Multistage Triaxial Tests Research Articles

Strength of Weakly Cemented Sands from Drained Multistage Triaxial Tests

Characterizing the strength of weakly cemented and sensitive soils in the laboratory is difficult because of the difficulty in obtaining high-quality replicate samples necessary for defining the failure envelope. Multistage triaxial tests have long been used to reduce the variability caused by testing multiple samples; however, traditional criteria used for transitioning from one loading stage to another often lead to destructuring or failure in sensitive or structured soils. The objective of this paper is to present a methodology for conducting multistage drained triaxial tests on weakly cemented sands and estimating the resulting shear strength parameters. Both multistage and single-stage drained triaxial tests were performed on artificially cemented samples of a silty sand at two levels of densities and cementation. The use of dεv/dεa=0 as a termination criterion to move on to the next stage of loading and εv=0 as the failure criterion for the final stage of the shear resulted in an average error of 6% and 5% in c′ and φ′, respectively, for the stress range considered in this study when compared with parameters obtained from the single-stage drained triaxial tests. Continuous shear wave velocity (Vs) measurements during shear showed that destructuring of the cemented samples did not occur by using the proposed termination criterion. The proposed method has the potential to be a cost-effective alternative to the testing of multiple samples for the characterization of the strength of weakly cemented and sensitive soils.

Stress-induced anisotropy in brine saturated shale

SUMMARY This paper reports the intrinsic and crack-induced anisotropic properties of a set of preserved, brine-saturated shale samples and their response to external stresses. We used undrained multistage triaxial tests to evaluate how the ultrasonic wave velocities and their anisotropy changed with increasing isotropic and differential stress conditions. In addition, the impact of stress orientation with respect to fabric orientation was evaluated. An array of ultrasonic transducers allowed to measure five independent wave velocities which were used to calculate the elastic properties of the shale. Results indicate that in this shale P- and S-wave velocities vary with stress in a different manner dependent on the maximum principal stress orientation with respect to the fabric. Where the maximum stress is normal to bedding, Vpv and Vs1 increase monotonically with increasing effective stress. However Vph and Vsh decrease during individual loading stages but increase from stage to stage as confining pressure increases. The reverse occurs when the microfabric is parallel to the maximum principal stress. Where the maximum stress is bedding normal, velocity anisotropy decreases as differential stress increases; when maximum stress is fabric parallel, anisotropy increases. Elastic anisotropy is related to the initial composition and the spatial distribution of the different minerals (fabric) in the sediment and the presence of microfractures, while changes in elastic anisotropy result from the applied stresses, their orientation with respect to the rock fabric and the degree of stress anisotropy.

Stress anisotropy and velocity anisotropy in low porosity shale

Shales are known for often marked intrinsic anisotropy of many of their properties, including strength, permeability and velocity for example. In addition, it is well known that anisotropic stress fields can also have a significant impact on anisotropy of velocity, even in an isotropic medium. This paper sets out to investigate the ultrasonic velocity response of well-characterised low porosity shales from the Officer Basin in Western Australia to both isotropic and anisotropic stress fields and to evaluate the velocity response to the changing stress field. During consolidated undrained multi-stage triaxial tests on core plugs cut normal to bedding, V pv increases monotonically with increasing effective stress and V s1 behaves similarly although with some scatter. V ph and V sh remain constant initially but then decrease within each stage of the multi-stage test, although velocity from stage to stage at any given differential stress increases. This has the impact of decreasing both P-wave (ε) and S-wave anisotropy (γ) through application of differential stress within each loading stage. However, increasing the magnitude of an isotropic stress field has little effect on the velocity anisotropies. The intrinsic anisotropy of the shale remains reasonably high at the highest confining pressures. The results indicate the magnitude and orientation of the stress anisotropy with respect to the shale microfabric has a significant impact on the velocity response to changing stress fields.

