Geophysical Characterization of Subsurface Suitability for Infrastructure at Ayat Real Estate Foundation, Addis Ababa

doi:10.21203/rs.3.rs-5108586/v1

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Geophysical Characterization of Subsurface Suitability for Infrastructure at Ayat Real Estate Foundation, Addis Ababa

https://doi.org/10.21203/rs.3.rs-5108586/v1

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This study provides a detailed geophysical assessment of subsurface conditions at the Ayat Real Estate site in northeastern Addis Ababa, employing electrical resistivity tomography (ERT), vertical electrical sounding (VES), and seismic refraction methods. Data from four VES points, dipole‒dipole ERT lines, and seismic refraction profiles reveal nearly vertical fractures and weak zones oriented NW–SE, which could impact foundation design, with competent hard formations suitable for construction located at depths of up to 12 m. The findings show increasing resistivity and P-wave velocity with depth, indicating progressively stronger subsurface materials, whereas low resistivity at shallow depths highlights physically weak zones. The subsurface consists of four layers: highly plastic clay with low resistivity (3.03–25 Ω-m) and P-wave velocities (< 0.42 km/s); a middle layer with moderate resistivity (30–42 Ω-m) and P-wave velocities (< 1.79 km/s); higher resistivity bedrock (38–65 Ω-m) with P-wave velocities (< 2.4 km/s); and a deeper, slightly weathered bedrock layer with resistivities of 86–206 Ω-m. Structural features and fractures indicate potential saturation or weathering. These findings highlight the need for drainage, compaction, and stabilization to manage soil expansion and geohazards. Geophysical data combined with borehole information confirm the site's suitability for multistory construction and provide key insights for foundation design.

Geophysics

Subsurface suitability

Electrical resistivity tomography (ERT)

Vertical electrical sounding (VES)

Seismic refraction

Infrastructure

The stability and safety of civil engineering structures such as buildings, bridges, and roads are critically dependent on the geotechnical properties of the underlying soils and rocks [2]. The foundations of these structures must efficiently transfer loads to subsurface materials, necessitating a comprehensive assessment of site-specific geotechnical and geophysical parameters [1]. The choice between shallow and deep footings is significantly influenced by variations in lithological layers and the presence of weak zones within the subsurface [10]. Inadequate investigations can result in structural deficiencies, especially in regions susceptible to natural hazards such as earthquakes, floods, and landslides. Given Ethiopia’s complex geological and tectonic framework, the country is particularly vulnerable to such geohazards [1].

In Addis Ababa, rapid urbanization has led to the development of numerous construction projects designed to meet rising housing demands and improve infrastructure. However, many of these projects depend on limited borehole data, which may not fully represent the complexity of subsurface conditions [14]. While detailed, traditional geotechnical techniques, including closely spaced drilling and sampling, are often costly and provide limited spatial coverage [39]. To address these challenges, geophysical techniques such as 2-D electrical resistivity imaging, vertical electrical sounding (VES), and seismic refraction are increasingly being used in urban development projects [13]. These methods can provide continuous profiles of subsurface conditions, thereby enhancing the understanding of soil–structure interactions and the identification of weak zones that may compromise structural integrity [3, 25].

The Main Ethiopian Rift is a prominent geological feature that significantly influences the geotechnical and geophysical landscape of Ethiopia [7, 8]. Magma intrusions, faulting, and volcanic activity in this region pose substantial risks to infrastructure, particularly in urban centers such as Addis Ababa, which lies in close proximity to active rift segments [5, 6, 11]. Seismic hazards are further compounded by complex interactions between tectonic and volcanic processes, making the region highly susceptible to earthquake activity [7, 17, 20].

In the case of Addis Ababa, subsurface investigations are essential for safe construction and urban planning. However, few studies have comprehensively assessed the integration of geophysical techniques with geotechnical data in this region [10, 14]. The use of advanced geophysical methods such as electrical resistivity tomography (ERT) and joint inversions of geophysical datasets can help overcome these limitations [8, 24, 29]. These techniques have been shown to effectively map subsurface heterogeneities and lithological variations that are critical for infrastructure development in complex terrains [12, 34, 40].

