The most important and essential scope in rock mechanics is measuring and determination of rock properties and behavior by using the recommended testing methods, procedures, and specifications. These include the engineering characteristics of rock such as its strength, mode of deformation, mode of failure, and modulus of elasticity.
A study upon rock in rock mechanics is one of civil subject disciplines which describe as oath-to-know or important as the rest of other mechanics related subject; soil, fluids, materials, and structures. Rocks are naturally occurring material where it is inhomogeneous and anisotropic, and also even-though it came from the same class or collected from the same spot, it still exhibit variations.
Generally there are two common category for testing of rock samples:
- Laboratory testing which is done at the lab with the rock samples obtained from the selected locations,
- Field or In-situ testing which is done by operating directly at the site itself.
Laboratory Testing of Rocks
* Recap: The testing of rock conducted for measuring and evaluating the changes of rock properties as well as its properties by applied loading or force. The properties will include physical, index and strength whereby the behavior include deformation and failure mode.
As stated before, the two most common methods of laboratory testing for rock are: 1) Index test, and Indirect Strength test; 2) Direct or Strength test. The types are generally based on the methods of testing and the nature or type of data obtained.
Index Test and Indirect Strength Test
Point-Load Index Test
Index test is relatively simple and rapid to conduct, but it does not provide fundamental property. The data obtained is just an indicator on property that being tested. The apparatus used are normally simple and portable which also allows the test to be conduct at site.
The tests may not require some detailed sample preparation where certain tests are non-destructive type and does not involve failure of samples (cost saving for sample could be reused). The data also not suitable for detailed design purposes but it is useful and valuable for preliminary or pre-feasibility assessments.
The tests for Index and Indirect Strength test include:
- Point-load index test
- Scmidt or Rebound hammer test
- Slake durability index test
- Sonic wave velocity test
- Uniaxial compressive strength test
- Brazilian or Inderect tensile strength test
Point-Load Index Test
It is a quick and simple test to conduct where the rock sample can be in core or irregular block. The equipment is easy to use and handle as test could be perform directly on site.
Scmidt or Rebound Hammer Test
Scmidt or Rebound Hammer Test
It normally test on surface hardness of rock sample using the L-type as it is also easy to use and handle. The sample can be in core or block shape and it is non-destructive type of test where the sample can be reused again.
Slake Durability Index Test
Slake Durability Index Test
This test is to assess the resistance of rock sample throughout weakening and disintegration when subjected to drying and wetting process, known as weathering process.
Sonic Wave Velocity Test
Sonic Wave Velocity Test
This test is non-destructive and the equipment is portable. The test involves transmitting of primary-wave through core rock sample and the data obtained is wave propagation velocity.
Brazilian or Indirect Tensile Strength Test
Brazilian or Indirect Tensile Strength Test using the UTM equipment.
The objective of this test is to measure uniaxial tensile strength of rock sample indirectly using Brazilian test.
Direct Test or Strength Test
The test procedure requires detailed preparation of sample in terms of standard shapes and finishing. The sample preparation process is equipment related and it is costly. The testing itself involving sophisticated and large equipment significant to the detailed testing procedures and may require complex analysis and this is also costly.
However, the data obtained is the fundamental property and would be the direct presentation of property being evaluated. The numbers of tests were limited due to its cost of operation and with this the data obtained can be use for detailed design.
The tests for Direct or Strength test include:
Hook's Cell used for Triaxial Compressive Strength Test
- Permeability of rock
- Modulus of deformation
- Uniaxial and Triaxial compressive strength test
- Shear strength test
Uniaxial Compressive Strength Test
It requires a preparation of sample as accordance to ISRM (International Society of Rock Mechanics). Uniaxial compressive strength (UCS) of rock material and deformation behavior under loading is verify by applying compressive load until failure using high capacity Universal testing machine (UTM).
Triaxial Compressive Strength Test
Triaxial compressive test is a 3 dimensional compression which used to evaluate the strength of rock under confinement condition; example rock samples obtained from deep seated rock mass.
Shear Strength Test
Shear Strength Test
Shear test is to evaluate shear strength and shear behavior of weakness plane in rock which is not shearing of the intact rock material. This is the most expensive laboratory strength tests, as it requires special kind of method for acquiring the samples from the site as fracture plane to be tested and utmost relatively complex testing procedures. The weakness planes shear strength, fractures and joints in rock mass is important for project which involves excavation in rock such as slope and tunnel.
Field or In-situ Testing of Rocks
* Recap: The testing approach is to access the rock properties at the site scene where it is found. It will include large-scale of direct strength test on site as the preparation and the equipment could be expensive, complex, and time-consuming.
In-situ strength tests are undertaken when properties of rock is very critical to the design and detailed assessment under the actual environment is consider essential. The cost involved in undertaking the test can be seen in the anticipated behavior of the unstable block with regards to nature of the project and the surrounding of rock mass.
The main advantages of field full-scale test are:
- It involves larger size of sample as inclusive of large-scale discontinuities,
- In-situ sample is undisturbed and more representative of the actual field condition.
In conclusion, the results obtained from either laboratory or field is very useful for projects involving excavation of tunnels, stability of slopes, and foundation of structures. Photos of equipment; credit to: Matest.
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Author: Mr. Vine | Creator: Civil Craft Structures | Subscribe: RSS or Email