Wellbore-Instability Predictions Within the Cretaceous Mudstones, Clair Field, West of Shetlands

Summary Some early high-inclination wells were impacted adversely by wellbore instability within Cretaceous mudstones overlying the Clair oil field. Detailed investigations of the possible reasons for borehole failure led to a decision to cut two cores within the unstable interval and carry out laboratory testing to investigate the failure mechanism. Analysis of rock failure based on a "planes of weakness" theory fitted previous drilling observations. This theory has been applied primarily for layered hard rocks (e.g., mining and tunneling) and is here extended to significantly weaker mudstones showing strength anisotropy. Minimum required mud weight predictions were made for a range of well configurations. The results of this study were used to plan an extended-reach-drilling (ERD) development well targeting a previously undeveloped part of the Clair field. The new well was successfully drilled, cased, and brought on stream within budget in 2009. This paper describes the basic geology of the field, rock properties of the Cretaceous mudstones, details of laboratory tests conducted, and analyses carried out. Cretaceous mudstones of the Clair field are characterized by low strength and minor anisotropy. Various rock tests were conducted on core to define the failure behavior: unconfined compressive strength (UCS), single-stage triaxial strength, and multistage triaxial strength tests were conducted, all at various angles to bedding. In addition, the effect of mud invasion was analyzed by performing a multistage triaxial test in drilling mud at high pressure. The analysis used commercial software to evaluate borehole instability in a rock medium that fails along weak planes. Minimum mud weight requirements derived from this analysis were compared with previous drilling experience and were used subsequently for predicting mud weight required to drill an ERD well. Lessons learned from the successful drilling of this well will be applied in future Clair development wells and may be of relevance elsewhere.

Multi-stage triaxial test on brittle rock

Multi-stage triaxial test on brittle rock

Time-Dependent Stress-Strain Behavior of Soft Hong Kong Marine Deposits

Abstract To investigate the time-dependent stress-strain behavior of Hong Kong marine deposits (HKMD), three types of triaxial tests have been conducted under both compression and extension states. The first is conventional undrained shear tests at different strain rates ranging from 0.0025 to 0.25%/min. The second type is stress relaxation tests. The last type is undrained creep tests. The results are reported in this paper. One order increment of logarithmic strain rate causes about 5 to 9% increase of undrained shear strength. In stress relaxation tests, the pore-water pressure increase is about 10% of the consolidation pressure under extension states. A power law equation is presented to calculate the decay of the deviator stress in relaxation. In multi-stage triaxial creep tests, the log(strain rate)-log(t) relationship of the lower stress level is linear. The slopes of these lines are dependent on current deviator stress as well as stress history.

Effect of chemical additives on the strength of sandstone

Chemical alteration of the strength of sandstone has been investigated to establish the fundamental knowledge for chemically assisted fracturing. If the rock strength can be chemically lowered, this technology would be useful to raise the fracturing efficiency. The purpose of the present study is to evaluate the effect of chemical solutions on the strength. To evaluate experimentally the effect, Brazilian tests and multi-stage triaxial compression tests have been performed on Gosford sandstone saturated with solutions of dodecyltrimethyl ammonium bromide (DTAB), polyethylene oxide (PEO) and aluminium chloride (AlCl 3 ). The tensile strength varied with the concentration of chemical additives, became the lowest at certain concentrations which are consistent with the zero zeta-potential concentrations. AE activity is the most active in dry specimen, and the least AE activity is found in the specimen saturated with chemical solution. The uniaxial and triaxial strengths did not vary significantly with concentration of the chemical solutions. In case of compression test where failure mode is dominated by shear failure, there is little effect of chemical solutions on the compressive strength.

Influence of stope geometry and blasting patterns on recorded overbreak

Chemical alteration of the strength of sandstone has been investigated to establish the fundamental knowledge for chemically assisted fracturing. If the rock strength can be chemically lowered, this technology would be useful to raise the fracturing efficiency. The purpose of the present study is to evaluate the effect of chemical solutions on the strength. To evaluate experimentally the effect, Brazilian tests and multi-stage triaxial compression tests have been performed on Gosford sandstone saturated with solutions of dodecyltrimethyl ammonium bromide (DTAB), polyethylene oxide (PEO) and aluminium chloride (AlCl 3 ). The tensile strength varied with the concentration of chemical additives, became the lowest at certain concentrations which are consistent with the zero zeta-potential concentrations. AE activity is the most active in dry specimen, and the least AE activity is found in the specimen saturated with chemical solution. The uniaxial and triaxial strengths did not vary significantly with concentration of the chemical solutions. In case of compression test where failure mode is dominated by shear failure, there is little effect of chemical solutions on the compressive strength.

