unloading compression test under low unloading rate > triaxial unloading compression test under high unloading rate, which indicating that unloading induces an obvious strength-weakening effect on fine-grained granite. In the triaxial unloading compression test with the same unloading rate, a higher confining pressure results in a greater rock bearing strength and stress drop. More energy will be stored in the rock, and the rockburst intensity will be more serious when the rock is destroyed. When the confining pressure is constant, the lower unloading rate is beneficial for improving the rock bearing strength, and more elastic energy will be accumulated and stored in the rock. Once rockburst occurs, the energy released by rock is larger and the degree of rockburst is stronger. The failure mode of fine-grained granite specimens in triaxial unloading compression tests transforms from shear-tension failure to tension failure with increasing confining pressure, which is the main reason for the weakening of rock strength. Establishing a strength criterion for 3D rock considering the unloading effect is a key for understanding the mechanism of rockburst.

Experimental investigation on rockburst behavior of the rock-coal-bolt specimen under different stress conditions

Coal and rock burst are one of the main dynamic disasters that affect coal mine production. In this paper, the burst structural model of the rock-coal-bolt (RCB) system and the burst tendency criterion are established on the background of deep thin coal seam mining. Uniaxial and triaxial mechanical tests under different stress states are carried out on RCB specimens with different angles. Combined with thermal imaging, the mechanical behavior of the inclined RCB specimen under uniaxial loading is discussed. The results show that the burst tendency of the RCB specimen increases with the angle. The stress-strain curves of some uniaxial and triaxial test specimens show two or more peaks, and the thermal imaging evolutionary process shows that the cracks of the coal and rock develop from shear to tension shear cracks. There is a further development of fracture and energy accumulation between the first and second peaks in the stress-strain curve of the specimen. Therefore, the failure degree of the second peak of the specimen may be stronger than that of the first peak. Additionally, the established stiffness coefficient and burst energy index can better describe the burst tendency of the RCB specimen under different stress states. The results show that the burst tendency of the RCB specimen under the triaxial test is much higher than that of the uniaxial test. In other words, it also explains that the essence of the burst failure of the surrounding rock in the roadway is the initial instability induced by the inside surrounding rock in the roadway. Moreover, the burst tendency is the largest when the rock and coal combination angle is 15°, and the burst damage range may also be increased by the failure of internal coal and rock mass.

Stress-strain Relationship in Homogeneous and Two-layered Triaxial Test Specimens

The stress-strain relationship of a homogeneous specimen, obtained from triaxial compression test, allows to determine stiffness parameters for numerical-method based analyses in common geotechnical software. Stiffness parameters are defined as the ratio of stress to strain along an axis. However, when a heterogeneous specimen is tested, the equivalent elastic modulus that represents a simplification of the nonlinear behavior is complex. This paper presents a study intended to contribute to the debate about the degree to which conventional soil mechanics approaches can be applied to layered specimens. Triaxial compression tests were carried out on both homogeneous and two-layered specimens under a low effective confining pressure of 30 kPa. The triaxial apparatus was chosen since the applied stress and specimen boundary conditions are well defined, and the repeatability of the test method is good. The behavior of both specimens was studied in terms of the stress-strain relationship and stiffness. The main differences were crucial to understanding the composite soil-aggregate interaction, which is discussed and compared. The results indicate that the interface between composite soil and aggregate is important to keep the stability of the layer of aggregate over the soft composite soil, and practical methods of achieving that are suggested.

Experimental Study on Characteristics and Mechanism of Macrography and Mesoscopic Failure of Deep Granite from Beishan

Because of its good stability, high strength and low permeability, granite is considered as an ideal surrounding rock for geological disposal of high-level radioactive waste, which is also the main candidate surrounding rock for high-level radioactive waste disposal project in China. Rock mechanics is one of the important problems for site selection, design, construction and operation of geological repository. It is of great theoretical and practical value to study the failure law of rock from the microcosmic aspect, to link the microcosmic failure morphology characteristics with the mechanical mechanism of rock macro fracture failure, and finally to establish a bridge between microcosmic failure mechanism and macro fracture analysis of rock. We carried out uniaxial compression test, triaxial compression test under different confining pressures, Brazilian splitting test and direct tensile test for the deep granite from the core of No. BS28 hole in Beishan area with depth of 560–680 m. SEM and EDS was carried out for the fracture surface under different test conditions. The macro and micro failure characteristics of granite under different test conditions are analyzed: It is basically the splitting failure phenomenon dominated by tensile failure after the uniaxial compression test, while the failure of the triaxial test specimen are basically all form a large inclined failure surface which show the shear failure phenomenon. It shows obvious brittle tensile failure under direct tensile test, while not simply tensile failure under Brazil split test. The micro structure and mineral composition of rocks are closely related to their macro mechanical properties. Due to the existence of other impurities, Beishan granite has a certain degree of discreteness, and because of the different composition, its fracture presents different forms. These studies provide the necessary basis for the design and construction of the geological disposal of high-level radioactive waste.

