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Engineering Geological Soil and Rock Characterization in the Mekelle Town, Northern Ethiopia: Implications to Engineering Practice
Gebremedhin Berhane
Department of Earth Sciences, College of Natural and Computational Sciences, Mekelle University, P.O. Box 1202, Mekelle, Ethiopia ([email protected] )
ABSTRACT The study was conducted to assess the index properties and characterize soils and rocks of Mekele town located in the northern Ethiopia having an area of 45 km2. Geological, engineering geological and geotechnical condition of the rocks and soils was studied on the bases of field description, in-situ geotechnical test and laboratory analysis. Four soil types were identified in the field: clay, silt, sandy clay/silt and clayey/silty sand soils. Laboratory result revealed that moisture content of the soils ranges from 15.8 to 40.9% for clay; 21.7 to 34.7% for silt; 6.6 to 20.5% for sandy silt/clay and 14.2 to 23% for clayey/silty sand soils. pH and electrical conductivity of the soils vary from 7.1-8.5 and 180-1930µS/cm, respectively. The liquid limit (LL) in percent ranges from 29-59, 50-67.4, 37.5-70.8 and almost non-plastic to 66 for clay, silt, sandy clay/silt and clayey/silty sand, respectively. Similarly, the plasticity index in percent (PI) of the soils ranges from 14-36.6, 13.3-37.4 and non-plastic -38.6 for clay, silt, sandy clay/silt and clayey/silty sand soils, respectively. The shrinkage limit in percent (SL) of the soils varies from 9.3 for clayey/silty sand to 30.9 for silt. The free swell of the soils is highly variable, from 0 to as high as 70%. The dolerite shows variable strength from weak rock mass strength (2Mpa) to very high rock mass strength (150Mpa); the sandstone and limestone-marl-shale intercalation show low or weak rock mass strength and the well-bedded limestone have generally high rock mass strength (60 to 160Mpa). The main geotechnical problems that affect design and related infrastructure development in the town are found to be presence of expansive soil, cyclic weak and strong rock units with depth, and variable weathering profile. Key words: Engineering geological, Liquid limit, Mekelle Town, Northern Ethiopia, Rock mass, Soils. 1. INTRODUCTION
Mekelle town is built-up over cyclic Mesozoic Sedimentary rocks and Quaternary sediments.
These materials are characterized with great diversity of genetic types and different engineering
geological behavior. Like most civil engineering structures (such as bridge, tunnels, etc) big
towns require detail knowledge of geological, geotechnical and engineering geological properties
of the foundation and the materials used for construction. Geotechnical and engineering
geological investigation and mapping mainly focus towards understanding the interrelationships
between the geological environment and the engineering situation; the nature and relationships
between the geological components, the active geodynamic processes and the prognosis of
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processes likely to result from the changes being made (UNESCO, 1976). From this perspective,
investigation of soils and rocks of a town, as a material that is used to build with or on and as a
material of the environment that may act in combination with other forces of nature (geodynamic
processes) or of civilization to affect landforms, structure, and the state of our environment is
extremely important. Collapsible and alluvial soils are found causing basic difficulties in
building and construction foundations (Vanushka Petrova and Jordan Evlogiev, 2003). The
cyclic sedimentary rocks of the Mesozoic rocks in the region are also found to be pervious and
weak in strength due to various types of discontinuities (Gebremedhin Berhane, 2010).
Mekelle was founded more than a century ago. The town is presently rapidly expanding.
Currently, many civil engineering structures such as multistory buildings, roads, bridges, etc are
under construction in the town. However as in many towns of Ethiopia, very little is known
about the soil and rock conditions or engineering geology of the town. The only research to be
mentioned is the work of Gebremedhin Berhane (2002), in which he has classified the rocks and
soils of the area based on their engineering properties in addition to geomorphological,
geological, and engineering geological and geotechnical mapping. This work was used as
baseline information for the present research.
In the light of the above point, the present research was aimed at assessing and evaluating the
engineering geological and geotechnical conditions of the town and to provide important
engineering geological data that may help to plan, design and maintain engineering projects.
