an evaluation of surface hardness of natural and modified rocks using schmidt hammer: study from...

AN EVALUATION OF SURFACE HARDNESS OF NATURAL AND MODIFIED ROCKS USING SCHMIDT HAMMER:

© The authors 2009Journal compilation © 2009 Swedish Society for Anthropology and Geography 179

AN EVALUATION OF SURFACE HARDNESS OF NATURAL AND MODIFIED ROCKS USING SCHMIDT

HAMMER: STUDY FROM NORTHWESTERN HIMALAYA, INDIA

BYVIKRAM GUPTA, RUCHIKA SHARMA AND MADHO PRASAD SAH

Wadia Institute of Himalayan Geology, India

Gupta, V., Sharma, R. and Sah, M.P., 2009: An evaluation of sur-face hardness of natural and modified rocks using Schmidt ham-mer: Study from northwestern Himalaya, India. Geogr. Ann. 91 A(3): 179–188

ABSTRACT. Four rock types (quartz mica gneiss,schist, quartzite and calc-silicate) located in theSatluj and Alaknanda valleys were used to testwhether a Schmidt hammer can be used to distin-guish rock surfaces affected by various natural andman-induced processes like manual smoothing ofrock surfaces by grindstone, surface weathering,deep weathering, fluvial polishing and blasting dur-ing road construction. Surfaces polished by fluvialprocess yielded the highest Schmidt hammer re-bound (R-) values and the blast-affected surfacesyielded the lowest R- values for the same rock type.Variations in R-value also reflect the degree ofweathering of the rock surfaces. It has been furtherobserved that, for all the rock types, the strength ofrelationship between R-values for the treated sur-faces (manual smoothing of rock surface by grind-stone) and the unconfined compressive strength(UCS) is higher than for the fresh natural surfaces.

Key words: Schmidt hammer, unconfined compressive strength,Satluj valley, Alaknanda valley, northwestern Himalaya, India

IntroductionSchmidt hammer is an instrument originally de-signed to measure in situ non-destructive surfacehardness of concrete (Schmidt 1951). When it im-pacts against a surface, the distance of rebound,known as Schmidt hammer rebound (R-) value,is measured. The instrument measures, therefore,the elastic recovery, which depends on the me-chanical strength of the surface (Day 1980). Overthe years, the hammer has been used in engineer-ing geology and geotechnical engineering forcomparing the surface hardness of different rocktypes (Deere and Miller 1966; Yaalon and Singer

1974; Barton and Choubey 1977; Day and Goudie1977; Day 1980; Selby 1980; Sheorey et al. 1984),inferring the extent of rock surface weathering(Day 1980; Matthews et al. 1986; Ballantyne et al.1989; Sjöberg and Broadbent 1991; McCarrolland Nesje 1993; White et al. 1998; Owen et al.2007), relative dating of moraines (Matthews andShakesby 1984; Sjoberg and Broadbent 1991;Evans et al. 1999; Winkler 2005; Shakesby et al.2006) and, more recently, distinguishing rock sur-faces influenced by different exogenic processessuch as aeolian, fluvial or glacial erosion (Ericson2004). Selby (1980, 1982) incorporated R-valuesinto the rock mass strength (RMS) classificationof rocks. There are also studies that exhibit the re-lationship between R-values and various engi-neering properties of rocks such as uniaxial com-pressive strength (Ghose and Chakraborti 1986;O’Rourke 1989; Cargill and Shakoor 1990; Ag-gistalis et al. 1996; Katz et al. 2000; Yilmaz andSendir 2002; Dincer et al. 2004; Aydin and Basu2005), Young’s modulus (Sachpazis 1990; Aggis-talis et al. 1996; Katz et al. 2000), Point load index(Aggistalis et al. 1996) and the Shore scleroscope(Yasar and Erdogan 2004).

Goudie (2006) has evaluated the advantages andlimitations of the Schmidt hammer and synthesizedthe contributions the hammer has made to engineer-ing geological studies. Surface texture and rough-ness of rocks have been considered a major source ofvariation in the R-values (Williams and Robinson1983). Other factors that might affect the results in-clude the presence of discontinuities, moisture con-tent, lichen growth, hammer variation and the dete-rioration of the hammer with age (McCarroll 1987;Sumner and Nel 2002; Shakesby et al. 2006). How-ever, with appropriate use of the instrument, these er-rors can be avoided. The instrument being robust,

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light, portable and user-friendly, and the rapiditywith which the data are obtained, are some of themain advantages to the use of this instrument.

