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The purpose of this study was to clarify the relationships between results of index tests and uniaxial compressive strength (UCS) in hydrothermally altered soft rocks of the Upper Miocene, which are typical of the soft rock found in northeastern Hokkaido, Japan. Index tests were performed using point load testing machine and needle penetrometer with irregular lump specimens under forced-dry, forced-wet, and natural-moist states. The relationships between irregular lump point load strength (IPLS) index and UCS, and needle penetration (NP) index and UCS were “UCS = approximately 19 IPLS index” and “UCS = 0.848 (NP index)
^{0.619}”, respectively, in soft rocks with a UCS below 25 MPa. These relationships could be applied to on-site tests of rocks with natural moisture content. The UCS could be calculated from IPLS and NP tests on soft rocks only when UCS was below 25 MPa, using the equations obtained as a result of this study.

The strength of rocks is generally evaluated based on uniaxial compressive strength (UCS). However, rock core pieces for UCS tests cannot always be obtained from outcrops of faulted, jointed, or heavily crushed rock masses. In these cases, the point load strength (PLS) or needle penetration (NP) test is a convenient and effective alternative to the UCS test because it can be done promptly using onsite testing equipment with various shaped small rock specimens taken from outcrops or floats. Provided that UCS can be estimated from a PLS or NP value, PLS and NP tests are more convenient and cheaper.

Many researchers have already studied the relationship between the PLS index and UCS. The representative relationships between the PLS index and UCS are shown in

Smaller samples are difficult to obtain even for PLS tests. In this case, the NP test is convenient and effective. Recently, relationship between the NP index and UCS of rocks was determined by Park et al. (2011) [

The purpose of this study was to investigate the relationship between the PLS index and UCS of hydrothermally altered soft rocks, which are typically found in

References | Equations | Maximum value of UCS (MPa) |
---|---|---|

D’Andrea et al. (1964) [ | UCS = 15.3 PLS + 16.3 | 350 |

Broch and Franklin (1972) [ | UCS = 23.7 PLS | 250 |

Bieniawski (1974; 1975) [ | UCS = 23 PLS | 350 |

Brook (1977; 1980) [ | UCS = 12.5 PLS | 300 |

Hassani et al. (1980) [ | UCS = 29 PLS | 200 |

ISRM Commission (1985) [ | UCS = 20∙∙∙25 PLS | 250 |

Hikita and Kikuchi (1988) [ | UCS = 12.3∙∙∙15.0 PLS | 200 |

Kahraman (2001) [ | UCS = 23.62 PLS − 2.69 | 150 |

Kahraman (2001) [ | UCS = 8.41 PLS + 9.51 | 150 |

northeastern Hokkaido, Japan (

Rock samples, which were collected primarily from the earth’s surface in ancient hydrothermal fields in northeastern Hokkaido, Japan, were hydrothermally altered volcaniclastic rocks, including fine tuff, medium tuff, pumice tuff, lapilli tuff, welded tuff, dacite, tuffaceous mudstone, tuffaceous sandstone, and tuffaceous conglomerate. The modes of occurrence of these hydrothermally altered rocks were examined in the field, and the hydrothermal alteration minerals in the rocks were identified primarily by X-ray powder diffraction (XRD) tests.

The IPLS test was conducted in accordance with ISRM Commission (1985) [^{2}, where D_{e} is the equivalent core diameter). The IPLS index can be represented by the formula:

I s ( 50 ) = F P D e 2 (1)

where F is the size correction factor, P is the peak load (failure load), and D_{e} is the equivalent core diameter. D_{e} is the diameter of a circle with an area equal to the minimum area of the cross sections containing the two loading points, and can be represented by the formula:

D e 2 = 4 W D ′ π (2)

where D_{e} is the equivalent core diameter, W is the specimen width, and D' is the distance between the two loading platens at the time of failure. The ISRM Commission (1985) [

D ′ = D − α (3)

where D' is the distance between the two loading platens at the time of failure, D is the distance between the two loading platens, and is the penetration distance of the conical platens. The distance between the two loading platens and the penetration distance of the conical platens were measured using slide calipers and a dial gauge (analog type), respectively. F can be represented by the formula:

F = ( D e 50 ) 0.45 (4)

where F is the size correction factor, and D_{e} is the equivalent core diameter.

In this study, irregular lump specimens were used for the IPLS tests (

The NP test was conducted in accordance to the methods proposed by Okada et al. (1985) [

NP index = P a (5)

where P is the penetration load, and a is the penetration depth.

The IPLS, NP, and UCS tests in this study were performed using a laboratory testing machine with specimens in forced-dry, forced-wet, and natural-moist states. The forced-dry and forced-wet states included absolutely dry and fully water-saturated specimens, respectively. The specimens were dried in an electric oven at a temperature below 60˚C for 4 days or more to achieve a constant mass and were saturated with water for 15 days or more to achieve a constant mass, respectively (Kohno et al. (2010) [

A total of 9 different rock types were sampled, and the total number of rock specimens tested was 2413 for the IPLS test, 180 for the NP test, and 262 for the UCS test (

Data points in the Figures 4(a)-(d) and

The relationships between the IPLS index and UCS in soft rocks with a UCS below 25 MPa are shown in

