Geological engineering description of the exposed surface rocks in wadi Dawa’an, Hadramout Governorate, Republic of Yemen

The present study deals with engineering geological characteristics of the rock mass encountered in and on sides of Wadi Dawa'an, Hadramout governorate. During the present investigation, detailed studies were carried out on lithology, discontinuities, etc. and basic Geotechnical Description (BGD) of rock mass. The results revealed that, the Limestone (Umm erRhaduma Formation) has been classified into three zones (Zone I to Zone III), and the exposed parts of Sandstone (Mukalla Formation) into two zones (Zone I and Zone II). The Limestone (very steep cliff) varies from dark brown and moderately weathered in bottom, to dark grey and fresh in the upper most layer. Furthe, the exposed parts of Sand stone Rocks vary from highly and completely weathered, very low compressive strength to moderately weathered and low compressive strength. The present work was initiated by preliminary investigation (desk study) that include data collection and extensive literature survey. This work was followed by reconnaissance stage to gather more knowledge on the lithology, topographic features, and land-use of the study area, in order to prepare work plan for field and laboratory studies. Firstly, 72 field stations on either side of Wadi Dawa’an were selected. The preliminary engineering geological study includes the detailed engineering geological studies and geotechnical study of the exposed surface rocks along the study area. Equipments used for this purpose are Brunton compass, Schmidt hammer, geological hammer, measuring tape, geological and topographical maps, GPS instrument, etc. The engineering properties of the exposed rocks were noted down on both sides of the WadiDawaan. Detailed studies on lithology (rock type, color, grain size, and texture), discontinuities (joint-orientation, spacing, aperture, persistence, roughness and infilling materials), block size and shape, and seepage were carried out. Rock samples were collected from the rock outcrops. geotechnical properties rock of weathering.


Introduction
Wadi Dawa'an, is located in SE Yemen mainly in Dawa'an district about 185km northwestof Mukalla city (Capital of Hadramawt governorate)( Figure 1).WadiDawa'an is the largest southern tributaries of Wadi Hadramawt and has catchment area of about 3394 km² and mainly comprises two wadis that identified as the wadi Dawa'an right and left (Liemen and Lieser at Hadramawt parlance) and sub wadis (WadiQaydoun,Wadi Alghabr, etc.). The Wadi is more than 60kms long and ranges in width from 102m and 2kms at some sections. The study part bounded by north latitudes of 1666800 to 1733418 and east longitudes of 201802 to 230971 ( Figure 1).Most archaeological, tourist towns and highly populated areas are occupied along the sides of the valley and on the surrounding terraces and plateaus.
Cretaceous to Paleogene sedimentary rocks are exposed on both side of the Wadi Dawa'an. These rocks are characterized with different engineering geological behavior. Figure1. Location map of the study area.
The geotechnical and engineering geological investigation and mapping mainly focus towards understanding the interrelationships between the geological environment and the engineering situation; the nature and relationships between the geological components, the active geodynamic processes and the prognosis of processes likely to result from the changes being made [19].
Due to complexity of rock mass with varying physico-mechanical in situ properties, numerous classification systems have been developed for the characterization, classification as well as to gain the knowledge on the rock mass properties and also to provide a quantitative valuation of rock mass [2,3].
The Basic Geotechnical Description of rock mass (BGD) technique introduced by the International Society of Rock Mechanics (ISRM) [14], was one of the major rock mass classification systems used for general geotechnical applications [18] including geotechnical conditions of exposed surface rocks.
From the geotechnical point of view, the rock masses in the Wadi Dawa'an has not been studied in the past. The present study was aimed at assessing and evaluating the preliminary engineering geological and geotechnical conditions and to provide important engineering geological data that may help to plan, design and maintain engineering projects and stability of slopes for the safety of the study area.
In order to accomplish these, 72 field stations from both sides of Wadi Dawa'an were selected, for geological mapping, discontinuity surveying and rock sampling for laboratory tests .