Since R-values are affected by surface rough-ness, which in turn reflect surface hardness, the ob-jective of this study was to evaluate surface hard-ness of different rocks exposed to various exogenic(natural and man-induced) processes such as man-ual smoothing of the rock surface by grindstone,surface weathering, deep weathering, fluvial ero-sion (water-polished surfaces) and blasting duringroad construction. The goal was to determinewhether, on a single lithology, mean R-value re-flects the influence of specific natural and man-in-duced processes on the rock surfaces.

Selection of the study areasThe study area is located in the northwestern higherHimalaya in two transects along Satluj andAlaknanda valleys and is briefly discussed below.

Satluj ValleyThe transect in the Satluj valley forms a part of theKinnaur district of Himachal Pradesh and is locat-ed between longitudes 77˚45' and 78˚22'E and lat-itudes 31˚28' and 31˚36'N (Fig. 1). The area underinvestigation is sandwiched between the MainCentral Thrust (MCT) in the west and the Teth-yan Thrust (TT) in the east. The dominant rocktype in the area is mainly quartz mica gneiss be-longing to the Wangtu Gneissic Complex(WGC) with a number of schist bands. The geo-logical setting of the area and its environs has beendescribed in detail by Sharma (1976), Tewari et al.(1978), Bassi and Chopra (1983), Kakar (1988)and Gupta (1998). Geomorphologically, the entirearea represents a highly immature topography asevidenced by high relief and active erosional proc-esses. The terrain is highly rugged and the slopesare generally steep with an angle of >50º. Amongthe geomorphic processes, glacial and fluvialprocesses have played a dominant role in shaping

Fig. 1. Location map of the study area along two transects in the Satluj and Alaknanda valleys. The figure also depicts the generalizedgeology of the area along with the sites for the Schmidt hammer measurements

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AN EVALUATION OF SURFACE HARDNESS OF NATURAL AND MODIFIED ROCKS USING SCHMIDT HAMMER


the present landscape. Most of the currently pre-vailing processes in the area are denudational.

Alaknanda ValleyThe transect in the Alaknanda valley forms a part ofthe Ruderprayag, Chamoli and Pauri Garhwal dis-tricts of Uttarakhand and is located between longi-tudes 78°45' and 79°35'E and latitudes 30°12' and30°50'N (Fig. 1). The area forms a part of the Lesserand the Higher Himalaya and mainly comprisesrocks of the Berinag Formation, Ramgarh Group,Munsiari Group and Vaikrita Group. The rocksconstituting the Berinag Formation are mainlyquartzite and are separated from the Ramgarh andMunsiari Groups by the Ramgarh and MunsiariThrusts. The rock constituting the Ramgarh Groupis mainly schist and quartzite, whereas the Mun-siari Group is dominantly composed of calc-sili-cates. Further north lies the Vaikrita Group com-posed predominantly of quartz mica gneiss andbands of schist. It is separated from the MunsiariGroup by the Vaikrita Thrust. The geological set-ting of the area has been described in detail bySrivastava and Ahmad (1979) and Valdiya (1980).Geomorphologically, this transect is also ruggedwith steep valley slopes. Glacial and fluvial proc-esses have played a dominant role in shaping thepresent landscape. Most of the currently prevailingprocesses are denudational, but none the less com-plementary depositional processes also occur.

Both transects are undergoing rapid develop-ment. Hydroelectric projects such as Nathpa-Ja-khri, Baspa stage II and Sanjay Jal Vidyut in theSatluj Valley and Bamri, Vishnu Prayag and Helangin the Alaknanda Valley are already operational.However, many are either at the initial stage of theirdevelopment or are planned. This resulted in theconstruction of dams, bridges, new road-cuts andnew townships, which often require the assessmentof strength characteristics of rocks.

Materials and methodsThe Schmidt hammer used in this study is the N-type, which is intended for testing concrete (Proceq1977). On each site, 50 measurements were takenrandomly across a matrix of a representative sam-ple area for each site, following the methodology ofDay and Goudie (1977).