Specifically, the objectives were to i) asses and analyze important engineering geological and
geotechnical properties of the soils and rocks of the town; ii) classify the soils and rocks of the
town into different engineering geological properties or units; and iii) produce geological and
engineering geological maps of the town.
2. DESCRIPTION OF THE STUDY AREA
2.1. Location
Mekelle is the capital city and commercial center of the Tigray National Regional State in the
northern Ethiopia (Fig.1). The town is located at 39033'E longitude and 13032'N latitude, situated
in the extension of the central highlands of Ethiopia. The altitude of Mekele is between 1965 m
and 2220m above sea level. The town is bounded by mountain ranges in the east and north.
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2.2. Climate
Climatically, the area is classified as "Woina Dega" (temperate) with an effective temperature
between 140C and 200C (Ethiopian Mapping Agency, EMA, 1981), which for most of the time is
comfortable. It has a moisture index (P/ET) ranging in between 0.25 and 0.5, which indicates
moderately dry area. Mekelle lies between 600mm to 700mm rainfall region. The mean annual
temperature ranges between 160C and 200C (Gebremedhin Berhane, 2002).
2.3. Topography
The altitude varies from 2220 m at eastern side to 1965 m in the northwestern side of the town
(lower reach of Illala River). The town has an overall tilt from eastern to western and
northwestern side. Most streams and tributaries are controlled by this tilt while others are
controlled by geological structures and underlying geology.
2.4. Geology
The first recorded geological work in the northern provinces of Ethiopia was done by Blanford
(1870) cited in Beyth (1971), who divided the Trap Volcanics of the Ethiopian highlands into
two units, a lower entirely basaltic Ashangi Series, and an upper Magdala Series which contained
many intercalation’s of trachyte. Then Dainelli and Marinelli (1912) and Merla and Minucci
(1943) as cited in Beyth (1971) proposed the transgression - regression phenomena to explain the
sedimentary history of the whole of the Horn of Africa, including Ethiopia. In 1970, Levitte
studied the geology of Mekele (Central part of Sheet ND37- 11) and he divided the rocks in the
area into four major units: Basement complex, Paleozoic - Mesozoic Sedimentary sequence,
Cenozoic Trap Volcanics and Sediments of the Ethiopian Rift.
Beyth (1972) done detail mapping of Northern Ethiopian provinces (Central and Western
Tigray). According to this work the history of the sedimentary basin in Tigray (Mekelle Outlier)
began in either the Ordovician or Carboniferous and probably ended in lower Cretaceous before
the eruption of the Trap Volcanics.
The main lithologic units in the Mekelle town are Quaternary sediments, dolerite, limestone-
marl-shale intercalation, sandstone and bedded limestone (Fig. 2). Description of each of the
rock units is presented below.
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2.4.1. Quaternary Sediments
These sediments consist of alluvial, colluvial deposits and residual soils. The alluvial deposit
ranges in grain-size from clay to sand with minor boulders. It is widely observed along streams
and northern and northwest of the town. It is dark to gray in color, loose to stiff and in places
stratified. Colluvial deposits are common along foot of steep slopes (east of the center of the
town). The residual soils range in grain-size from clay to sand with some inclusions of angular
boulders, mainly yellowish in color, are found in areas of gentle slopes.
Figure 1. Location map of Mekelle town (including 1:10,000-scale aerial photographs showing
two selected points within the study area).
Main Road
Palace
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Figure 2. Geological Map of Mekelle Town (including location of test pits and major road net
works, modified after Gebremedhin Berhane, 2002). 2.4.2. Dolerite
It outcrops on the eastern side of the town (forming steep cliff) and near the center of the town
(Fig. 2). It is black, fine to medium grained and is characterized by spheriodial weathering. This
rock unit is jointed (vertically and horizontally). It shows differential weathering from place to
place and in many localities remnant corestones are common especially in foundation
excavations.
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2.4.3. Limestone-Marl-Shale Intercalation
This intercalation unit covers large part of the area forming gentle slopes. It is variegated,
stratified, fine grained, friable and laminated. The limestone layer is black and in places light
yellow in color; it is stronger than the other layers. The shale and marl layers are generally gray
and light yellowish in color. In places swarms of dolerite dykes and sills are observed.
