Correlations Between Petrography and Some Engineering Properties of Coralline Limestone: A Case Study Along the Red Sea Coast of Jeddah, Saudi Arabia

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1 JAKU: Earth Sci., Vol. 24, No. 1, pp: (2013 A.D. / 1434 A.H.) DOI: / Ear Correlations Between Petrography and Some Engineering Properties of Coralline Limestone: A Case Study Along the Red Sea Coast of Jeddah, Saudi Arabia Alqahtani, M.B. and Abu Seif, El-Sayed Sedek 1 Department of Engineering and Environmental Geology, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia ( mqahtani@kau.edu.sa) 1 Geology Department, Faculty of Science, Sohag University, Egypt, (elsayed_71@yahoo.com) Received: 2/5/2012 Accepted: 3/2/2013 Abstract. Jeddah city is located in the western region of Saudi Arabia, where Quaternary coralline limestone covers extensive areas. Three distinct facies were recorded; foraminiferal biosparite; algal biosparite and ostraodal biosparite. The allochems are mainly composed of fossils including foraminifera, algae, ostracods, gastropods, and pelecypods. The original structures of bio-skeletons show partial dissolution and recrystallization of high Mg-calcite and replacement occurring after deposition as daigenesis processes under meteoric vadose sub-environment. During the growth of Jeddah city, many structures were constructed over the coralline limestone. Constructing on coralline limestone is associated with some problems, due to some special geotechnical properties of this type of soft and highly porous calcareous marine sediments. The studied coralline limestones are generally poor foundation materials in their natural state due to weak mechanical strength and heterogeneity of their both physical and mechanical properties. The meteoric diagenetic modifications of the original coralline limestone as well as organism borings affect greatly their physical and mechanical behavior when buildings are constructed on this rock type. The total porosity of the studied coralline limestone samples ranges between 3.0 and 39.0%. According to test results of the compressive strength and point load, the studied coralline limestone is classified as very low strength. A strong correlation existed between compressive strength and both point load 99

2 100 Alqahtani, M.B. & Abu-Seif, E.S. strength index and drilling rate but weak correlation was found between compressive strength and both total porosity and absorption. Keywords: Coralline limestone, Meteoric diagenesis, Borings, Compressive strength, Red Sea Coast, Saudi Arabia. 1. Introduction The Governorate of Jeddah (latitude: 21 29' N and longitude: 39 13' 9" E) is located in the western region of Saudi Arabia. The city increased in area during the last decades to cover most of the low-lying areas surrounding the old city where coralline limestone covers extensive stretches which are parallel to the Red Sea coast. During the growth of the city, many structures were built on the coralline limestone. The limestone is exposed at the ground surface and is extended downward to various depths. It is one of the sedimentary rocks which are complicated in their textural characteristics owing to the diagenetic processes which they suffer. The coralline limestone is a heterogeneous rock type in terms of fabric, since porosity and sedimentary structures show distinct vertical variations. From the geotechnical point of view, the coralline limestone is composed mainly of soft rock causing problems during construction. Very limited information is available on correlations between coralline limestone petrography and its physical and mechanical properties. This work represents a detailed correlation between the geological and engineering properties of the coralline limestone. 2. Geological Setting Jeddah is located on a low lying narrow coastal plain nearly oriented more or less north south, along the Red Sea to the west and a chain of Pre-Cambrian hills to the east. The coastal plain is approximately 10 km wide and 80 km in length in this district between Jabal At Tawilah in the south and Al Kura in the north. The old city of Jeddah was built on relatively higher grounds that rise 7 to 13 m above sea level (Alqahtani, 1999). This natural setting protected it locally against floods hazard as well as groundwater rising in the past. The general geology of Jeddah was based on the geology of Makkah Quadrangle mapped by Moore and Al-Rehaili (1989). The hills and mountains east of Jeddah consist of metamorphosed layered rocks and intrusive rocks of the Arabian Shield while the coastal plains consist of alluvial sand, sabkha and coralline

