A new technique for non‐destructive field measurement of rock‐surface strength: an application of the Equotip hardness tester to weathering studies

Abstract

Tafone-like depressions have developed on the Aoshima sandstone blocks used for a masonry bridge pier in the coastal spray zone. A thin layer of partial granular disintegration was found on the surface in depressions. To evaluate quantitatively the strength of the thin weathered layer, the hardness was measured at the surface of the sandstone blocks using both an Equotip hardness tester and an L-type Schmidt hammer. Comparison of the two testing results indicates that the Equotip hardness value is more sensitive in evaluating the strength of a thin layer of weathered surface rock than the Schmidt hardness value. By applying two methods, i.e. both the repeated impact method and the single impact method, the Equotip tester can evaluate the strengths of fresh internal and weathered surficial portions of rocks having a thin weathering layer. Comparison of the two strengths enables evaluation of strength reduction due to weathering. Figure 2. Measurement ranges of Equotip hardness tester, Schmidt hammer, needle-type penetrometer (model SH-70, Maruto, Tokyo) and cone penetrometer (model YH62, Yamanaka, Tokyo).Figure 7. (a) Aoshima sandstone block used for calibration of Equotip rebound value of weathered rock surface (area A) and fresh surface (area B): the split surface was cut by a diamond saw. (b) Sectional view of the edge of split surface.Sandstone is mainly composed of quartz grains having a diameter of 0·1-0·2 mm. Micro-undulation due to protruding sand grains (white arrows) and cracks (black arrows) seems to be formed by granular disintegration due to salt weathering. The photograph shows that the weathering layer has a thickness of about 0·5-0·7 mm. Figure 8. Rebound values of Equotip (L-value) on weathered surface and split surface of Aoshima sandstone block.impact method (10 individual impacts on different points). Figure 8 shows the results. The rebound values obtained, from both the weathered and unweathered parts of the sample, increase as the number of impacts increases. It should be noted here that the first impact values of the weathered and unweathered portions are clearly different (443 for the former and 551 for the latter), but L max , the mean of the largest three values during the measurement, shows similar values (716 for the former and 720 for the latter). The value of L max thus defined designates the hardness of the unweathered interior of the rock. A new technique for non-destructive field measurement of rock-surface strength Figure 10. Relationship between estimated L-value of fresh rock (L intact -value) and measured L surface -value.value of L surface , which results in a higher δ -value. When considering the strength reduction due to weathering, therefore, the minimum value in the data cluster of each zone is of significance. Figure 9 shows that δ = 0·67 for the spray zone and 0·76 for the intertidal zone. The results indicate that the strength of a block in the spray zone has been reduced to 67% of the strength of the unweathered fresh part, and in the intertidal case it has been render...

Chapter 12 The rock coast of Japan

“…If coastal rocks are weathered, then we must replace S c in equation (5) with reduced strength owing to weathering. The strength of rocks subjected to weathering has been measured by many researchers using Schmidt hammers (references cited in Aoki & Matsukura 2007). Weathering of a coastal rock mass without discontinuities begins at the rock surface and penetrates into the interior with time, which results in the formation of strength gradient, that is, weathering profile, with the weakest part being located at the surface forming a thin layer.…”

“…Weathering of a coastal rock mass without discontinuities begins at the rock surface and penetrates into the interior with time, which results in the formation of strength gradient, that is, weathering profile, with the weakest part being located at the surface forming a thin layer. For testing this thin weathered layer a Schmidt hammer is not a suitable device (Aoki & Matsukura 2007), rather an Equotip hardness tester or a needle-type penetrometer is used. Using the Equotip tester, Aoki & Matsukura (2007) investigated weathering-induced strength reduction of sandstone blocks used for a masonry bridge pier at Aoshima, described before, and reported that the surface strength of the blocks located in the inter-to supratidal environment reduced to c. 70% of that of fresh blocks.…”

“…Attempts have been made to examine the relationship between the Equotip reading and compressive strength (e.g. Verwaal & Mulder 1993;Aoki & Matsukura 2008) and to apply this instrument to geomorphological studies (Aoki & Matsukura 2007;Viles et al 2011). A needle-type penetrometer (e.g.…”

“…Tuffaceous sandstone of the landward cliff at the MEM measurement site is considerably weathered: a fingertip can easily rub off sand grains from the rock surface. Aoki & Matsukura (2007) have reported the usefulness of the Equotip hardness tester in evaluating the strength of the surficial part of weathered rocks. Application of the Equotip tester to the tuffaceous sandstone very close to the MEM measuring point indicated that compressive strength, based on the empirical relationships of Aoki & Matsukura (2008), is 4.0 MPa, which is less than a half that of the fresh rock strength (9.5 MPa).…”

Schmidt hammer tests across a recently deglacierized rocky coastal zone in Spitsbergen - is there a "coastal amplification" of rock weathering in polar climates?

“…However SHRT should be treated only as a preliminary reconnaissance and the study on influence of coastal processes on rock breakdown in polar climates should be supported by application of more advanced methods (e.g. Equotip, MEM, Terrestrial Laser Scanning, digital photogrammetry, GIS modelling) to re− duce the risk of misinterpretation (Lim et al 2005(Lim et al , 2010Aoki and Matsukura 2007;Viles et al 2011). The major weakness of this study is above all the lack of information regarding the spatial distribution of permafrost and the difference in development of the active layer between coastal and inland outcrops, which could have a significant impact on rock stability and degree of weathering.…”