First EAGE International Conference on Engineering Geophysics

Of-late geophysical techniques are becoming powerful tools in the geotechnical/ civil engineering and engineering geological applications for exploring the subsurface ground conditions. These provide one of the early means of identifying and acquiring information on the potentially dangerous geologic structures at site. The sources of civil engineering hazards are from the undetected shallow subsurface in-homogeneities/ weak features like faults, shear zones, karstic features/ cavities/ solution features etc. which substantially influence the foundations for structural loadings. The geophysical surveys also give an over all appreciation of the dynamic properties of the geo-mass which helps in the characterization of rock masses for different purposes like assessments of foundations, excavatability and evaluation of geo-hydrological potentials etc. The paper discusses a case study of the application of surface geophysical/ surface seismic refraction technique for an assessment of the excavatability of the relatively soft destructured (Class D/C) rock mass/ intermediate geo-materials (IGMs). No standard means or definitions are in vogue for differentiating the soft rock/ weathered rock mass for quantification in excavations. These intermediate materials/ IGM, between the well defined soil and rock mass, in general have vague layer boundaries differentiating from the soil and rock layers, which often brings in/ raises disputes in classifying and quantifying the material from the excavatability point of view. The present study was carried out as an attempt to address this problem logically at the instance of the contractor involved with the construction of an interchange in Dubai. The construction works were accomplished before the present study. In view of a pending case of remeasurement of rock excavations with the authorities the study was carried out to assess the excavatability of the rock mass already excavated at site, based on the geotechnical investigation data available from previous site investigations and investigations carried out at five trial areas deploying drilling, rock mechanic testing’s and geophysical/ surface seismic refraction profiling, besides consultation of literature on excavation of rock mass for a realistic assessment of the parameters and estimating the excavatability/ rippability with respect to CAT D8R equipment, a yard stick for the authorities.

Geophysical Surface Techniques have been widely used at various stages of a site investigation program ( Clayeton et.al., 1995 ) for engineering structures (New construction or Restoration), as indirect means to define the subsurface conditions, the location, depth and the lateral extent of adverse strata areas representing cavities and weak zones within the underground strata up to the desired depth of the investigation; and whether they pose a risk to the proposed development or to an existing building. In urban areas, site constraints, access and ambient noise strongly influences the validity of employing surface geophysical techniques to investigate subsurface conditions at construction sites. The employment of borehole geophysical technique represents an alternative solution. Electrical Resistivity Tomography is a well known geophysical technique routinely used in site investigations. Cross Hole Resistivity Tomography in which both current and potential electrodes are places in boreholes, can provide detailed 3D information about resistivity distribution of underground strata. In this study, Cross Hole Electrical Resistivity Tomography technique was employed for a multistory building construction site to define the subsurface conditions and to locate adverse high risk strata within the underground up to 40m depth with an aim to increase the resolution in mapping of geological structures and hazards.

The presence of cavities and buried falajs (ancient irrigation system) in Al Ain city has direct impact on human's activities, especially when geo-hazard maps are unavailable. Therefore, a 3D presentation of such geological features will provide an exact location in any study area considered for future site development. Most cavities in Al Ain City are generated in the Limestone formations, which are the most predominant geological formation in Al Ain region. The study area represents the investigation of a subsidence under a recently constructed villa, which is 26m × 24m. The geotechnical boreholes drilled in the site did not reveal the presence of any subsurface weak zones. However, several months after finishing the construction of villa, a subsidence started to affect the whole structure of the villa and cracks appeared on the walls. Because Al Ain City is known for its Limestone formations and many cavities have been located in the city, therefore it was decided to conduct a geophysical survey to investigate the reasons for the subsidence. Based on the drilled geotechnical boreholes in the site, the geological formations in area are mainly weathered Limestone to Silty-Sandy and Clays.

