Civily
    Technical reference

    Soil & rock testing

    Classification and preparation, dynamic testing, shear, consolidation, and triaxial tests.

    6 learning resources
    Discipline

    Soil & rock testing

    06 resources
    GUIDESoil & rock testing

    Soil consolidation and compressibility

    How pore pressure dissipates and how it relates to settlement under loading.

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    Concept

    • In saturated soil, loading can create excess pore-water pressure. As drainage occurs, stress transfers to the soil skeleton and settlement develops.
    • An oedometer compresses a laterally confined specimen to study deformation and consolidation rate under load increments.

    Specimen and sequence

    • An undisturbed sample better preserves structure and moisture; handle it carefully and record orientation and depth.
    • Trim the specimen to fit the ring, mount it between porous elements, and apply the inundation condition specified.
    • Apply load increments and record deformation over time until each stage meets its criterion; unloading and swelling stages may also be included.

    Reading the curves

    • The load-deformation relationship characterizes compressibility; the deformation-time curve describes the rate of the process.
    • Derived parameters help estimate settlement and time, but depend on the model, stress history, and specimen quality.

    Points to consider

    • Distinguish consolidation from immediate deformation and secondary compression; do not attribute all settlement to one mechanism.
    • The source also describes CRS and suction methods. Select a procedure for the soil, geotechnical question, and applicable standard.
    GUIDESoil & rock testing

    Dynamic soil testing

    Small-strain stiffness, seismic waves, and cyclic soil response.

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    Two response scales

    • Dynamic tests span very small strains and larger cyclic loading; stiffness and damping depend on strain level.
    • Maximum shear modulus Gmax describes very-small-strain response and relates to shear-wave velocity and soil density.

    Main methods

    • Bender elements generate and receive shear waves through a specimen; travel time is used to estimate wave velocity and Gmax.
    • A resonant column excites the specimen and measures resonant frequency to estimate stiffness and damping at small strains.
    • Cyclic triaxial and cyclic simple shear apply repeated loading histories to study degradation and deformation and, under suitable conditions, liquefaction susceptibility.

    Preparation and procedure

    • Trim or remould a suitable specimen and mount transducers without damaging it or distorting the signal.
    • Saturate and consolidate to represent the expected stress state when required; then apply the defined stress and strain sequence.
    • Control alignment, frequency, amplitude, confining pressure, and sensor response; check signal quality before interpreting results.

    Geotechnical interpretation

    • Compare modulus and damping at equivalent strain levels; do not treat Gmax as constant stiffness for every load.
    • Use data for dynamic and seismic analysis within the tested range, together with soil characterization and the geotechnical model.
    GUIDESoil & rock testing

    Rock classification and specimen preparation

    Intact rock versus rock mass, discontinuities, and specimen preparation.

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    Intact rock and rock mass

    • Intact rock is the material between discontinuities; a rock mass also includes joints, faults, fractures, weathering, and infill.
    • Rock-mass strength depends on discontinuity orientation, persistence, roughness, infill, and condition, as well as the rock matrix.

    Specimen preparation

    • Select a representative sample, record source and orientation, and inspect weathering or fractures before cutting.
    • Core, trim, and grind faces to the method’s geometric tolerances; poor preparation affects failure mode.
    • Rock properties vary. Document defects and do not treat rock as if it were homogeneous concrete.

    Characterizing discontinuities

    • A joint profilometer allows roughness comparison with reference profiles and estimation of a JRC index.
    • Rebound on a joint wall can support hardness assessment; a tilt test provides an empirical observation of friction angle.
    • Direct shear across a joint evaluates sliding resistance under a defined normal stress.

    Durability and application

    • Water-durability or abrasion tests study degradation relevant to fill, rockfill, excavation, and building stone.
    • Interpret results with geology, project scale, and rock-mass structure; one specimen value does not represent an entire rock mass.
    GUIDESoil & rock testing

    Uniaxial and triaxial rock testing

    Compressive strength, confinement effects, and failure-envelope analysis.

