CIVE 1105 – ROCK MECHANICS
ROCK POINT-LOAD TESTING
Course Coordinator: Dr. Gang Ren
NAME :LAU ZHI XIAN
STUDENT ID :S3481975REPORT DUE : 09/04/2018
The aim of this rock point-load testing is to determine the point load strength index for a specimen and classify them into a group depending on their strength. This test can be done either at field or laboratory with a proper apparatus and procedure. A peak load/failure load will be obtained from this test can be used to classify rock strength. In the test, the direction of loading depends on rock shape. If it is cylindrical core, the loading can be either diametral or axial. For diametral loading, the sample length to diameter ratio must be greater than 1, this is the equation to calculate uncorrected poind load strength Is for diametral loading. P is the peak load which will be obtained from the test, D is the distance between platen contact point which will be measured using a rule and it is recorded after rock failure. For example, before the test, D is 30mm and after it fails, there are indention on the roc, so the D is getting smaller and become to 29mm, hence the D is used to calculate Is , not the initial D. Is can be calculated easily after D and P are known. Is(50) equation is then used to calculate the point load strength index. For the axial loading, the sample length to diameter ratio must be between 0.6 to 1.The equation used to calculate Is is the same as diametral, D needed to convert to De , the equivalent core diameter using this equation. A is the area through platen points, D is measured after rock fails, W can be measured by using rule or calliper. De is then used to calculate Is(50). In this test, axial load is not able to be carried out due to the sample is not cut properly, top and bottom is not flattened. The point load test concentrates on a sample in form of rock core or irregular lump. The rock core is checked with diametral or axial test and the irregular lump is checked with irregular lump test. The point load test is started by compressing the sample in between a pair of conical steel plate until the sample fails.
This report mainly highlights the use of a point load test technique to determine the strength of rock specimen in the laboratory. The specimen is coming from either irregular lump or rock core test by using a machine to break the specimen by applied force by a pair of conical platens. To classify this rock into some group, the result of point load test is used to calculate the point load strength index (Is(50)). The test is very simple and set up of apparatus is not complicated. The test should follow the right procedure to ensure the end result is adequate. Lastly, the relationship between point load strength index and uniaxial compressive strength can be archived through point load testing. The purpose of this test is to determine the index value of specimen and categorise them into the right group.
Apparatus and Materials
This apparatus is taken from Australian Standard, Method of testing rock for engineering purposes, AS4133.4.1-2007, accessed by RMIT University Library on 30th March 2018.
The following apparatus is required:
A loading device comprised of a loading frame, pump, ram and platens. The device shall have the following essential features:
The device shall be adjustable to accept and test available rock specimen, e.g., in the size range 25mm to 100mm, for which a loading capacity of up to 50kN is commonly required.
A quick-retracting ram, to help minimize delay between tests, with a low ram fiction so as not to impair the accuracy of the load measurement.
Conical platens, of included angle 60° with a 5mm radius spherical tip, to transmit the load to the specimen. The platens shall be hardened to a Rockwell hardness of 30 HRC. They shall be accurately aligned so that each is coaxial with the other and the device shall be rigid to ensure that the platens remain aligned during testing.
A spherical seat or other non-rigid component shall not be permitted in the loading system.
A system capable of measuring the failure load (P) to an accuracy of ±5% irrespective of the strength of the specimen tested. The system shall incorporate a maximum-indicating device such that the reading is retained and can be recorded after specimen failure. It shall be resistant to hydraulic shock and vibration so that the accuracy of readings is maintained during testing.
A distance-measuring system capable of indicating the distance (D) between platen contact point to an accuracy of ±0.5mm. it shall be designed to allow zero check and adjustment, shall be sufficient robust to ensure that accuracy is maintained during testing.
Figure SEQ Figure * ARABIC 1: Shape proportions and equivalent core diameter of test specimen
While carrying out for the diametral and axial tests, there are few steps we should take into account in accordance to Australian Standard, AS4133.4.1-2007;
Measure the diameter and length of the rock specimen chosen to be tested.
