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Integrated Geological, Geophysical, and Geotechnical Site Characterization of Ajima Dam Site, Amhara Region, Ethiopia: Implications for Seepage and Foundation Stability

This study presents an integrated geological, geophysical, and geotechnical characterization of the Ajima Dam site in Ethiopia, revealing critical foundation stability and seepage risks due to insufficient bearing capacity and high permeability, and recommending specific mitigation measures such as deep excavation, curtain grouting, and seismic-resistant design modifications.

Original authors: Tadesse Demisie Fenta, Nigus Kebde Asmerom, Leulaem Shano Bako

Published 2026-07-02

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Tadesse Demisie Fenta, Nigus Kebde Asmerom, Leulaem Shano Bako

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Building a House on a Shaky Foundation

Imagine you want to build a massive, heavy house (the dam) that will hold back a huge lake. Before you pour the concrete or lay the bricks, you need to know: Is the ground underneath strong enough to hold it? Will water leak through the floor? And will the ground shake apart if an earthquake hits?

This paper is a detailed "medical checkup" of the ground at the Ajima Dam site in Ethiopia. The researchers used three different "doctors" to examine the site:

Geologists (who look at the rocks and map).
Geophysicists (who use sound waves and electricity to see underground without digging).
Geotechnical Engineers (who drill holes and test the soil strength).

The Diagnosis: Three Major Problems

1. The "Weak Top Layer" (Bearing Capacity)

The Problem: The top 5 meters (about 15 feet) of the ground is like a soggy sponge or a pile of loose sand. It is too weak to hold the weight of the dam.
The Analogy: Imagine trying to park a heavy truck on a mattress. The truck (the dam) weighs 1.154 MPa (a measure of pressure), but the mattress (the top soil) can only handle 1.1 MPa. If you park the truck, the mattress collapses.
The Finding: The ground is currently unsafe. The "Factor of Safety" is less than 1.0, meaning a collapse is likely if they build right now.
The Fix: They recommend digging out (excavating) the top 5 to 6 meters of this weak material to reach the harder ground underneath.

2. The "Leaky Bucket" (Seepage)

The Problem: Water is very good at finding cracks. The study found that the ground on the left side of the valley and the valley floor is full of cracks and holes, acting like a sieve.
The Analogy: Think of the dam foundation as a bucket. If the bucket has holes in the bottom and sides, the water will leak out before it can fill up. The researchers found "Lugeon values" (a measure of how fast water leaks) as high as 75 in some spots. This is like a bucket with a giant hole punched in it.
The Specifics:

Left Side & Valley Floor: Very leaky (like a colander).
Right Side: Much better, mostly solid rock (like a solid plastic bucket).
Hidden Dangers: They found natural caves in the red soil (laterite) on the right side. These are like hidden tunnels that water could rush through, washing away the soil from the inside (internal erosion).
The Fix:
Grout Curtain: Imagine injecting a giant wall of cement glue deep underground (30 meters deep) to seal all the cracks. They plan to do this in 2 or 3 rows to make sure no water slips through.
Upstream Blanket: They will lay a thick, waterproof mat (clay) on the ground in front of the dam to force water to take a longer, harder path to get through, slowing it down.

3. The "Earthquake Risk" (Seismic Hazard)

The Problem: The dam is built in a tectonic rift zone (where the Earth's plates are pulling apart). This area is prone to earthquakes.
The Analogy: Imagine a stiff wooden board (the deep bedrock) covered by a layer of jelly (the soft top soil). If you shake the board, the jelly wobbles violently and amplifies the shaking.
The Finding: The soft top layer will amplify earthquake waves, making the shaking feel twice as strong as it would on solid rock. There is also a risk that the soft soil could turn into liquid (liquefaction) during a quake, causing the dam to sink or tilt.
The Fix: The dam must be designed specifically to withstand these stronger shakes (using a "PGA of 0.20g" standard) and the weak soil must be treated so it doesn't turn to jelly.

The "Good News" (What's Deep Down?)

While the top layer is a mess, the deeper you go, the better it gets.

0–5 meters: Weak, weathered, leaky.
5–35 meters: Getting better, but still cracked.
Below 35 meters: The ground turns into "Fresh Ignimbrite" (a type of volcanic rock). This is like hitting solid concrete. It is strong, has very few cracks, and won't let water through.

The Final Prescription (Recommendations)

Based on this "checkup," the researchers give a specific recipe to save the project:

Dig it out: Remove the top 5–6 meters of weak soil in the valley.
Glue it shut: Drill deep holes and inject cement (grout) 30 meters down to seal the leaks. Use special chemical glue for the areas with caves.
Cover it up: Lay a waterproof blanket 30 meters long in front of the dam.
Build it strong: Design the dam to handle the specific shaking of this earthquake zone.

Summary

The Ajima Dam site is a tricky location. The ground is currently too weak to hold the dam, too leaky to hold water, and too shaky for a standard design. However, the study proves that if you dig out the weak top layer and seal the deep cracks with cement, the solid rock underneath is strong enough to support a safe, long-lasting dam. Without these fixes, the dam would likely fail, leak, or collapse during an earthquake.