Our sites are distributed essentially on quartzmica gneiss in the Satluj Valley transect, and quartzmica gneiss, schist, quartzite and calc-silicate in the

Alaknanda Valley transect. Different natural andmodified rock surfaces, namely rock surfacessmoothed by grindstone, weathered surfaces, deep-weathered surfaces, surfaces polished by fluvial ac-tion and blast-affected surfaces were selected for theSchmidt hammer measurements. Grindstone pro-vided with the Schmidt hammer is used for surfacesmoothing. Weathered surfaces are classified intofresh surface with no sign of weathering, slightlyweathered surfaces having little discoloration andmoderately weathered surface having discolorationand grussy surface. Surfaces polished by fluvial ac-tion are mostly confined along the palaeo-drainagechannels and are generally smooth and shiny,whereas the blast-affected surfaces lie along thefresh road-cuts. All the measurements were takenwith the hammer held horizontally, perpendicular tothe intact test surface, at least 6 cm away from anyjoints or cracks and on surfaces free from lichens,dirt and flakes. In order to control operator variation,all the measurements were taken by the same oper-ator. All the measurements of the R-value were doneon bedrocks; therefore the entire hammer’s impactenergy is returned to the instrument. A total of 6500measurements were taken for the present study, outof which 4000 measurements were used for thecomparison of fresh natural and treated rock surfac-es (manual smoothing of rock surface by grind-stone) on four rock types (quartz mica gneiss, schist,quartzite and calc-silicate); 500 measurements wereused for the comparison of fresh natural and water-polished surfaces on quartz mica gneiss; 1500measurements were used for the comparison offresh and weathered rock surfaces on quartz micagneiss, quartzite and calc-silicate; and 500 measure-ments were used for the comparison of natural andblast-affected surfaces on the quartz mica gneiss. Inorder to statistically analyse the data, minimum,maximum, mean and the lower and upper 25% quar-tile of the 50 measured R-values for different sur-faces were calculated and plotted as box plot.

For measuring the unconfined compressivestrength (UCS) of the rocks, cylindrical cores of2.54 cm diameter and 4.5 to 5 cm length weredrilled from the chunks of bedrock collected fromthe field. UCS of all the cores was measured in ac-cordance with the ISRM (1981).

ResultsFresh natural surfaces versus treated surfacesFour rock types (quartz mica gneiss and schist be-longing to the higher Himalaya and quartzite and




calc-silicate belonging to the lesser Himalayansequence of the Alaknanda Valley) were used tocompare R-values from fresh natural surfaces andsurfaces treated with grindstone for about fiveminutes. It should be noted that the variability ofthe R-values for the fresh natural surfaces is sub-stantially higher than for the treated surfaces forthe same rock type (Fig. 2). This can be explainedby the variation in factors such as surface rough-ness and length of exposure to exogenic condi-tion. It should also be noted that in the case ofquartzite and calc-silicate composed dominantlyof single mineral, the spread of R-values is lesscompared to the quartz mica gneiss and the schist(Fig. 2).

The unconfined compressive strength (UCS) ofquartz mica gneiss, quartzite and calc-silicate hasalso been calculated and correlated with the R-val-ues for the fresh natural surfaces and the treatedsurfaces. Since it was not possible to obtain cylin-drical cores of the required size from schist, it wasnot included for the correlation. It may be observedthat, for all rock types, the strength of the relation-ship between R-values and UCS is higher for thetreated surfaces than for the fresh natural surfaceand the UCS (Fig. 3a–c). It is 0.90, 0.89 and 0.58for the treated surface of quartz mica gneiss,

quartzite and calc-silicate; and 0.72, 0.54 and 0.49for the fresh natural surfaces, respectively.

Fresh natural surfaces versus water-polished surfacesWater-polished bedrock surfaces are found frequent-ly along the palaeo-drainage channels. These areshiny and smooth (Fig. 4). Schmidt hammer R-val-ues from these surfaces have been compared with theR-values of the same rock located in the adjoining ar-ea. The rock type studied was quartz mica gneiss inboth of the valleys. It should be noted that the R-val-ues on the water-polished surfaces are much higherwith an average of 68 (Range 62–69) than on thefresh natural surfaces, which have an average R-val-ue of 54 (Range 47–57) (Fig. 5). Furthermore, thevariability of R-values on the water-polished surfac-es is much less than the R-values on the fresh naturalsurfaces. This is mainly due to the removal of pro-jections of the grains of quartz and feldspar, resultingin the smooth and polished surfaces.