Effect of strain level on strength evaluation of date palm fiber-reinforced sand

Conventional researches on the behavior of fiber-reinforced and unreinforced soils often investigated the failure point. In this study, a concept is proposed in the comparison of the fiber-reinforced with unreinforced sand, by estimating the strength and strength ratio at different levels of strain. A comprehensive program of laboratory drained triaxial compression test was performed on compacted sand specimens, with and without date palm fiber. The fiber inclusion used in triaxial test specimens was form 0.25%-1.0% of the sand dry weight. The effect of the fiber inclusion and confining pressure at 0.5%, 1.0%, 1.5%, 3.0%, 6.0%, 9.0%, 12%, and 15% of the imposed strain levels on the specimen were considered and described. The results showed that, the trend and magnitude of the strength ratio is different for various strain levels. It also implies that, using failure strength from peak point or the strength corresponding to the axial strain of approximately 15% for evaluating the enhancement of strength or strength ratio, due to the reinforcement, may cause hazard and uncertainty in practical design. Therefore, it is necessary to consider the strength of fiber-reinforced specimen at the imposed strain level, compared to the unreinforced specimen.

Triaxial Compression Test Results on DMC Samples Prepared by Using Different Soil Types

In this study, the effects of cement, fly ash and super plasticizer, improver materials by adding into the grout, on soil-binder mixing columns called as deep mixing columns (DMC) are investigated. Fly ash is a waste product emerging from burning in the thermal power plant. By evaluating waste materials in this way, environmental pollution will be reduced directly and cement usage and carbon emissions caused by cement production will be reduced indirectly. By using super plasticizer into the grout, less porous and permeable elements with more structural strength will be manufactured. In order to achieve these goals, an experimental program was developed using statistical and experimental design methods. It is desired to determine the optimum grout quantity and consistency required to maximize the strength of the column manufactured with DMM in silty soils and silty sands. For this purpose, the amounts of fly ash (0-40%), cement (3-11%), super plasticizer additive (0.5-2%) and water/binder percentage (0.5-1.25%) were chosen as variables to form grouting material. Experimental studies have been carried out using Taguchi method, which is a powerful optimization technique, using 5-parameter and 4-level L16 design table. Undrained-unconsolidated triaxial test specimens were prepared in PVC tubes with diameter of 47 mm and length of 100 mm for each design for curing time of 28 days. As a result of the experiments, deviator stress of the soil-binder mixture was found for each design.

Horizontal stiffness evaluation of geogrid-stabilized aggregate using shear wave transducers

Lateral restraint resulting from the interlock between geogrid and aggregate is recognized as a primary mechanism governing the load-bearing behavior of a geogrid-stabilized pavement base course. However, the level of geogrid–aggregate interlock and the local stiffness enhancement due to the lateral restraint has not been adequately quantified. In this paper, a new experimental method is proposed to evaluate the stiffness enhancement provided by the interlock of the geogrid–aggregate composite system using shear wave transducers. Repeated load triaxial tests were conducted to determine the resilient modulus and deformation characteristics of both geogrid-stabilized and unstabilized base course aggregates. The stabilized test specimens were evaluated for two geogrid types with rectangular and triangular apertures. For the shear wave measurements, three pairs of bender elements fixed at each mounting base were installed diametrically on the triaxial test specimens at three different locations above the mid-height level, where the horizontal shear modulus profiles of the geogrid-stabilized and unstabilized specimens were determined. The experimental results indicate that the shear modulus profiles obtained as a function of confinement changed significantly based on the geogrid inclusion and type, whereas there were no considerable changes in the resilient moduli from the different specimens, as they were only influenced by the applied stress states. The shear moduli estimated in the vicinity of the geogrid were greater than those at locations farther away from the geogrid, which was installed at the mid-height of the specimen. The shear modulus profiles varied according to the confining stress, and the shear modulus ratio of the stabilized to unstabilized specimens clearly demonstrated the stiffness enhancement provided by the two different geogrids. Accordingly, the shear modulus profiles estimated from the horizontal shear wave measurements of the bender element can be effectively used to determine the mechanically stabilized layer characteristics of a geogrid, and therefore quantify the local stiffness enhancement provided by the geogrid–aggregate interlock.