If length/diameter ratio is greater than 1.0, follow the Diametral Test loading (refer to Figure 1).
If length/diameter ratio is of 0.6 to 1.0, follow Axial Test loading (refer to Figure 1).
The Diametral Test is carried out for this rock specimen.
Place the rock in the loading device and close the platens so that they align to the centre.
Make sure that the distance (L) from the contact points to the nearest free end is at least more than half the diameter (D).
The load is then increased until failure and the distance between platens is taken into measurement.
The failure load (P) and failure mode for each specimen is recorded.
Point Load Test Result
Rock type Sand Stone
Length/Diameter Ratio 2.06 ? 1
(manual measurement) L1 91.70mm
Average L 92.43mm
(manual measurement) D1 44.78mm
Average D 44.83mm
Platen separation, D
(before failure) 45mm
Peak Failure Load, P
Platen separation, D
(after failure) 36mm
Figure 2: D is measured using a calliper Figure 3: Peak failure load of rock specimen
In accordance to Australian Standard, AS4133.4.1-2007, the point load strength and point load strength index for Diametral Test is as follow:
The uncorrected point load strength (Is) is calculated using the following equation:
Is=P×1000D2 =23.753×1000362 =18.33 MPawhereIs = uncorrected point load strength, in megapascalsP = load at failure, in kilonewtonsD = platen separation, in millimetres
The point load strength index (Is(50)) is calculated using the following equation:
Is(50)=Is×D500.45 =18.33×36500.45 =15.81 MPawhereIs(50) = point load strength index
Is = uncorrected point load strength, in megapascalsD = platen separation, in millimetres
Calculate the strength anisotropy index (Is(50)) as the ratio of corrected strength indices for tests perpendicular and parallel to planes of weakness.
I?(50)=IS(50)IS(50)=normal to bandingparallel to bandingAccording to Bieniawski, Z. T, he had plotted an index-to-strength conversion factor versus core diameter chart that could be referred to in determining a sample’s uniaxial compressive strength (figure 6). As such, the equation is as follow:
?c=k×Is =21.8×11.33 =246.99 MPawhere?c = uniaxial compressive strength, in megapascalsk = index-to-strength conversion factor
Is = uncorrected point load strength, in megapascals6.1 Unconfined Compressive Strength
There is a variety range of the rock-strength test. One of them is Uniaxial Compressive Strength (UCS) test. This test will break the sample in confined compression to calculate the stress.
Figure 4: Rock-Strength Tests
Figure 5: Point Load Test Interpretation
Figure 6: Size correlation graph for index-to-strength ratio (Bieniawski,1974)
From the Figure 6 above shown, it composes of generalized conversion factors that does not take into consideration the understanding of the rock masses response behaviour. Based on present studies, depending upon mineralogical and textural factors, the relationship between point load index and uniaxial compressive strength varies between different rock types. (Singh, Kainthola, Vankatesh, 2011). For example, according to Rusnak and Mark (1999), the conversion factor for sandstones should be 20 Is while Bieniawski (1974) suggested that 23Is would be a more appropriate value for its compressive strength. Taking into consideration both of these studies, it is concluded that in this case, the conversion factor of 21.8 would give better results.
