Delineation of Near-Surface Structure in the Southern Part of 15th of May City, Cairo, Egypt Using Geological, Geophysical and Geotechnical Techniques

Abstract

The integration of geological, geophysical, and geotechnical interpretation at the southern part of 15th of May City, have been used to evaluate the subsurface stratigraphy, especially the clay layer which may cause serious danger to construction. Those techniques have been used to delineate the subsurface structures as normal faults, which play a critical role on the stability of buildings. Geological setting of the area has been evaluated through the construction of a geological map from different geological sections and samples obtained from more than 30 observation points. Geophysical tools such as vertical electrical soundings (12 VESs), 2-D dipole–dipole array (7 sections), P-wave shallow seismic refraction (31 profiles) and multiple channel analysis of surface waves (31 MASW profiles) have been carried out to image the subsurface situation. Geotechnical evaluation using 26 boreholes, samples, laboratory tests and geotechnical parameters has been done at the area of interest. The geological setting demonstrates that the city had been constructed on the second and third members of Qurn Formation (Upper Eocene) composed of argillaceous limestone, marl and shale. Two normal faults are passing through the area were observed. The resistivity (VES and dipole–dipole) and seismic (P-waves and MASW) results reflect the presence of the two normal faults cross the study area, affecting the obtained section of marl, clayey marl and limestone layers. The geotechnical information indicate the presence of the normal faults and the existence of clay layer with swelling ability reaching 140%, which may cause cracks in the upper layers and/or subsidence.

1 Introduction

The aim of the current study is to evaluate the near surface layers (lithologically and structurally) and to determine the sources which cause the recorded cracks, fractures and soil subsidence at Quarters (buildings) 25, 26 and 27 at the study area. In order to diagnosis the causes of the mentioned phenomena, a detailed geological study, electrical resistivity measurements (VES and ERT), seismic survey (P-wave and MASW) and geotechnical investigation have been carried out and interpreted in integration manner.

Geophysical methods have been widely applied to help solve many problems in engineering geology. The probability of a successful result increases if appropriate methods, based on the principles of information complementarily and coherency, are selected. Such a strategy is mostly advisable in delicate environments, where the adoption of absolutely noninvasive geophysical methods is the only possibility for target identification, prior to exploration (Domenico and Sergio, 2009). With the growing demand for space utilization, there are also an increasing number of incidences of structural damage which can be accompanied by collateral losses. Uncertainties associated with the design and planning of structures play a role in such failures (Akintorinwa and Adesoji, 2009). Design uncertainties related to unknown soil properties are among the most important (Bremmer, 1999). The non linear behavior of soil under stress, the difficulty in estimating soil properties in undisturbed or in situ conditions, and high spatial variability, all make it impossible to predict the exact behavior of soil in time and space.

The 15th of May City is located south of the Nile Delta and east of Helwan city (Fig. 1) which is built on Eocene strata. The city was constructed to solve the problem of insufficient accommodation, where recent extensions had taken place. The city is subjected to quarry blasts from the two quarries; one is located 1.5 km to the east and the other is situated 3.5 km to the south. Some buildings experienced cracks, fractures and soil subsidence.

Fig. 1

Location map of study area

Geophysical applications have been used for imaging the geotechnical problems at some localities of many cities in Egypt. Many authors have used the geophysical tools to delineate the subsurface stratigraphy and structural elements. Tealeb et al. (2000), investigate the stress level for the ground beneath the 15th of May City. Abdel-Hafez (2004), apply the geophysical and geotechnical studies in pharaonic and urban areas in Egypt, especially at the 15th of May and Mokattam cities, and he documents structural discontinuities in the subsurface (joints and faults), which could be a part of the geotechnical problems observed at the study areas. Sultan (2010), has used the geoelectrical and shallow seismic refraction for detecting faults, fractures, caves and clay layer. Atya et al. (2010) apply the electromagnetic imaging of the near surface dynamics and its impact on the foundation stability at Quarter 27 of 15th of May City.

