Physical Property Measurements
Unconfined Compressive Strength (UCS)UCS is one of the most basic parameters of rock strength, and the most common determination performed for boreability predictions. It is measured in accordance with the procedures given in ASTM D2938, with the length to diameter ratio of 2 by using NX-size core samples. 3 to 5 UCS determinations are recommended to achieve statistical significance of the results. If the sample length to diameter ratio was greater or less than 2, ASTM recommends a correction factor that is applied to the UCS value determined from testing. UCS measurements are made using an electronic-servo controlled MTS stiff testing machine with a capacity of 220 kips. Loading data and other test parameters are recorded with a computer based data acquisition system, and the data is subsequently reduced and analyzed with a customized spreadsheet program.
The unconfined compressive strength of the specimen is calculated by dividing the maximum load at failure by the sample cross-sectional area:
sc = Unconfined Compressive Strength (psi)
F = Maximum Failure Load (lbs)
A = Cross-sectional area of the core sample (in2)
Indirect (Brazilian) Tensile Strength
Indirect, or Brazilian, tensile strength is measured using NX-size core samples cut to an approximate 0.5 length-to-diameter ratio, and following the procedures of ASTM D3967. BTS measurements are made using an electronic-servo controlled MTS stiff testing machine with a capacity of 220 kips. Loading data and other test parameters are recorded with a computer based data acquisition system, and the data is subsequently reduced and analyzed with a customized spreadsheet program.
BTS provides a measure of rock toughness, as well as strength. The indirect tensile strength is calculated as follows:
sT = Brazilian Tensile Strength (psi)
D = Diameter of the core sample (in)
F = Maximum Failure Load (lbs)
L = Length of the core sample (in)
In bedded/foliated rocks, particular attention needs to be given to loading direction with respect to bedding/foliation. The rock should be loaded so that breakage occurs in approximately the same direction as fracture propagation between adjacent cuts on the tunnel face. This is very important assessment in mechanical excavation by tunnel boring machines.
Cerchar Abrasivity Index (CAI)
This test measures rock abrasivity for determining cutter wear and costs. A series of sharpened steel pins of a known hardness alloy steel are pulled across a freshly broken surface of the rock. The average dimensions of the resultant wear flats are related directly to cutter life in field operation. The geometry of the planned excavation then allows calculation of the expected cutter wear per unit distance of cutter travel. This test can be performed on irregular rock pieces an inch across.
The rock sample is fixed in a holder with the fresh surface facing upward. A conical 90° hardened steel pin, fastened in a 15 lb. (7.5 kg) head, is set carefully on the fresh surface and drawn 0.4 in. (1 cm) across it in 1 second. This is repeated for a total of five pins The tips of the pins then are examined under a reticular microscope and two perpendicular diameters of the resulting wear flat are recorded for each pin. Coating the pin tips with machinist’s blue dye prior to testing makes the wear flat more visible.
The Cerchar abrasivity index (CAI) is then calculated by:
di = pin diameter ( micro in.)
Punch Penetration Test
In this test, a standard conical indentor is pressed into a rock sample that has been cast in a confining steel ring. The load and displacement of the indentor are recorded with a computer system. The slope of the force-penetration curve indicates the excavatibility of the rock, i.e., the energy needed for efficient chipping. This is affected by the stiffness, brittleness, and porosity of the sample.
A program written in Excel macro is used to reduce the data. The program calculates and plots the following slopes:
- 45-degree slope (a baseline reference)
- Average slope (average of slope of a floating point)
- Peak slope (from origin to peak load)
- Energy slope (area under the curve)
Acoustic Velocities and Dynamic Elastic Constants
This measurement is performed in accordance with the procedures recommended by ASTM D2845, usually on core samples prepared for UCS testing. The velocities of compressive and shear ultrasonic waves through the core sample are measured and used to calculate the elastic modulus and Poisson's ratio. This method indicates the competency of the rock. Other factors such as anisotropy affect the results, however, and a minimum of five measurements is recommended.