6.3 Field Estimates of Uniaxial Compressive Strength
Grade* Term Uniaxial Compressive Strength (MPa) Poind Load Index (MPa) Field Estimate of Strength Examples
R6 Extremely Strong ; 250 ; 10 Specimen can only be chipped with a geological hammer Fresh basalt, chert, diabase, gneiss, granite, quartzite
R5 Very Strong 100 – 250 4 – 10 Specimen requires many blows of a geological hammer to fracture it Amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, limestone, marble, rhyolite, tuff
R4 Strong 50 – 100 2 – 4 Specimen requires more than one blow of a geological hammer to fracture it Limestone, marble, phyllite, sandstone, schist, shale
R3 Medium Strong 25 – 50 1 – 2 Cannot be scraped or peeled with a pocket knife, specimen can be fractured with a single blow from a geological hammer Claystone, coal, concrete, schist, shale, siltstone
R2 Weak 5 – 25 ** Can be peeled with a pocket knife with difficulty, shallow indentation made by firm blow with point of a geological hammer Chalk, rocksalt, potash
R1 Very Weak 1 – 5 ** Crumbles under firm blows with point of a geological hammer, can be peeled by a pocket knife Highly weathered or altered rock
R0 Extremely Weak 0.25 – 1 ** Indented by thumbnail Stiff fault gouge
-1381008840*Grade according to Brown (1981)
Based on the table above, the specimen sample with a 246.99MPa is belonging to rock Grade 5 which is very strong. The specimen requires many blows of a geological hammer to fracture it. At the same time, the sample will ring when experience hit. The example for the specimen is amphibolite, sandstone, basalt, gabbro, gneiss, granodiorite, limestone, marble, rhyolite, tuff. Therefore, this specimen can be classified into a group of igneous rocks.
The rock as shown above is the rock sample that used for rock point-load testing. Rocks consist of minerals which are formed by the combination of nature process. In general, rocks could be classified into three type namely igneous rock which is formed by magmatism, metamorphic rock which is formed by metamorphism, and lastly sedimentary rock which is formed by sedimentation. The different types of sedimentary rocks together with its identification could be seen in table 1.
There are five geological factors in rock masses which are intact rock, discontinuities/rock structure, in situ stress, pore water and time influence. The orientation of the rock fracture above can be described as the dip and dip direction of the line of steepest declination in the plane of discontinuity. The alteration ring around the fracture (the thin and lighter zone at the base of the fracture as shown above) indicates that some alteration has occurred because of circulating water or other fluids. This texture on the fracture running from top left to bottom right, represents a “slickensided” surface which occurs when the rock surfaces have moved over one another. Therefore, fluid has travelled through this fracture and there has been shear movement on the fracture. These features indicate a connected rock fracture system in which the rock blocks have been moved about. Thus, the rock blocks could be well developed and hence more likely to be unstable.
Sandstone, which is the rock that was used in this test, falls into the sedimentary rock category. These types of rock got its name due to it being formed by sedimentation of material at the Earth’s surface and within bodies of water. Basically, particles, or sediment, are accumulated by weathering and erosion in a source area, in which will then be transported by water, wind, mass movement, or glaciers to a deposition place. Hence is the process of how sedimentary rocks are formed (Zhang, 2016). Unlike igneous and metamorphic rocks, a sedimentary rock usually contains very few different major minerals, and it has lower strengths and higher porosity.
Referring back to sandstone, its varies from fine-grained to coarse grained and could be determined without the use of experimental procedures. Mature sandstones or quartz sandstones are light-coloured and majorly consist of rounded and well-sorted quartz grains. Sandstones that contain angular grains of several different minerals are referred to as immature sandstones or graywackes. In general, sandstones could be found in the colour of white, red, grey, pink, black, or brown. From observation, it could be seen that the sandstone tested could be categorized as a mature sandstone.
Table SEQ Table * ARABIC 1: Scheme for sedimentary rock identification.
Rock Strength Characteristics
Table 2: SKM
The table above shows the strength of material had been classified into eight categories for individual properties such as uniaxial compressive strength. When the value of UCS is more than 1MPa, the rock is separated from called soil. Based on the table above, we can classify that the term for the tested rock is a group of extremely high due to the value of UCS is more than 250MPa. Therefore, the rock can only be chipped with a geological hammer and produce a ring when been hit.
When classifying the strength characteristics of rock, it is recommended to make the classification based on the uniaxial compressive strength rather than the point load strength itself as could be seen in table 3 (Deere and Miller, 1966). This is because the strength ranges in table 1 are more internationally recognized, unlike those of the point load index which differs from each author that uses different designation and size correlations (Bieniawski,1974).
Also, using the unconfined compressive strength of intact rock, the Rock Mass Rating (RMR) could be concluded based on table 4 (Zhang, 2017) which is almost similar to table 1 but with slight modifications. The RMR or the Geomechanics Classification System was initially developed for tunnels. However nowadays, it could be used to approximate the strength of in-situ rock masses.
