Unconfirmed Compressive Strength

[Ken Reels]

TheUnconfined Compression Test is a laboratory test used to derive the Unconfirmed Compressive Strength (UCS) of a rock specimen. Unconfirmed Compressive Strength (UCS) stands for the maximum axial compressive stress that a specimen can bear under zero confining stress. Due to the fact that stress is applied along the longitudinal axis, the Unconfined Compression Test is also known as Uniaxial Compression Test. UCS is a parameter widely used in geotechnical design, but may not represent the strength in-situ. On a large scale, the rockmass properties are highly affected by other factors including discontinuities, faults and weathering.

Samples are retrieved by drill cores and are selected cautiously in order be representative of the original rock formation. The minimum diameter of a specimen must be at least 47 millimeters and 10 times larger than the size of the largest mineral grain (or 6 times larger for weaker rocks e.g. sandstones, marlstones).

Loading Device: The loading device must be designed to consistently apply load at the required rate until the end of the test. The test may be stress- or strain-controlled. It is pointed out that only strain-controlled devices can capture the post-failure behavior of a material.

Strain measurement devices: The axial and lateral deformations are measured by various devices (e.g. Linear Variable Differential Transformers (LVDTs), Compressometers, Electrical Resistance Strain Gages).

The two plates shall be carefully cleaned before the specimen is placed in the testing chamber. The load should be continuously applied at a rate of 0.5 MPa/s to 1.0 MPa/s (in case of a stress-controlled load device) and failure must occur in approximately 10 minutes. Stress and deformation data can be recorded through an electronic system that has the appropriate accuracy specifications. The maximum load is recorded in Newtons within a 1% accuracy.

A typical stress-strain diagram deriving from a Uniaxial Compression Test of an undisturbed specimen of basalt is presented in Figure 1. The UCS is the peak value of the diagram and is equal to 44.7 MPa. Photos of the specimen before and after the test are presented in Figure 2. During the failure process, cracks propagated from the bottom to the top of the specimen, shearing off a large piece of the sample.

The modulus of elasticity (Young's modulus) E which represents the ratio between axial stress and axial strain can be derived via several methods. Usually, it is calculated at stress-strain level of about 50% of the maximum load.

According to ASTM, the preferable sample L/D ratio is 2.0. Therefore, a correction formula is applied for larger ratios (smaller rations are unacceptable). In particular, the Uniaxial Compressive Strength is re-calculated as:

ASTM D7012-14e1, (2014). Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures, ASTM International, West Conshohocken, PA.

ISRM, (1979). Suggested Methods for Determining the Uniaxial Compressive Strength and Deformability of Rock Materials. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts. 16, 2.

Unconfined compressive strength is a standard geotechnical test performed on cohesive soil samples in construction materials testing laboratories. Intact, remolded, or reconstituted cylindrical soil specimens are axially loaded to failure on a laboratory load frame as stress forces vs. strain (deformation) values are recorded. Straightforward sample preparation and a rapid, uncomplicated procedure make unconfined compression tests cost-effective when fundamental strength values are adequate for design. Shear strength is one-half of the compressive stress at failure. Unconfined compression is not a substitute for triaxial shear methods that replicate in-situ lateral confining pressures more accurately.

Most intact cylindrical specimens for unconfined compression are prepared from Shelby tube samples, obtained, and preserved following methods in ASTM D1587. Sample preparation is straightforward, using a hydraulic extruder to reduce disturbance to the soil.

Extrude the sample in a smooth, continuous motion in the same direction that the soil entered the tube. Tube samples of intact soil need only be trimmed to length and have the ends squared in a miter box to be ready for testing. Field-excavated intact block or cylindrical samples acquired following ASTM D7015 practices need more extensive preparation using a soil trimmer, and wire saw to produce a specimen ready for testing. Trimmings from this process can be used for soil moisture and specific gravity determinations. Avoiding moisture loss and sample disturbance is key to producing valid test results throughout the sample preparation process.