2 Geology of the Area

The study area is a part of 15th of May City and includes the Quarters 25, 26 and 27. The geologic setting of the area is evaluated by constructing a new geological map based on Landsat satellite images, an aerial photograph (scale 1:20,000), and described geological sections at more than 30 observation points distributed to cover the area of interest. Figure 2, shows the description of five geological sections in addition to the composite section. For map accuracy, these observation points are controlled using traditional survey. Some of these points are located on the geological map (Fig. 3). The constructed geological map reveals that most of the area consists of deposits of Pliocene, Upper, and Middle Eocene. The Pliocene deposits are represented by wadi deposits, which are composed of compacted sandstone of medium to coarse grains; these deposits occupy the southwestern part of the 15th of May City. The Upper Eocene deposits are represented by Wadi Garawi and Qurn Formations, while the Middle Eocene deposits are represented by Observatory formation. Wadi Garawi Formation is distributed at the south and southwestern parts of 15th of May City and consists of marl and marly limestone with clay intercalation at the upper part of the formation with thicknesses ranging from 50 to 80 m. Qurn Formation belongs to the Upper Eocene, covers most of 15th of May City, and is composed of five units. The first unit (at the base) consists of massive crystalline limestone inter-bedded with argillaceous limestone. This unit occupies the northeastern part of the area (Fig. 3). The second unit includes argillaceous limestone, marl and shale and occupies the north, northwestern and central parts of the area (Fig. 3). The third unit of Qurn Formation outcrops at the northwestern, central and eastern parts of the interested area and is made up mainly of marl and shale. The fourth unit is exposed at the northeastern and southwestern parts and is represented by limestone with claystone bands. The fifth and last unit (at the top) of Qurn Formation which consists of limestone and shale and is found as small patches at the eastern, central and southwestern parts of the area (Fig. 3). The Observatory formation of Middle Eocene characterized by highly fractured limestone and caves. This formation is represented by a small part at the northwestern corner of the area (Fig. 3). Moustafa et al. (1985); Farag and Ismail (1959) investigated the surface structures for the whole city and state that the area has been dissected by sets of faults trending NW–SE, E–W, and NE–SW. In the current study, the surface structures are executed through detailed descriptions for the different observation points and are confirmed by geophysical and geotechnical studies. They indicate that the area was dissected by two essential sets of faults trending NW–SE (Fig. 3). On the other hand, the authors delineate secondary sets of faults trending NW–SE passing through Quarters 25, 26 and 27 at the studied area (Fig. 4).

Fig. 2

Detailed description of the geological sections at different observation points; a observation point 265, b observation point 260, c observation point 287, d observation points 288 and 290, e observation point 290 and f complete composite section for the interested area

Fig. 3

Geological map of the 15th of May City

Fig. 4

Photos show a normal fault where the layers of the downthrown are dipping southward; a at the observation point (293) and b at observation point (269)

3 Geophysical Data

3.1 2-D Electrical Resistivity Tomography (Dipole–Dipole Array)

In the dipole–dipole array, the space between current electrodes and potential electrodes are equal (a) and the two current electrodes (AB) are kept fixed while the potential electrodes (MN) are moved collinearly to the current ones. The spacing between current electrodes and potential electrodes is a multiple of the electrode spacing (a). The depth of penetration is a function of spacing (a) and the dipole separation factor (n) (Edwards, 1977). The processing and interpretation of the obtained data have been done using RES2DINV, (2003) program, which produces an image of the electrical resistivity distribution in the subsurface based on a regularization algorithm (Loke and Barker, 1996a). The 2-D inversion model consists of a number of rectangular cells. The arrangement of the cells approximately follows the distribution of the data points in the apparent resistivity pseudo-section. The regularized least-squares optimization method with cell-based model is sufficiently flexible to represent almost any subsurface structures with an arbitrarily resistivity distribution (Loke et al., 2003). The dipole–dipole array was successfully used in archaeological prospecting at Al Ghouri Mausoloum, Islamic Cairo, Egypt (Sultan, 2004). Also, the geoelectrical technique was used for solving some geotechnical problems at two sites in Greater Cairo (Sultan et al., 2004). In the present study, the electrode spacing is 8, 9 and 10 m in order to investigate the shallow subsurface stratigraphy and structures. The apparent resistivity (ρ_a) is calculated using the following equation:

$$ {\pi}\, {\text{an}}(n {\text{ + 1}})(n {\text{ + 2}})\Updelta V/I $$

(1)

where n is the number of dipoles separating current electrodes from potential electrodes (maximum n = 7, in this work). Seven dipole–dipole sections of different lengths ranging between 140 and 550 m are applied on the study area (Fig. 5). These sections are P1–P1′, P2–P2′, P3–P3′, P4–P4′, P5–P5′, P6–P6′ and P7–P7′. Each section consists of a number of spreads and each spread consists of seven values for each electrode spacing at different depths (from n = 1 to n = 7). The dipole–dipole cross sections (Fig. 6) exhibit large variation of resistivity values corresponding to lateral variation in the subsurface lithological units. The inverted models for the seven sections indicate that the subsurface consists of different geological units represented by limestone, marl and clayey marl. Also, the dipole–dipole section along profile P1–P1′ reveals a fractural zone at the end of the section which exhibits high resistivity values.

Fig. 5

Location map of the geophysical measurements and the boreholes

Fig. 6

Seven 2-D dipole–dipole sections (ERT) along different profiles at the study area

3.2 1-D Vertical Electrical Sounding (VES) Using Schlumberger Array

Twelve Vertical Electrical Soundings (VES) were carried out using Schlumberger configuration with an AB/2 spacing ranging from 1 to 400 m (Fig. 5). The main objective was to detect clayey layers and the different sub-surface successions. There are several methods for VES data interpretation: graphical (manual) and analytical methods. The authors used a graphical interpretation, which depends upon matching the plotted field curves with the standard curves and the generalized Cagniard graphs (Koefoed, 1960). The results obtained were used as initial models for the analytical methods. The quantitative interpretation was made using the IPI2WIN program which deals with VES curves in a human-computer interactive regime and draws theoretical and field curves on a display screen together with a Rho (z) model curve. The quantitative interpretation has been applied to determine the thicknesses and the true resistivities of the stratigraphic units beneath each VES station and in the construction of five geoelectrical cross-sections (Fig. 7a–e). The geoelectrical cross-sections indicate that the shallow subsurface section of the study area consists of three geological units which belong to Qurn formation. The first geological unit is composed of marl which is distributed through the area at different depths ranging from near surface to a depth of 50 m. This unit exhibits moderate resistivity values ranging from 7 to 39 Ωm. The second geological unit is composed of clayey marl which reveals low resistivity values ranging from 2 to 11 Ωm; usually this unit lies beneath the unit of marl except at the VES number 9 for the geoelectrical cross-section B–B′ (Fig. 7b) which lies beneath the limestone unit. This unit represents the main problem in the study area, where the marl and limestone are overlain by a clayey marl unit which causes the most engineering problems due to volume changes in swelling clays as a result of man-made activities that modify the local environments (Velde, 1995) causing most fractures on the limestone and marl. The third geological unit is limestone unit which have high resistivity values ranging from 15 to 6,547 Ωm according to the hardness and fractures.

Fig. 7

Five geoelectrical cross-sections, a A–A′, b B–B′, c C–C′, d D–D′ and e E–E′, from the twelve VESes