Topography
Wadi Dawa'an represents one of the southern tributaries of Wadi Hadramawt that forms deeply cut water course through Paleocene limestone in the eastern plateau region ( Figure 2). It runs from south to north, in the south mainly consisted of right and left wadis (Liemen and Lieser at Hadramawt parlance) and begins about 102 m width at the upper part reaches Wadi Lieser and 112 m in the upper part reaches Wadi Liemen.It becomes progressively wider toward the north until it is about 2 km nearby AL-Mashhad area and the catchment area covers an area of about 3394 km². Many sub wadis reach their inflow to the main wadi. The range of the bed Wadi Dawa'anbeing from 763 m above the sea level in Al-Mashhad area in the north to 1024 m in Wadi Lieser and 1076 m at Wadi Liemen in the south. The highlands of the wadi are a rather broad, barren and pebbly plateau which named (Jawl Dawaan, Jawl Khuribah, Jawl Hah, etc), averaging at about 965 m to 1480 m above sea level.Contour banks on top of the plateau will gently direct the runoff water towards the wadi top head cut(Figure3).
Geomorphologically, the drainage pattern system can be classified as dendritic, flat-topped cliffs that drop vertically to the wadi bottoms. This causes the runoff to be extremely rapid and may become flood. Especially in the spring and autumn seasons when heavy rain falls dominate. At times, the floods can cause enormous devastation to crops and buildings; and many people may get killed.

Climate
Wadi Dawa'an is U-shape Wadi in a mountainous area at Hadramawt plateau with hot dry climate. The annual average temperature ranges from 9-43c, the annual average of relative humidity range from 30 -50% and the annual precipitation ranges from50 to 125 mm/year [1,7]covered with considerable vegetative cover.
The monthly average of wind speed was (0.9 m/s) for the period 1980 -2004 and the maximum monthly average is 1.1 m/s in July, while the minimum was 0.7 m/s in November and December [1].

Geological setting
Geological mapping with a scale of 1:100000 was conducted for the study area ( Figure 4). The exposed surface rocks on either side of Wadi Dawa'an are very steeply dipping Tertiary rocks underlain by gently dipping Cretaceous rocks covered with scree.The exposed surface rocks range in age from Upper Cretaceous to Upper Paleocene. These rocks are represented (19) Mukalla Formation, (2) Umm erRadhuma formation [6].
(1) Mukalla formation is the deepest formation encountered in the subsurface at Al-Hajjrein Well-1 (within the study area) with 1164 m thickness [15]. It generally consisted of Sequence of Clastics primarily of coloured, friable, and current bedded sandstones, with pebbly horizons and with subordinate marls and siltstones. This formation is outcropping in limited area few metersin height and mostly covered with scree.
This formation consists of a basal dolomite overlain by paper shale's and nodular limestone, succeeded by a thick sequence of massive, dark-brown and dark grey commonly dolomitic limestone. Toward the top of the formation, a thick zone of white and grey, with dolomitic, chalky, nodular and cavernous limestone, due to scaly disintegration and exfoliation slumped sections, are common.
In the study area, the Umm erRhaduma formation is underlain unconformable by the upper Cretaceous Mukalla formation clastics, where the Sharwayan formation is absent [15].

Structural setting
The study area is located to the east of the Arabian shield as a part of Arabian shelf [6;5]. It lies in Say'un Sector, that part of Say'un-Almasila basin, that is one of sedimentary basins, which was formed at Mesozoic period in Yemen. The Mesozoic rift basins were formed due to the breakup of Gondwana and separation of India/Madagascar from Africa-Arabia during the late Jurassic/early Cretaceous [4].
The dominant structures in the study area are faults, joints and bedding planes. The faults are interpreted from aerial photographs (verified in the field), while the other structures are observed and measured. The study revealed NNW-SSE, ENE-WSW and NNE-SSW with minor NE-SW, NW-SE and E-W striking faults ( Figure 5). The joints represent the second common structures in the study area. The strikes of the joints are generally parallel to the faults of the area. Most of the joints are vertical. Figure 5, shows rose diagram of joints measured in the study area. Mainly two major joint sets are observed (NNW & ENE trending) with some minor sets (NE & NW and E-W strike). The third geological structure is bedding plane, which is predominantly horizontal.