For remolded unconfined compression specimens, soil from the previously tested intact specimen or unused sample material is recovered and manually kneaded in a latex membrane before compacting into a two-part mold for forming. Density and moisture content should be close to the original values. Remolded specimens help predict soil characteristics when compacted during construction operations such as embankment or structural fills.

The test method specifies a length-to-diameter ratio (L:D) between 2:1 and 2.5:1. So, the length of a 2.8in (71.1mm) diameter sample should be between 5.6in and 7in (144.2mm to 177.8mm). Samples under compression generate friction between the machine platens and the soil, restraining movement at the sample ends. Samples that are too short have an exaggerated reaction to this restraint and may exhibit a higher apparent compressive strength. On the other hand, samples that are too long may buckle in the middle rather than fail in true axial compression. Two-part miter boxes in popular sample diameters have required lengths.

The test method recommends selecting a rate of strain between 0.5% and 2.0% that will load the sample to failure in no more than approximately 15 minutes. Doing so entails some assumptions, simple calculations, and guesswork. Soft soils deform more before failure and should be tested at a higher rate. Stiff soils exhibit less deformation and should be tested at a lower rate. A 1% strain rate for a 5.60in (142.2mm) long sample requires advancing the platen of the load frame at 0.056in (1.42mm) per minute. For a softer sample of the same size, a 2% strain rate would need a platen speed of 0.112in (2.84mm) per minute.

Center the prepared test specimen on the lower platen of the load frame and adjust the upper platen so that it just contacts the specimen. Adjust all the instruments measuring load, time, and deformation to their proper positions, ready to take readings. Start applying the load at the calculated strain rate. The time intervals to record load and deformation values should be sufficient to produce a well-defined stress-strain curve of at least 10 to 15 points. The test is complete when load values decrease or strain values reach 15%. Record the slope angle of the failure and condition of the sample using a hand-drawn sketch or photograph. If trimmings were not collected earlier, use the failed sample for a moisture content determination.

In this paper a new empirical relation between the unconfirmed compressive strength of intact rock and the end bearing capacity of drilled shafts in rock is developed. In addition, an analytical relation between rock mass strength and end bearing capacity is developed to explicitly consider the effect of discontinuities. Specifically, a database of 39 load tests is used in this paper to derive the new relation between end bearing capacity and unconfirmed compressive strength of intact rock. The derived relation indicates that the end bearing capacity factor, Nc, which is the ratio of the end bearing capacity, qmax, and the unconfined compressive strength, 8sc, of intact rock, decreases with increasing 8sc. This is in contrast to many existing relations assuming constant Nc values. Since this new empirical relation is derived from the results of load tests, the effect of discontinuities is implicitly considered. To explicitly study the effect of discontinuities, an analytical relation based on the Hoek-Brown strength criterion that considers the effect of discontinuities is developed. The new analytical and empirical relations are in good agreement and are thus combined in a simplified form for predicting the end bearing capacity of drilled shafts socketed into rock masses. A comparison with two examples from instrumented test shafts indicates that the recommended relation produces satisfactory predictions.

N2 - In this paper a new empirical relation between the unconfirmed compressive strength of intact rock and the end bearing capacity of drilled shafts in rock is developed. In addition, an analytical relation between rock mass strength and end bearing capacity is developed to explicitly consider the effect of discontinuities. Specifically, a database of 39 load tests is used in this paper to derive the new relation between end bearing capacity and unconfirmed compressive strength of intact rock. The derived relation indicates that the end bearing capacity factor, Nc, which is the ratio of the end bearing capacity, qmax, and the unconfined compressive strength, 8sc, of intact rock, decreases with increasing 8sc. This is in contrast to many existing relations assuming constant Nc values. Since this new empirical relation is derived from the results of load tests, the effect of discontinuities is implicitly considered. To explicitly study the effect of discontinuities, an analytical relation based on the Hoek-Brown strength criterion that considers the effect of discontinuities is developed. The new analytical and empirical relations are in good agreement and are thus combined in a simplified form for predicting the end bearing capacity of drilled shafts socketed into rock masses. A comparison with two examples from instrumented test shafts indicates that the recommended relation produces satisfactory predictions.

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