3 Correlation Between Petrography and Some Engineering 101 limestone. The limestone is approximately 10 km wide with an elevation less than 15 m above sea level. Small occurrences of the limestone are found as small mounds in disseminated areas. The limestone was also observed close to the ground surface east of Jeddah, which is possibly because of faulting and uplifting (Fig. 1). Fig. 1. Geological map of Jeddah modified by Alqahtani, (1999) after Moore and Al-Rehaili (1989). 3. Site Investigation and Laboratory Tests A site investigation program is always required before building any engineering structure. A detailed site investigation program involves drilling boreholes and tests, in situ and/or laboratory testing of the materials encountered. The site investigation program was done to rotary drilling and sampling in ten boreholes extending along NW-Jeddah

4 102 Alqahtani, M.B. & Abu-Seif, E.S. Governorate (Fig. 1). The drilling and sampling was done to depth ranging from 10 to 15 m. More than 253 samples were selected and put in specially designed wooden boxes for further testing in the Faculty of Earth Sciences, King Abdul-Aziz University. Topographic maps and GPS were used to locate the drill hole sites. The core samples were tested in the field and in laboratory to determine their physical and mechanical properties. Thin sections were also made for petrographic study under the microscope. The classification of coralline limestone was done according to Folk (1959 and 1962). The physical properties of these samples included absorption percentage, voids ratio, porosity and density. The mechanical properties included unconfined compressive strength and point load strength index. Drilling rate was also recorded during drilling processes. The unconfined compressive strength and the point load strength index tests were done according to ASTM D The results of these tests are summarized and listed in Table 1 for representative 72 samples selected from the 253 tested samples. Table 1. Summarized physical and mechanical properties of all studied samples (253 samples). Compressive Strength (MPa) Frequency Percent Point load (MPa) Frequency Percent Dry Density (γdry) Frequency Percent < < < > > Total % Total Porosity (n) Frequency Percent Void Ratio (e) Frequency Percent Absorption % Frequency Percent < < < Total %

5 Correlation Between Petrography and Some Engineering Results and Discussion The physical properties; dry density, porosity, absorption and void ratios as well as mechanical properties; compressive strength, point load strength index and drilling rate of coralline limestone play a vital role in its geotechnical characteristics. This is discussed as follow: 4.1. Facies Associations of the Studied Coralline Limestone The microscopic investigations of more than 50 samples of the coralline limestone indicated the presence of three main components; allochemes (grains), matrix (mostly micritic) and cement (spary calcite). These investigations show that similar petrographical, mineralogical and faunal assemblage. According to compositional classification of Folk (1959 and 1962), three distinct facies were recorded; foraminiferal biosparite; algal biosparite and ostraodal biosparite. The allochems are mainly composed of fossils including foraminifera, algae, ostracods, gastropods, and pelecypods. The composition of the coralline limestone samples as well as their fabrics indicated shallow marine conditions and early diagenetic processes (Basaham, 1998). These skeletal components are embedded in micritic fine groundmass or cemented by sparitic cement. The coralline limestone in the studied area can be classified into three rock units Pale-Yellow Shelly Weak Coralline Limestone This coralline limestone unit has animal borings which were partially filled with brown sand. White pectin, molluscs shells, gastropods and other skeletons are weakly cemented by white calcareous binding materials in the form of lumps. This mode of formation makes it easy to excavate (very low drilling rate). This type of coralline limestone is characterized by large amount of organisms-borings and low unconfined compressive strength. This rock unit was recorded at three depth intervals; in the top most part of the study area (0.0m to 3m depth), in the middle part (from 4.5 m to 7.5m depth) and in the lowermost part (from 12.0 to 13.5 m depth). Large organism-borings could be seen in the hand specimens (Fig. 2). The main allochems are composed mainly of broken shells and algae which are partially cemented by micritic and sparitic materials (Fig. 3A).

