Engineering bedrock depth estimation and ground response analysis of the northern Jeddah urban area, western Saudi Arabia

1 Introduction

The evaluation of the spatial changeability of the engineering bedrock depth in urban expansion zones using multichannel analysis of surface waves (MASW) survey is important for various applications to allocate the input parameters for response spectra. Through geotechnical investigations, the bedrock identification has great importance to adopt the kind of foundation for appropriate structure. Both of peak ground acceleration (PGA) and spectra at a particular site are evaluated for bedrock and ground surface levels including local site response effects. The main objective of our work is the engineering bedrock depth estimation for northern Jeddah area based on shear-wave velocity, V_s, measured through MASW survey. MASW becomes a widespread seismic method applied for geotechnical classification of shallow sediments (Miller et al., 1999; Park et al., 1999; Xia et al., 1999; Park and Miller, 2005; Kanli et al., 2006; Kanlı, 2010; Rehman et al., 2016). Furthermore, MASW is more efficient for determining shallow subsurface properties (Park et al., 2001; Zhang et al., 2004). Aldahri et al. (2017) conducted site soil classification for surface soil in Ubhur area according to the national earthquake hazards reduction program (NHREP) recommendations where, the greatest part of the study area falls in site class C while class B and D covered limited areas in the western and the eastern parts respectively.

The results of MASW include vertical and cross-section of vs profiles. MASW was applied through the study area for 76 sites for shear wave velocity assessment. The estimated values were analyzed and then the engineering bedrock surface depth was classified. The engineering bedrock can be defined as the layer having shear-wave velocity about 700 m/s (Miller et al., 1999; Santamarina et al., 2001; Ryden, 2004; Nath, 2007). While Ansal and Tonuk (2007) indicate that the bedrock layers have Vs varying from 700 to 750 m/s. moreover the program NEHRP (2009) and (Akin et al., 2013), illustrated the value of 760 m/s corresponds to the bedrock.

Depending on the majority of studies, the depth equivalent to Vs of 760 m/s is taken as the engineering bedrock depth in this study. Then, the identified bedrock depths have been mapped illustrating the engineering bedrock depth surface in the northern Jeddah urban area.

2 Study area and local geology

The studied area lies sideways the Red Sea coast to the north of Jeddah city, west Saudi Arabia (Fig. 1). The study area is the northern expansion part of Jeddah area, which has been subject to moderate earthquakes (Fnais et al., 2010). Some of these earthquakes have been felt through the Jeddah region. The maximum moment magnitude (M_w) was 7.2 in 1967 occurred through the Red Sea axial trough and affected the study area (Ambraseys et al., 1994). Unfortunately, the surface soil of the study area ranges from very soft to massive/stiff sediments or rocks. Some of sediments have poor geotechnical properties (e.g., the sabkhah deposits) and consequently cause the damages of buildings and structures in the case of strong earthquakes.

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Fig. 1

Location map of the study area (Yellow circles are the locations of MASW sites).

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The surface geology of northern Jeddah was evaluated by Moore and Al-Rehaili (1989) and differentiated as follows (Fig. 2): Ubhur Formation of early Miocene which consists of green siltstone, and limestone. These Tertiary units have been tilted and overlain northward by alkaline basalt of the Miocene to Pliocene Rahat Group. The Quaternary deposits have been divided into seven lithological units. The oldest is a raised reef limestone that outcrops in the western part followed by undifferentiated alluvial, eolian, and sabkha deposits, which are the youngest. These alluvial fan deposits are divided into two types: terraced and non-terraced deposits. The terraced deposits are composed of poorly sorted, coarse-grained gravels and beds containing a high proportion of cobbles and boulders. The other type is composed of gravel and sand. The reef limestone is widespread westward and raised 3–6 m above sea level and not exposed because it is covered sabkha. Moreover, there are small sabkha deposits detached westward.

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Fig. 2

Geological map for the study area.

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3 Data acquisition and processing

3.1

3.1 Engineering bedrock depth estimation

MASW field data in this study were acquired using Geode seismograph equipped by 24 vertical geophones with 4.5-Hz. Seismic waves were produced by an impulsive source of weight drop. The recorded waves were analyzed using the SurfSeis software to produce a 1-D or 2-D Vs data through three processing steps as follows: i) field-data preparation, ii) dispersion-curve construction, and finally, iii) inversion process. The selected sites for MASW are distributed throughout the study area and cover the identified geologic units. Seventy-six of 1-D MASW surveying profiles were conducted in the investigated area. Field data were collected with 1-m geophone spacing. The energy source was sustained at a distance of 10 m. This source distance is recommended for recording high quality signals within very soft, soft, and hard soils. Then, shear-wave velocity model was assessed through the inversion process based on least-squares fitting algorithm (Xia et al., 1999).

The engineering bedrock depth were estimated for each site of one dimensional velocity profiles of the MASW measurements and the corresponding Vs values (Figs. 3 and 4). Then, shear wave velocity represents the key for calculating seismic hazard at a certain site where the average shear wave velocity for the depth “d” of soil is referred as V_H as follows; the average shear wave velocity up to a depth of H (V_H) is computed according to Kanli et al. (2006) as;$$V_H=\sum d_i/\sum \left(d_i/v_i\right)$$where H = $\sum d_i$  = Cumulative depth in meters.