Shear-strength characteristics of a residual soil

Shallow landslides in natural residual soils slopes are common all over the world. The slip surfaces associated with these landslides are often situated above the groundwater table. Therefore, it is important to quantify the contribution of negative pore-water pressure to the shear strength of soil. The shear-strength characteristics of residual soil from the Jurong Formation in Singapore were assessed using multistage, consolidated drained triaxial tests. These tests involved shearing under either a constant net confining pressure and varying matric suctions or under a constant matric suction and varying net confining pressures. An extended form of the Mohr–Coulomb equation was used to interpret the test results. The test results show that for matric suctions up to 400 kPa, the angle of internal friction associated with the matric suction, [Formula: see text], is similar to the effective angle of internal friction, [Formula: see text], which averages 26° for the residual soil of the Jurong Formation. The residual soil can maintain a high degree of saturation for matric suctions as high as 400 kPa. Examples involving stability analyses of a residual soil slope with varying pore-water pressure profiles indicate that soil suction contributes significantly to the factor of safety, particularly for shallow slip surfaces. Key words : residual soil, unsaturated soil, matric suction, shear strength, multistage triaxial test, slope stability.

Strength Correlation Factor for Residual Soils

Complete weathering of parent rock has resulted in the formation of residual soils that cover most of the land area of Sri Lanka. The physiography of the island demands that new engineering structures be located on sloping ground, thus posing many problems. The residual soils are heterogeneous due to variable weathering of the jointed rock mass. The heterogeneity of residual soils is due to the influence of relict joints, presence of boulders, and variability of the soil matrix. The repercussion of this heterogeneity is that properties of small samples of residual soils are unrepresentative of the mass, thus rendering conventional methods of field testing, laboratory testing, and analysis procedures for slope stability unsuitable. Heretofore, there have been no testing and analytical models available to represent residual soils. This paper describes an alternative method to predict shear strength parameters of residual soils by a correlation factor obtained from a multistage triaxial test and conventional slope stability analysis. Three landslides were investigated and analyzed to obtain the field shear strength parameters using the back analysis technique. On comparison with laboratory strength parameters, obtained from multistage triaxial tests, a correlation factor was ascertained. This correlation factor can be used to modify laboratory shear‐strength parameters to obtain field shear‐strength values of residual soils for design purposes.

Stress path considerations in multistage triaxial testing : Schoenemann, M R; Pyles, M R Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P732–739. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

The particular sequence of axial load and confining-stress change employed in a multistage triaxial test can dramatically influence the stress paths imposed on triaxial specimens, as can isotropic and anisotropic consolidation. The cumulative radial strain in isotropically consolidated specimens can be as much as three times greater than that in anisotropically consolidated specimens at the end of the third stage of a test. Cumulative axial strain was about equal in isotropically and anisotropically consolidated specimens, even though the isotropically consolidated specimens apparently experienced less total compressive strain by the end of a test. While increased strain did not appear to influence the results from multistage triaxial tests on relatively insensitive residual soils, the influence on tests results for more sensitive soils may be significant.

Results and interpretation of multistage triaxial compression tests : Soranzo, M Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P353–362. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

Results and interpretation of multistage triaxial compression tests : Soranzo, M Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P353–362. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

Application of multistage triaxial test to Kuwaiti soils : Saeedy, H S; Mollah, M A Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P363–375. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

Application of multistage triaxial test to Kuwaiti soils : Saeedy, H S; Mollah, M A Proc Symposium on Advanced Triaxial Testing of Soil and Rock, Louisville, 19–20 June 1986P363–375. Publ Philadelphia: ASTM, 1988 (ASTM Special Technical Publication 977)

Multistage triaxial test for unsaturated soils : Ho, D Y F; Fredlund, D G Geotech Test J, V5, N1–2, March–June 1982, P18–25

Multistage triaxial test for unsaturated soils : Ho, D Y F; Fredlund, D G Geotech Test J, V5, N1–2, March–June 1982, P18–25

A Multistage Triaxial Test for Unsaturated Soils

Abstract A triaxial testing procedure is presented for measuring the increase in shear strength resulting from soil suction in an unsaturated soil. Necessary modifications on a conventional triaxial cell are described. A simple graphical method is presented to interpret the test data in accordance with the shear strength equation for unsaturated soils.