6 104 Alqahtani, M.B. & Abu-Seif, E.S. Fig. 2. Typical subsurface profile and animal borings of the studied coralline limestone Light-Brown Massive Weak Coralline Limestone This unit is composed mainly of massive coralline limestone and has the highest unconfined compressive strength values as compared with other coralline limestone units. The organisms-borings are of pencil-size which are filled with brown sand. Shell-fragments as well as large organisms-borings were not observed. The thin sections show it is composed mainly of coral-skeletons with partial dissolution. This unit is recorded at three depth intervals from 3.0m to 4.5m depth, from 9.0m to 12.0 m depth and from 13.5m to 15.0m depth Bright-White Weak Coralline Limestone This unit has intermediate characters between the two above mentioned rock units. It is composed mainly of coralline limestone with

7 Correlation Between Petrography and Some Engineering 105 pectin and gastropods shells with less borings. The thin sections show foraminiferal algal biosparite facies with calcite cement material which are probably formed during dissolution and re-precipitation processes (Fig. 3B). This unit is present at depth interval from 7.5m to 9.0m Evidence of Meteoric Diagenesis in the Coralline Limestone Diagenesis processes (compaction, dissolution, cementation, recrystallization and dolomitization) are of special importance when studying carbonate sediments because these processes modify the texture, structure and composition of the original sediments. Diagenetic effects in carbonate rocks may create high-permeability zones. Theses diagenetic modifications of original carbonate-rocks may greatly affect their mechanical properties. Two carbonate minerals predominate: Aragonite and calcite, in particular aragonite, being metastable, is invariably replaced by calcite (Evans, 1987). The coralline limestones in Jeddah are composed of high Mg-calcite and aragonite with minor amount of low Mg-calcite and traces of dolomite. The low Mg-calcite, is formed by the meteoric digenesis of high Mg-calcite and aragonite (Behairy, 1980, Behairy and El- Sayed, 1984, Dullo and Jado, 1984, Dullo, 1986 and Basaham, 1998). The original structures of bio-skeletons show partial dissolution and recrystallization of high Mg-calcite and replacement occurring after deposition as daigenesis processes under meteoric vadose subenvironment (Fig. 3B). The dissolution of the original high Mg-calcite coralline limestone is due to the higher concentration of CO 2 in the meteoric water and the precipitation of thin layers of sparite. Under the conditions of meteoric water environment, aragonitic bio-skeletons were partially dissolved and reprecipitated as sparry low Mg-calcite and partially filling the pores (Tucker and Wright, 1990, Fig. 3C). The dissolution and precipitation processes indicate diagenesis under meteoric vadose sub-environment. High Mg-calcite was converted into low Mg-calcite and aragonite was partially dissolved. Sparites are developed within skelatal cavities of foraminiferal and mollusc chambers (Basaham, 1998 and Andrea, 2005). The sparry low-mg calcite and dolomite appear in some thin sections (Figs. 3D and 3E). The presence of such dolomite inclusions indicates an original high Mg-calcite (Ross, 1991). The originally coralline polyp (halysitid corallum) were dissolved and filled by sparry low Mg-calcite (Fig 3F).

8 106 Alqahtani, M.B. & Abu-Seif, E.S. Fig. 3. Photomicrograph of thin section of the studied Quaternary coralline limestone in cross-polarized light showing; (A): Coralline algae (Amphiroa sp.), the cement is fine sparite partially filled pore spaces. (B): Foraminiferal coralline limestone, the cement is fine sparite partially filled pore spaces and foraminiferal moulds. (C): Dissolution of originally aragonite ostracodal shells (Xestoleberis rotunda) and filling molds and shell cavities by low Mg-calcite. (D): Foraminiferal coralline limestone, the cement is fine sparite partially filled pore spaces and foraminiferal moulds. Note the small rhombic areas of fine calcite spar. (E): Dissolution of coralline algae (Amphiroa sp.) and sparry low Mg-calcite replaced algae moulds. (F): Dissolution of originally coralline polyp (halysitid corallum) and filling by sparry low Mg-calcite.