3.3 P-wave Shallow Seismic Refraction

Thirty-one shallow seismic refraction spreads for longitudinal waves (Fig. 5) were carried out to cover Quarters 25, 26 and 27. The field survey has been carried out using a seismograph model StrataView (of 1 ms. sensitivity) and manufactured by Geometrics Company (Underwood, 2007). The acoustic waves were generated by using a hammer of 15 kg weigh as a seismic source. Forty-eight geophones for seismic spread (of 94 m length) were used with a spacing of 2 m. Five shots are carried out; the first shot is the normal one at 5 m before the first geophone, the second shot at the half distance between geophones 13–14, the third one is the midpoint between the geophones 24 and 25, the fourth one between geophones 36–37, and the last shot is the reverse one at 5 m after the last geophone. The data have been processed and interpreted by using SIPT2 (RIMROCK, 1992), SEISREFA (1991) and SEISIMAGER (2009) software packages. The interpretation of data is based on iterative ray-tracing technique, in which the ray propagation will be simulated through a model (Scott, 1973). The results of the interpretation for 21 spreads reveal four layers but 10 spreads exhibit three layers. The P-wave average velocity of the first layer is ranging between 230 and 320 m/s and corresponding to clay and sand of thickness ranging between 0 and 3 m. The second layer has an average velocity value ranging from 400 to 1,300 m/s which consists of fractured marl and limestone of thickness ranging from 0 to 10 m. The third layer reveals a velocity value ranging from 780 to 2,000 m/s and thickness ranging from 1.5 to 22 m corresponding to marl. The Fourth layer exhibits a high velocity value ranging from 1,900 to 3,800 m/s corresponding to clayey marl. According to the fault criteria, the seismic profiles 7, 11, 20, 22, 28 and 29 demonstrate the effects of normal faults. Figure 8 shows a representative example of the P-wave model which affected by the normal fault.

Fig. 8

P-waves geoseismic section along spread No. 11; a time-distance curves, b interpreted (processed) section and c geological section

3.4 1-D Multiple Channel Analysis of Surface Waves (MASW)

Another technique to determine the shear wave velocities is used. This is the most common type of multi-channel analysis of surface Waves (MASW) survey (Park et al., 1999). The maximum depth of investigation that can be achieved, is usually in the 10–30 m range, but this can vary with sites and types of active sources used.

A fairly heavy sledge hammer was used as a source of active MASW. Vertical stacking with multiple impacts can suppress ambient noise significantly and is therefore always recommended, specially, if the survey takes place in an urban area. Low-frequency (of 4.5 Hz) geophones are used. The length of the receiver spread is directly related to the longest wavelength that can be analyzed, which in turn determines the maximum depth of investigation. On the other hand, (minimum if uneven) receiver spacing is related to the shortest wavelength, and therefore, the shallowest resolvable depth of investigation. A 1 ms of sampling interval is most common with a 2 s total recording time (T = 2 s).

The MASW spreads were carried out along the same spreads of longitudinal waves (Fig. 5) using 24 geophones. Each spread is of 46 m length. One shot was applied using a 15 kg hammer at a distance of 5 m before the first geophone. The processing and interpretation are carried out by using the SeisImager software package for 1-D MASW, passing through the following steps: surface wave imaging (overtone), constructing the dispersion curve, preparing the initial model (from the boreholes and the P-wave model) and applying the inversion technique (Fig. 9). The MASW technique is one of the seismic surveys for determining the shear wave velocity and consequently, evaluating the elastic condition (stiffness) of the ground for geotechnical engineering purposes. The obtained shear wave velocity of the first layer is ranging between 140 and 240 m/s, for the second layer is ranging between 215 and 720 m/s, with respect to the third layer reveals a velocity of range 450–1,020 m/s and the fourth layer has a shear-wave velocity ranging between 980 and 2,100 m/s.

Fig. 9

The surface waves interpretation along spread No. 17; a the surface wave image (overtone), b the dispersion curve obtained from the overtone, c the initial model deduced from the boreholes and P-wave seismic model and d the final 1-D shear wave velocity model using the inversion technique

4 Geotechnical Data

The geotechnical information obtained from the results of the 26 boreholes of maximum depth 20 m using air rotary drilling which forces air down the drill pipe and back up the borehole to remove the drill cuttings. These boreholes distributed at the area of study as well as Quarters 25, 26 and 27 (Fig. 5), field and laboratory tests and the deduced parameters from the seismic study, may reflect the behavior of the subsurface layers and consequently, help us to provide a reliable solutions.

