Consolidated Quick Shear Test: Principle, Procedure, Standards, and Applications

The consolidated quick shear test (CQ test) is a laboratory geotechnical procedure used to determine the shear strength parameters—cohesion and internal friction angle—of fine-grained cohesive soils under consolidated-undrained, total stress conditions. In this test, a soil specimen is first consolidated under a normal stress in a direct shear apparatus and then sheared at a rapid, controlled rate so that excess pore water pressure cannot dissipate during loading. This article explains the test principle, applicable standards, sample requirements, operating procedure, data interpretation, and typical engineering applications of the consolidated quick shear test.

Technical Overview

The consolidated quick shear test, commonly abbreviated as the CQ test, is a variant of the direct shear test performed on cohesive soils. A cylindrical soil specimen confined in a metal ring is placed in a shear box consisting of an upper and a lower half. The specimen is first allowed to consolidate fully under a prescribed vertical (normal) stress, after which a horizontal shear force is applied at a rapid, constant rate of displacement. Because shearing is completed within only a few minutes, excess pore water pressure generated during shear cannot dissipate in low-permeability soils, and the measured strength corresponds to a consolidated-undrained condition expressed in terms of total stress.

The test yields the shear strength parameters of the Coulomb failure criterion, τ = c + σ·tan φ, where τ is the shear stress on the failure plane, σ is the applied normal stress, c is the cohesion intercept (denoted c_cq for this test), and φ is the angle of internal friction (denoted φ_cq). By testing at least four identical specimens under different normal stresses and plotting peak shear stress against normal stress, a linear failure envelope is obtained through regression, from which c_cq and φ_cq are read directly.

In China, the procedure is codified in the national standard GB/T 50123, Standard for Geotechnical Testing Method. Comparable direct shear equipment and procedures are described in international standards such as ASTM D3080, BS 1377 Part 7, and ISO 17892-10, although the quick (undrained) shearing variant is specifically defined in the Chinese standard for fine-grained soils of low permeability. Testing is routinely performed in accredited third-party laboratories and in institutional geotechnical laboratories supporting civil engineering design and construction.

• Strength model: total-stress Coulomb envelope providing c_cq and φ_cq
• Apparatus: strain-controlled (or stress-controlled) direct shear apparatus with proving ring, dial gauges or digital transducers, and consolidation loading frame
• Standard specimen: circular ring, 61.8 mm in diameter and 20 mm in height
• Standard quick shear rate: 0.8 mm/min, producing failure within approximately 3 to 5 minutes
• Applicable soils: clays, silty clays, and other fine-grained soils of low permeability

Testing Samples

Undisturbed Cohesive Soil Specimens

The preferred test material is undisturbed soil obtained by thin-wall sampling, piston sampling, or block sampling from boreholes, test pits, or exploration shafts. Samples must be carefully sealed with wax and protective casings, transported without vibration or shock, and stored in moisture-controlled conditions to preserve natural structure, density, and water content. Test specimens are trimmed into the standard cutting ring with minimal disturbance, and trimmings are used to determine natural water content.

Remolded and Compacted Specimens

For engineered fills, laboratory research, or quality verification of treated ground, remolded specimens may be prepared at a specified dry density and water content, or compacted according to a specified compaction effort. Remolded specimens allow controlled comparison of strength before and after ground improvement, such as surcharge preloading or consolidation under staged construction.

Specimen Dimensions and Quantity

Each specimen is prepared in a ring of 61.8 mm internal diameter and 20 mm height, giving a shear plane area of approximately 30 cm². A minimum of four specimens, and preferably four to five, is required from the same soil layer and sampling location, each assigned a different normal stress so that a reliable failure envelope can be established. All specimens in a group should share similar physical properties; index tests (water content, density, Atterberg limits, and particle size distribution) are performed on representative material for correlation.

Testing Items

The consolidated quick shear test delivers the following principal results:

• Peak shear stress under each applied normal stress (50, 100, 150, 200, 300, or 400 kPa, selected to cover the anticipated field stress range)
• Shear stress versus horizontal displacement curves for each normal stress level
• Vertical deformation versus time records during the consolidation stage
• Cohesion intercept c_cq of the consolidated-quick (total stress) failure envelope
• Angle of internal friction φ_cq of the consolidated-quick (total stress) failure envelope
• Post-peak softening behavior where shearing is continued beyond the peak value
• Post-test water content and bulk density of the sheared specimen

Testing Methods

Specimen Preparation and Installation

The specimen is trimmed flush with the cutting ring, weighed together with the ring, and its wet density computed. The specimen, confined by the ring, is placed into the shear box between saturated porous stones and filter papers. The shear box is aligned, the proving ring and displacement gauges are mounted, and the contact between the loading cap and the vertical loading piston is established. Water is added to the shear box container when saturated (or field-saturation) conditions are to be simulated.