9 Correlation Between Petrography and Some Engineering Organism-Borings in the Coralline Limestone The coralline limestone in Jeddah area is composed mainly of shelly limestone with pencil size organisms-borings (dwelling structures). These borings are mainly filled with brown sand-size material. The volume of these borings is about % of the total volume of the limestone. In general, the borings can displace more than 50% of the lithified framework of the coralline limestone with relatively large organismfilled voids (Moore and Shedd, 1977). In the investigated coralline limestone, the majority of borings were made by bivalves. These borings are of synsedimentary origin. The bivalvia borings play a major role in the breakdown of hard coral to calcareous silts (Gohar and Soliman, 1969). These borings are oriented generally parallel and in some cases, perpendicular to the bedding plane of the limestone and are pencil size in diameter (Fig. 2) Dry Density (γ dry ) The dry density of coralline limestone specimen was determined according to ASTM D Its value varies from 1.37 (g/cm 3 ) to 2.26 (g/cm 3 ) with an average value 1.74 (g/cm 3 ), (Table 1). These values are much less than that of calcium carbonate, because of the macro-voids seen both in hand specimens and microscopic images (Fig. 2) Absorption and Total Porosity Water absorption is an important rock index property depending on mineralogy and porosity of rock. Absorption relates to the particle's ability to take in a liquid. Porosity is one of the governing factors for the permeability. Porosity provides the voids for water to flow through in a rock material. The total porosity of the studied coralline limestone samples ranges between 3.0 and 39.0% with an average value of 21.6%. More than 83% of the studied samples have total porosity exceeding 10%. It is widely understood that diagenetic processes play a key role in controlling porosity and permeability within coralline limestone. Petrographical analyses of the studied samples indicate that the diagenetic processes increase the total porosity of the studied coralline limestone.

10 108 Alqahtani, M.B. & Abu-Seif, E.S Unconfined Compressive Strength The unconfined compressive strength, the most frequently used strength test for rocks is their ability to withstand crushing under direct pressure, as in blocks and columns (Fox, 1923). Unconfined compression strength is one important method for determining rock strength (Farmer, 1983 and West, 1994). The tests are carried out on dry core samples perpendicular to bedding plane according to ASTM D The studied coralline limestones range in their compressive strength from 2.2 to 32.9 MPa, thus classified according to Deere and Miller (1966) into very low strength and as very weak to moderately weak rocks (Piteau, 1970). It is found that, more than 95% of the tested samples have compressive strength less than 15 MPa. Hence, it is safe to say that the compressive strength values of the study samples are less than 15 MPa with 95% confidence Point Load Strength Index (IS 50 ) The point load test has been reported as an indirect measure of the compressive or tensile strength of the rock (Akram and Bakar, 2007). The point load test, developed by Broch and Franklin (1972) for classifying and characterizing rock material, is a relatively simple test for estimating rock strength. The International Society of Rock Mechanics standardized and established and used it for geotechnical study for over twenty years (ISRM, 1985). The point load strength index can be used to predict other strength parameters because it correlates closely with compressive strengths (Broch and Franklin, 1972 and ISRM, 1985). The point load values of the studied samples are less than 1.0 MPa with 95% confidence, the studied coralline limestone is characterized by very low shear strength (Broch and Franklin, 1972). 5. Vertical Variation of Physical and Mechanical Properties Figure 4 shows vertical variation of some physical and mechanical properties of the study coralline limestone. It is clear that the studied coralline limestone has distinct lowest values of unconfined compression strength, point load strength index and drilling rate in the depth ranging from ground surface to 3.0 m, where this rock unit has highest values of void ratio, total porosity and absorption. That may be due to presence of