Shear-Induced Failure and Static Liquefaction in Reinforced Soil and Triaxial Tests

In the Federal Highway Administration (FHWA) laboratory, a phenomenon was identified that reduces soil bearing capacity. This phenomenon arises in common situations, for example, reinforced soil, triaxial tests, mat foundations on sand over bedrock, and other stratified media. In reinforced soil, this reduction is based on reinforcement spacing S and soil particle diameter D. This study provided a generic derivation showing that Coulomb’s failure criterion is triggered by shear stress associated with not only S and D, but also the other dimensions of a soil structure. Although identified and validated in experiments, the S:D instability was verified by finite-element analysis when the number of elements equals the number of soil particles. Static liquefaction was identified with instability and collapse of force chains. Although analysis was performed in the idealized context of unfaced soil structures, FHWA test data demonstrated violent effects on faced structures when the S:D stability limit was exceeded. For narrow structures, this study also evaluated the S:B instability, where B is the breadth of the reinforcement, triaxial test specimen, or foundation mat.

Boundary effects in discrete element method modeling of undrained cyclic triaxial and simple shear element tests

Cyclic triaxial and simple shear tests are both commonly used to study the behavior of granular assemblies subjected to dynamic loading. These two stress paths have also been simulated with the discrete element method (DEM) using varying boundary conditions. In this work, stiff boundaries and periodic boundaries were implemented in DEM simulations of constant volume cyclic triaxial and simple shear tests. The model response was analyzed at the specimen scale by comparing stress paths, decrease in mean effective stress, and number of cycles to failure across simulations. Particle scale analyses, including quantification of the contact force network, entropy of the local void ratio distribution, and spatial variability in the distribution of void ratio, contact number, and normal contact force are used to provide insight into model response. Simulation results imply that periodic boundaries are more appropriate for consolidating specimens to a homogenous state and minimizing boundary effects during cyclic loading. Significant uneven “squeezing” where particles are pinched between the horizontal and lateral plates in the corners of the specimen with stiff boundaries was observed in the cyclic simple shear test. After liquefaction initiation, increased entropy indicated a more disordered particle assembly in all cases. In addition, specimens in cyclic simple shear tests and cyclic triaxial tests behave significantly differently at the microscale. With the same level of deviatoric shear strain amplitude, the simple shear specimens liquefied in fewer cycles than the triaxial specimens. Cyclic triaxial tests are not able to produce pure shear waves, while cyclic simple shear tests do, micromechanically indicating the better suitability of cyclic simple shear tests to simulate shear wave induced liquefaction. The findings from this work will have implications for the future modeling of cyclic element tests using DEM.

Bender Elements Successfully Quantified Stiffness Enhancement Provided by Geogrid–Aggregate Interlock

Lateral restraint is a primary mechanism of geogrid base reinforcement contributing to the performance improvement of flexible pavements, and the interlocking between the geogrid and aggregate is responsible for stiffness enhancement in a zone formed around the geogrid. A novel application of bender elements is introduced: as shear wave transducers for quantifying local stiffness increase in the vicinity of a geogrid. Several triaxial test specimens of a dense-graded granite-type aggregate were prepared at two moisture contents for resilient modulus testing. Reinforced specimens also included a punched and drawn geogrid piece placed at specimen midheight. Two pairs of bender elements installed on the membrane at two heights enabled measurement of shear waves horizontally across the specimen. Shear wave velocities and axial resilient strains were recorded under the applied stress states. The test results show that the resilient modulus of the reinforced specimen was similar to that of the unreinforced one tested at the same moisture content. In contrast, the shear moduli obtained at midheight of the reinforced specimen were always greater than those obtained from the unreinforced specimen, indicating a local stiffness increase in the vicinity of the geogrid. In the reinforced specimens, the shear moduli obtained near the upper end were always less than those obtained from the specimen midheight. The small-strain shear modulus determination by bender elements was effective for evaluating the stiffness enhancement provided by geogrid–aggregate interlock.