Weathering is more intensive in shale beds than limestone beds.
2.4.4. Sandstone
It is white to light yellowish in color, friable, less cemented, bedded and weathered. On hand
specimen quartz grains are dominant, with some dark minerals. It is fine to medium grained, in
places interbeds of siltstone are observed, shows weak to fairly strong effervescence upon a drop
of 10% hydrochloric acid solution indicating the presence of calcite as cementing material.
2.4.5. Bedded Limestone
It is well bedded, black and yellowish in color, crystalline and slightly weathered. Traces of
fossils and shell fragments are observed in some hand specimens. Bed thickness is variable (1m
to 3m). Thin layers of shale and marl are found in this unit, i.e. alternating limestone and thin
beds of shale and marl.
2.5. Geological Structures
The dominant structures in the Mekelle town are faults, joints and bedding planes. The faults are
interpreted from air photographs (verified in the field), while the other structures are observed
and some measurements were taken in the field. The study revealed WNW-ESE, N-S and NNE-
SSW with minor E-W striking faults (Fig. 3). Joints are other structures common in the study
area. The strikes of the joints are generally parallel to the faults of the area and seldom
perpendicular. Most of the joints are vertical and some are horizontal, parallel to bedding planes
in sedimentary rocks. Figure 3 shows rose diagram of vertical joints measured in the study area.
Two major joint sets are observed (NNE & WNW trending) with some minor sets (NE & NW
strike, Fig. 3). The third geologic structure is bedding plane, which is considered as discontinuity
in engineering geological investigations and could facilitate landslide or slope instability. In
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large part of the town beds are horizontal, but inclined beds are also observed resulted from
dolerite intrusion and faulting.
Figure 3. Rose diagram of joints (Gebremedhin Berhane, 2002).
3. METHODOLOGY
The research work involved a number of fieldworks in different seasons (2001 - 2005 and 2008).
During these fieldworks rock and disturbed soil samples were collected and analyzed for various
index and engineering parameters. Existing geotechnical and engineering geological data were
collected from different organizations and individuals and preliminary photo-geological map
were produced by photo interpretation using stereo-pairs of panchromatic black and white aerial
photographs of scale 1: 10,000 which were flown in 1994.
Continuous rock and soil descriptions, test pit excavation (up to a depth of 6m) for in-situ
observations and sample collection, and discontinuity measurements were conducted. Pocket
penetrometer test and Schmidt hammer rebound test on soil and rock respectively were carried
out. Samples were analyzed in Mekelle geotechnical laboratory according to procedures and
methods proposed by American State Testing Materials (ASTM). The rocks and soils of the
town were classified on the basis of their engineering behavior according to Unified Soil
Classification Systems (USCS) and classification proposed by International Association of
Engineering Geologists (IAEG, 1981).
N,360
Strike Direction: 15 ° classes
90270
180
Num total: 114
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4. RESULTS AND DISCUSSION
4.2. Description, index properties and classification of soils
4.2.1. Lithological / Textural Description of the Soils
Even though portion of the existing buildings in Mekelle town are dominantly founded on the
limestone-marl-shale intercalation, limestone and dolerite units, large part of the town is covered
by soil of up to more than 10 m thick. The soils are lithologically grouped into clay, silt, sandy
clay/silt and clayey/silty sand soils.
Clay soils are generally dark / black in color, mainly observed in the northern part of the town
and partly in the central part. The silt soils are variable in color (dark, gray and yellowish), found
in the northern and northwestern part of the town.
The sandy clay/silt soils are mainly residual in origin and found in the north and to southern part
of the town. Its color is yellowish to dark-gray. The clayey/silty sand soils have very limited area
coverage, as pockets with light gray to dark color.
4.2.2. Natural Moisture Content (NMC)
For coarse and fine-grained soils, water content can have a significant effect on the soils
behavioral properties when used for construction purposes and foundations. Moisture content
affects the settlement (consolidation) condition; shear strength and suitability of soil for
compaction. Moreover, the swelling-shrinkage condition of a particular soil is related to its
moisture content and its change with time. Consistency of a fine grained soil also depends
largely on its moisture content. Samples were collected and immediately submitted to determine
natural moisture content (NMC) of soils. The result (Table 1) shows that the clay soils have a
moisture content of 15.8 – 40.9%, silt soils have 21.7 – 34.7%, sandy clay / silt have 6.6 – 20.5%
and the clayey / silty sand soils have 14.2 –23%.
