Effects of lithofacies and environment on in situ and laboratory Schmidt hammer tests: a case study of carbonate rocks

Hasan Arman, Osman Abdelghany, Waheed Hashem and Ala Aldahan
Quarterly Journal of Engineering Geology and Hydrogeology, 50, 179-186, 31 March 2017, https://doi.org/10.1144/qjegh2016-049

Abstract

The Schmidt hammer (SH) test, which is quick, inexpensive, non-destructive and simple to use, has been widely applied to determine hardness and to assess the unconfined compressive strength of rocks. Lithofacies (composition and texture) and environmental conditions (mainly climate) influence the mechanical properties of rock masses as shown by the investigation presented here. To evaluate these effects, a range of carbonate rocks from Al-Ain arid region in the United Arab Emirates were used for extensive in situ and laboratory SH tests. The results expose a wide range of Schmidt hardness rebound (SHRcor) values for both the in situ and the laboratory tests, but show comparable correlation coefficients between the in situ and laboratory natural surface data. This indicates that field SH measurements can be a simple and reliable tool to obtain data on rock mechanical properties; however, caution should be taken in generalization even in a small locality or a region. Although our data are within the wide range of SH values in the published literature, it is apparent that they represent unique values for the rock type investigated and the environment and thereby provide better constraints for engineers in planning for the safety aspects of constructions.

The measurement of rock strength and the characterization of discontinuities (mainly fractures) are essential tasks in engineering applications. In general, rock strength is defined as the ability to withstand stress or deformation; the quality of materials by which they encounter the application of force without breaking. In engineering terms, rock strength may be defined as the inherent strength of an isotropic rock under specific conditions, notably wet or dry. However, rock masses are anisotropic and, as a consequence, the strength of rocks is influenced by the presence of impurities, weak zones and/or discontinuities (Hawkins 1998).

Extensive (metre-scale) changes in carbonate rock structures (layering, fractures, faults, folds), texture (grain size, fossil content, cementation, dissolution) and even mineralogy (dolomitization, per cent of clay minerals, silicification) have always created problems in generalizing overall bulk physical properties such as rock strength, porosity and permeability. Thus, mass homogeneity may exist only at a small scale (in a cube of a few decimetres) whereas rock failure problems are faced by oil companies, construction engineering companies and environmental agencies when the size of the rock mass is larger. The results of such extensive change in the physical properties cause unexpected rock failure or formation of karst (surface and subsurface caves and sinkholes) that are sometimes difficult to predict. Changes in rock mass homogeneity become even worse when they are combined with water, which significantly lowers the strength and stiffness of rocks, specifically in sedimentary rock (Erguler & Ulusay 2009; Yilmaz 2010).

In situ measurements of rock strength, when possible, can provide a relatively accurate and simple estimate of changes in rock physical properties. If these data are combined with complete description of the measurement site in terms of rock texture and mineralogy, stratigraphic and structural position, sequence boundary, fracturing spatial and temporal distribution, fracture filling material and surface roughness, then the approach will provide a vital source of information for engineers in their construction and environmental planning (Arman et al. 2013a, b).

Climatic and environmental conditions when linked to rock weathering also contribute to the rock hardness, in particular of carbonate rocks (Moses et al. 2014). Extensive carbonate dissolution under wet climatic conditions may result in a deterioration in rock hardness owing to karstification. In dry climates, particularly in arid regions, rock expansion and contraction owing to daily heat differences together with occasional humidity, storm rain events and thin soil cover may induce fracturing that can diminish rock hardness and enhance weathering (Viles & Goudie 2013; Joo et al. 2016).

Laboratory determination of either compressive strength or deformability of a rock material is time consuming and relatively expensive. Generally, a great number of specimens should be tested and destructive tests are necessary to produce a representative value for a large rock exposure. For these reasons, replacement of these tests with a quick, non-destructive Schmidt hammer (SH) test would be valuable for at least the preliminary stage of designing a structure or for rock exposure zoning. SH tests have acceptable reliability for engineers as they provide data as good as those from uniaxial compressive tests (Sachpazis 1990; Arman et al. 2013a, b).

