Geotech Properties of Rus Fm in Dammam Dome Saudi - Abdullatif

8/13/2019 Geotech Properties of Rus Fm in Dammam Dome Saudi - Abdullatif

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July 2010 The Arabian Journal for Science and Engineering, Volume 35, Number 2A 173

GEOMECHANICAL PROPERTIES AND ROCK MASS

QUALITY OF THE CARBONATE RUS FORMATION,DAMMAM DOME, SAUDI ARABIA

Osman Abdullatif*

Earth Sciences Department, King Fahd University of Petroleum & Minerals

Dhahran 31261, Saudi Arabia

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*Corresponding Author:

E-mail: [email protected]_____________________________________________________________________________________________

Paper Received June 13, 2009; Paper Revised November 25, 2009; Paper Accepted December 9, 2009


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The Arabian Journal for Science and Engineering, Volume 35, Number 2A July 2010174

ABSTRACT

This study investigates the geomechanical properties, rock mass quality, and correlation relationships of the

carbonate Rus Formation in the Dammam dome. The rock mass classification systems for tunneling purposes, the

rock mass rating (RMR), and rock mass quality index (Q) were used to classify the rock quality from outcrop

exposures. Rock mass quality is controlled by variable lithology, strength, weathering, multiple discontinuities, and

solution caverns. The caverns are present and controlled by the fractures in the dome. Physical and geomechanicalproperties vary and the lower Rus shows lower average properties than the middle Rus. The RMR system classifies

the rock masses into fair, good, and very good, while the Q system classifies the rock quality into poor, fair, and

good. Both systems indicate that the lower Rus has lower rock mass quality than the middle Rus. The rock quality

classification indicates that different support requirements might be needed for excavation, construction, and

tunneling design in the lower and middle Rus. Characterization of rock quality on a regional scale within the dome

can help to avoid potential hazards and possible problems in engineering structures or any expensive repair

remedies.

Key words:carbonates, rock mass classification, solution caverns, dome structure, Saudi Arabia


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GEOMECHANICAL PROPERTIES AND ROCK MASS QUALITY OF THE

CARBONATE RUS FORMATION, DAMMAM DOME, SAUDI ARABIA

1. INTRODUCTION

The Dammam dome is located in the eastern province of the Kingdom of Saudi Arabia and is bounded

approximately by longitudes 50 and 50 14N and latitudes 26 12 and 26 26E .The dome forms a local topographic

high along a generally flat area in eastern Saudi Arabia. It is an oval shape structural body trending northwest and

covering an area of 150 km2(Figures 1 and 2) The dome encompasses Dhahran, Khobar, and part of Dammam city

[1]. The study area is part of the Arabian Gulf region where the impact of engineering construction and rockexcavation has increased the pressure on a fragile environment which already suffers from inferior inherent ground

conditions with problematic variable material properties and foundation conditions [24]. Construction work,

development, and excavation within the dome are continuing at an advancing rate. Previous studies have indicated

instability problems and solution cavities. In addition, a collapse of rocks was encountered during excavation within

the Dammam dome. Solution cavities were found to be related to the predominant rock structure orientation. They

mainly form along joints in the rock formation [59]. Information on rock geomechanical properties in the dome has

been lacking. Understanding the deposition, lithological variation, and structure of the dome formations has

important implications in understanding the impact on rock material and mass properties and problems related toconstruction or engineering structures [10,11]. The objective is to describe and characterize the geomechanical

properties and rock mass quality of the carbonate Rus Formation within the Dammam dome.

Figure 1: Location map of the study area in Dammam dome, Saudi Arabia


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Figure 2: Simplified geological map for the tertiary succession in Dammam dome area modified after [1]

2.GEOMORPHOLOGY

The geomorphology of the area has been shaped by geologic and climatic elements since Mesozoic times. The

area is located at the western edge of the Arabian Gulf. The drainage in the eastern region is eastward, where underpresent day dry conditions, the wadis play a minor role. However, during past periods, different climatic conditions

affected and shaped the landscape [16]. The main geomorphic features in the area include (1) the Gulf and the

coastal region, (2) the Dammam dome, (3) the Lidam and Al Summan escaparpment to the west, and (4) the central

area between the eastern edge and the Lidam escarpment to the west. A brief account is summarized here below

[1,16].

1) The Arabian Gulf is a shallow sea with a maximum depth of 110 m. The elevation rises gradually inland tothe west to an altitude of about 200m in the westernmost part. . From north to south, rock benches 1.5 to 2

high containing shells and reef occur along the Gulf coastline and indicate higher stands of the sea in past

times [16]. Salt flats and subkhas are also common features and they consist of sands, silts, mud, and salt toa depth of several meters. Locally, sand sheet, drifting sand, and sand dunes are also common.