Fresh surfaces versus weathered surfacesOn the basis of visual observation, three stages ofweathering for the sample surface were identified

Fig. 2. Box plot summarizingthe calculated R-values (shapeof distribution, median and var-iability) on the fresh naturalrock surfaces and manuallysmoothed rock surface bygrindstone for quartz micagneiss, schist and quartziterocks of the Alaknanda Valley




in the field. This is presented in Table 1. Weather-ing class I is equivalent to the fresh surfaces,whereas weathering classes II and III are slightlyand moderately weathered respectively. Schmidthammer tests were undertaken on each weather-ing class on the quartz mica gneiss, quartzite andcalc-silicate. The results are presented in Fig. 6. Itshould be noted that, for all the studied rock types,there is a slight difference in R-value (in the rangebetween 2 and 3) between weathering classes Iand II. The mean of the R-value decreases with in-creasing weathering. The difference betweenweathering classes II and III, particularly for thequartz mica gneiss, is in the range 3–13, whereasfor quartzite and calc-silicate the difference is inthe range 4–10. This is mainly because the higher

weathering class exhibits rough and undulatingsurfaces, thus exhibiting higher variability andlower mean R-values.

Blast-affected surfacesIn the Himalayan region, roads are cut by the con-ventional method of uncontrolled drill and blast.Thus, blast-affected rock surfaces are common allalong the road-cuts. These are evidenced by thepresence of drill marks on the rock surface (Fig. 7).Schmidt hammer rebound (R-) values were takenon the blast-affected surface of the recent road-cuts. It should be noted that, for all rock types, themean R-value is lower than for any other surface,even surfaces of weathering class III (Fig. 6).

Fig. 3. The relationship betweenthe Schmidt hammer rebound (R-)values and the unconfined com-pressive strength (UCS) for thefresh natural (dashed line) andtreated surfaces (solid line) for (a)quartz mica gneiss (b), quartziteand (c) calc-silicate rock of theAlaknanda Valley




Fig. 4. A view of a water-polishednatural rock surface. Note the freshnatural rock surface at the top rightcorner

Fig. 5. Box plot exhibiting Schmidthammer rebound (R-) values forthe fresh natural weathered surfaceand the water-polished surface




DiscussionIt is well known that R-value for different rocktypes varies to a great extent. The present study ex-plains some of this variability, in this case for rocktypes affected by various natural and man-inducedprocesses, namely manual smoothing of rock sur-face by grindstone, surface weathering, deepweathering, fluvial polishing and blasting duringroad construction. For example, there are distinctdifferences in the R-values measured on the freshnatural surfaces and on the surfaces treated with thegrindstone, and also between the fresh surfaces anddifferent weathered surfaces. It has further beennoted that the strength of the relationship betweenthe R-value and the UCS is stronger for the treatedsurfaces than for fresh natural rock surfaces. Fur-thermore, the variability of the R-values increases

as the weathering increases. This is mainly becausethe surface roughness plays a crucial role whentesting with Schmidt hammer as the presence ofmicro-cracks and the protruding micro-grains in-fluence the R-values. A more weathered and roughsurface is characterized by protruding grains whichconsequently dissipate the energy of the blow of theimpact plunger.

There are studies that exhibit the simple andcomplex empirical relationship between R-valuesand the UCS without referring to the type of rocksurfaces used (Deere and Miller 1966; Aufmuth1973; Singh et al. 1983; Ghose and Chakraborti1986; O’Rourke 1989; Cargill and Shakoor 1990;Sachpazis 1990; Katz et al. 2000; Yilmaz and Send-ir 2002; Yasar and Erdogan 2004; Shalabi et al.2007). These are presented in Table 2. In the

Fig. 6. Box plot exhibiting Schmidthammer rebound (R-) values fordifferent classes of weathering andthe blast-affected surface for therocks of quartz mica gneiss (SatlujValley), quartzite and calc-silicate(Alaknanda Valley)

Table 1. Different classes of weathering observed in the study area.

Weathering class Field description

I (Fresh Surface) Fresh, smooth and completely intact surface with no visible sign of weathering

II Slightly weathered surface with some discoloration, light cracking, protruding grains,original rock surface and material fabric still intact

III Moderately weathered and discoloured surface, original mass surface and material fabric still intact but the surface is grussy




present study, it has been noted that the strength ofrelationship between the R-value and the UCS ishigher for the treated rock surfaces than for thefresh natural rock surfaces; therefore is necessaryto grind the surface to make it sufficiently smoothbefore measuring R-values for the purpose of es-tablishing the relationship between R-value and theUCS.

Katz et al. (2000) compared measurements onseven different rock types and on three types of sur-faces: naturally weathered surfaces, rock surfacespolished manually by the grinding stone, and rocksurface polished with an electrical grinder. Theirstudy showed that the R-values and repeatability ofthe hammer readings increase with intensity of pol-ishing.