The SH test is not only non-destructive and quick but also can be easily performed under both field and laboratory conditions. Since the early 1960s, it has been used in rock engineering and numerous studies have been carried out on the relationships between the Schmidt hardness values and both strength and the deformability characteristics of various rock types (Table 1). A summary of the advantages and disadvantages of the SH test in evaluating rock hardness and correlation to weathering and landform development was given by Goudie (2006), who found that SH data provide a rather good representation of the rock uniaxial compressive strength and Young's modulus of elasticity. Katz et al. (2000) and Kahraman (2001) evaluated the mechanical properties of different types of rock and found that correlation factors of equations can be used only to estimate the relevant mechanical properties in the field and laboratory. All reports of SH tests point out that uncertainty in the data may also stem from the rocks’ surface roughness, geometry and orientation during testing. Generally, there are significant non-linear correlations between the UCS and different test results, suggesting that rock types and test conditions are the dominant factors in prediction and correlation (Yagiz 2009). Nazir et al. (2013) found that unreliable UCS prediction may be encountered in the case of extensively weathered rock. Investigating the statistical relationship between rock hardness and physicomechanical properties of constructional and cover rocks indicated strong correlation and can be used to compare with calculated values from different empirical equations (Yasar & Erdogan 2004; Aydin & Basu 2005; Buyuksagis & Goktan 2007; Shalabi et al. 2007).

View this table:
Table 1.

Previous studies on the correlations between Schmidt hardness and uniaxial compressive strength and modulus of elasticity

In the investigation presented here, extensive field and laboratory SH tests were conducted on different types of carbonate rocks collected from an area that is affected by slope instability and the common occurrence of landslides, karst and cave development. The study area is located in an arid climatic region, namely in the locality of Al-Ain city, which is one of the fastest growing cities in the United Arab Emirates (Fig. 1). The geology of the area is relatively well documented in terms of lithology, structural deformation and major stress–strain fields. In addition, most of the area was accessible for the extensive data collection fieldwork. The main objective of the SH test is to demonstrate and discuss the effect of particular geological environments on in situ and laboratory SH test results for different types of carbonate rocks in climatically dry desert conditions.

Fig. 1.
Fig. 1.

Location map of the study area.

Geological setting, composition and texture of the rocks

The geological setting of the study area considered here is dominated by the Jabal Hafit anticline, which is about 29 km long and 5 km wide with a maximum elevation of about 1240 m above sea-level. Structurally, Jabal Hafit is a huge doubly plunging highly asymmetric anticline that is developed over two large thrust faults underlying its eastern and western limbs. The beds on the eastern limb dip steeply (70 – 90°) toward the NE, whereas those on the western limb dip gently (20 – 30°) toward the SW (Fig. 2). The faults and fractures are generally trending between NW–SE and east–west (orthogonal to the fold's hinge line), in addition to a subordinate NE–SW trend (Sirat et al. 2016). Some of these structures are thought to have been rejuvenated 25 – 15 myr ago (Warrak 1996; Noweir 2000; Styles et al. 2006).

Fig. 2.
Fig. 2.

Geology of the northern part of Jabal Hafit and studied locations (arrows) (modified from Abdelghany (2002)).

Jabal Hafit provides a rare outcrop sequence of Tertiary carbonates that includes three main rock units (Figs 2 and 3). The oldest unit is the Rus Formation (Early Eocene; 55 – 49 Ma) and consists of about 200 m of greyish white, thick-bedded to massive limestone. The lower part of the formation has subordinate brownish grey layers of dolomite and abundant chert nodules and bands that are coloured grey, white and brown. The overlying rock unit is the Dammam Formation (Middle to Late Eocene; 49 – 34 Ma), which is about 600 m thick. The lower part of the formation is composed of greyish thick-bedded fractured and rarely cavernous limestone that changes locally to become chalky or dolomitic and contains soft marl beds. The upper part is composed of yellowish thinly interbedded nummulitic limestone and marl. The youngest rock unit in the sequence is the Asmari Formation (Early Oligocene; 34 – 29 Ma), which is about 140 m thick and shows variations in lithology between two parts. The lower part is 30 – 40 m thick and consists of greenish, gypsiferous mudstone and brownish nummulitic marly limestone. The upper part is composed of thick-bedded chalky limestone with some layers of dolomitic limestone and marl (Arman et al. 2013a, b).

Fig. 3.
Fig. 3.

Generalized stratigraphic sequence of Jabal Hafit in Al Ain area.

Microscopic examinations of thin sections of the rock samples used for the SH and unconfined compressive strength (UCS) measurements indicate textures varying between skeletal grains embedded in partially recrystallized micritic groundmass and/or grains of variable sizes. In many of the studied samples, microfractures that are either empty or partly filled with secondary calcite and dolomite, forming veins, may occur. These textural features may influence the mechanical properties of rock masses. In this investigation, however, no numerical correlation was performed between the different textural features and the SH tests; this could be an interesting subject for future investigations.

