correlations between petrography and some engineering...
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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,
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
Alqahtani, M.B. & Abu-Seif, E.S. 102
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
Correlation Between Petrography and Some Engineering… 103
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).
Alqahtani, M.B. & Abu-Seif, E.S. 104
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
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.
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).
Alqahtani, M.B. & Abu-Seif, E.S. 106
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.
Correlation Between Petrography and Some Engineering… 107
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.
Alqahtani, M.B. & Abu-Seif, E.S. 108
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
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
Alqahtani, M.B. & Abu-Seif, E.S. 110
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.
Correlation Between Petrography and Some Engineering… 111
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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