Methodology
The present work was initiated by preliminary investigation (desk study) that include data collection and extensive literature survey. This work was followed by reconnaissance stage to gather more knowledge on the lithology, topographic features, and land-use of the study area, in order to prepare work plan for field and laboratory studies. Firstly, 72 field stations on either side of Wadi Dawa'an were selected.
The preliminary engineering geological study includes the detailed engineering geological studies and geotechnical study of the exposed surface rocks along the study area. Equipments used for this purpose are Brunton compass, Schmidt hammer, geological hammer, measuring tape, geological and topographical maps, GPS instrument, etc.
The engineering properties of the exposed rocks were noted down on both sides of the WadiDawaan. Detailed studies on lithology (rock type, color, grain size, and texture), discontinuities (joint-orientation, spacing, aperture, persistence, roughness and infilling materials), block size and shape, and seepage were carried out. Rock samples were collected from the rock outcrops.
In the present study, the BGD classification systems [4] were applied for the evaluation of engineering geotechnical properties [18]. The BGD classification system is based on a number of parameters that are combined to give acceptable judgments to provide the general picture of the geotechnical properties of the rock mass zones of the study area. Each zone is provided with its rock name, followed by the standard symbols for rock strength, discontinuity spacing and grade of weathering.

Engineering geological properties and classification of the exposed surface Rocks
The engineering geological properties of the exposed surface rocks in Wadi Dawa'an were determined based on the field observations, measurement, and laboratory tests which has been carried out for the first time. The description of rock material and mass characteristics were based on [13].

Field investigation and data collection
After the recognition of the lithological units in the area, discontinuity surveys from different rock masses were performed in seventy two selected field stations to determine the rock mass characteristics and for the purpose of the classification according to [13]. Some of the field examinations included using Schmidt rebound hammer and geological hammer for estimating compressive strength.
A total of 51 samples were collected from the field for laboratory testing and location of samples are plotted on a geological map of scale (1: 100,000);Figure4.

Physical and mechanical properties of the exposed rocks
Physical and mechanical properties include bulk density, water absorption, specific gravity, uniaxial compressive strength; shear strength parameters of the material within the discontinuities by direct shear tests and tensile strength test were tested in the laboratory for Paleocene limestone only. The result range values of physical and mechanical properties are illustrated in Table (1). Determination of Physical and mechanical properties of the sandstone rock mass (Mukalla Formation) during geotechnical examination works in situ was disabled because of the sandstone rock mass friable behavior. It was difficult to get undisturbed samples because of rock mass disintegration in highly (W4) to completely weathered (W5) sandstones with thin siltstone; the significant is also a sudden degradation and disintegration of moderate weathered (M3) sandstone, after removing of geostatic loads and exposed to air and water during preparation. Small number of undisturbed samples from moderately weathered sandstone for density was tested in laboratory by using waxtreatment. The unconfined compressive strength of moderately weathered sandstone was estimated in the field by geological hammer and described according to As-saruri et.al. [13]. The range value from (1-5 MPa) for highly and completely weathered and (5 -25 MPa) for moderately weathered and the mean values was (3 -15 MPa) respectively.
For determination of the mechanical properties (cohesion, friction angle, tensile strength, compressive strength) of sandstone (Mukalla Formation), the parameters used for the Hoek-Brown criterion [16], the uniaxial compressive strength of intact rock (sigci), material constant of intact rock (mi).The Geological Strength Index (GSI) and distribution factor (D)based on the information obtained from the observation of outcrops in the field were introduced into RocLab program [11 ;12].
For the moderately weathered sandstone, the field test data was confirmed by the Rock data program in which the characteristics of the rock were determined as (Sigci =15, mi = 9, GSI = 37, D = 0) and the result of the program shows that, shear strength parameters (c = 0.58 MPa and φ = 25.83º), tensile strength (-0.01MPa) and uniaxial compressive strength (0.411.MPa) (Figure6).
And for the highly to completely weathered sand stone,the field test data was confirmed by the RockData program in which the characteristics of the rock were determined as (Sigci = 3, mi = 19, GSI = 19, D = 0) and the result ofthe program shows that, shear strength parameters (c = 0.10 MPa and φ = 26.44º), tensile strength (-0.00035 MPa) and uniaxial compressive strength (0.02MPa) (Figure7). Table 1. The results of physical and mechanical properties of the exposed surface rocks in WadiDawa'an