This study explores the application of these geophysical techniques in urban settings, with a specific focus on the Main Ethiopian Rift and Addis Ababa. Additionally, we review recent advances in the use of seismic and electrical methods for site characterization in geohazard-prone areas [19, 31, 37]. By integrating geophysical data with traditional geotechnical approaches, a more comprehensive understanding of subsurface conditions can be achieved, ensuring the stability and longevity of civil structures in Ethiopia’s rapidly developing urban environments.

Location of the study area

The study area is located in Ayat Real Estate Zone 8, northeastern Addis Ababa, Ethiopia (Fig. 1). Situated near the western margin of the Main Ethiopian Rift (MER), this region experiences a complex geological setting characterized by rapid urbanization and significant volcanic and tectonic activity (Fig. 1) [9, 11]. Addis Ababa, positioned on a plateau surrounded by volcanic formations, is influenced by the MER, which is a divergent boundary between the African and Somali tectonic plates (Fig. 1) [7, 11].

The proximity to MERs presents substantial geophysical and geological challenges, such as volcanic activity and tectonic faulting [7, 8, 9, 17, 32]. These factors underscore the necessity for a thorough geophysical assessment to inform construction practices and mitigate associated risks [15, 16]. The site’s location adjacent to the Yeka Ayat condominium construction area and along a major road connecting Lege Tafo and Legedadi highlights the urgent need for detailed subsurface investigations to ensure safe and resilient infrastructure development [10]. Given the dynamic geological environment and the rapid pace of urbanization in the region, conducting a comprehensive geophysical assessment is crucial [2, 12]. This assessment will support the development of robust infrastructure and address potential stability challenges posed by the region’s complex geological conditions [36, 38].

Previous studies

Geophysical and geotechnical investigations in Addis Ababa have significantly enhanced the understanding of subsurface conditions, which are critical for infrastructure development [28]. Studies employing electrical resistivity, seismic refraction, and magnetic surveys have assessed various sites, such as Addis Ababa Science and Technology University and Akaki Kality subcity (Fig. 2). These investigations provided valuable insights into the region’s geological and geotechnical characteristics, forming a crucial foundation for urban planning and construction projects [14]. Further research has extended this understanding by focusing on the soil and rock properties essential for determining construction suitability. Specifically, these studies highlighted the volcanic nature of the geology of Addis Ababa, which is dominated by tuff, rhyolite, ignimbrite, trachyte, and basalt formations (Fig. 2) [38]. Figure 2 shows that the Ayat-Meri area, adjacent to the current study site, was found to be primarily underlain by ignimbrite and tuff units [15]. This provides a valuable geological context for evaluating subsurface conditions and ensuring the safety and stability of civil structures [12].

Groundwater research in Addis Ababa has also been instrumental in understanding subsurface conditions. Groundwater generally flows from north to south across cities, with notable variations in porosity and bulk density [40]. The northern areas exhibit greater water retention due to the volcanic nature of the rocks [10]. Additionally, studies of groundwater recharge highlight the role of subsurface geological formations in controlling water flow and availability [39, 40].

Several studies have applied electrical resistivity tomography (ERT) and vertical electrical sounding (VES) to investigate the subsurface geology of Addis Ababa. These techniques are crucial for identifying geological units and assessing groundwater potential in urban settings [25, 39, 40]. ERT and VES have been effectively used to map lithological variations and detect faults or fractures that influence groundwater flow [29, 40]. These geophysical methods are also employed to understand seismic hazards, as the volcanic nature of a region contributes to seismic risk [21, 28].

