Correlations Between Petrography and Some Engineering...

JAKU: Earth Sci., Vol. 24, No. 1, pp: 99-114 (2013 A.D. / 1434 A.H.)

DOI: 10.4197 / Ear. 24-1.6

99

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 (E-mail: [email protected])

1Geology Department, Faculty of Science, Sohag University, Egypt,

([email protected])

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

Alqahtani, M.B. & Abu-Seif, E.S. 100

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

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


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 2938–95. 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

< 5 18.9 < 0.3 9.7 < 1.5 10.7

5-10 35.6 0.3-1.0 84.9 1.5–1.75 53.4

10-15 39.6 1.0-3.0 5.4 1.75–2.0 25.7

15-20 3.2 2.0–2.25 7.1

20-25 1.9 >2.25 3.1

> 25 0.8

Total % 100 100 100

Total Porosity

(n)

Frequency

Percent Void Ratio (e)

Frequency

Percent Absorption %

Frequency

Percent

< 5 7.8 < 10 14.6 < 10 36.3

5 - 10 8.7 10-20 17 10-20 58

10 -15 11.3 20-30 32 20-30 5.7

15 - 20 21.6 30-40 26.9

20 - 25 24.8 40-55 7.1

25 - 30 21.4 50 - 60 2.4

30 – 35 3.7

Total % 100 100 100


4. 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.

4.1.1. 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).


Fig. 2. Typical subsurface profile and animal borings of the studied coralline limestone.

4.1.2. 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.

4.1.3. 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


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.

4.2. 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 sub-

environment (Fig. 3B). The dissolution of the original high Mg-calcite

coralline limestone is due to the higher concentration of CO2 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).


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.


4.3. 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 10-30 % 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 organism-

filled 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).

4.4. Dry Density (γdry)

The dry density of coralline limestone specimen was determined

according to ASTM D5030-04. Its value varies from 1.37 (g/cm3) to 2.26

(g/cm3) 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).

4.5. 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.


4.6. 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 2938–95.

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.

4.7. Point Load Strength Index (IS50)

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


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


load strength index (IS50) and drilling rate of the studied coralline

limestone samples (R2-value is 0.412 and 0.96 respectively). Similarly,

strong relationships are observed between void ratio and both total

porosity and absorption (R2-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.


7. 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.

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