Material and testing methods

Schmidt hammer (L-type) tests

Seven field sites were used for the in situ SH measurements and encompass the three rock units described above. The sites within the Rus Formation are M1 and M2. Three sites were selected within the Dammam Formation (DM1, DM2 and DM3) and two sites within the Asmari Formation (O1 and AS on the eastern and western fold limbs) (Figs 2 and 3). It is important to mention that each of these sites was represented by extensive SH measurements, totalling 3300 measurements with a range of 210–1130 measurements at a single site. The range of measurements at each site depends on the site conditions including the site size, surface structures (layering and fracturing) and location (different elevations) (Fig. 4). All sites were exposed to daily sunlight and have negligible surface weathering. In addition, 4 – 6 representative rock block samples (c. 0.25 × 0.25 × 0.25 m in size) were collected from each site depending on site characterization. Then, NX core specimens, c. 54 mm in diameter, were drilled in each rock block sample with a length to diameter ratio of c. 2:1. The UCS tests were performed on 24 core samples.

Fig. 4.
Fig. 4.

In situ SH measurements on site, Jabal Hafit.

The examined rock blocks of the Rus Formation (M1 and M2) are composed of dolomitic limestone that is relatively hard, thick-bedded to massive, and ranges between grey and white in colour, with subordinate reddish ferruginous-rich bands. These rocks commonly contain lenses and concretions of white to brownish chert. Dolomitization of these carbonate rocks is a characteristic property around this area and in this stratigraphic interval. The rock type is dominated by paleokarstic features, which developed along fracture planes and were filled later forming calcite veins and geodes of variable sizes and shapes in many locations (Fig. 3).

The rock sequence of the Dammam Formation is thicker than the others and has more lithological variability, thus three sites were selected for the study of this rock unit. The sampled rock blocks (DM1, DM2 and DM3) are composed of limestones that are interbedded with layers of marl. The limestone beds are slightly chalky or dolomitic, white to orange in colour, relatively hard and partly cavernous (Fig. 3).

Two sites (O1 and AS) were selected within the outcrops of the Asmari Formation where the rock blocks are chalky limestone, bedded to massive, white in colour and highly fossiliferous. The most characteristic feature of this interval of the Asmari Formation at both sites is the abundance of cavities, which have sizes varying between a fraction of a metre to over 5 m. The obtained samples and measurements for these sites were collected from beds inside one of the relatively large caves (Fig. 3).

In situ measurements

A total of 3300 SH measurements were performed at seven selected test sites (Figs 2 and 4). Three of these sites are within the Damman Formation, which is marly limestone (nummulitic highly fossiliferous to rare fossiliferous surfaces), with a total of 210 SH measurements. Two sites, with 1780 SH measurements, are within the Asmari Formation, which is chalky limestone, bedded, fossiliferous to highly fossiliferous, and with white chert concretions. The final two sites encompass 1310 SH measurements within the Rus Formation, which is dolomitic limestone (no fossils, with brown chert nodules). All SH measurements were carried out on natural, fresh (open to atmospheric conditions) and dry rock surfaces, and during the testing obvious (macro) fractures, discontinuities and edges were excluded. In some cases, but infrequently, rock surfaces were smoothed with an abrasive grinding wheel. Generally, the position of the rock hammer was horizontal, except in some rare cases where a vertical position was used. The average corrected Schmidt hardness rebound (SHRcor) value for each test site was determined and reported according to the guidelines outlined in ASTM D5873-95 (ASTM 2001).

Laboratory measurements

All representative rock block samples (33 samples) collected for laboratory SH measurements were saw-cut in the laboratory to create flat surfaces, but to keep the natural size as much as possible, which was a cube with a height of 0.25 m. Laboratory SH measurements (about 570) were performed on selected rock block samples having either natural (19 samples) or saw-cut (14 samples) surfaces (Fig. 5). Precautions were taken to avoid obvious (macro) fractures, discontinuities or proximity to edges during performance of SH measurements. The position of the rock hammer was vertically downward and at least 2 – 3 cm away from the block edge to avoid boundary effects. The SHRcor value of each test was determined and reported according to ASTM D5873-95 (ASTM 2001).

Fig. 5.
Fig. 5.

Laboratory SH test sample from Jabal Hafit.