Fig. 7. A view of the blast-affectedsurface

Table 2. Empirical relationship between the unconfined compressive strength (UCS) and the Schmidt hammer rebound (R-) value asreported by various workers

Sr. no. Equations Strength Lithology References

1 UCS = 1246SHR -34890 0.880 basalt, diabase, dolomite, gneiss, granite, Deere and Miller 1966limestone, marble, quartzite, rocksalt,sandstone, schist, siltstone and tuff

2 UCS = 6.9 × 0 10[1.348log(γSHR)+1.325] ---- 25 lithological units Aufmuth 19733 UCS = 2 SHR 0.72 30 sedimentary units Singh et al. 19834 UCS = 0.45 SHR – 3.6 0.94 20 lithological units Sheorey et al. 19845 UCS = 0.994 SHR – 0.383 0.70 10 lithological units Haramy and DeMarco 19856 UCS = 0.88 SHR -12.11 0.87 Coal Ghose and Chakraborti 19867 UCS = 702 SHR – 1104 0.77 anhydrite, siltstone, sandstone and limestone O’Rourke 1989

8 UCS = 4.3 × 10-2(SHR γd)+1.2 sandstone Cargill and Shakoor 1990UCS = 1.8 × 10-2(SHR γd)+2.9 carbonates

9 UCS = 4.184 SHR – 65.792 0.93 carbonate rocks Sachpazis 199010 UCS = 1.31 SHR – 2.52 0.55 gabbro and basalt Aggistalis et al. 199611 UCS = 2.208 e0.067 SHR 0.96 chalk, limestone, sandstone, marble, Katz et al. 2000

granite and syenite12 UCS = e(0.059 SHR + 0.818) 0.98 gypsum Yilmaz and Sendir 200213 UCS = 4 × 10-6 (SHR)4.2917 0.89 limestone, marble, basalt and sandstone Yasar and Erdogan 200414 UCS = 3.201 SHR – 46.59 0.76 dolomite and dolomitic limestone Shalabi et al. 2007




In the present study, blast-affected surfaces ex-hibit the lowest R-value for a particular rock type.This is because the gunpowder is known to changethe crystalline structure within the bedrock (Meur-man 2000). The lowest variability in the R-valuesis found in the hard rock surfaces and the fluvialpolished surfaces. It is mainly due to the polishingof the bedrock giving it a smoother surface.

ConclusionsThe Schmidth hammer rebound (R-) value and theunconfined compressive strength (UCS) for dif-ferent rock surfaces of quartz mica gneiss, schist,quartzite and calc-silicate were measured. Thestudy suggests that the Schmidt hammer rebound(R-) value may be used to distinguish rock surfac-es affected by various natural and man-inducedprocesses like manual smoothing of rock surfacesby grindstone, surface weathering, deep weather-ing, fluvial polishing and blasting during roadconstruction. The following conclusions may bedrawn:

1 Surface polishing by fluvial processes yieldedthe highest Schmidt hammer rebound (R-) val-ues.

2 The mean R-value for blast-affected surfaces,for all rock types, is lower than for any other sur-face, even surfaces of the highest weatheringclass III.

3 Variations in R-value are reflected by the degreeof weathering of the rock surfaces for a particu-lar rock type.

4 For all rock types, the strength of relationshipbetween R-vaule and the UCS is stronger for thetreated surfaces (manual smoothing of rock sur-faces by grindstone) than for the fresh naturalsurfaces. In order to estimate the strength pa-rameter of the rock with Schmidt hammer, it istherefore, necessary to grind the surface withgrindstone before measuring the rebound value.

AcknowledgementsThe authors thank the Director, Wadia Institute ofHimalayan Geology, Dehra Dun, for his kind per-mission to publish this paper. We also thank theanonymous reviewer for his critical comments. Apart of the study has been supported by a DST-funded project titled “Rock Properties Laboratory– A National Facility” which is gratefully acknowl-edged.

Vikram Gupta, Ruchika Sharma and Madho Pras-ad Sah, Wadia Institute of Himalayan Geology, 33General Mahadeo Singh Road, Dehra Dun –248 001, IndiaE-mail: [email protected]@wihg.res.in

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Manuscript received Dec. 2008 revised and accepted March2009.


an evaluation of surface hardness of natural and modified rocks using schmidt hammer: study from...

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