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Table 1. Natural Moisture content, pH, electrical conductivity and percent dispersion of soils of Mekelle town.
4.2.3. Electrical Conductivity (EC) and pH of Soils
Chemical tests are normally preformed on soils to ascertain whether the soil is acidic, alkaline or
neutral (Abramson et al, 1996). pH and electrical conductivity of soils of Mekelle town were
analyzed in the laboratory from extracts (Table 1). The electrical conductivity (opposite to
specific electrical resistance) of the soil is one of the most important factors in determining soil
aggressiveness. As the electrical conductivity of a soil increases its aggressiveness increases
(Abramson et al, 1996). Soils pH values vary from 7.1 (about neutral) to 8.5 (alkaline). This
shows that further chemical tests will be important in design and planning of engineering
structures in the alkaline soil. The EC value of the soils was found to vary from 190 to 1930 µS
/cm. The value is generally high signifying its corrosive nature.
4.2.4. Dispersion of Soils
Dispersive clays are those, which deflocculates or disintegrate when exposed to water.
Dispersive clay soils are identified by various methods. In this work the double hydrometer
method was adopted. It is similar to the normal hydrometer test, except that neither mechanical
agitation nor chemical dispersing agent is applied in this method. The percent dispersion of soils
Pit Depth (m)
Lithologic type
Moisture content (%)
pH EC (µS/ cm) % dispersion
TP1 1.5 Clay soils
24.3 8.5 250 TP4 1.0 24.9 8.2 180 TP8 2.0 40.9 7.4 900 TP9 3.0 15.8 0.016 TP11 1.5 18.3 0.013 TP3 0.5
Silt soils
34.7 7.1 1930 TP3 3.0 27.9 7.3 1840 TP6 2.4 21.7 7.6 530 TP2 0.5
Sandy clay/silt soils
20.5 7.7 190 TP10 1.5 9.95 TP12 1.5 6.6 14.33 TP12 4.0 10.4 TP13 2.0 10.6 55.03 TP14 2.0 14.7 TP5 2.0
Clayey /silty sand soils
15.3 7.8 250 TP6 1.4 23 7.5 900 TP13 1.2 14.2
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is the ratio of percent passing 5µm size without applying dispersing agents and percent passing
5µm by applying dispersing agents.
When the percent dispersion is nearly 100% it indicates a completely dispersive soil. In general
dispersive clays are highly erosive, have high shrink-swell potential and have low permeability
in an intact state. Dispersive soils are troublesome in terms of slope stability (in natural slopes
and embankments); the underlying soil mass of the slope often suffers from internal erosion
when subject to localized seepage zones. From the results (Table 1) of the test the soils seems
non-dispersive.
4.2.5. Grain-size Analysis
Two methods were used to find the particle-size distribution of the soil samples: sieve analysis,
for particle sizes larger than 0.075 mm (No. 200) in diameter; and hydrometer analysis for
particle-sizes smaller than 0.075 mm in diameter. During hydrometer analysis sodium
hexametaphosphate (NaPO3) or Calgon was used as a dispersion agent, and all analysis was
determined based on ASTM (D 421 & 422) procedure. The results of grain-size analysis are
presented in table 2. The diameter or size range is adopted from ASTM as follows: >4.75 mm
(No.4) gravel; 4.75 – 0.075 mm (No.200) sand; 0.075 – 0.002 mm silt and < 0.002 mm clay.
Table 2. Specific gravity and grain-size analysis results of the soils of Mekelle town area.