Al-Ain City is located in the eastern part of Abu Dhabi Emirate and is considered as one of the most urbanized cities in the UAE (Fig. 1). The rocks outcropping around the city are made up of carbonate rocks, mainly limestone and interbeds of marl. These rock types represent the foundation bedrocks in most of Al-Ain City. They also form the rock slopes surrounding many new developing sites to the south of the city like the road to the summit of Jabal Hafit and the eastern border of Al-Ain Wildlife Park & Resort (AWPR), whereas some engineering problems appeared. Therefore, it requires knowing the geological and geotechnical characteristics of these carbonate beds. This would overcome any uncertainties that might occur at any type of engineering applications such as design of structures either upon or inside the rocks, slope instability and others. The overall rock properties are considerably controlled by their geologic setting as stratigraphy, mineralogy, petrography and tectonic structures. So, the present study aims to find the geological and geotechnical properties of these rocks which are outcropped in Jabal Hafit and extend northward under Al-Ain City. The stratigraphy of these carbonate rocks were studied in detailed by many authors among them are Abdelghany (2002), Hunting (1979), Cherif and Deeb (1984), Noweir ( 2000), Anan et al. (1992) and Boukhary et al. (2005). The rocks build up the Hafit Mountain (Fig. 1) which represents a large doubly plunging highly asymmetric anticline that developed over a thrust fault underlying its eastern limb (Noweir, 2000; Warrak, 1996 and Ali et al., 2008). Numbers of other interesting research papers were already pointed out to the relation between the geological and geotechnical properties with respect to various rock types (Willard and McWilliams, 19969; Merriam et al., 1970; Basu et al., 2009; Tsiamboas and Sabatakakis, 2004; Sabatakakis, 2008 and Arman et al., 2007).

High resolution reflection seismic methods have proven to be useful tools for locating fracture zones in crystalline rock. Siting of potential high-level nuclear waste repositories is a particularly important application of these methods. Although ambient noise conditions in areas such as the Fennoscandian and Canadian shields are generally low, industrial noise can be high in some areas, particularly at potential sites suitable for spent nuclear fuel repositories since these are often located close to existing infrastructure. In addition, the presence of such infrastructure limits the choice of sources available to the geophysicist. Forsmark, located about 140 km north of Stockholm, has been identified as the site where Sweden will store its spent nuclear fuel [1]. Reflection seismic surveys were an important component in characterizing the site and in localizing the planned repository [2]. Existing infrastructure at the Forsmark site includes nuclear reactors for power generation and a lowlevel waste repository. For site characterization it was particularly important to investigate structures below the existing nuclear reactors. In the vicinity of the reactors, it was not possible to use an explosive source due to permitting restrictions. Instead, a VIBSIST system consisting of a tractor mounted hydraulic hammer was used [3, 4]. By repeatedly hitting the pavement, without breaking it, at predefined sweeps and then stacking the signals, shot records comparable to explosive data could be generated [5, 6]. These shot records were then processed using standard methods to produce stacked sections along 3 profiles within the reactor area. Results from these 3 profiles are presented here and how these results correlate with observed fracturing in an existing borehole.

The study area is located between Wadi Al Dawasir and Aflaj cities, and about 690 km south of Riyadh city. The investigated area is found within the Wadi Tathlith quadrangle. About two thirds of the quadrangle is underlain by basement rocks of the Arabian Shield, much of it consists of an almost flat pediment surface. The remaining part of this quadrangle is in the southwest is underlain by almost horizontally bedded Phanerozoic sedimentary rocks. Wadi Al Dawasir Area forms part of the interior homocline structural province Al-Faifi (2005). The study location is divided into four sites (Fig. 1). Seismic methods have been widely used in detecting and mapping subsurface features, especially the layered sedimentary sequences in search of oil and gas reservoirs (Burger et al. 1992). Advantages of seismic methods over other geophysical techniques are due to their high accuracy, high resolution, deeper penetration, and the amount of information that can be extracted including mapping of the structures, faults, and compactness of various layers Sheriff and Geldart (1995). Recently, these methods, including the high-resolution seismic reflection method, have been applied to map shallow subsurface structures, depth of water tables, and identification of engineering related problems Kearey and Brooks (1984). Since all the engineering and environmental problems are located at shallow depths (near surface), seismic reflection method is an excellent choice to achieve high-resolution images from this domain. Keeping in view the usefulness of this method, four high-resolution seismic reflection profiles have been conducted in the study area. The main objective of this study is to provide estimates of the depth to bedrock and in detection the geological faults.