Consolidation Stage

The selected normal stress is applied vertically, and vertical deformation is recorded at timed intervals, typically every hour. Consolidation under the normal stress is considered complete when the rate of vertical deformation falls below 0.005 mm per hour, indicating that primary consolidation under that load has essentially finished. This stage reproduces the field condition of a soil that has consolidated under existing overburden or a sustained load before a rapid change in loading occurs.

Rapid Shearing Stage

After consolidation is complete, a small gap (approximately 0.5 to 1.0 mm) between the upper and lower shear boxes is introduced with the adjusting screws so that the two halves do not rub against each other, and all gauges are set to zero. The horizontal shear displacement is then applied at the standard quick rate of 0.8 mm/min. Shear force readings are recorded at regular displacement increments until failure. Failure is taken as the peak of the shear stress-displacement curve; if no distinct peak occurs, shearing is continued to a total displacement of 6 mm, and the shear stress at 4 mm of displacement is adopted as the failure value. The specimen is then unloaded, the shear box drained and dismantled, and the specimen weighed and sampled for final water content.

Data Processing and Strength Envelope Determination

The shear stress at each displacement is computed from the calibrated proving ring constant divided by the specimen cross-sectional area. For each of the four or more normal stress levels, the peak (or designated failure) shear stress is determined. These failure points are plotted on a τ–σ diagram, and a straight line is fitted by least-squares regression in accordance with the Coulomb criterion. The intercept of the fitted line on the shear stress axis gives the cohesion c_cq, and the inclination of the line with respect to the normal stress axis gives the internal friction angle φ_cq. The regression coefficient is reported to assess the consistency of the specimen group, and outliers attributable to sampling disturbance or material variability are identified and retested where necessary.

Applications

The consolidated quick shear test models the common field situation in which a cohesive soil has fully consolidated under sustained loading—such as overburden, embankment weight, or reservoir water pressure—and is subsequently subjected to a rapid change in load that does not allow drainage. Consequently, its strength parameters are widely used in total stress stability and capacity analyses, including:

• Short-term stability analysis of slopes and excavations in clays subjected to rapid loading or unloading
• Rapid drawdown stability assessment of dam upstream slopes and reservoir banks
• End-of-construction stability of embankments, levees, and staged fills on clay foundations
• Bearing capacity evaluation for footings and structures on cohesive soils under rapid loading conditions
• Earth pressure calculation for retaining walls, sheet piles, and braced excavations
• Verification of strength gain in soft ground improvement works, such as surcharge preloading and staged construction
• Input parameters for limit equilibrium and numerical analyses in geotechnical design reports

FAQ

What is the difference between quick shear, consolidated quick shear, and consolidated slow shear tests?

The quick shear test neither consolidates nor drains the specimen, representing unconsolidated-undrained conditions. The consolidated quick shear test consolidates the specimen first and shears it rapidly without drainage, representing consolidated-undrained conditions in total stress terms. The consolidated slow (drained) shear test both consolidates the specimen and shears it slowly enough for full drainage, providing effective stress strength parameters.

Why is rapid shearing able to prevent drainage?

Fine-grained soils such as clays and silty clays have permeability coefficients many orders of magnitude lower than those of sands. At the standard quick shear rate of 0.8 mm/min, the specimen fails within roughly 3 to 5 minutes, a period far too short for significant pore water migration, so the undrained condition is effectively achieved in practice.

How many specimens are required for one test group?

A minimum of four specimens from the same soil layer, each consolidated and sheared under a different normal stress, is required to define the failure envelope. Using four to five points improves regression quality and allows detection of outliers.

Which soils are suitable for this test?

The test is intended for low-permeability fine-grained soils, primarily clays and silty clays. Free-draining sands and gravels are unsuitable because drainage cannot be prevented during rapid shearing; for such soils, drained direct shear testing in accordance with ASTM D3080 or equivalent procedures is appropriate.

How does the consolidated quick shear test compare with the consolidated-undrained triaxial test?

Both tests simulate consolidated-undrained conditions, but the direct shear apparatus forces failure along a horizontal plane, is simpler and faster to operate, and requires a smaller specimen, whereas the triaxial test applies a more uniform stress state, permits pore pressure measurement, and therefore yields both total and effective stress parameters. The direct shear CQ test is often selected for routine projects, with triaxial testing reserved for critical or complex conditions.

How is the quality and reliability of test results assured?

Reliability is ensured through careful undisturbed sampling and sealed transport, verification of specimen saturation, use of calibrated proving rings and displacement transducers, adherence to the consolidation completion criterion and the standard shear rate, and execution of the work by accredited third-party laboratories or qualified institutional laboratories in accordance with GB/T 50123 and related international standards.