الفهرس 
Introduction and MethodologyOnly 14 pages are availabe for public view
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Abstract
This study aims to evaluate the physical and engineering properties of the Upper
Cretaceous- Eocene carbonate rocks, and assess the hazards related to these rocks. Also,
the integration between geologic data, field measurements with the engineering
properties of the carbonate rocks are the targets to achieve a better result about the
nature and strength of the foundation carbonate rocks under El Galala city. The
carbonate rocks of the present study are exposed at the upper part of El Galala El
Bahariya plateau.
The Upper Cretaceous- Eocene carbonate rocks that forming the cap unit of the
eastern part of El Galala El Bahariya plateau (Northern Galala) are selected to be the
target of the present study, where there exist many investment projects, such as El
Galala city that is currently constructed on the top surface of this plateau. El Galala El
Bahariya is a high flat topped plateau and is one of the most impressive topographical
features in the northern part of the Eastern Desert. The study area is located in the
northwestern part of Gulf of Suez, which includes the eastern edge of El Galala El
Bahariya plateau. The study area is defined by Latitudes 2915 and 29°35N and
Longitudes 32°15and 32°35E.
New Galala road constructed ascending El Galala El Bahariya from north to
south that facilitated the present study and sampling process. Coastal and new Galala
road are connected in the middle latitude of El Galala El Bahariya plateau by a NE
oriented pumping stations road.
from the field study and measurements of many sections, a composite
stratigraphic section was measured in the eastern scarp of El Galala El Bahariya plateau.
The total thickness of these units is about 900 m. The section begins at the base with
the Paleozoic units (Rod El Hamal, Abu Darag and Ahemir Formations) that followed
upward by Permo-Triassic (Qiseib Formation), Jurassic (sandstone and marl), Lower
Cretaceous (Malha Formation) and Upper Cretaceous (Galala Formation and
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carbonates) and finally all these rocks are capped by more than 200 m of hard, massive
dolomitic limestone and dolostone rocks that are belonging to Eocene rocks that are
charged by kartisified caves.
The petrography and XRD analyses of the present carbonate rocks indicate that
most of the 46 rock sample are dolomitic limestone and dolostone with some chert
bands. The identification proved the petrographical characteristics that all samples are
composed mainly of dolomite, calcite and quartz, respectively in a descending order.
There are four main carbonate facies which are dolopelmicrite, dolobiomicrite,
dolosparite, and dolointrasparite. In addition to 2 specimens that identified as biosparite.
There are various kind and amount of allochems (skeletal fragments,
terrigenous particles and pellets or peloids) and orthochem (micrite and sparite). In
addition to a diversity of diagenetic processes that play an important role in the rock
strength, such as micritization; recrystallization; and cementation with (iron oxide,
silica, calcite and dolomite crystals) that decrease the porosity by filling the pore spaces
which lead to increase the rock strength and making the rock more dense and hard. On
the other hand, the dissolution process increases the porosity.
Also, most of the carbonate rocks have low porosity value (0-5%), although the
dissolution process affected the rocks. According to intensive diagenetic processes that
closed most of the pore spaces and give the rocks high strength. Some specimens have
high porosity values up to 28% that led to reduce the strength of the rocks. Most of the
porosity spaces resulted from the dolomitization process which give intercrystalline
porosity (secondary porosity) in addition to moldic and fracture porosity in some
samples.
The geotechnical properties of rocks are studied in detail in order to understand
the engineering problems related to the foundation bedrocks. The physical properties of
these carbonate rocks include the bulk density, effective porosity, the insoluble residue,
and initial moisture content. As, their mechanical properties include a direct method
with uniaxial compressive strength (UCS) and indirect method with Schmidt hammer
rebound number.
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The insoluble residue (non-carbonate) values range between 0.5 % and 14 %,
with an average of 4.5 %, whereas the carbonate content values range between 86% and
99.5% with an average 95.5%. These values indicate that most of the carbonate rocks
in the study area are nearly pure carbonate containing little amount of non-carbonate
component. The bulk density values range from 1.75 gm/cm3
to 2.8 gm/cm3 with an
average of 2.46 gm/cm3
, initial moisture content varies from 0.01% to 0.34%, with an
average of 0.1%. In contrast, the porosity values range from 0.25 % to 28.3 %, with an
average of 6.34 %. Most of the samples having low bulk density values are related to
those having the high porosity.
The measured values of the dry UCS values of the Upper Cretaceous- Eocene
carbonate rocks range between 15.7 MPa and 110 MPa, with an average value of 51.7
MPa. According to the Egyptian Code (2001) classification, the study carbonate rock
samples are classified as medium hard to very hard rocks, with hard class as an average.