Seismic refraction and reflection techniques have further contributed to urban geotechnical investigations in Addis Ababa [21]. These methods are essential for determining subsurface seismic velocity profiles, which are crucial for assessing the dynamic properties of soils and rocks [19]. By utilizing seismic refraction, geophysicists can estimate subsurface layer depths and identify potential risks to infrastructure development [10].

Urban planning in Addis Ababa must also consider the risk of volcanic and tectonic activity. Volcanic activity has shaped the region’s subsurface structure, with the main Ethiopian rift influencing the geology of Addis Ababa and surrounding areas [7]. Off-axis volcanic activity has been documented in areas such as Goba-Bonga, adding complexity to the local geology [8]. Furthermore, frequent pulses of deformation associated with magmatic activity have been observed in the Main Ethiopian Rift [4].

In conclusion, the integration of geophysical methods such as electrical resistivity, seismic refraction, and magnetic surveys has greatly enhanced the understanding of subsurface conditions in Addis Ababa (Fig. 1). These findings provide crucial information for urban development, ensuring that infrastructure is built on stable ground and mitigating potential geohazards such as seismic events and groundwater depletion [1].

Geological setting of the study area

The study area is predominantly volcanic in nature and features formations such as ignimbrite, tuff, and basalt (Fig. 2). These volcanic rocks define the geological framework, which is characterized by highland terrains and intervening valleys, resulting in a diverse range of subsurface conditions. The area is located on the periphery of the Main Ethiopian Rift (MER), which contributes to its complex geological setting, including volcanic rock formations and potential fault zones [5, 8, 20, 36].

The site’s proximity to a major transport route and its significance for urban development highlight the need for comprehensive geophysical and geotechnical assessments. Such evaluations are critical for addressing potential stability challenges and mitigating risks associated with a region’s dynamic geological environment. Integrating geophysical data with existing geotechnical information is essential for developing resilient infrastructure, ensuring long-term stability in this complex geological context [14, 19, 40]. The area's topography and geology suggest the presence of weathered volcanic deposits and fractured bedrock at varying depths, emphasizing the necessity of thorough subsurface characterization (Fig. 2) [10, 39].

Geophysical methods such as seismic refraction and electrical resistivity tomography have proven effective in characterizing subsurface features in volcanic and rift environments [3, 25, 29]. These methods help delineate lithological units and identify potential fault zones [6, 12], as well as assess the extent of weathering and fracturing of the bedrock [13, 16]. The integration of these techniques, along with geological mapping and remote sensing [22], provides a comprehensive understanding of the subsurface conditions necessary for sustainable urban development [15].

In regions such as the Main Ethiopian Rift, the presence of magmatic activity also complicates subsurface investigations [4, 17, 26], necessitating frequent monitoring and updating of subsurface models to mitigate geohazards [1, 27, 31]. Figure 2 illustrates the geological setting, showing the diverse lithological units and potential fault zones that may influence construction and infrastructure development (Fig. 2).

Geophysical methods

The geophysical investigation aimed to assess subsurface conditions by employing vertical electrical sounding (VES), electrical resistivity tomography (ERT), and seismic refraction techniques (Fig. 3) [23, 37]. These methods were selected on the basis of site accessibility, geological context, and the presence of buildings and construction materials that influence the positioning of survey lines [15, 19]. The study involved four VES points, ERT lines, and seismic refraction profiles to characterize the subsurface (Fig. 3) [22, 24].

Electrical resistivity tomography (ERT)

ERT was applied to map subsurface structures by detecting variations in electrical resistivity influenced by lithology, porosity, and fluid content [25, 29]. Electrodes were deployed along linear profiles to measure resistivity at multiple depths [18, 24]. The collected data were processed via Res2DInv inversion software to generate two-dimensional resistivity models, revealing detailed stratigraphy and structural features (Fig. 4) [40].

Vertical electrical sounding (VES)

VES was conducted at four strategic locations to acquire one-dimensional resistivity profiles, revealing vertical resistivity variations [3, 30]. The survey identified four distinct layers (Fig. 5), including a surface layer with low resistivity values corresponding to loose topsoil, followed by higher resistivity layers related to compacted materials and potential groundwater aquifers [10, 38].

