Point Load Laboratory Report U1011770 Mihai Solomon
University of East London CE 1203 Module Leader: Prof. Richard Freeman Date of the experiment: 19th of November 2010
Contents 1.
Introduction ............................................................................................... 3
2.
Research Methodology ............................................................................. 3 2.1 Apparatus ............................................................................................... 3 2.2Procedure ................................................................................................ 4
3.
Data readings/results ................................................................................ 4
4.
Analysis of data/errors .............................................................................. 5
5.
Discussion (data interpretation)................................................................. 7
6.
Conclusion ................................................................................................ 7
7.
References................................................................................................ 8
8.
Bibliography .............................................................................................. 8
9.
Appendices ............................................................................................... 9
2
1. Introduction
The data for this report was gathered during a Rock and Soils laboratory, on the 19th of November 2010 at the University of East London. It was requested to test five irregular lumps of rock, and fi nd their point load strength index.
2. Research Methodology 2.1 Apparatus
Ruler
The apparatus (Figure 1) comprises of two metal cones, the upper one fixed to the steel frame, the lower one connected to a hydraulic jack. A small display on the right records the maximum force, while a measuring device (ruler) is attached to measure the distance between the two cones, as the rock tested breaks. A pressure release valve is present in the lower part of the apparatus. It is there to be used in case of accidents, or when the pressure has to be released very fast.
NOTE: the apparatus is not the exact representation of the one used in the
Pressure
Digital Force
release
Display
valve
Figure 1 – Apparatus Sketch
UEL laboratory. It was simplified to that a clear sketch could be designed
3
2.2Procedure
The aim of the test was to find the strength of five pieces of “Terracota”, a red, man-made rock. Each of the five pieces of rock was measured and placed between the two metal cones. The failure force (the force at which the rock broke) was recorder. The rocks were measured both pr ior to the experiment and afterwards, to make sure that the area of the breaking cross-section was accurate. The readings are illustrated in Table 1.
NOTE: Even though the rocks were measured in the lab, only the Failure Area is present in the table.
3. Data readings/results
Description of
Force at
Failure Area
rock
Failure (kN)
( )
Rock 1
3.43
1498.49
Comments
Man made “Terracotta” rock
Rock 2
1.86
1184.69
Man made “Terracotta” rock
Rock 3
1.55
723.84
Man made “Terracotta” rock
Rock 4
3.91
1159.03
Man made “Terracotta” rock
Rock 5
2.59
1255.88
Man made “Terracotta” rock
Table 1 – Data collected in the lab
4
4. Analysis of data/errors
After the data has been collected, calculations to find out the Uncorrected Point Load Strength (Is) can begin.
Is
P
De
2
Eqn 1 – Uncorrected Point Load Strength formula P = failure force De
2
4A
Because the lumps the experiments were conducted on were not perfect conical structures (they were irregular lumps), a size correction factor has to be applied to the Is. The corrected PLS (point load strength) index, Is(50) is defined as the value of Is that would have been measured with a diametral test on the same rock with D = 50 mm. The most reliable method is to carry out diametral test on the rock at, or close to D = 50 mm and therefore, no correction is necessary. However, if this is not possible, there are two methods of finding the size correction factor.
Method 1 – Graphical method- test the sample over a range of D values, and draw a log-log graph of De^2 against P. The value of P50 (corresponding to De = 50 mm) can be found by interpolation (or extrapolation). Any points on the graph that do not follow a straight line can be ignored but must be plotted. The Is(50) was calculated using this method in the appendices. (GRAPH 1)
Method 2 – Mathematical method – The size corrected PLS index (Is(50)) can be calculated from the equation below (EQN 2)
5
() F x Is Where F
(
De
50
)
0.45
EQN 2
-the formula is applied to all of the rock samples, and an average is calculated (the highest and lowest value are not taken into account)
The formulas above have been applied to the data collected in the lab, and the results were put together in TABLE 2.
Rock
Force at
Failure
no.
Failure (kN)
area ( )
Is
F
Is 50
1
3.43
1498.49
1908.297
1.797414
0.941042
1.691442
2
1.86
1184.69
1508.679
1.232867
0.892581
1.100434
3
1.55
723.84
921.7956
1.681501
0.798926
1.343394
4
3.91
1159.03
1476.001
2.649049
0.888194
2.352871
5
2.59
1255.88
1599.338
1.61942
0.904378
1.464568
TABLE 2
By ruling out Rocks no. 4 and no. 2, the Average Is 50 value is 1.49981 ~ 1.5
6
5. Discussion (data interpretation)
The results obtained, seem to be wrong for Rock no 4. It has the second lowest failure area, but the highest Force at Failure value. This might have happened because the rock broke into four pieces, and the failure area recorded may be flawed. Rock no 4 was ruled out of the calculations for the average Is (50) value.
The average size corrected Point Load Strength Index of 1.5 describes the rock as a moderately weak rock. It is used for many things, but particularly, terracotta of this strength is used in pavements and walkways.
The Is (50) value recorded through the graphical method is very different and unrealistic compared to the value recorded by the mathematical method. However, as the graphical method is more prone to error (there is a lot of interpretation going on to find the Is (50) value), this experiment will base its interpretation on the mathematical method.
However, because this experiment only tested five rocks, the r esults should not be applied to other rocks of the type. More strength tests should be conducted to reach a viable result.
6. Conclusion
To conclude this report, it can easily be noticed that the mathematical method is much more accurate (the rock rock tested was indeed moderately weak, parts of it could have been scrapped of with the finger-nail) and the authors believe it is the best line of approach when calculating the PLS index of a rock.
7
7. References
1) Freeman, R.(2010), Rock Classification Lecture Notes, UEL:London
8. Bibliography 1) Brown, E.T., Brady H.G.B. (2006), Rock mechanics: For underground
mining, 4th. Ed, Springer: Dordrecht, The Netherlands.
2) Jaeger, J.C. et al ( 2007), Fundamentals of Rock Mechanics, 4th Ed., Blackwell Publishing Ltd: Oxford, UK
8
9. Appendices
9