Also, according to the International Society for Rock Mechanics (ISRM, 1978), these
carbonate samples are classified as weak rocks to very strong rocks. The Schmidt
rebound numbers’ mean values and the indirect estimated uniaxial compressive strength
of the study carbonate samples. The uniaxial compressive strength values range
between 20 MPa and 384 MPa, with an average of 77.3 MPa. According to the
International Society for Rock Mechanics (ISRM, 1978), these carbonate samples are
classified as hard rocks. According to the Egyptian Code (2001) classification, the study
carbonate rock samples are classified as medium hard to extermely hard rocks, with an
average of hard class. There is tare crusts (removing small pieces of rocks produced
during failures) that characterized most of the dolomitic rocks according to
dolomitization process and presence the secondary porosity and the vuggy texture.
The present study aims to understand the mode of failure of rock specimens
under uniaxial compression. After completion of the cubical rock sample, the
compressive strength test done to get the various modes of failure at the maximum
pressure for the failure. Four various types of failure modes exist, such as simple shear
along one plane, splitting, Y-shaped crack, and cone splitting. These failure cracks may
exist in single, double and multi numbers.
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Failure modes of carbonate rocks under uniaxial compression were examined
based on our observations and analyses of the failure modes concerning corresponding
strengths, and physical and petrographical characteristics; the following conclusions are
concluded:
There is a strong inverse relationship between the porosity and density. The
mineral composition of the present study samples is mainly calcite and dolomite, so the
porosity, particularly the secondary type, controlled the present carbonate density.
High density samples that have actually low porosity values produced cone,
splitting and Y-shape modes in the form of single, double and multi patterns.
Low density and high porosity samples gave splitting and shear failure modes
having mainly single and double patterns.
Tight similarities exist between the modes and number of failures resulted by
the high density and high UCS samples (cone and Y-shape modes along double and
multiple planes). This similarity supports the direct relationship between UCS and
density values.
The relationship exists between failure modes under uniaxial compression and
corresponding UCS values can broadly be viewed in terms of damage shape and
intensity. Most of the weak carbonate rocks gave shearing along single planes whereas
the hard samples showed intense damages (cone and Y-shaped fractures having single,
double and multi patterns).
Most of dolomicrite rocks that have fine textures and lack of diagenetic criteria
gave shearing and splitting along single planes. On the other hand, most of dolosparite
rocks have coarse crystalline textures and diagenetic criteria resulted in splitting, cone
and Y-shaped fractures along single (mainly) and double (minor) patterns.
The nature of the principal failure mode varies from shearing along single plane
to axial splitting along single to multiple planes, and to Y-shaped and Cone splitting in
rock specimens as uniaxial compressive strength (UCS) increases.
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Chemical deterioration test of the present carbonate aggregate (particles) carried
out to simulate the actual carbonate deterioration at the foundation levels and in the civil
concrete structures when using the carbonate aggregates including the measurements of
solubility affinity of the carbonate samples belonging to Upper Cretaceous-Eocene
rocks in different types of water (tap water and sewer water). In addition to study the
behaviors of major cations and anions during the carbonate dissolution in both water
types. These two types of water were selected because they have been found actually
on contacts with the carbonate rocks in different engineering projects.
The pH of the present tap water was around 7.8 while the sewer water have a
relatively lower pH values, around 7.2. EC in the tap water was around 387 µS/cm, it is
much lower than EC value in sewer water (around 828 µS/cm) owing to the high ionic
concentrations in sewer water. TDS of the water ranges from 238 mg/l (tap water) to
498 mg/l (sewer water). Four major cations were determined in water samples include
(K+
, Na+
, Mg2+ and Ca2+). Sodium ion is concentrated in tap and sewer water at 35 mg/l
and 114 mg/l, respectively. Potassium ion exists in tap and sewer water at 8 mg/l and
15 mg/l, respectively. Concentration of calcium ion in tap and sewer water are 29.95
mg/l and 48.26 mg/l, respectively. Magnesium ion concentrations in tap and sewer
water are 12.13 mg/l and 11.12 mg/l, respectively. Major cations in water samples
include Cl-
, CO3
2-
, SO4
-2
and HCO3
-
. Chloride ion concentrations in tap and sewer
water are 30 mg/l and 125 mg/l, respectively. Sulphate ions concentrations found in tap
and sewer water at 44 mg/l and 50 mg/l, respectively. The concentrations of bicarbonate
ion in in tap and sewer water are158.6 mg/l and 268.4 mg/l, respectively. We can
conclude that the concentrations of cations and anions are higher in sewer water than
those in tap water except Mg+2, both have nearly the same concentrations. So sewer
water has higher chemical activity than tap water.
After immersing the carbonate aggregates in both tap and sewer water for 6
months, the solubility in term of weight loss for different carbonate types are calculated.