11 Correlation Between Petrography and Some Engineering 109 secondary diagenetic porosity in addition to the original porosity of the studied coralline limestone. Fig. 4. Vertical variation of some physical and mechanical properties in different studied sites. 6. Correlation Between Petrography and Some Physical and Mechanical Properties Many researchers have shown that the mechanical properties of rocks are greatly affected by their fabric and petrography. The effect of grain size and shape (Haney and Shakoor, 1994), porosity and mineralogical composition (Sabatakakis et al., 2008) on strength and crack propagation were analyzed for many lithologies. Several relationships and good correlations have been established between the porosity and engineering properties for different rock types (Al-Harthi et al., 1999). Figure 5 shows the correlation between some physical and mechanical properties of the studied coralline limestone. Strong relationships are observed between compressive strength and both point

12 110 Alqahtani, M.B. & Abu-Seif, E.S. load strength index (IS 50 ) and drilling rate of the studied coralline limestone samples (R 2 -value is and 0.96 respectively). Similarly, strong relationships are observed between void ratio and both total porosity and absorption (R 2 -value is 0.63 and 0.69 respectively). In the same time a weak correlation between compressive strength and both total porosity and absorption. This may be due to more or less homogeneous nature and high complexity of the pore structure of the studied coralline limestone. The studied coralline limestones are characterized by different types of porosity (both primary and secondary diagenetic porosity) and other complex pore size distributions owing to the organisms-boring. Fig. 5. Correlation between some physical and mechanical properties.

13 Correlation Between Petrography and Some Engineering Conclusion Based on the obtained geological information and the engineering property results in both the field and the laboratory of the studied coralline limestone in Jeddah, we could be able to formulate the following conclusions: 1- The coralline limestone in the study area is relatively soft and can easily be broken due to its compositional heterogeneity. 2. A typical subsurface profile shows that the coralline limestone has a distinct heterogeneity in physical properties. 3. The mechanical properties of rock are greatly influenced by the composition, fabric (arrangement of intraclasts of skeletons and voids) and the diagenetics processes. 4. The studied coralline limestones are mainly made up of soft carbonate materials which are weakly and poorly cemented and contain large amount of cavities and borings. 5. Certain concern is to be given to depths between 3.0m to 7.0m below ground surface because water loss during drilling was recorded indicating large cavities or other voids in this coralline limestone rocks. 6. When soft rock contains high amount of voids and soft fragments, settlement is likely to occur and continue until full consolidation is reached. Acknowledgments The authors are deeply grateful to the Editor in Chief Prof. A.A. Sabtan and the two anonymous reviewers for insightful comments and criticism that improved this manuscript. References Akram, M. and Bakar, M.Z.A. (2007) Correlation between uniaxial compressive strength and point load index for Salt-Range Rocks. Pakistan Journal of Engineering and Applied Science, 1: 1-8. Al-Harthi, A.A., Al-Amri, R.M. and Shehata, W.M. (1999) The porosity and engineering properties of vesicular basalt in Saudi Arabia. Eng. Geol. 54: Alqahtani, M.B. (1999) Engineering Geology of Greater Jeddah Metropolitan Area. Ph. D. Thesis, Faculty of Earth Sciences, King Abdulaziz University, Saudi Arabia, 397 p. ASTM (1995) Soil and Rock, Building Stone, Peats, Annual Book of ASTM. Standards, Part 19. ASTM D (2012) Standard Test Method for Density of Soil and Rock in Place by the Water Replacement Method in a Test Pit. Soil and Rock, I: D420 - D5876.