Electrical resistivity tomography (ERT) results

The electrical resistivity tomography (ERT) survey delineated three primary subsurface layers (Fig. 4). The uppermost layer is characterized by low resistivity values, indicative of highly plastic clay, which is consistent with similar findings in comparable terrains (Fig. 4). The intermediate layer displayed moderate resistivity values, reflecting a combination of soil and weathered rock, which aligns with observations in other volcanic regions (Fig. 6). The deepest layer has high resistivity values, indicating slightly weathered bedrock, corroborating prior research on resistivity variations in analogous geological contexts. The deepest layer has high resistivity values, which is consistent with previous studies in similar geological contexts [40]. The ERT survey also revealed structural features, including fractures and saturated zones, which are important for foundation stability assessments (Fig. 4).

In addition to stratigraphic details, ERT models revealed significant structural features, including fractures and saturated zones, which are critical for assessing site stability and construction suitability (Figs. 4, 5). The ERT surveys were performed via a dipole‒dipole configuration with 5-meter electrode spacing, enabling detailed resistivity imaging to depths of up to 20 meters. The data were processed with Res2Dinv software, generating 2D resistivity sections that highlighted the stratigraphic sequence and identified potential weak zones (Fig. 4).

Vertical electrical sounding (VES) results

The VES data confirmed the vertical resistivity variations identified via ERT, delineating a surface layer of plastic clay underlain by weathered rock and bedrock (Figs. 4, 5). The resistivity profiles revealed by VES provided further detail on the lateral extent of these layers, supporting the findings from ERT [13]. Deeper layers suggest transitions to weathered rock and bedrock, corroborating the findings from ERT and providing additional insights into subsurface variations (Fig. 7).

Seismic Refraction Results

The seismic refraction survey, which uses a 24-channel seismograph and a 5 kg hammer as the energy source, identified three subsurface layers on the basis of P-wave velocities (Fig. 6). The top layer exhibited low velocities, which was consistent with highly plastic clay, whereas the intermediate layer corresponded to weathered rock. The deepest layer exhibited high velocities, indicating slightly weathered bedrock [23]. The seismic data corroborated the resistivity findings and provided additional insight into subsurface stability. Seismic refraction surveys complemented the ERT findings by identifying three primary subsurface layers on the basis of P-wave velocity measurements (Fig. 6). The intermediate layer displayed moderate velocities, indicative of weathered rock, whereas the deepest layer was associated with high velocities corresponding to slightly weathered bedrock (Figs. 7 and 8). These results align with resistivity data, providing a comprehensive characterization of subsurface conditions.

Seismic refraction surveys utilized a 5 kg hammer for energy generation and a 24-channel seismograph for data recording. The travel-time data were interpreted via the Plus‒Minus method, which allowed for accurate delineation of subsurface layer depths and velocities. This integrated approach reinforced the reliability of the geophysical characterization (Figs. 6, 8).

Integration of Geophysical and Geotechnical Data

The integration of electrical resistivity tomography (ERT), vertical electrical sounding (VES), and seismic refraction data with geotechnical information, such as borehole and trench data, provided a comprehensive characterization of the subsurface at the Ayat Real Estate Zone 8 site. This multidisciplinary approach enabled the validation and refinement of geophysical interpretations and subsurface models, enhancing the precision of the findings [30]. Critical structural features, such as fractures and fault zones, which may affect foundation stability, were identified. These features, along with the presence of highly plastic clay and weathered rock, require specific engineering considerations, including adequate drainage solutions (Figs. 6, 7, 8).