Besides that, rock strength may also be characterise using the International Society of Rock Mechanics (ISRM) classification which could be seen in table 5 (ISRM, 1978c). In this classification, the rock may range from extremely weak to extremely strong depending on the unconfined compressive strength or the approximate field identification. It is to be noted that the ISRM classification may yield ambiguous results for any unconfined compressive strength below 25 MPa.
Thus, based on all 3 tables, it could be seen that based on the unconfined compressive strength of 246.99MPa, the rock sample in this test is of very high strength (table 3), has a rating of 12 (table 4), and its field identification is that it can only be chipped with the geological hammer (table 5).
Table 3: Strength classification of rock materials (Deere and MIller,1996)
Table 4: Rating of intact rock masses using unconfined compressive strength as parameter (Zhang, 2017)
Table 5: Engineering classification of rock by strength (ISRM, 1978c)
Interpretation of the Point Load Test
Point Load Test is considered as a technique used to determine the strength and fracture roughness of a specimen. The test value is used to classified the specimen into a group depend on the strength significance. This test can be done at field or laboratory with lower cost, appropriate, fast and easiness of sample preparation. The result from the test is used to calculate point load strength index (Is(50)) of rock specimen and then find the anisotropy index (Is(50)) which is the ratio of point load strength in a direction which gives the highest value and lowest value. The specimen is broke in between a pair of conical plates either the rock is core or irregular lumps.
In this experiment, the point load test was conducted to determine the strength characteristic of rock, which is an important geotechnical parameter. The rock is initially a round cylindrical shape that is intact. During the Diametrical testing, the rock was split roughly through the middle with a peak failure load of 23.753kN. Before the testing the platen distance was 45mm but once failure of rock was achieved, the platen shows a distance of 36mm, a reduction of 9mm of its initial value. Using the experiment data and calculation formula from resources online, it is concluded that the Is is 18.33MPa while the Is(50) is 15.81MPa. As for the unconfined compressive strength, a value of 246.99MPa was calculated. Since this value is more than 250 MPa, based on table 5, it could be concluded that this rock is of very strong strength with grade 5. This highlights that only the hammering of a geological hammer can put a dint on it.
The value of point load strength index in this experiment is 15.81MPa. It clearly shows that the increase in the diameter of the rock will decrease the value of point load strength index. In addition, the bigger the value of P, the value of point load strength index will increase. The value for anisotropy is not calculated because it is not needed.
The point load testing is an effective technique to determine the rock strength by calculating the UCS value using a test result. This test is very suitable for a drill core sample. In the view of geotechnical evaluation, this test is very acceptable. When calculating the point load test result to find a value for UCS, the engineering skill is used to produce the result. At the same time, the point load test has required an engineer to fully utilise the data interpretation. Last but not least, this point load test is proving very convenient and use minimal cost compared to another complicated test.
Australian Standard, Method of testing rock for engineering purposes, AS4133.4.1-2007, accessed by RMIT University Library on 30th March 2018.
Encyclopedia. (2017). Igneous rocks facts, information, pictures| Encyclopedia.com articles about Igneous rocks. Online Available at http://www.encyclopedia.com /earth-and-environment/geology-and-oceanography/geology-and-oceanography/igneous-rocks Accessed 3rd April 2018.
Dr Gang Ren. Week 2 Lecture. ONLINE Available at: https//lms.rmit.edu.au/bbcswebdav/pid-8005670-dt-content-rid-18596184_1/courses/CIVE1105_1710/Week2_Lecture%20Notes_Geological%20Setting.pdf.Accessed 6th April 2018
Dr Gang Ren. Week 4 Lecture. ONLINE Available at: https//lms.rmit.edu.au/bbcswebdav/pid-8005672-dt-content-rid-18596191_1/courses/CIVE1105_1710/Week4_Lecture%20Notes_Rock_mechanics_principles_pt1. Accessed 6th April 2018