Pit Depth (m)
Lithologic type Specific gravity
Gravel Sand Silt Clay Origin
(%) TP1 0.2 -1.5
Clay soils
2.70 0 13 58 29 Residual TP4 0.15 -1.0 2.84 0 14 66 20 Residual TP8 0.3 -2.0 2.45 0 13 60 27 Alluvial TP9 0.3 -3.0 2.70 0 10 46 44 Alluvial TP11 0.2 -1.5 2.75 0 14 36 50 Residual TP3 0.2 -0.5
Silt soils 2.51 0 11 45 44 Alluvial
TP3 1.0 -3.0 2.51 0 12 56 22 Alluvial TP6 1.4 -2.4 2.60 0 25 33 42 Residual TP2 0.1 -0.5
Sandy clay/silt soils
2.71 0 28 40 32 Residual TP10 1.0 -1.5 2.70 0 38 16 30 Alluvial TP12 0.2 -1.5 2.70 0 34 40 26 Alluvial TP12 1.5 -4.0 2.75 0 18 39 43 Alluvial TP13 1.2 -2.0 2.70 0 34 38 28 Alluvial TP5 1.0 -2.0
Clayey/silty sand soils 2.90 2 73 16 8 Alluvial
TP6 0.2 -1.4 2.51 0 51 13 36 Residual TP13 0.2 -1.2 2.65 3 62 23 12 Alluvial
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4.2.6. Atterberg (consistency) Limits and Plasticity Index
The consistency of a fine-grained soil is the physical state in which it exists; it is related to a
larger extent to water content. Consistency denotes degree of firmness of the soil that is indicated
by tests in the field as soft, firm, stiff or hard. Even though it is not possible to interpret the
Atterberg limits and plasticity characteristics in fundamental terms, these parameters are of great
practical use as index properties of cohesive soils. The engineering properties (uses) of fine-
grained soils are, generally, related to these index properties. The more plastic a soil means the
more compressible, higher shrinkage-swell potential and the lower is its permeability will be
(Abramson et al, 1996).
The Atterberg limit of selected soil samples was determined in the laboratory (Table 3). From the
results of plastic (PL) and liquid limit (LL) plasticity index (PI) of the soils was calculated.
Plastic index is important in classifying fine-grained soils and is fundamental to the Casagrande
plasticity chart. The larger the plasticity index, the greater will be the engineering problems
associated with using the soil as an engineering material, such as foundation support for
residential building and road sub grades (Bowles, 1992).
The clay soils have LL: 29-59%, silt soils 50-67.4%, sandy clay / silt soils 37.5-70.7% and fine
fraction of clayey/ silty sand soils non-plastic to 66%. The PL of these soils ranges 15-25.9%,
30.2-37.9%, 22.3-33.4% and non- plastic to 27.4% respectively (Table 3). Most of the soils of
the study area fall in intermediate to high plasticity type, except some of the clayey / silty sand
soils which fall in soils of low plasticity type. Comparison of the LL and PL with the NMC
(Table 1) shows that most of the clays were in plastic phase, while others were in semisolid or
solid state (their NMC were below their plastic limit at the time of sampling). The PI of clay
soils ranges from 14 to 36.5 %; silt soils 12.1 to 32.6%; sandy clay / silt soils 13.3 to 37.4% and
clayey/silty sand soils non-plastic to 38.6%, respectively.
According to description of plasticity of fine soils in terms of range of plasticity index given by
IAEG (1983), the clay, sandy clay/silty and clayey/silty sand soils are moderately to extremely
plastic and the silty soils are moderately to highly plastic type. In general, most of the plasticity
values of the soils of Mekelle town fall in the highly plastic range (17-35%).
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Table 3. Consistency limits, plasticity index values and activity of the soils of Mekelle town.
Figure 4. Plot of soils of Mekelle town on activity chart.