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Fig. 3

Flowchart showing the methodologies applied in this study.

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Fig. 4

Depth to the engineering bedrock in the study area.

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For 30 m average depth, shear wave velocity is written as (Boore, 2004):$$V_{s\left(30\right)}=\frac{30}{{\sum }_{i=1}^N\left(d_i/v_i\right)}$$where d_i and v_i denote the thickness (in meters) and shear-wave velocity in m/s of the ith layer, respectively.

The estimated engineering bedrock depth is shown in Fig. 4. Notably, the corresponding velocity of the overlying soil spans from 200 to 752.05 m/s down to the engineering bedrock level. The average Vs values ranges from 200 to 400 m/s, which can be categorized as medium to dense soil. Whereas the engineering bedrock depth varies between 0 and 36.2 m.

4 Results and discussion

The estimated shear-wave velocity at 76 MASW locations used to identify the engineering bedrock depth at each site (Fig. 4). The recorded depth of engineering bedrock for northern Jeddah urban area varies from zero (exposed on the ground surface) to approximately 36.23 m below ground surface. This depth in the western part is less than 15 m while increased eastward where it ranges between 20 and 36 m depth. This indicates that the engineering bedrock in the western zone obtained at a shallow depth than the rest of the studied area. These results correlated with the borehole data of Aldahri et al. (2017).

The ground motion frequency is of utmost importance where the PGA value alone cannot describe the surface ground motion. So, the response spectra are widely accepted parameter to specify the frequency content. Accordingly, the integrated influences of acceleration, amplitude, and frequency components can be expressed well the effects of ground motion. In this study, the ground surface response spectra for 76 locations were plotted with 5% critical damping. Fig. 5 shows examples of ground response analysis at the MASW site Nos. 1, 13, 22, 34, 52, and 73. Site No. 1 presents two PGA peaks of 16.53 cm/s² and 17.64 cm/s² which occurred at 1.25 Hz and 10.37 Hz, respectively. Site No. 13 shows two PGA peaks of 9.9 cm/s² and 17.0 cm/s² at 1.5 Hz and 10.7 Hz, respectively. Furthermore, site No. 22 shows two PGA peaks of 24.6 cm/s² and 18.3 cm/s² at 2.0 Hz and 10.5 Hz, respectively. Moreover, site No. 34 shows two PGA peaks of 15.5 cm/s² and 17.9 cm/s² at 1.53 Hz and 10.6 Hz, respectively. In addition, site No. 52 shows two PGA peaks of 24.9 cm/s² and 19.5 cm/s² at 2.0 Hz and 10.75 Hz, respectively. Finally, site No. 73 shows two PGA peaks with the same value of 18.5 cm/s² at 1.3 Hz and 10.9 Hz. According to these values, it can be stated that there is vertical variation in the density of the subsurface materials where these sediments vary from dense to very dense at different depths which is reflected in the presence of two fundamental resonance frequencies at the MASW measurement sites.

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Fig. 5

Ground response spectra for 5% damping at MASW sites in the study area.

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The spectral acceleration (SA) values for all MASW stations of measurements at 1.5, 3, 5, 8, and 10 Hz were computed. These frequencies represent the frequency range for single-story to multistory buildings (Day, 2001). The Peak spectral acceleration (PSA) and corresponding frequency of each site were calculated. PSA varies from 16.8 to 62.6 cm/s² (Fig. 6). The eastern study area has higher spectral acceleration compared to the western part. Table 2 shows that the frequency corresponding to the PSA varies from 1.05 to 14.28 Hz.

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Fig. 6

Maximum peak spectral ground acceleration with 5% damping at the study area.

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Most of the urban area characterized by low-rise buildings and the frequency of the soil cover can be close to their fundamental frequency of vibration. According to Parolai et al., (2006), when the fundamental frequency of vibration of a building is higher than that the fundamental frequency of soil fo it may, however be, close to the frequency of higher modes. Higher modes are expected at frequencies fn = (2n + 1) f_o where n = 1,2,3…… and f_o is the fundamental frequency. The H/V spectral ratio provides the lower frequency threshold from which ground motion amplification due to soft soil can be expected (Aldahri et al., 2018). Therefore, it cannot exclude that in the study area, such soil amplification of ground motions may occur at higher mode frequencies close to the fundamental frequency of vibration of low-rise buildings, even if it is smaller than that at the fundamental frequency of the sedimentary cover (Parolai et al., 2006).

5 Conclusions

Based on the aforementioned, it is highly recommended that the sites of unconsolidated sediments should be treated from an engineering perspective. Although the value of the spectral acceleration is not high, it may be hazardous where the risk lies in the value of the frequency corresponding to the spectral acceleration, which may cause severe damage to facilities and infrastructure. This point should be considered before designing important engineering facilities in the study area. Results of this study should be forwarded to civil engineers, land-used planners, and decision makers during the design of either new buildings or rehabilitation of pre-existing structures.