2) The Damman dome consists of gently sloping topography where small hills stand out from the landscape,reaching a maximum of 150 m at Jebel Umm Er Rus at the top of the dome. The slopes are small,

decreasing gradually further outward. Three rocks exposed within the dome are gently dipping Tertiary

rocks covered locally with Quaternary deposits [1,16].

3) The Lidam and Al Summan escarpments border the Arabian Gulf region on the west and form a long, flat,hard-rock plateau trending north-south. The elevation reaches 200m along the western margin of the

plateau. The rocks consists of contrasting lithologies made of limestone, sandstone, mudstone, and shale ofmarine and fluvial origin.

4) The central region in the middle between the western Lidam escarpment and the dome is generally flat,locally marked by rock outcrops and interrupted by inland sabkhas and aeolian deposits. These deposits

consist of mobile barchans dunes and less thick sand sheet deposits, which are generally fixed byvegetation[16].


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3. GEOLOGIC SETTING

The geology of the eastern province has been dealt with by several authors [1216]. Recently, Weijermars [1]

documented the surface geology of the Dammam dome and discussed the distribution and stratigraphic and

sedimentologic aspects of the Tertiary outcrops of the Dammam dome (Figure 2). The Dammam dome is an

anticlinal structure with a core excavated by erosion with a major axis trending N 3040 west and covering an area

of about 60 sq. miles. It originated from episodic upward movement of deep-seated infra-Cambrian Hormuz salt.According to Weijermars [1], movement started in the Jurassic period in response to general tectonic extension on

the Arabian plate. The tectonic extension was further accelerated in the Miocene era. The uplift rate of the Dammam

dome continued during the Neogene period (7.5 m/Ma) [1].

The rocks exposed within the Dammam Dome are very gently dipping Tertiary rocks covered with Quaternarydeposits locally developed into sand dune and sheet deposits or extensive sabkhas. The exposed rocks of the

Dammam dome range in age from Paleocene to Middle Miocene. The rocks sequence from oldest to youngest and

consist of (1) Umm er Radhuma Formation, (2) Rus Formation, (3) Dammam Formation, (4) Hadrukh Formation

and Dam Formation, and (5) Quaternary coastal deposits, sabkha plains and aeolian sands (Figure 2).

(1) The Umm er Raduma Formation consists of very fine to medium crystalline dolomite with siliceous

inclusions up to 15 cm. Quartz geodes and chert nodules are present throughout the section. The fauna foundsuggest a shallow marine environment [14]. The total thickness of Umm er Raduma is 243.1 m [13].

(2) The deposition of the lower Eocene Rus Formation carbonates and its equivalents in eastern margins of the

Arabian Peninsula extends from southern Iraq in the north to Yemen and Oman in the south. This has

produced a shallow marine platform that covers an area approximately 450 thousands km2

[17]. The Rus

Formation is divided lithostratigraphy into three members: lower, middle, and upper Rus. The lower Rus

shows alteration of marls and thin dolomitic limestone beds, abundant slumps, and geodes. The middle Rus

consists of a vuggy weathered, well-jointed dolomite to dolomitic limestone with abundant mud balls. The

upper Rus consists of fine grained chalky limestone with few marls and clay layers at the top. The

depositional environment of the Rus Formation consists at the base of sabkha, which is subjected episodically

to shallow marine incursions and changes towards the top of the formation into regressive facies, lagoonal

associated with continental facies [1]. The lower Rus is composed of thinly to moderately interbedded

dolomitic marls and thin, hard dolomitic limestone with abundant slumps, geodes, and solution caverns. The

rock underwent different grades of weathering. In contrast, the Middle Rus shows thickly bedded grey buff,

fractured, and well-jointed dolomitic limestone that shows vuggy weathering and solution caverns. Based ona composite section at KFUPM and Saudi Aramco [13,14], the lower, middle, and upper Rus thicknesses are

estimated at 21 m, 10 m, and 25 m, respectively.

In the subsurface, the Rus Formation consists of interbedded anhydrite parasequences. Many solution cavernshave been reported [68]. Anhydrite in Rus Formation thins or is absent above many structural highs, including

much of Qatar Arch, Ghawar field, Dammam dome, and other large anticlines. The Rus anhydrite is an important

seismic reflector. In the basins, the reflector is strong and continuous, indicating that the anhydrite is thick and

bedded. In contrast, the reflector is weak and scattered along the crests of many large anticlines, suggesting that the

anhydrite is not present or that it is not continuously bedded, or both. In addition, individual beds can be correlated

among closely spaced wells using wire line logs for hundreds of kilometers. In the subsurface, the absence ofanhydrite within the Rus Formation in some areas is attributed to (a) non-deposition because the structures were

moving or facies change, (b) deposition followed by erosion, and (c) extensive subsurface dissolution of the

carbonate and sulfate evaporate under mid-Pleistocene climatic conditions [18]. In the subsurface of the Rub' Al-

khali basin, the Rus Formation consists of anhydrite interbedded with carbonate strata and it ranges in thickness from400 to 800 feet [13,19].