The integration of these diverse data sources provides a holistic understanding of the subsurface environment, which is crucial for informed infrastructure development and foundation design decisions (Figs. 6, 7, 8). The analysis revealed four distinct subsurface layers: the uppermost layer, characterized by low resistivity (3.03–25 Ω-m) and low P-wave velocities (< 0.42 km/s), corresponding to highly plastic clay; the middle layer, characterized by moderate resistivity (30–42 Ω-m) and intermediate P-wave velocities (< 1.79 km/s), indicative of soil and weathered rock; the bedrock layer (38–65 Ω-m, P-wave velocities < 2.4 km/s); and a deeper, slightly weathered bedrock layer, with resistivity values ranging from 86 to 206 Ω-m (Figs. 6, 7, 8). The geophysical investigation at the Ayat Real Estate site identified subsurface layers of plastic clay, weathered rock, and bedrock, highlighting key structural challenges such as fractures and weak zones that require tailored engineering solutions for stable construction (Table 1).

Interpretation

The integration of ERT, VES, and seismic refraction data provides a comprehensive understanding of the subsurface conditions at the Ayat Real Estate site. The combined data delineate four primary subsurface layers with distinct resistivity and P-wave velocity values, which are indicative of clay, weathered rock, and bedrock. Structural features such as fractures suggest potential challenges, including soil expansion, weathering, and saturation, which could affect construction stability (Figs. 6, 7, 8). On the basis of these findings, recommendations include effective drainage systems, soil compaction, and stabilization techniques to mitigate the identified risks [13].

Electrical resistivity tomography (ERT)

ERT effectively delineates subsurface layers by identifying resistivity variations (Fig. 4). Electrodes placed along linear profiles provided resistivity measurements at varying depths. These measurements were processed into two-dimensional resistivity models, which produced detailed subsurface images. ERT identifies lithological units on the basis of resistivity contrasts, which reflect differences in lithology, porosity, and fluid content [24].

Vertical electrical sounding (VES)

VES produced one-dimensional resistivity profiles at four key locations (Fig. 5). This method revealed vertical variations in subsurface resistivity, which is crucial for understanding geological structures. Curve-matching techniques applied to the VES data identified four distinct subsurface layers, including a surface layer of low resistivity, an intermediate layer of moderate resistivity, a higher resistivity bedrock layer, and a deeper, slightly weathered bedrock layer (Figs. 7, 8).

Integration of ERT and VES

The VES results supported the ERT findings by identifying the same four subsurface layers (Fig. 5). This consistency confirms the heterogeneity and variability of the subsurface in terms of layer thickness and resistivity, which is essential for accurate site characterization.

Velocity Interpretation

Seismic refraction data, acquired along profiles parallel to the ERT lines, utilized a 24-channel spread with a total length of 48 meters, including a 5-meter beyond-shot extension. The resulting velocity models (Figs. 6 and 8) reveal three distinct layers with P-wave velocities ranging from 0.4 to 2.4 km/s. The first layer, approximately 2–3 m thick, exhibited low compressional wave velocities between 0.29 and 0.3 km/s, indicative of dark clay‒silty clay soil (Figs. 7 and 8).

The geophysical investigation at the Ayat Real Estate site offers critical insights into subsurface conditions, informing construction practices and foundation design. By applying electrical resistivity tomography (ERT), vertical electrical sounding (VES), and seismic refraction methods, we characterized subsurface layers and identified potential challenges. The findings reveal a stratified subsurface, including a top layer of highly plastic clay, an intermediate layer of weathered rock, and a deeper layer of slightly weathered bedrock. These results are consistent across methods and identify key structural features such as fractures and saturated zones, which pose challenges related to soil stability and geohazards.

Integration with existing geotechnical data supports the site's suitability for multistory construction, highlighting the need for tailored engineering solutions such as effective drainage, compaction, and stabilization to address risks associated with soil expansion and weak zones. The observed increase in resistivity and P-wave velocity with depth indicates improved subsurface competency, suggesting the potential for deeper, more stable foundations.