Rock type | Irregular lump point load strength test | ||
---|---|---|---|

Forced-dry state | Forced-wet state | Natural-moist state | |

f Tf | 302 (12) | 426 (19) | 401 (10) |

m Tf | 25 (1) | 22 (1) | - |

pm Tf | 195 (6) | 129 (6) | 81 (2) |

lap Tf | 28 (1) | 102 (3) | 66 (2) |

weld Tf | 76 (3) | 66 (3) | 50 (1) |

tfMs | - | 15 (1) | 46 (1) |

tfSs | 117 (3) | 96 (3) | 100 (2) |

tf Cg | 10 (1) | 12 (2) | - |

Dac | 23 (1) | 25 (1) | - |

Rock type | Needle penetration test | ||

Forced-dry state | Forced-wet state | Natural-moist state | |

f Tf | - | - | 100 (10) |

m Tf | - | - | - |

pm Tf | - | - | 20 (2) |

lap Tf | - | - | 20 (2) |

weld Tf | - | - | 10 (1) |

tfMs | - | - | 10 (1) |

tfSs | - | - | 20 (2) |

tf Cg | - | - | - |

Dac | - | - | - |

Rock type | Uniaxial compressive strength test | ||

Forced-dry state | Forced-wet state | Natural-moist state | |

f Tf | 35 (12) | 63 (19) | - |

m Tf | 1 (1) | 1 (1) | - |

pm Tf | 34 (6) | 32 (6) | - |

lap Tf | 1 (1) | 10 (3) | - |

weld Tf | 16 (3) | 16 (3) | - |

tfMs | - | 3 (1) | - |

tfSs | 19 (3) | 17 (3) | - |

tf Cg | 2 (1) | 2 (2) | - |

Dac | 5 (1) | 5 (1) | - |

fTf: Fine tuff, m Tf: Medium tuff, pm Tf: Pumice tuff, lap Tf: Lapilli tuff, weld Tf: Welded tuff, tfMs: Tuffaceous mudstone, tfSs: Tuffaceous sandstone, tf Cg: Tuffaceous conglomerate, Dac: Dacite.

( ): Numbers of sampling sites.

of specimens required for the coefficient of variation or those that have only one specimen were eliminated in IPLS and UCS tests, respectively; they were not included in the analysis. The correlations between the IPLS index and UCS in the forced-dry and forced-wet states were linear. The line drawn through the data points is the best fit, determined by the method of least squares regression. The equations and correlation coefficients for the forced-dry state were

UCS = 17.8 × (IPLS index), and R = 0.90 (

And those for the forced-wet state were

UCS = 21.7 × (IPLS index), and R = 0.95 (

Here, R is the correlation coefficient. The correlation coefficients for the forced-dry and forced-wet states were 0.90 and 0.95, respectively, indicating a strong correlation. We attempted to combine the forced-dry and forced-wet states. The equation and correlation coefficient for the line were

UCS = 18.9 × (PLS index), and R = 0.93 (

Where R is the correlation coefficient. The scatter in the data points was lesser at low strengths, and slightly higher at higher strengths (

The relationships between the NP index and UCS in soft rocks with a UCS below 25 MPa are shown in

UCS = 0.848 × (NP index)^{0.619}, and R = 0.74 (

Where R is the correlation coefficient. On comparing this equation to that proposed by Okada et al. (1985 [^{0.929}), there were differences observed in slope of the graph. One of the reasons why the equation in this study and that proposed by Okada et al. (1985) [

The discrepancies in the IPLS and UCS tests were calculated using a coefficient of variation:

C v = S x × 100 ( % ) (6)

where C_{v} is the coefficient of variation, S is the standard deviation, and x is the average of the IPLS (or UCS) test results. The coefficient of variation can be used to determine the number of specimens required for IPLS testing. The number of specimens required to obtain results within ϕ = 25% of the average value over a one-sided confidence interval at a 90% level of confidence was 5, 7, and 10 for a C_{v} of 20%, 30%, and 40%, respectively (dashed line in

The following is a summary of our findings related to the UCS estimates of hydrothermally altered soft rocks from northeastern Hokkaido, Japan, based on our IPLS test and NP test results.

1) The relationships between the IPLS index and UCS and the NP index and UCS were “UCS = approximately 19 × (IPLS index)” and “UCS = 0.848 × (NP index)^{0.619}”, respectively in soft rocks with UCS below 25 MPa.

2) In soft rocks, the relationships between the IPLS index and UCS in the “forced-dry and forced-wet states” and “natural-moist state” were similar. Therefore, it can be concluded that it is also possible to apply the relationship to onsite tests of soft rocks in the natural-moist state, which is intermediate between the forced-dry and forced-wet states.

3) We need to choose either the equation proposed in this study (soft rocks) or that proposed Okada et al. (1985) [

4) The number of tested specimens satisfied the accuracy requirements based on the coefficient of variation. The IPLS was strongly correlated with the UCS. Therefore, the relationships between IPLS and UCS established in this study were highly precise.

5) The IPLS and NP tests were convenient and effective because they could be performed promptly using onsite and laboratory testing equipment for various shaped small rock specimens taken from outcrops or floats.

Kohno, M. and Maeda, H. (2018) Estimate of Uniaxial Compressive Strength of Hydrothermally Altered Soft Rocks Based on Strength Index Tests. Geomaterials, 8, 15-25. https://doi.org/10.4236/gm.2018.82002