Results and discussions

The results of in situ SHRcor on carbonate rocks indicate a wide range (Ncor = 16 – 55) of distribution even for data from a single site. The maximum SHRcor value is recorded in the Mubazarah test-site area (M2), which belongs to the Rus Formation dolomitic limestone. The high values of SHRcor recorded at this site may relate to the nodular nature of the dolomitic limestone and the occurrence of chert bands. The minimum SHRcor value was recorded at test site O1, which represents the chalky limestone of the Asmari Formation. It is also interesting to recognize that the mode value of the chalky limestone is almost similar to some of the dolomitic limestone. The distribution of all SHRcor values based on rock types for all test sites is presented in Figure 6, which shows that the data are highly scattered with respect to rock types.

Fig. 6.
Fig. 6.

Distribution of in situ corrected SHR (Ncor) with rock types for different locations.

To be able to describe the relationships between the mean SHRcor value (mean Ncor) and the UCS values for 24 core samples, regression correlation was performed for the three carbonate rocks investigated here. The equation of the best-fit line and the correlation coefficient (Fig. 7) indicate R2 values that are low and moderately significant (0.22 – 0.69) for the dolomitic and chalky limestone. The R2 value for the relationship in the marly limestone is significantly higher (0.99), indicating that the rock mechanical properties in the marly limestones can be generalized for this rock type in the region. When all the data for the carbonate rocks investigated here are plotted as a single domain, the R2 value becomes 0.52 (Fig. 8) and suggests a weaker relationship than in the chalky and marly limestones (Fig. 7). These data indicate that the rock mechanical properties vary widely even for a well-specified rock type and within a relatively small area. Thus, generalization of the rock mechanical properties for a locality or a region should be made with a great care.

Fig. 7.
Fig. 7.

Relationship between in situ mean SHRcor value (mean Ncor) and the UCS for three types of rocks.

Fig. 8.
Fig. 8.

Relationship between in situ mean SHRcor value (mean Ncor) and the UCS for carbonates.

The results of laboratory SHRcor on carbonate rocks also display a wide range (Ncor = 16 – 58) of distribution (Fig. 9) comparable with the in situ SHRcor. The relationships between the SHRcor (Ncor) and the UCS values for natural surface (R2 values 0.61 – 0.80) and saw-cut surface (R2 values 0.06 – 0.90) of the carbonate block samples are shown in Figures 10 and 11. When considering the whole dataset, the R2 value becomes 0.56 (Fig. 12) for the natural surfaces and 0.83 (Fig. 13) for the saw-cut surfaces. These correlations show that there is considerable agreement between the in situ and the natural surface laboratory measurements for the whole dataset, which indicates that errors introduced by operators, if they exist, are manageable. The improvement in the correlation coefficient when saw-cut surfaces are used may indicate removal of irregularities caused by the geological environment (lithology, weathering, surface fractures, surface filling, etc.) that reduce durability of the rocks.

Fig. 9.
Fig. 9.

Distribution of laboratory corrected SHR (Ncor) with rock types for different locations.

Fig. 10.
Fig. 10.

Relationship between laboratory natural surface, SHRcor (Ncor) and UCS for three types of rocks.

Fig. 11.
Fig. 11.

Relationship between laboratory saw-cut surface, SHRcor value (Ncor) and UCS for three types of rocks.

Fig. 12.
Fig. 12.

Relationship between laboratory natural surface, SHRcor (Ncor) and UCS for limestone.

Fig. 13.
Fig. 13.

Relationship between laboratory saw-cut surface, SHRcor value (Ncor) and UCS for limestone.

Conclusions

The results of extensive in situ and laboratory SH measurements on samples collected from seven selected sites in Al-Ain region, United Arab Emirates, within carbonate-dominated rocks, indicate variable relationships between the mean SHRcor value and UCS. This variability is attributed to compositional and textural heterogeneity of the rocks as well as environmental conditions that influence the mechanical properties of rock masses. The results also show a wide range of SHRcor values for both the in situ and the laboratory tests, but show comparable correlation coefficients between the in situ and laboratory natural surface data. This feature indicates that field SH measurements can be a simple and reliable tool to obtain data on rock mechanical properties; however, caution should be taken in generalization even in a small locality or region. Although our data are within the wide range of SH values in the published literature, they represent unique values for the rock types investigated and the environment, and therefore provide better constraints for engineers in planning for the safety aspects of constructions.

Acknowledgements

We thank the undergraduate students who participated in the field and laboratory measurements.

Funding

This research was supported by the United Arab Emirates University research project number FOS/IRG-16/11.

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