(3) The Dammam Formation rests unconformably above the Rus Formation. It is subdivided into five members.

It consists from bottom to top of shale, limestone, dolomitic marls, and dolomitic limestone. These facies

were deposited within sabkha, lagoonal to shallow marine environments. Based on composite section after

[1,13] the total thickness of the Dammam Formation is 50 m.

(4) The Miocene Hadrukh Formation consists of sandy marl at the base overlain by intercalated thin limestone

beds, and sandstone. Towards the top, the facies changes into calcareous sandstone and shales interbeddedwith minor amounts of marl and gypsum. The Hadrukh Formation represents continental to shallow marine

facies [13, 1]. The Hadruch Formations estimated thickness is 84 m [13].

(5) The Miocene Dam Formation unconformably overlies either Rus or Dammam rocks, depending upon

location. The basal Dam Formation consists of a sandy conglomerate overlain by stromatolitic limestone

passing upwards into clastic limestone intercalated by of geodes filled with microcrystalline calcite. The


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upper Dam is capped by massive reef limestone. The Dam Formation indicates a major marine transgression,

and it shows a shallow marine environment of subtidal to intertidal environment [1,20]. The estimated

thickness from the outcrop section is 40 m and at the Lidam area 50 km west of Dhahran, total thickness is

estimated at 90.80 m [1,13].

4. TECTONICS AND STRUCTURAL FEATURES

Study of fractures on the Dammam dome revealed three sets of fractures:

(a) regional (major) fractures, (b) local (minor) fractures, and (c) very small-size localized fractures. This

classification is based on the fracture size, trend, and extension within the dome and these characteristics can be

observed and measured on the dome rocks. In addition, the mode of fracture, filling materials opening and fracture

spacing are all noted and recorded [21,22]. All of the above are criteria that can be observed in the field and bywhich different types of fractures can be distinguished.

Field study also indicated two fracture modes are present within the Dammam dome: Mode I and Mode III.

Mode I fractures are extension or tension fractures and are characterized by a lack of displacement in both the

horizontal and vertical directions. Mode III fractures are normal faults where movement is parallel to the fracture

surface and in the vertical direction. Field investigation indicated that regional fractures are the most prominent or

dominant type of fractures within the dome. They are characterized by their large size, long extension (>500m

1000 m), and systematic trends compared to the local and very small-size localized fractures. These characteristicshelp to determine the relationship of fractures to the tectonic doming and structural settings.

Patterns and trends of those fractures were determined in several locations within the Dammam dome (Figure

3a). The resulting pattern confirmed the relationship of those fractures to the doming in the area (Figures 3a and 3b).

Concentric and radial trends characterize the fracture pattern that coincides with a pattern produced by an ellipticalshape dome [21,22].

Figure 3a: Major fracture trends within the Dammam dome and the expected major trends [21,22]


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Figure 3b: The proposed fracture trends associated with elliptical shape dome [22]

Lineament studies for the Dammam dome indicated that the general distribution pattern is compatible andconcordant with the proposed and expected fracture pattern for an oval shape dome (Figure 3b). The general

dominant trend of the lineaments also coincides with the fracture trends observed in the field. These observations

further support the idea that lineaments traced from landsat images of the Dammam dome most probably reflectfractures generated during dome emplacement. [21,22]. Major lineament trends are almost northerly, north-

northwest and north-northeast, and south-southwest. These trends were observed in the field at several locations

(Figure 3a). This general pattern may also indicate that the dominant lineament trend within the dome is related tothe dome emplacement.

The regional fractures are more important than the other local types concerning the engineering, environmental,

and hydrocarbon implications. For example, the solution cavern (sinkhole) occurrences at KFUPM, Aramco, and

RSAF Military Base tend to occur along a north-western trend, most possibly located along and controlled by one of

these regional fractures [7,21]. Table (1) summarizes solution caverns and their characteristics in the Dammam area.

Table 1. Summary of Solution Caverns (Sinkholes) Characteristics Recorded

in Previous Studies in Dammam Area


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5. METHODS

Methods consisted of field work, outcrop survey, and laboratory measurements. The field work collected data on

intact rock and made rock mass descriptions and classifications on the lower and middle Rus Formation. Twenty-

three exposed and excavated outcrop exposures, 11 from the middle Rus and 12 from the lower Rus, were described

and classified in this study. The rock mass and discontinuity survey data sheets were recommended by the

Engineering Group Working Party [23] and were used for data collection. Data collected on the sheets includedinformation related to rock strength, fracture spacing, joint orientation, joint condition, and weathering. This

information was used to assess the rock mass quality. Then the NGI Quality Index (Q) and the CSIR Rock Mass

Rating (RMR) systems were used to classify all rock masses in terms of rock mass quality. In addition, rock samples

were collected from the lower and middle Rus Formation to determine their geomechanicaland physical properties.