Engineering classification of the rock mass
Numerous rock mass classification systems have been developed for special purposes such as tunneling, mining, etc.The BGD classification system is one of the important systems, it intent to characterize the various zones that constitute a rock mass in a simplified form.
In the present study, a preliminary geotechnical investigations on the exposed surface rocks in 72 selected field stations along sides of Wadi Dawa'an using the technique (BGD) of ISRM [14] was performed.
Based on the results of BGD, the Limestone (Umm erRhaduma Formation) has been classified into three zones, and the exposed part of the Sandstone (Mukalla Formation) has been classified into two zones (Table 2, Figure8).Each zone is provided with rock name followed by standard symbols. Table 2, summarized the average of the geotechnical characteristics of these rock formations at the surface, besides some general comments are given in the following paragraphs.
The uniaxial compressive strength was determined from Laboratory tests (dimensions of rock samples 5x5x5cm³)for limestone and, estimated by Schmidt hammer hardness readings through the approximate correlations of [9]. For Sandston the uniaxial compressive strength was estimated by geological hammer and approximated based on ISRM [13] and also estimated by Schmidt hammer.
The friction angle values for limestone was determined by direct shear test for material within the discontinuities, and also using Inclined Plane method (the rock samples in dimension5x5x2.5cm³). For Sandstone, using the Rock Data program (Rock Science group) [11].

Limestone (Umm erRhaduma Fn.)
Limestone is the most widespread rock mass in study area. This rock unit has variable properties that resulted from nature structural discontinuities (Joints), bedding planes and weathering status. According to BGD classification system, this rock unit has been classified into three distinguished zones. These zones are The general geotechnical description (BGD) of this subunit in the study area can be designated as: Light Yellowish Brown and White, slightly to moderately weathered nodular LIMESTONE L2-3, F1, S2, A3. The third zone (ZIII): Dark Brown and moderately weathered nodular Limestone (L1-2, F1, S2, A3) This rock represents the base of the steep cliff of limestone rock (Umm erRhaduma Formation) along the study area, covered by sediments in most places. Its Dark brown in colour, medium to fine grained, moderately weathered (W3) and the layer thickness (L2) is between (60 -200 cm),in some places the average thickness is >200 cm(L1).
Two sets of joints are dominant, with exceptional three sets of joints in some sites. The joints are closed in the bottom and open towards the top, generally without filling material, occasionally contain little of soil and litho-fragments rocks; mostly with rough undulating surface. The fracture intercept is very wide in average (F1). The averages of Schmidt hammer hardness and the density of this rock type are 72.9 MPa and 30.1KN/m3 (3.07gm/cm3) respectively which indicate high compressive strength (S2). The averages of friction angle is 34º which indicated high friction angle (A3). This rock contains numerous of cavern cavities and noticed the drops of water are falling from ceiling of caves.
The general geotechnical description (BGD) of this type in the study area is: Dark Brown and moderately weathered nodular LIMESTONE (L1-2, F1, S2, A3)

Discussion
The BGD of the rock units shows slight difference between the suggested subunits in the study area. The significant features recognized these units are the physical properties (table 2). That meansthe limestone could be divided into three subunits primarily due to its geological setting and inherited physical properties, and secondly due to its geotechnical properties.The same classification words can be supposed for the geotechnical characteristics of the Sandstone subunits.

Conclusions
Engineering geological features of rock masses and Geotechnical characteristics of the exposed surface rock onboth side of Wadi Dawa'anhave been studded by engineering geological rock mass description and BGD classification system.
Further specific studies, for foundations, slope stability and site selectionis necessary and required for safety of the future engineering projects.
The results show that, the Limestone (Umm erRhaduma Formation) has been classified into three zones (Zone I to Zone III), and the exposed parts of Sandstone (Mukalla Formation) has been classified into two zones (Zone I and Zone II).