The subsurface at the Ayat Real Estate site is characterized by four distinct layers: the uppermost layer has low resistivity values (3.03–25 Ω-m) and low P-wave velocities (< 0.42 km/s), which are indicative of highly plastic clay. Beneath this layer, moderate resistivity (30–42 Ω-m) and intermediate P-wave velocities (< 1.79 km/s) suggest a composition of soil and weathered rock. A deeper layer has higher resistivity (38–65 Ω-m) and elevated P-wave velocities (< 2.4 km/s), reflecting weathered bedrock. The deepest layer, identified as slightly weathered bedrock, has resistivity values ranging from 86 to 206 Ω-m.

The identification of critical structural features, such as fractures and fault zones, underscores potential challenges related to foundation stability. These features, combined with the presence of highly plastic clay and weathered rock, necessitate targeted engineering solutions, including effective drainage and soil stabilization measures. This comprehensive geophysical assessment provides a robust framework for guiding the development of infrastructure at the Ayat Real Estate site. Future research should continue monitoring and conducting additional geotechnical investigations to ensure long-term stability and safety.

Conflict of interest

The authors have no conflicts of interest with the editors of the International Journal of Geo-Engineering.

Data availability statement

The geophysical data supporting this study's findings are available from the corresponding author upon reasonable request.

Author contributions

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

Funding Declaration

The authors received no financial support for the research, authorship, and/or publication of this article.

Acknowledgement

The authors would like to thank the School of Earth Sciences, Addis Ababa University, for financial support in conducting this research. We would also like to thank the anonymous reviewers for their valuable comments and suggestions.

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Table 1 The geophysical investigation at the Ayat Real Estate site identified subsurface layers

Method	Subsurface Layer	Resistivity (Ω-m)	P-Wave Velocity (km/s)	Characteristics	Comments
Electrical Resistivity Tomography (ERT)	Layer 1	3.03–25	<0.42	Highly plastic clay	Low resistivity; weak material
	Layer 2	30–42	<1.79	Moderate resistivity; soil and weathered rock	Intermediate resistivity; transition zone
	Layer 3	38–65	<2.4	Bedrock	Higher resistivity; competent hard formation
	Layer 4	86–206	Not specified	Slightly weathered bedrock	Deepest layer; strong material
Vertical Electrical Sounding (VES)	Surface Layer	3.03–25	<0.42	Highly plastic clay	Confirms ERT findings on surface layer
	Intermediate Layer	30–42	<1.79	Weathered rock and soil	Supports ERT findings on intermediate layer
	Higher Bedrock Layer	38–65	<2.4	Competent hard bedrock	Corroborates ERT findings on higher bedrock layer
	Deeper Bedrock Layer	86–206	Not specified	Slightly weathered bedrock	Confirms ERT findings on deeper bedrock
Seismic Refraction	Top Layer	Not specified	Low velocities	Highly plastic clay	Low P-wave velocity; consistent with ERT findings
	Intermediate Layer	Not specified	Moderate velocities	Weathered rock	Moderate P-wave velocity; confirms weathered rock layer
	Deepest Layer	Not specified	High velocities	Slightly weathered bedrock	High P-wave velocity; aligns with ERT results

The authors declare no competing interests.

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Geophysical Characterization of Subsurface Suitability for Infrastructure at Ayat Real Estate Foundation, Addis Ababa

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Version 1

Abstract

Figures

Introduction

Location of the study area

Previous studies

Geological setting of the study area

Methodology

Geophysical methods

Electrical resistivity tomography (ERT)

Vertical electrical sounding (VES)

Results and Discussion

Electrical resistivity tomography (ERT) results

Vertical electrical sounding (VES) results

Seismic Refraction Results

Integration of Geophysical and Geotechnical Data

Interpretation

Electrical resistivity tomography (ERT)

Vertical electrical sounding (VES)

Integration of ERT and VES

Velocity Interpretation

Conclusion

Declarations

References

Tables

Additional Declarations

Status:

Version 1