Geomechanical properties studied included point load strength (Is), uniaxial compressive strength (UCS), Youngs

modulus and Poissons ratio, Schmidt hammer rebound hardness, dry and relative density, water absorption,

porosity, and permeability. Tests were conducted according to procedures described by Brown [24] and Johnson and

DeGraff [25].

6. ROCK MASS AND MATERIAL DESCRIPTION

A generalized composite lithological section for the lower and middle Rus Formation is shown in Figure 4. Thelower and middle Rus vary in their stratification, lithology, fracture pattern, and weathering. The lithologic

succession thickens upward. The lower Rus consists of thinly- to moderately-bedded carbonate strata dominatedby dolomitic marl, dolomite, and dolomitic limestone (Figure 5). In contrast, the middle Rus is thickly-bedded and

dominated by fine-grained marly dolomitic limestone. Both regional and local fractures cut the lower and middle

Rus carbonate strata.

Figure 4: Generalized composite stratigraphic section of Rus Formation Dhahran study area showing lithological variation

and structures of lower and middle Rus and samples at outcrop locations


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Figure 5: Excavation in Rus Formation at KFUPM campus showing regional fracture

and solution caverns in the lower and middle Rus

Karst features and solution caverns in Rus Formation are common and appear to be inherited from post-

depositional erosion and drainage. These formed during the late Tertiary. Solution caverns vary in size and range

from a few to several meters in size. Karst is developed best in limestone and dolomite where the strata is in both

competent and fractured rocks with UCS 30100 M Pa. Karst is more pronounced in competent rocks than in

weaker rocks. A wet tropical climate likely influenced and lead to development of the karst landforms [10]. A

number of factors were probably involved; these include: (1) generally hard competent rocks of the Rus Formation;

(2) well-developed fractured patterns due to doming; (3) variable lithologies and well developed bedding planes; (4)

the presence of interbedded evaporates such as anhydrite; and (5) repetitive periods of wet and dry climate,especially during the late Tertiary to Quaternary times, that provided water recharge and drainage that acted to

dissolve the Rus carbonates.

Outcrops on the Dammam dome show that solution caverns in the Rus carbonates intersect underground

fractures. This is especially evident at exposed outcrops and at rockheads beneath soil (Figures 5, 6, and 7). Figure 5

shows an excavation in the Rus Formation at KFUPM campus where a regional fracture cuts across lower and

middle Rus. Solution caverns (suffusion type) are shown in lower Rus. The cavern contains unconsolidated graveland sand matrix which occlude the cavern. These were washed down by percolating water during wet periods.

Solution caverns are associated with fractures in the Rus Formation. Gravel and sand matrix fills the cavern and

gypsum is lining the cavern walls (Figures 6a, b). Figure 7 shows solution cavern (A) and rock collapse (B) in

middle Rus. Figure 7c shows land subsidence in lower Rus underlain by solution caverns containing gravel and sand

matrix. A buried solution cavern more than 1 m in diameter containing gravel and sand matrix is shown in Figure 7d.

The cavern wall is lined with gypsum. Dissolution caverns recognized within the Rus Formation may be classified,based on Waltham and Fookes [10] classification, as (a) dissolution sinkhole, (b) collapse sinkhole, (c) suffusion

sinkhole, (d) buried sinkhole, or (e) sand pipes caverns (Figures 5, 6, and 7).


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Figure 6a: A gravelly to sandy matrix fills the regional fracture and extends down through the middle and lower Rus

Figure 6b: Fracturing is associated with solution caverns developed in the lower Rus.

Infilling material is made of gravelly, silty sand, and gypsum is lining the cavity wall (arrows).

Solution caverns, subsidence, and rock instability are recorded in the subsurface. They tend to be present and

pronounced along discontinuities that intersect lithological variations such as anhydrite. The wide regional

distribution of the Rus Formation in the Arabian Peninsula [17,18,19] suggests that rock instability and engineering

hazards may be encountered elsewhere beyond the Dammam dome, especially on structurally high areas similar to

the Dammam dome. Therefore, understanding fracture pattern from outcrop combined with remote sensing and GIS

techniques, could aid engineering geological investigation and characterization studies [20,21].

Summary characteristics of the Rus Formation and its lithological and structural variation in the outcrops

studied are shown in Figure 8. The middle Rus consists of massive dolomite (A) which is thickly bedded and widelyjointed (B); and moderately Vuggy (C) to highly cavernous dolomite (D). The lower Rus is thinly to moderately

interbedded marl, dolomite, and dolomitic limestone (E-H). Solution caverns are lined with gypsum (F). Regional

fractures (arrows) cut across thinly interbedded marls and dolomite in lower Rus (H). Petrographically, the lower

Rus ranges from fine- to medium-grained dolomite to dolomitic limestone. Porosity typically reaches a maximum of

20%, whereas the middle Rus is composed of fine-grained, hard crystalline dolomite with very low porosity around8% (Figure 9). X-ray diffraction analysis indicates that mineralogy of the Rus Formation is dominated by dolomites(9199%), calcite (17%), and minor quartz (16%) and clay minerals (12%).