A behavioural framework for fibre-reinforced gravel

The mechanical behaviour of granular materials is known to be influenced by the addition of fibres. However, most previous research has been carried out on materials with relatively small grains (sands), and its application to larger grains is not well documented. This paper reports an investigation into the mechanical behaviour of a fibre-reinforced granular material with a relatively large grain size corresponding to one-third and one-fifth scale railway ballast. The investigation was carried out by means of triaxial tests incorporating a full-field, image-based deformation measurement technique; this enabled detailed observations to be made of each triaxial test specimen during shearing. The test data demonstrate the benefits of random fibre reinforcement for aggregates that have a relatively large grain size. Analysis taking into account the effect of fibre tension on the effective stresses experienced by the granular skeleton provides new insights into the mechanisms of reinforcement in larger sized granular materials.

Recognition of the Stress-Strain Curve Based on the Local Deformation Measurement of Soil Specimens in the Triaxial Test

Abstract A digital image technique was developed and used to measure the deformation distribution over the entire surface of soil specimens in triaxial tests. The measured deformation process shows that the specimen apparently exhibits three states during the test, i.e., pre-failure, in-failure, and post-failure, in correspondence to the different features of its deformation. The deformation feature in each state is then analyzed by the displacement and strain contours. Additionally, the stress level, S, is calculated to determine whether the soil is in failure at a point (representative element volume (REV)) on the surface of the specimen. Next, the failure zone, namely the shear band, was considered to be enveloped by the curve defined by S = 1 on the stress-level contour map. The stress level is calculated based on the strain, Young's modulus and Poisson's ratio. According to analysis of the deformation feature and the failure process, we recognized the following properties: (i) the deformation of the specimen was approximately uniform in the pre-failure state; (ii) failure occurred from a point and developed gradually until the shear band cleaves the specimen; (iii) in the post-failure state, deformation was exclusively due to the blocks of the specimen sliding along the shear band; (iv) the deformation feature in the shear band was quite different from that outside the shear band. In conclusion, the stress-strain curve of the specimen revealed a structural response, not an elementary response, especially in the in-failure and post-failure states, in which the deformation features of different corner points were different, and the observed deformation for the entire specimen may be the combination of local deformations. Therefore, it is not appropriate to build the constitutive model for soil according to the stress-strain curves of the entire deformation process and to take the specimen as a uniform element in the entire process.

Unloading Elastic Behavior of Sand in Cyclic Triaxial Tests

In this paper, digital image technology was used in the surface-deformation measurement of soil, making it possible to monitor the axial, radial, and volumetric deformations and the deformation distribution over the entire surface of specimens in triaxial tests. A series of cyclic triaxial tests on silica powder and Fujian sand were conducted to investigate the stress-strain behavior of sand in pre-failure deformation, particularly in the unloading elastic behavior. The sand gradually tended toward the stable state of deformation after it was subjected to a certain number of cyclic loadings. The sand appeared to be entirely nonlinear elastic, with unchanged and closed hysteresis loops of the stress-strain curve. In addition, the elastic parameters—Young's modulus and Poisson's ratio—of the specimens during the unloading process of the stable state of deformation were carefully studied. The test results suggested that the unloading tangent Young's modulus of sand was determined by the stress state and void ratio but was not affected by the stress amplitude of cyclic loading. However, Young's modulus depended more definitely on the axial stress than on the lateral stress when the deviatoric stress increment was applied in the triaxial stress condition. Moreover, the unloading average Poisson's ratio was dependent on the stress state and void ratio.

Direct and indirect observations of local deformation properties of saturated sand specimens in undrained cyclic triaxial tests

Direct and indirect observations of local deformation properties of saturated sand specimens in undrained cyclic triaxial tests

Using Blends of Construction & Demolition Waste Materials and Recycled Clay Masonry Brick in Pavement

In Egypt and many developing countries, the use of Construction & Demolition (C&D) waste and Recycled Clay Masonry (RCM) brick materials as Unbound Granular Materials (UGMs) in pavement construction is generally very limited. There has been reluctance to adopt recycled materials in pavement construction. Despite the fact that there are allocated land fill areas for dumping this kind of waste, in Egypt C&D waste is usually dumped on the sides of canals, minor roads and at the entrance of the cities and towns. This causes environmental problems and affects the daily lives of people. Therefore, blends of C&D waste materials with RCM were evaluated for use as UGMs. The C&D waste and RCM materials were obtained from some dumps on the sides of roads and canals around Mansoura City, Egypt. The collected materials were crushed and mixed in the laboratory. The proportions of the C&D/RCM blends were 100/0, 90/10, 80/20, 70/30, 55/45, 40/60, 20/80, and 0/100% of the total aggregate mass. The characterization of C&D/RCM blends as well as the response to cyclic loading by Repeated Load Triaxial Testing (RLTT) was covered in this research paper. Static triaxial shear tests and RLTT were conducted using a Universal Testing Machine. RLTT and static triaxial shear test specimens were prepared at the optimum moisture content and compacted according to modified Proctor. RLTT were performed in accordance with the AASHTO T307 on triplicate specimens. RCM materials (100%) as UGMs failed to meet the requirements of Los Angeles Abrasion (LAA) test of 50% maximum as specified by the Egyptian Code of Practice (ECP). RLTT data showed that all C&D/RCM blends yielded high resilient moduli however, no significant variation in resilient moduli values were observed between the different C&D/RCM blends.