Technical Summary: Integrated Geological, Geophysical, and Geotechnical Site Characterization of Ajima Dam Site

Problem Statement
The Ajima Dam, a proposed 48m-high earth fill structure in the Amhara Region of Ethiopia, presents significant engineering challenges due to its location within the tectonically active Main Ethiopian Rift. Previous dam failures in the region, such as the Katar Dam, have been attributed to inadequate pre-construction geological and geotechnical investigations. The primary concerns at the Ajima site include potential foundation instability, uncontrolled seepage, and seismic vulnerability. Specifically, the study addresses the need to characterize the complex lithological setting (dominated by tuff, ignimbrite, basalt, and laterite), assess the bearing capacity of the foundation relative to the dam's design load, and identify permeability anomalies that could lead to reservoir leakage or internal erosion.

Methodology
The study employs a multidisciplinary approach integrating geological mapping, geophysical surveys, and geotechnical testing to create a comprehensive site characterization.

Field and Subsurface Investigation: Nineteen boreholes were drilled to a total depth of 633m (17 at the dam foundation). Field tests included Standard Penetration Tests (SPT) for soil strength and Packer (Lugeon) tests to determine rock mass permeability.
Geophysical Surveys: Electrical resistivity surveys (Vertical Electrical Sounding and Electrical Profiling) and seismic refraction surveys were conducted to map subsurface lithology, overburden thickness, and velocity contrasts.
Laboratory Testing: Soil samples underwent grain size analysis, Atterberg limits, and permeability tests. Rock samples were tested for Unconfined Compressive Strength (UCS).
Data Integration: Rock Mass Rating (RMR) and Rock Quality Designation (RQD) were calculated to classify rock mass quality. Data from all three disciplines were cross-validated to develop a geo-conceptual model and engineering geological map.

Key Results

Foundation Stability and Bearing Capacity: A critical load-stress mismatch was identified. The allowable bearing capacity of the upper 5m of the valley floor is less than 1.1 MPa, which is insufficient for the applied dam load of 1.154 MPa. This results in a Factor of Safety (FOS) of less than 1.0, indicating a high risk of deformation or failure without intervention.
Permeability and Seepage: The site exhibits high heterogeneity in permeability.
- Valley Floor and Left Abutment: These zones are highly permeable, with Lugeon values reaching up to 75 LU (equivalent to $7.5 \times 10^{-6}$ m/s). Approximately 66% of tests in the left abutment and 75% in the valley floor exceeded 1 LU, indicating significant seepage potential.
- Right Abutment: This area shows lower permeability, with 76% of tests below 1 LU.
- Soil Permeability: Alluvial soils are generally impervious ( $10^{-8}$ m/s), whereas laterite soils are pervious ( $10^{-5}$ m/s).
Rock Mass Quality: Rock quality improves with depth. The upper weathered zones (0–5m) are classified as "Poor" (RMR 21–40) with RQD values of 25–50%. Below 35m, the rock transitions to "Good" quality (RMR 61–80) with RQD values of 90–100%, characterized by fresh, massive ignimbrite with UCS values up to 99.2 MPa.
Structural Geology and Hazards: A major fault crosses the dam axis (N86°W), and natural caves were identified in the laterite soils of the right abutment downstream. These features pose risks of internal erosion and slope instability.
Seismic Hazard: The site is located in Seismic Zone IV (PGA 0.20g). Geophysical data reveals a significant velocity contrast between the soft overburden ( $V_s < 1000$ m/s) and the stiff bedrock ( $V_s > 3500$ m/s), suggesting a site amplification potential of up to 2.0. Liquefaction analysis indicates a Factor of Safety < 1.0 for the upper 5m.

Recommendations and Mitigation Strategies
Based on the integrated findings, the authors propose specific remedial measures to ensure dam safety:

Excavation: Removal of the upper 5–6m of highly weathered valley floor materials to reach competent bearing strata.
Grouting: Installation of a 30m-deep grout curtain with 2–3 rows spaced at 1.5–2.0m to seal the highly fractured and permeable zones (particularly in the valley floor and left abutment).
Seepage Control: Construction of an upstream impervious blanket extending 30m from the dam toe to lengthen the seepage path through the shallow pervious zones.
Cave Treatment: Use of chemical grouting (rather than cement) to treat the caves found in the laterite soils of the right abutment.
Seismic Design: Implementation of seismic-resistant design modifications accounting for the Zone IV classification and site amplification effects.

Significance and Contribution
The paper claims to provide the first comprehensive characterization of the Ajima Dam site, filling a critical gap in Ethiopian dam investigations where insufficient geological assessment has historically led to failures. The study's primary contribution is the identification of a critical "load-stress mismatch" in the upper foundation layers, a finding that necessitates immediate design modification. Furthermore, the work demonstrates the efficacy of integrating geological, geophysical, and geotechnical data to map permeability heterogeneity and structural hazards (faults, caves) that single-method investigations might miss. The quantitative design recommendations (excavation depth, grout curtain parameters) serve as a template for foundation treatment in similar volcanic terrains within the Main Ethiopian Rift, offering a risk-mitigation framework for future infrastructure projects in the region.