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Figure 7: Solution caverns in the middle Rus (A,B) and the lower Rus (C,D) at KFUPM campus

7. PHYSICAL AND GEOMECHANICAL PROPERTIES

The geomechanical and physical analyses included point load strength (Is), uniaxial compressive strength (UCS),

dry density, porosity, and permeability tests. Results are shown in Table 2. A total of 10, one-inch diameter

specimens were selected for determining Young's modulus and Poisson's ratio by static test on the lower and

middle Rus (Table 3 and 4). Schmidt hammer rebound hardness tests were also conducted on outcrop sections of the

lower and middle Rus as shown in Figures 10a,b. The average, minimum, and maximum values of geomechanical

and physical properties for the lower and middle Rus are shown in Table 5. The average point load strength values

for the lower and middle Rus are 5.20 and 6.90 megapascals (Mpa), respectively. The minimum and maximum pointload strength values for lower and middle Rus Formation are 0.56 to 24.30 Mpa and 0.50 to 13.21 Mpa, respectively.

Average porosity in lower Rus is 10.39% (poor to fair) and it ranges from 5 to 16% (poor to fair/good). In

comparison, the average porosity in middle Rus is 8.83 (negligible to poor) and it ranges from 5 to 14% (poor tofair). For dry density, the lower and middle Rus show average values of 2.28 and 2.56 g/cm

3, respectively. Dry

density in lower Rus ranges from 1.7 to 2.52 g/cm3and in middle Rus it ranges from 2.14 to 2.86 g/cm

3. The average

permeability in lower Rus is 84.57 milli Darcy (mD) (good) and it ranges from 5.08 to 316.27 (poor/fair to very

good). In contrast, the middle Rus has an average permeability of 58.35 mD (moderate/good), and it ranges from

2.98 mD (poor to fair) to

278.72 mD (good). Schmidt hammer tests indicate that the middle Rus has higher values

than those encountered in the lower Rus. In the middle Rus, 75% of the measurements are greater than 50 (SHV) and25% are between 40 and 50 Schmidt hammer rebound values (SHV), whereas in the lower Rus, only 16% of the

measurements are greater than 50 (SHV), 25% are greater than 40%, and 50% of the measurements are less than 30

(SHV) (Figures 10a,b). In the lower Rus, Youngs modulus values range from 4 to 183 gigapascals (Gp) with an

average of 42 Gp. In contrast, values for middle Rus range from 13 to 207 Gp and average 80 Gp. The lower andmiddle Rus have similar Poissons ratio values that average 0. 21 and range from 0.10 to 0.38 (Table 4).


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Table 2. Rock Geomechanical and Physical Properties for Lower and Middle Rus Formation


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Figure 8: Shows massive (A) and fractured thickly bedded (B), moderately vuggy (C)

and highly cavernous dolomite (D) in the middle Rus. Thinly interbedded marl and dolomite (E),

cavernous (F,G) and regional fractures (arrows) (H) are shown in the lower Rus.

8. Rock Mass Classification

The Rock Mass Rating (RMR) and the Quality Index (Q) rock classification systems were used to classify the

rock masses of Rus carbonates. The RMR system is a sum of six properties: (1) uniaxial compressive strength; (2)

rock quality designation (RQD); (3) spacing of discontinuities; (4) condition of discontinuities; (5) groundwatercondition; and (6) orientation of discontinuities [2730]. The Q system consists of five parameters: (1) RQD;

number of joints sets (Jn), (2) roughness of joints (Jr), (3) joint alteration number (Ja), (4) joint water reduction

factor (Jw), and (5) stress reduction factor (SRF) [31].

For both classification systems, a number of points or rating are allocated to each range of values for each

parameter, and the overall rating of the rock mass is made by adding the ratings of the parameters, and the rock mass

is given a class representing the rock mass quality. For all rock masses, the intact strength was estimated by using

point load strength measurements. The spacing of joints, conditions of joints, and the groundwater condition were

estimated from the rock mass and discontinuity data sheets. The rock quality designation (RQD) values wereestimated from the average frequency of discontinuity using Priest and Hudson's formula, and the rock quality

designation, RQD, is given by


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RQD = 100 e 0.1 (0.1 + 1) [31]

where is the discontinuity frequency per meter. For the lower and middle Rus, the calculation of the Rock Mass

Rating and Quality index is shown in Table 6. The rock mass quality rating according to the RMR system is

classified into three categories: (1) fair; (2) good; and (3) very good. Results of the quality index according to the

NGI quality system are shown in Table 6. The lower and middle Rus were classified into three categories: (1) poor,

(2) fair, and (3) good. Figure 11 shows different categories of rock mass quality for the lower and middle Rusestimated from the Rock Mass Rating (RMR) and Quality Index (Q) rock mass classification systems. The Rock

Mass Rating (RMR) and Quality Index (Q) for both lower and middle Rus are plotted in Figure 12. The rock mass

quality data indicates that the lower Rus carbonates have lower rock mass quality compared with the middle Rus.