14 112 Alqahtani, M.B. & Abu-Seif, E.S. Basaham, A.S. (1998) The composition and diagenetic features of the inland Quaternary coralline limestone, South-Sharm Obhur, Red Sea Coastal plain of Saudi Arabia. Journal of King Abdulaziz University, Marine Sciences, 9: Behairy, A.K.A. (1980) Clay and carbonate mineralogy of the reef sediments, west coast of Saudi Arabia, Bull. Fac. Sci., KAU, Jeddah, 4: Behairy, A.K.A. and El-Sayed, M.K. (1984) Carbonate cements in modern Red Sea reef, north of Jeddah, Saudi Arabia. Mar. Geol., 58: Broch, E. and Franklin, J.A. (1972) The Point Load Strength Test: International Journal of Rock Mechanics and Mineral Sciences, 9: Deere, D. and Miller, U. (1966) Engineering classification and index properties for intact rock, Technical Report No. AFWL-TR , Air Force Weapons Lab., Kirtland Air Base, New Mexico. Dullo, W.C. (1986) Variation in diagenetic sequences: An example from pleistocene coral reefs, Red Sea, Saudi Arabia. In: Schroeder, J. H. and Purser, B. H. (ed). Reef diagenesis Springer-Verlag, pp: Dullo, W.C. and Jado, A.R. (1984) Facies, zonation pattern and diagenesis of Pleistocene reefs on eastern Red Sea coast. In: Saad M. A. H. (ed) Proc. Symp. on Coral Reef Environment of the Red Sea, Fac. Mar. Sci., K.A.U., Jeddah, Saudi Arabia. pp: Evans, K.M. (1987) A model study of the end bearing capacity of piles in layered carbonate soil. Ph. D. Thesis, Keble College, University of Oxford. Farmer, I. (1983) Engineering behaviour of rocks: London, Chapman and Hall London. 208p. Folk, R.L. (1959) Practical petrographic classification of limestones, Bull. Am. Assoc. Petrol. Geol., 43: Folk, R.L. (1962) Spectral subdivision of limestone types. In: Ham, W. E. (ed.) Classification of Carbonate Rocks. Mem. Am. Assoc. Petrol. Geol., 1: Fox, C.S. (1923) Civil Engineering Geology, Crosby Lockwood and Son, London. Gohar, H.A.F. and Soliman, G.N. (1969) On Two Mytilids Boring in Dead Coral. ibid: Haney, M.G. and Shakoor, A. (1994) The relationship between tensile and comprehensive strengths for selected sandstones as influenced by index properties and petrographic characteristic. Proc. 7 th IAEG Cong. Lisbon, Portugal, II: ISRM (1985) Suggested methods for determining point load strength: International Journal of Rock Mechanics and Mineral Sciences Geomech., 22: Kramadibrata, S., Made, A.R., Juanda, J., Simangunsong, G.M. and Priagung N. (2001) The use of dimensional analysis to analyze the relationship between penetration rate of Jack Hammer and rock properties and operational characteristics. Proc. Indonesian Mining Conference and Exhibition, Jakarta, Indonesia. Moore, C.H. and Shedd, W.W. (1977) Effective rates of sponge bioerosion as a function of carbonate production. Proc. 3 rd International Coral Reef Sym., Miami, Ha. pp: Moore, T.A. and Al-Reaili, M.H. (1989) Geologic Map of the Makkah Quadrangle, Sheet 21D, Kingdom of Saudi Arabia, Ministry of Petroleum and Mineral Resources, Deputy Ministry for Mineral Resources Publication, Jeddah, KSA. Piteau, D.R. (1970) Engineering geology contribution to the study of stability in rock with particular reference to De Beer s Mine, Ph.D. Thesis, Faculty of Science, Witwatersrand University. Sabatakakis, N., Koukis, G., Tsiambaos, G. and Papanakli, S. (2008) Index properties and strength variations controlled by microstructure for sedimentary rocks. Eng. Geol. 97: Tucker, M.E. and Wright, V.P. (1990) Carbonate Sedimentology. Blackwell Scientific Publications, Oxford, London, pp West, T.R. (1994) Geology Applied to Engineering: Englewood Cliffs, N.J., Prentice Hall, 560p.

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