Reply to the discussion by Mesri and Wang on “Test fill on soft plastic marine clay at Onsøy, Norway”

The author is grateful for the clarification given in the discussion regarding the Cc and C values. The comparison of the Ko-values from the empirical equations referred to in the Discussion with the hydraulic fracturing Ko-value given in the article is interesting, although it is perhaps a bit unusual to take ′-values from DSS tests, where ′ is the friction angle, when triaxial test values are available. The 24 h virgin compression line for the CRS oedometer tests was determined by drawing a line parallel with the CRS virgin compression line (stress in log scale) through the end point where the stress has been kept constant overnight. I agree that the 24 h void ratio – effective vertical stress relationship for 20 mm thick oedometer specimens with a maximum drainage path of 20mmcannot be expected to be exactly the same as the e– v ′ relationship for a 110 mm high triaxial test specimen with a maximum drainage path of 55 mm, but in my opinion the difference should not be very pronounced. Equation (D3) in the Discussion is the same one used at the Norwegian Geotechnical Institute (NGI) to compute the coefficient of permeability, k, fromCRS oedometer tests. In the paper by Sandbaekken et al. (1986) this equation is referred to as originating from Wissa et al. (1971). The k-values calculated for a particular CRS oedometer test are found to group along a straight line (the k-line) when plotting logk versus vertical strain, 1, for the virgin part of the stress–strain data. The calculated k-values for stresses below the apparent preconsolidation stress, pc ′, are usually significantly higher than corresponding to the values from the k-line. A possible reason for this may be that CRS oedometer tests at NGI are carried out without back pressure. The k-values from the k-line usually agree well with constant-head permeability tests when performed at stresses above pc ′, and are therefore normally used to calculate cv-values, also below pc ′. To perform permeability tests below pc ′ in the oedometer cell is considered to be a bit risky due to the possibility of getting by-pass of water between the stiff oedometer ring and the specimen in this stress region. For the Onsoy clay with pores with humus components and black spots (indicating high concentrations of iron–sulfur in the spots) down to a depth of about 8 to 10 m, it was suspected that the k-line values from CRS tests could be too low for stresses below pc ′. Constant-head permeability tests on triaxial test specimens with an initial height mostly about 110 mm (initial diameter mostly about 54mm) with back pressure about 300 kPa were therefore carried out. The permeability tests on the triaxial test specimens could be performed at effective stresses below pc ′ because no fluid by-pass can take place due to the soft lateral boundary in these tests. The constant-head permeability values measured on triaxial test specimens, shown in Fig. 20 and especially in Fig. 21 in the paper under discussion, are seen to be significantly higher than the oedometer k-line values. However, the difference between the two sets of k-values varies considerably; k-values back-calculated from fieldmeasurements were therefore used to correct the k-line values when estimating rate of settlement in the field.

Strength and dilation of jointed granite specimens in servo-controlled triaxial tests

We investigated the complete stress–strain response of artificially jointed granitic rock specimens with two joint sets. A series of confined compressive tests on these jointed rock specimens were performed in a fully servo-controlled press. Elastic, strength and post-failure parameters were calculated and analysed, with special attention paid to the dilation angle. The results were compared to those for corresponding intact specimens investigated in a previous study. Although the peak strength of the jointed cores was much lower than that for the intact cores, the residual strength envelopes were practically equal. The apparent elastic and drop moduli tended to be lower for jointed specimens than that for intact specimens. At low confining stress levels, the jointed rock tended to dilate less than the intact rock, but at higher confinement dilation behaviour converged. Another finding with respect to post-yield behaviour was that the dilation angle of the jointed specimens could adequately be represented by an existing plastic-shear–strain and confinement-dependent model developed for intact specimens.