Dolosparite rocks have average weight loss values of 0.21 gm (0.21%) and 0.31 gm
(0.31%) in both tap and sewer water, respectively. Dolopelmicrite have average weight
losses in both tap and sewer water at 0.46 gm (0.46%) and 0.66 gm (0.66%),
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respectively. Dolointrasparite have average weight loss values at 0.13 gm (0.13%) and
0.25 gm (0.25%) in tap and sewer water, respectively. Dolobiomicrite have weight loss
values at 0.16 gm (0.16%) and 0.63 gm (0.63%) in tap and sewer water, respectively.
The analysis of the solubility results in respect to the carbonate microfacies concludes
that:
For all carbonate types, the solubility values in sewer water are greater than
those occurred in tap water. This solubility differences are related to the high chemical
activity of the sewer water, owing to its higher anions and cations concentrations (TDS);
in addition to its lower PH value.
Microfacies components and textures controlled the solubility differences of the
present carbonate samples. Generally, the coarse interlocked crystalline dolosparite and
dolointrasparite facies have lower solubility potential than those occurred in the fine
crystalline dolopelmicrite and dolobiomicrite facies.
Dolopelmicrite and dolobiomicrite carbonate types have the greatest solubility
capacities in both water types owing to their fine textures, containing shell fragments,
pellets, peloids, pores and cavities due to dissolution and dolomitization, as well as
having rare cementaion and recrystallization. These textural and diagenetic properties
increased mostly their porosity values that allowed water flow inside the micrite
samples faster than those occurred in the sparite samples.
The greatest solubility difference in both water types is observed in the
dolobiomicrite (about 4 times). This could be related to the fast dissolution of biofragments (shell tests, skeletons) instead to the fine carbonate crystals in the ground
mass of the rocks.
Sparite facies (dolosparite and dolointrasparite) have the lowest solubility
potentialities in both water types as well as solubility difference exists between the two
water types. The closed textures, rock components, and diagenetic processes made these
two microfacies have mainly low porosity values. These two facies are characterized
by interlocked coarse crystals and high cementation, recrystallization, and compaction.
Dolomitization process is responsible only on the formation of pores and cavities inside
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these rock samples. So, porosity of these samples was controlled by the degree of
dolomitization process.
from the correlations between solubility (weight loss, density, porosity and
UCS) we can conclude that Porosity is considered as one of the most important factors
that affected dissolution process, the high porosity aggregates produced more solubility
(weight loss) in both types of water. Also, solubility of carbonate samples is associated
with decreasing in density and increasing in porosity. The more stable carbonate
samples have lower weight loss and higher UCS.
The solubility affinity of the Na+
, SO4-2
, and Clions in tap and sewer water are
more than the other cations, owing to the high solubility of its Halite and Gypsum
minerals in water rather than the calcite and dolomite minerals that containing the other
cations.
The concentrations of the Ca+2, Mg
+2 and HCO3
-
(Calcite and Dolomite minerals)
in tap and sewer water are higher in the dolomicrite facies than those in the dolosparite
facies. This is verified that, the dolopelmicrite and dolobiomicrite have higher solubility
than those in the dolosparite and dolointrasparite owing to the relatively high porosity
values of the two micrite facies.
It was expected that the activity of Mg+2 in the water is more than that of Ca+2
,
but high Ca+2 activity is due to the presence of Ca-bearing salts which increase the
concentration of Ca+2 in tap and sewer water.
The strong positive correlations between Mg+2 and both CO3
-2 and HCO3
-1 reflect
the dominance of dolomite mineral in the study carbonate aggregates instead of calcite
mineral as supported by the weak relationships exist between Ca+2 and both CO3
-2 and
HCO3
-1
.
There are significant strong positive relationships between Ca+2 , SO4
-2
and Cl-
,
that support the presence of calcium sulphates (Gypsum) and calcium chloride minerals
in the carbonate rocks that dissolved in the water. The strong correlations exist between
the Cland SO4
-2 on one side and the Na+
and K+ on the other side reflect the high
solubility affinity of sodium chloride (NaCl), sodium sulphate (Na2SO4), potassium
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chloride (KCl) and potassium sulphate K2SO4. These salts are associated with the
carbonate rocks in low quantities less than 5% that not identified on XRD analysis.
Also, the Halite and Gypsum minerals are observed as thin veins and layers among the
carbonate beds.
Finally, one of the most significant features in the Eocene carbonate rocks exist
in El-Galala El-Bahariya plateau are caves with stalactite and stalagmite that distributed
with different sizes along both sides of new Galala road. The presence of these caves
has a great evidence that some carbonate rocks have high solubility potentiality in water
either tap or sewer.