From the waves’ velocities and the sample bulk density, the dynamic elastic modulus and dynamic Poisson’s ratio are then calculated:
VS= Shear wave velocity (in./s or m/s)
VP= Compressive wave velocity (in./s or m/s)
E= Elastic modulus (psi or Pa)
m= Poisson’s ratio
r= Density (lb/in3 or kg/m3)
Point Load Test
The Point Load Strength test is intended as an index test for the strength classification of rock materials. It may also be used to predict other strength parameters with which it is correlated, for example the unconfined compressive and the tensile strength. It is measured in accordance with the procedures recommended in ASTM D5731, usually with NX-size core samples. The testing machine consists of a loading frame, which measures the force required to break the sample, and a system for measuring the distance between the two platen contact points. Rock specimens in the form of either core, cut blocks, or irregular lumps are broken by application of concentrated load through a pair of spherically truncated, conical platens.
The applied force at failure of the sample and distance between the platen tips are recorded in order to calculate the point load index as follows:
IS = Point load index (psi)
F = Failure load (lbs.)
De =Distance between platen tips (in.)
De2 = D2 = for diametrical test
= 4A/p = for axial, block and lump test
A = W.D = minimum cross-sectional area of a plane through the platen contact points
Thin-Section Petrographic Analysis
The thin section petrographic analysis provides useful information about microscopic features of rock, which might significantly impact its boreability behavior. Numerous field case histories exist where the TBM failed to achieve its expected performance because of one or more unusual characteristics exhibited by the rock formation, which could not have been foreseen unless petrographic analysis data were available. These “unusual” features include grain suturing/interlocking, certain alignment/orientation of hard minerals, tight matrix, micro fractures, etc.
Abrasive Mineral Content and Moh's Hardness
Cutter life also can be estimated from the relative percentages of minerals of several Moh's hardness classes (>7, 6, 4 to 5, and <4). This is determined by hand-lens examination of fresh rock surfaces. The higher the percentage of harder minerals, the more abrasive the rock and the shorter the cutter life.
Schmidt Hammer Hardness
This test measures the rebound hardness of a rock specimen by the rebound of an impacting piston striking rock, which is held by a steel anvil. The piston is driven by a set of springs within the hammer, which store and release energy while pressing the hammer on the sample by hand. The test was originally developed as a quick measure of compressive strength of concrete and later was extended to estimate the hardness of rock.
Shore Sceleroscope Hardness
This test measures the rebound hardness of a rock specimen by the impact and rebound of a small diamond tipped impactor. The impactor free falls through a smooth tube to strike the rock sample, which is held by a steel anvil. The height of the impactor rebound is a function of the rock rebound hardness.
This test determines the abrasivity of a rock specimen by a mechanical abrasion device. A rock disc is rotated under an abrasive grinding wheel for a fixed number of revolutions. The weight loss of the rock disc and the wheel are used to calculate the abrasion hardness of the rock.
Static Elastic Modulus
This determination can be performed during UCS testing, and consists of measuring and recording the axial/lateral deformation history of the sample in addition to its load history. It is measured in accordance with the procedures given in ASTM D3148.
The loading data, recorded at the same rate, are converted to stress by:
s = stress (psi or Pa)
P = Load (lbf or kN)
A = Cross-sectional area of core sample (in2 or m2)
The displacement data are converted to strain by:
el = Longitudinal strain
er = Radial strain
L = Sample length (in. or mm)
DL = Change in length (in. or mm)
D = Sample diameter (in. or mm)
DD = Change in diameter (in. or mm)
The stress-versus-strain curve for the axial and lateral direction is plotted. Average slope of the more-or-less straight line portion of the stress-strain curve is used to calculate the average elastic modulus. And the Poission's ratio is calculated by:
Triaxial Strength Testing
This type of test simulates the behavior of rock underground, because it applies significant confining pressure to the sample during loading. It is performed according to ASTM D2664. Five tests are conducted, each at a different confining pressure. The resulting sample strengths are plotted on a stress difference versus axial strain curve, and on a Mohr circle, to determine the failure criteria for the rock type.
In addition to the tests described above, the following tests can be performed on request, with costs determined on a case by case basis:
Direct shear test (ASTM 5607)
Moisture content (ASTM D2216)
Rock Quality Designation, RQD (ASTM D6032)
Mineral identification by X-ray diffraction
Slake durability index
Sieve Analysis (ASTM D2487)