The RQD (rock quality designation) test at most of the boreholes (up to 10 m depth) demonstrates the very poor to poor soil category. The swelling test for the claystone samples reflects its swelling ability reaching a value of 140% specially, at Quarters 26 and 27. The swelling phenomenon of claystone at the studied area may causes cracks in the overburden and consequently, cracks in the buildings and/or subsidence. The mechanical analysis of the limestone samples (dry and immersed in water for 48 h) reveals partial and total failure of the limestone by immersion. That behavior of limestone reflects how the water from the garden irrigation and any leakage from the drinking water and drainage networks will affect the limestone and overburden. The presence of clay minerals in a fine-grained soil will allow it to be remolded in the presence of some moisture without crumbling. If the clay slurry is dried, the moisture content will gradually decrease, and the slurry will pass from a liquid state to a plastic state. With further drying, it will change to a semisolid state and finally to a solid state. These limits are the liquid, plastic and shrinkage limits. Those limits are generally referred to as the Atterberg limits. These limits were determined at the boreholes (Quarters 25, 26 and 27) and demonstrate a class of clay to rich clay (CH) according to the USCS classification (Astm, 2006).

The thickness of the yellow sandy silt clay (marl) layer varies from 1.4 m at the borehole 13 which drilled at building 40 (Quarter 27), to 4.0 m at borehole 14, which is drilled at building 41, indicated that there is a normal fault dissects Quarter 27 passing between the two buildings 40 and 41 and reflecting some cracks at building 40 (Fig. 10a–c).

Fig. 10

The normal fault effects on the buildings at Quarter 27; a Land-sat image shows the 2 buildings 40 and 41, b the oriented cracks at building 40, c crack separate between the 2 buildings 40 and 41, d zoming for one of the oriented cracks and e the soil description of the 2 boreholes 13 and 14 at buildings 40 and 41

5 Discussion

The integration for geological, geophysical and geotechnical data are used to define the properties and characteristics of the different subsurface layers and to delineate the structural elements which could have direct effects on construction in the 15th of May City. Geological study, which includes field work and geological descriptions at many observation points, indicates that the study area consists of different geological units belonging to different geological formations. The main formation in the study area is Qurn Formation which is composed of five geological units, occupying most of the study area. Most of these units consist of marl intercalated with clay. The marl intercalated with clay (clayey marl) represents the main geotechnical problem. Two sets of faults of NW–SE were investigated through detailed field geology at the area. The geoelectrical data, which include electric resistivity tomography (ERT) represented by dipole–dipole sections and 1-D VES using Schlumberger array, indicate that the study area consists of different geologic units according to the resistivity values, where the quantitative interpretation for the 1-D and 2-D resistivity data indicate that the subsurface section consists of different geologic units such as fractured limestone, clayey marl, marl and limestone where these units coincident with the geologic description. Also, shallow seismic refraction interpretation for P-waves and surface waves indicate that the subsurface sections divided into four layers according to the velocity values, these layer are sand and clay (soil), fractured limestone, clayey marl and marl which are compatible with geological observation and the results of resistivity data. Added, the location of VES 6 lies at the beginning of seismic spread no. 11, where the interpretation for both data, are coincident for the subsurface layers and fault element. Also, the comparison of results of VES 5 which lies at the beginning of dipole–dipole section at profile P1 indicate good correlation, where the inverse section reflects marl and clayey marl as shown in Figs. 6, 7. The geotechnical data for the boreholes 13 and 14 at the buildings 40 and 41 respectively, refers to the existence of a normal fault. This normal fault reveals variation in the thickness of the marl layer and cracks at building 40.

6 Conclusion

From the integrated interpretation for geology, geoelectric, shallow seismic refraction and geotechnical data we can conclude that the study area consists of four subsurface layers, the first layer is the sand and clay (soil), the second layer is composed of fractured limestone. The third layer is clayey marl and the fourth layer is marl. A number of normal faults trending NW–SE were delineated at different locations as shown in the geoelectrical, and geoseismic cross-sections. The limestone layer at shallow depths exhibits fractures. Some buildings have been suffered oriented cracks, due to the effects of the fault elements, which dissect the study area.