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    Uniaxial and triaxial compression

    • In uniaxial compression, a cylindrical specimen is loaded axially without lateral confinement to study strength and stress-strain response.
    • In triaxial compression, a cell adds lateral pressure; repeating the test at different confinements shows how strength changes.

    Preparation and instrumentation

    • Use specimens with dimensions, faces, and parallelism that meet the method; record lithology, orientation, defects, and moisture.
    • Measure load and axial strain; strain gauges or other local sensors can provide radial strain and Poisson’s ratio.
    • For triaxial tests, check the sleeve seal, cell alignment, and stability of confining pressure.

    Failure and envelope

    • Identify peak strength and strain; where control permits, observe post-peak response or residual strength.
    • A series of confining pressures can define failure points and estimate a strength envelope within the tested range.
    • The Brazilian test loads a disc diametrically to estimate indirect tensile strength; geometry and contact must meet the procedure.

    Safety and application

    • Rock may fail brittly and eject fragments. Install guards and keep people outside the projection zone.
    • Interpret results alongside discontinuities and rock-mass scale; intact-rock strength is not automatically rock-mass strength.
    GUIDESoil & rock testing

    Direct shear and residual strength

    Shear strength, normal-stress effects, and residual soil strength.

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    What it measures

    • In direct shear, two halves of a box impose a defined sliding plane while a normal load is applied.
    • Shear stress against displacement shows peak resistance and, after repeated movement, may reveal residual strength.

    Typical sequence

    • Prepare a specimen of known dimensions, place it in the box, and record its condition, density, and moisture.
    • Apply the selected normal stress and allow consolidation where required by the method.
    • Move the box halves at the specified rate and record shear force, displacement, and vertical deformation.
    • Repeat at several normal stresses to construct a strength envelope.

    Residual strength and related methods

    • During back-and-forth cycles or large displacement, strength may stabilize at a residual level; interpret each cycle, not just the initial peak.
    • Ring shear can maintain the shear area over large displacement, although it commonly uses remoulded material.
    • A laboratory vane is useful for a rapid strength assessment in soft clay, following its own procedure.

    Limitations

    • The shear plane is imposed by the box and stress distribution is not uniform; consider these limitations when applying parameters to the ground.
    • Shear rate must allow the required drainage condition. Report stresses, rate, corrected area, and strength criterion according to the standard.
    GUIDESoil & rock testing

    Soil triaxial testing

    Saturation, consolidation, and shearing under confinement: UU, CU, and CD tests.

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    What the test can show

    • A cylindrical specimen receives lateral pressure in a cell and axial load; stresses are controlled and, depending on instrumentation, pore pressure and volume change are measured.
    • The test helps evaluate strength, deformation, and response under stress and drainage conditions relevant to geotechnical problems.

    Common stages

    • Prepare and mount the specimen with membrane, porous stones, and sealed connections; remove air from lines and transducers.
    • Saturate when required and check the specified saturation criterion.
    • Consolidate under the intended effective stress until volume or pressure response meets the completion criterion.
    • Shear at a rate compatible with the drainage condition and measure load, axial deformation, pore pressure, and volume where applicable.

    UU, CU, and CD

    • UU: no consolidation and no drainage during shear; used to study rapid, short-term conditions.
    • CU: consolidate first, then prevent drainage during shear; measuring pore pressure can provide total- and effective-stress responses.
    • CD: consolidate and allow drainage during shear; the rate must be slow enough to dissipate pore pressure.

    Analysis and control

    • Relate principal stresses to strain; a series of specimens at different confining pressures can define a failure envelope.
    • Select the test type for the duration of the problem and field conditions. Do not interchange total and effective parameters without checking the analysis.
    • Durations in the source are illustrative: actual time depends on soil, drainage, specimen size, and the standard criterion.

    Learning guides based on technical presentations, written to make civil engineering materials and test methods easier to understand.