Measurement of Swelling, Hydraulic Conductivity, and Shear Strength in a Multistage Triaxial Test

AbstractThe multistage triaxial (MST) test described here measured swelling on saturation, hydraulic conductivity, isotropically consolidated‐undrained strength, and drained and undrained compressibility. Peak strengths were evaluated by application of a hyperbolic stress‐strain model and by extrapolation of stress‐strain curves to failure. Strength parameters obtained from MST and conventional triaxial (CT) tests agreed reasonably well for three compacted soils of widely varying properties. Both drained and undrained compression modulii from MST tests increased logarithmically with increasing confining pressure. Initial undrained compression modulii from CT and MST tests agreed fairly well, although there was a definite tendency towards reduced compressibility in the later stages of MST tests. Volumetric compressibilities during the drained phases of MST tests were markedly lower than those obtained in K0 compression tests.

Multistage triaxial testing of rocks : Kim, M M; Ko, H Y Geotech Test J, V2, N2, June 1979, P98–105

Multistage triaxial testing of rocks : Kim, M M; Ko, H Y Geotech Test J, V2, N2, June 1979, P98–105

Strength and Deformability of Highly Fractured Rock

This paper reviews procedures used to determine properties of the greywacke sandstone forming the foundation of a concrete dam. Multistage triaxial testing of carefully reassembled core specimens provided a series of shear-strength versus normal-stress curves for fractured rock; the lowest of these was adopted as representative of the strength of the rock mass. Deformability was measured during the triaxial tests and in the field using a borehole jack in 3-in. (76-mm) drill holes. The latter yielded a value for the normal stiffness of the fractures in the rock. Introducing this stiffness value together with any given fracture spacing then determined the rock-mass modulus of elasticity corresponding to the assigned fracture spacing. Longitudinal and shear-wave velocity measurements in cross-hole configuration determined modulus of elasticity values considerably higher than those measured with the borehole jack, as expected in fractured rock.

Multistage Triaxial Testing of Rocks

Abstract To determine the constitutive properties of the components in a rock structure, triaxial tests are commonly carried out on cylindrical test specimens. The conventional triaxial test (designated the single-stage triaxial test) involves obtaining a peak strength for the specimen and, after its breakage, a residual strength. In the multistage triaxial test, however, several peak and residual strengths can be obtained from a single specimen. This technique enables the experimenter to conserve samples and to obtain more consistent strength data by eliminating sample variabilities for the construction of failure envelopes. The feasibility of this test is examined by comparing strength results for single-stage and multistage tests in terms of the cohesion and the friction angle, which are calculated from the failure envelopes constructed by the two different types of test data. The feasibility is confirmed for Pierre shale and Raton shale, but is doubtful for Lyons sandstone, which is very brittle.

Multistage Triaxial Testing of Lightly Overconsolidated Clays

Abstract A comparison has been made between consolidated undrained tests on separate soil specimens and multistage testing of a single specimen. Three series of samples were prepared, two of the series consisting of remolded kaolin in each of which the specimens were lightly overconsolidated under K0 conditions, and one series consisting of undisturbed specimens of soft silty clay from a site at Fulford near York in England. It is concluded from these tests that although some differences arise from the two methods, the results from multistage tests are adequate for practical purposes both in the prediction of failure conditions and in the prediction of deformations and pore pressure response under working loads. In view of the number of advantages accruing from multistage testing, it is recommended that this method should be used in the routine triaxial testing of soft lightly overconsolidated clays of low sensitivity.