Especially in urban areas, tunneling is the method of choice to built new pathways to improve the infrastructure, for e.g. rail tracks, roads or power cables. In this context, safety threads are not limited to the tunneling construction itself but can occur years later. Cavities or fracture zones that can be weakened by the tunneling are a serious risk, both for the stability and integrity of the tunnel tube and buildings on the surface. A collapse of a cavity can result in sudden load peaks possibly overpowering the stability of the tunnel casing and subsidence damage to buildings and buried gas pipelines (Fig. 1). Seismic tomography is a useful tool to detect such anomalies in the vicinity of the tunnel tube while the tunneling progresses or after completion. In order to do so, seismic receivers can be placed at the tunnel wall or at anchors behind the tunnel wall. The seismic wave field is excited by a hammer blow applied to the tunnel wall. Basis for such a tomography is a profound understanding of the seismic wave propagation in the complex surrounding of a tunnel which can be gained from seismic modeling. We, therefore, investigate the influence of the excavation damaged zone (EDZ) that is usually present as a side effect of the tunneling and the topography of the tunnel wall. Both features will significantly effect the seismic waves excited at the tunnel wall. The modeling is done by the parallel elastic 3-D finite difference (FD) modeling code SOFI3D using a Cartesian coordinate system (Bohlen 2002). Later, we insert an anomaly close to the tunnel wall. This paper will now primary focus on the realistic description of a tunnel example and the accurate modeling of seismic waves with respect to this model.

g the Strategic Tunnel Enhancement Programme (STEP) to improve the sewerage systems in Abu Dhabi city. STEP comprises a deep sewer tunnel, link sewers and an underground pumping station. The city has experienced vigorous economic and social growth since the 1970’s, when the majority of its sewer system was designed and constructed. Today this growth continues under the framework of Abu Dhabi’s long term master plan, referred to as ‘Plan Abu Dhabi 2030’. The growth is acute in the metropolitan area, where large scale development is occurring on an aggressive timescale and the population is expected to double within ten years. As a result, ADSSC sees many of its sewerage assets being overloaded and approaching the end of their intended service life. Two key drivers for ADSSC’s consideration of a deep tunnel sewer system were limited available land within Abu Dhabi for development and the need to build an efficient, cost-effective and reliable sewerage infrastructure to ensure it could support long term population and industrial growth. STEP is intended to have a design life of 80 years. The deep sewer tunnel is designed to be self-cleansing and to meet the hydraulic design criteria the tunnel alignment will fall at a gradient of 1 in 1300 from north to south. At the southern end of the alignment, a new pump station (PS) will lift the sewerage and discharge into an independent sewerage treatment plant (ISTP), where it will be treated and reclaimed as treated sewage effluent (TSE) for return to the metropolitan area for irrigation purposes.

In soil science, agriculture and gardening, hardpan is a general term for a dense layer of soil, residing usually below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. Hard pans restrict root growth and make it difficult for water, air, other gases plus soil organisms to move through the soil. They are often caused by tilling or ploughing to a particular depth. The hardpan is more likely to cause severe problems if it is: closer to the surface, thicker and/or harder and particularly if it is present during a dry time or present during any other period that stresses plants. Another major determinant is the soil particle size. Clay particles are some of the smallest particles commonly found in soils. Due to their structure the spaces between individual clay particles is quite small and already restricts the passage of water, negatively impacting drainage. Soils with high clay content are also easily compacted and affected by man-made discharges. Clay particles have a strong negative electrostatic charge and will readily bond to positively charged ions dissolved in the soilwater matrix. Common salts such as sodium molecules contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types. This is a common cause of septic system failure due to the prevention of proper drainage in field. Hardpan can be a problem in farming and gardening by impeding drainage of water and restricting the growth of plant roots.

Several roads and buildings in ABU DHABI mainland area were subjected to sever damages in the form of cracking, subsidence and settlement,(Fig. 1) as a result of the adverse subsurface conditions. It is necessary to investigate the depth and lateral extent of these adverse strata before any maintenance or rebuilt operations can be carried out .This requires a considerable amount of investigation into foundation conditions at the proposed sites, by examining materials taken from various depths at a number of discrete points over the site (Clayton et.al., 1995). As the number of boreholes and trial pits increases, the cost of investigation becomes prohibitive and the period to accomplish the work becomes very lengthy. This is an important consideration when investigating localized features such as cavities, weak and contaminated zones, or where detailed subsurface profiles are required. These problems have led to the use of geophysical, non-destructive techniques integrated with geotechnical techniques for site investigations to optimize the identification and location of problematic underground conditions promoting distress. In this study the recent geophysical techniques used in civil engineering investigations and their integration into the overall geotechnical information in relation to detailed design, Road & Building construction, are discussed and illustrated with a case study.