Table 3. Youngs Modulus and Poissons Ratio for the Lower and Middle Rus Formation Carbonates____________________________________________________________________________

Sample No. Youngs Modulus Poissons Ratio

MPa_________________________________________________________________________________________

Lower RusL-4 5708 0.10

L-6 183236 0.32

L-7 16129 0.38

L-10 4624 0.11

L-11 4047 012

Middle Rus

MR-6 86790 0.10

MR-7 13879 0.17

MR-9 19479 0.11

MR-11 207503 0.33

MR-13 73961 0.35_______________________________________________________________________________

Table 4. Average, Minimum and Maximum Values of Youngs and Poissons Ratio

of Lower and Middle Rus Formation Carbonates__________________________________________________________________________________________________________

Formation Youngs Modulus Mpa Poissons Ration

Average Minimum Maximum Average Minimum Maximum__________________________________________________________________________________________________________

Lower Rus 42748 4047 183236 0.22 0.10 0.38

Middle Rus 80322 13879 207503 0.21 0.10 0.35__________________________________________________________________________________________________________

Table 5. Summary of Laboratory Testing Results for the Lower

and Middle Rus Formation Carbonates Studied in Outcrops_____________________________________________________________________________________________________________________________________

Formation Lower Rus Middle Rus

Geomechanical

Average Minimum Maximum Average Minimum Maximum

Parameter_____________________________________________________________________________________________________________________________________

Point load Strength I 5.20 0.56 24.3 6.90 0.5 13.21

Index (Mpa)

Uniaxial Compressive 100.10 12.12 506 152.92 15 288.86

Strength (Mpa)

Porosity (%) 10.39 5 16 8.83 5 14

Dry Density (g /cm3) 2.28 1.7 2.52 2.56 2.14 2.86

Permeability (md) 84.57 5.08 316.27 58.35 2.98 278.72

Youngs ModulusE(Mpa) 42748 4047 183236 80322 13879 207503

Poissons Ratio 0.22 0.10 0.38 0.21 0.10 0.35

Schmidt Hammer 33 12 54 50 36 60_____________________________________________________________________________________________________________________________________


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Figure 9: Microphotographs showing euhedral to anhedral dolomite in lower Rus (A) and middle Rus (B)

Figure 10: Schmidt hammer rebound values; lower Rus shows lower values than middle Rus

Table 6. Rock Mass Quality Classification of the Lower and Middle Rus Formation Carbonates Using the

Quality Index (Q) and Rock Mass Rating (RMR) Classification Systems Studied in Outcrops

Rock Mass # Quality Index Rock Mass Rock Mass Rating Rock Mass

(Q) Classification (Q) (RMR) Classification (RMR)

Middle RusMR 1 20 Good 80 Good

MR 2 12.9 Good 77 Good

MR 3 7.5 Fair 72 Good



MR 6 5 Poor 76 Good


MR 8 10 Fair 66 Fair

MR 9 5 Poor 76 Good

MR 10 19.4 Good 92 Very Good


Lower Rus

LR 12 7 Fair 77 Good

LR 13 4.8 Poor 68 Fair








LR 21 6.5 Fair 68 Fair

LR 22 6.2 Fair 54 FairLR 23 6.7 Fair 59 Fair


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Figure 11: Relationship between rock mass ratio (RMR) and quality index (Q) for lower Rus (triangles) and middle Rus (circles)

Figure 12:Correlation and curve fitting for RMR and quality index (Q) data for lower Rus (triangles) and middle Rus (circles)

9. CORRELATION BETWEEN ROCK MASS RATING AND QUALITY INDEX CLASSIFICATION

SYSTEMS

The Rock Mass Rating system (RMR) considers orientation of discontinuities and material strength. These arenot directly included in the Quality Index (Q) classification system. Rather, the Q system considers rock stress and

the joint set number, which are indirectly considered in the Rock Mass Rating system. Both systems include

conditions for discontinuities and groundwater conditions. The two systems are the most practical schemes for rock

mass classification where tunneling is to be reformed. Previous studies indicated that the Rock Quality Index

classification system (Q) yields a better forecast of support quantities [3335]. Figure 11 shows the relationship

between Rock Mass Rating (RMR) and the Quality Index (Q) for lower and middle Rus. The relationship betweenRMR and Q is given by RMR = 15.5 x (ln Q) + 39.7.

Various correlations between the RMR and Q rock mass classification systems have been found and these are

summarized in Table 7 [28,3537]. The relationships between the RMR and Q classification systems are in the form

of RMR = A ln Q + B, where A is between 7 and 14 and B is between 36 and 49. As shown from the linear

relationship in Figure 11 and the power relationship in Figure 12, the two classification systems RMR and Q index

indicate that the lower Rus carbonates have lower rock mass quality than the middle Rus carbonates. Figure 12shows the correlation between RMR and Q has a correlation coefficient of 92%.


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Table 7. Relationships Between Rock Mass Rating (RMR) and Quality Index (Q)

Established by Previous Studies

Author Relationship between RMR and Q

Bieniawski [28,30] RMR = 9 ln Q + 44

Kaiser and Gale [35] RMR = 8.7 ln Q + 39

Al-Harthi [37] RMR = 9 ln Q + 49Tugrul [38] RMR = 7 ln Q + 36

This study RMR = 15.5 ln Q + 39.7

The engineering hazards presented by karst features include failure of carbonate rocks over voids created bydissolution, or failure due to downward washing of soil into old stable voids, or both [10]. Cave stability is related to

the cave width and the rock mass quality Q value (Figure 13) [31]. The Q data of the lower and middle Rus shows

poor to good rock mass quality (Table 6). Figure 13 shows cave stability related to cave width and rock mass quality.

The envelope of the field for limestone caves is derived from observations of caves around the world. The labeled

fields of stable, supported, and unstable are based on Q quality methods [10]. Cave width observed from Rus outcrop

exposures ranges from 1 to 5 m. The data indicate that the field for cave stability lies within the observed cave widthvalues. Structural support is needed in areas where cave width increases to values greater than 520 m [10]. On the

basis of this relationship, caves in the lower Rus carbonates are less stable than are caves in the middle Rus. Figure

14 compares the results of rock mass quality classification (this study) with other studies (Table 6). The results ofthis study are similar to other studies [30,3538].

Figure 13: Cave stability related to cave width and rock mass quality. The data for the lower Rus (red)

and middle Rus (yellow) both lie within the stable field, modified after [10].

Figure 14: Comparison of the result of this study and other studies (adapted after 38)


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10. CORRELATION RELATIONSHIPSAMONG INTACT ROCK PROPERTIES

Regression analysis was applied to obtain relationships between geomechanical and physical properties measured

from lower and middle Rus. Bivariant correlation provides a means for summarizing the relationship between two

variables [39]. Previous studies indicated relationships between index properties and mechanical properties.

Various regression equations are proposed for predicting mechanical properties and provide an indirect way of

estimating the mechanical properties. Various types of regression, in addition to linear regression, power (y = axb

)relationships between geomechanical and physical properties were investigated. Figure 15 shows some of these

relationships and the best fit curves between different parameters. The relationships show rather higher correlations

between point load (Is) and uniaxial compressive strength (UCS) (Figure 15a). Moderate correlations were obtained

between UCS and permeability and between point load and permeability (Figures 15b,c). A relatively higher

correlation was obtained between porosity and dry density. This is shown in Figure 15d. Low correlations were

obtained between point load and permeability, and between porosity and permeability.

Figure 15a shows the relationship between the point load strength and uniaxial compressive strength and the best

fit curves for lower and middle Rus samples. It can be seen that the linear relationship between Is and UCS yields a

correlation coefficient (R= 99%). The power relationship between Is and permeability yields a rather lower

correlation coefficient of 51%. The relationship between UCS and permeability as shown in Figure 15c also shows alower correlation with a correlation coefficient of 50%. The relationship between porosity and dry density as shown

in Figure 15d shows rather moderate correlation with correlation coefficients of 61%. Figures 15e-j show therelationships among some physical properties and Schmidt rebound hammer values.

The correlation between porosity and dry density conforms to those relationships reported for carbonate rocks by

Bell [40]. However, some of the dry density values in lower and middle Rus are rather low. As indicated earlier,this might be attributed to the lithological heterogeneity of the Rus in terms of type, composition, and degree of

weathering, such as a wide range of lithologies, including weathered marls, marly and muddy limestone, and muddy

dolomitic limestone. Bell [40] reported dry density values for different carbonate rocks ranging from 2.16 to 2.58

g/cm3. Dearman [41] noted that density of carbonate rocks increases with age, whilst porosity is reduced, and

diagenetic processes mainly allow for lower porosities of carboniferous and magnesian limestone. William and

McNamara [42] mentioned that both dolomitization and dedolomitization can give rise to increased porosity. Tugrul[38] reported mean values of absolute and effective porosity for Cretaceous limestone which are lower than those

of Jurassic limestone. The amount of clastic non-carbonate sediments in marine environment means that carbonate

sediments may grade into terrigenous clastic rocks. The Rus Formation is deposited within a shallow marine

environment associated with continental facies [1]. Al Jassar and Hawkins [43] showed that the engineeringproperties of several lithological types in Carboniferous limestone are related to lithology, and that age often has an

influence on the strength and deformation characteristics of carbonate rocks. Bell [40] recorded that the dry densityof marl decreases progressively with increase of degree of weathering. At weathering grades I and II, the dry density

of marls ranges from 1.9 to 2.4 g/m3, and at high weathering grade of IV, the dry density ranges from 1.4 to 1.8

g/cm3.

IS vs. UCS

IS = 0.047 UCS - 0.3287

R2= 0.9909

0

5

10

15

20

25

30

0 100 200 300 400 500 600

UCS

IS L Rus

M Rus

Figure 15a: Point load strength vs unconfined compressive strength


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IS vs Perm

IS = 16.156 Perm-0.4539

R2= 0.5143

0

5

10

15

20

25

30

1 10 100 1000

Perm (mD)

IS L Rus

M Rus

Figure 15b: Point load vs permeability

UCS vs Perm

UCS = 347.24 Perm-0.4249

R2= 0.5055

0

100

200

300

400

500

600

1 10 100 1000

Perm

UCS L Rus

M Rus

Figure 15c:Unconfined compressive strength vs permeability


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Porosity vs Dry density

Porosit y = -10.88 Dry Density + 36.383

R2= 0.61

0

2

4

6

8

10

12

14

16

18

20

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1

Dry Density

Porosity

L Rus

M Rus

Figure 15d: Dry Density vs porosity

Figure 15e: Point load strength (Is) vs Schmidt hammer rebound number


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Figure 15f: Unconfined compressive strength (UCS) vs Schmidt hammer rebound number

Figure 15g: Porosity (%) vs Schmidt hammer rebound number


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Figure 15h: Grain density vs Schmidt hammer rebound number

Figure 15i: Log permeability vs Schmidt hammer rebound number11. DISCUSSION AND CONCLUSIONS

The rock mass quality classification using the Rock Mass Rating (RMR) and the Quality Index (Q) systemsindicate that the lower Rus has lower rock mass quality than that of the middle Rus. The variation in rock mass

qualities in the Rus Formation suggests that different support requirements and designs are needed for excavation

and engineering structures in the lower and middle Rus carbonates. This conclusion conforms to the results of intact

rock geomechanical properties, where the lower Rus carbonates have lower average values than those of the middle

Rus, as shown in Table 5.

The heterogeneity of the intact rock and rock masses are influenced by a number of factors, including lithological

variation and the presence of multiple discontinuities, including fracturing and bedding, weathering, and dissolution

caverns. The Rus Formation is characterized by fractures of regional to local extent. The regional fractures are

related to dome emplacement; whereas, the local fractures probably formed in response to minor block movements.Dissolution caverns vary in type and size and are typically present along discontinuities and changes in lithology.


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The Rus Formation varies in its physical and geomechanical properties. The lower Rus has lower strength,

lower density, lower Schmidt hammer rebound values, and relatively higher water absorption and higher average

porosities than the middle Rus. Bivariant correlation relationships between geomechanical and physical parameters

were determined by this study. For example, a high correlation was obtained between point load strengths (Is) anduniaxial compressive strengths (UCS), Is and porosity, and porosity and dry density. Moderate correlations were

obtained between UCS and permeability. Low correlations were obtained between Is and permeability and between

porosity and permeability. In conclusion, generally the intact rock material properties indicate that the lower Rus

has lower physical and geomechanical properties than the middle Rus Formation.

On the basis of observed cave widths (1 to 5 m) and Q value relationships, the data indicate that the field of cave

stability of the Rus Formation lies within the stable field under observed cave width values. However, support is

needed in areas where cave width increases to values greater than 520 m [10]. The lower Rus carbonates are

expected to show less cave stability than those in the middle Rus.

In conclusion, both intact rock properties and rock mass quality classification indicate that lower Rus carbonates

have lower geomechanical properties and lower rock mass quality than those of middle Rus carbonates.

The complexity of geology in the Dammam dome area indicates that there is a need for an engineering

geological data base. Such a data base is important for land use, planning, and design of local and extendedengineering works and structures. Outcrop surveys and measurements aided with geophysical surveys can help

characterize the rocks and establish a susceptibility zonation that might need further investigation or application ofcertain mitigation measures [10,11,44].This may help others to avoid potential hazards, foundation instability

problems, or expensive repair remedies on the Dammam dome.

ACKNOWLEDGMENTS

The author would like to thank Dr. Hariri for his help and support, and to acknowledge KFUPM and SABIC for

their support for this project under SABIC Grants (Project # 2002-06). The author thanks AJSE reviewers for their

valuable comments and suggestions which improved the original manuscript.

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Geotech Properties of Rus Fm in Dammam Dome Saudi - Abdullatif

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