Pit Depth (m)
Lithologic type LL PL PI Activity
(%) TP1 02 -1.5
Clay soils
29 15.05 13.95 0.481 TP4 0.15 -1.0 40 21.5 18.5 0.925 TP8 0.3 -2.0 54.5 25.85 28.65 1.061 TP9 0.3 -3.0 59 22.85 36.51 0.83 TP11 0.2 -1.5 40.8 22.3 25.70 0.514 TP3 0.2 -0.5
Silt soils
67.4 34.85 32.55 0.74 TP3 1.0 -3.0 50 37.94 12.06 0.548 TP6 1.4 -2.4 52 30.18 21.82 0.52 TP2 0.1 -0.5
Sandy clay/ silt soils
55.5 23.9 31.60 0.988 TP10 1.0 -1.5 37.8 24.25 13.55 0.452 TP12 0.2 -1.5 45.41 22.85 22.56 0.868 TP12 1.5 -4.0 48 22.3 25.70 0.598 TP13 1.2 -2.0 70.75 33.4 37.35 1.334 TP5 1.0 -2.0
Clayey /silty sand soils 21 16 5
TP6 0.2 -1.4 66 27.36 38.64 1.073 TP13 0.2 -1.2 28.5 20.23 8.27 0.689 TP14 0.5 – 2.0 34.37 18.52 15.85
A C T I V E S O I L S
N O R M A L S O I L S
I N A C T I V E S O I L S
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Table 4. Summary of shrinkage limit results and calculated shrinkage indexes and ratio.
Pit Depth (m) Lithologic type
Shrinkage limit
Shrinkage Index
Shrinkage ratio
Liquidity index (LI)
Consistency Index (CI)
(%) TP1 02 -1.5
Clay soils
10 5.05 0.6631 0.3369 TP4 0.15 -1.0 19.23 2.27 1.63 0.1838 0.8162 TP8 0.3 -2.0 8.69 17.16 1.92 0.5253 0.475 TP9 0.3 -3.0 20.41 2.44 1.75 -0.195 1.195 TP11 0.2 -1.5 15 7.3 1.45 -0.216 1.216 TP3 0.2 -0.5
Silt soils
12.76 22.09 2.04 -0.005 1.005 TP3 1.0 -3.0 30.95 6.99 1.35 -0.833 1.833 TP6 1.4 -2.4 22.78 7.4 1.25 -0.388 1.388 TP2 0.1 -0.5 Sandy clay/
silt soils 15.55 8.35 1.88 -0.1076 1.1076
TP10 1.0 -1.5 -1.0554 2.0554 TP12 0.2 -1.5 11.11 11.74 1.93 -0.7203 1.7203 TP12 1.5 -4.0 0.016 22.28 2 -0.4630 1.4630 TP13 1.2 -2.0 20.33 13.07 1.84 -0.61 1.61 TP5 1.0 -2.0
Clayey /silty sand soils
1.76 TP6 0.2 -1.4 9.3 18.06 1.87 -0.113 1.113 TP13 0.2 -1.2 13.33 6.9 1.97 -0.729 1.729 TP14 0.5 - 2.0 7.26 11.26 1.96 -0.241 1.241
4.2.7. Activity of the soils
The activity of the soils of Mekelle town was determined from consistency limit tests and grain-
size analysis (Table 3). In geotechnical work the term activity indicates the percentage of clay in
the fraction of soil used for Atterberg limits and the potential swell and shrinkage (volume
change) of a soil, with larger values indicating an increasing potential. Skempton (1953) cited in
Bell (1983) suggested three classes of activity (active, normal and inactive, Fig. 4). Kaolinitic
and illitic clays are usually inactive whilst montmorillonitic clays range from inactive to active.
In terms of potential expansiveness soils with activity less than 0.75 are low, 0.75-1.25 medium
and those with greater than 1.25 are highly expansive (Fig.4).
4.2.8. Shrinkage Limit (SL)
The shrinkage limit of soils of the study area was determined in laboratory (ASTM D-427). The
shrinkage parameters frequently used in soil engineering, shrinkage index and shrinkage ratio,
were also calculated from the laboratory results. Liquidity index (LI) of the soils (the nearness of
its water content to its LL) and consistency index (CI) (firmness of a soil), were also calculated
from results of LL, PL and NMC (Table 4).
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Table 5. Free swell values of the soils of Mekelle town.
Pit Depth (m) Lithologic type Free swell (%) TP1 02 -1.5
Clay soils
20 TP4 0.15 -1.0 30 TP7 0.2 -2.0 40 TP8 0.3 -2.0 70 TP9 0.3 -3.0 70 TP11 0.2 -1.5 40 AF5 1.4 60 AF23 1.5
Silt soils 60
AF29 1.3 60 TP3 0.2 -0.5
Sandy clay/ silt soils 20
TP3 1.0 -3.0 50 TP6 1.4 -2.4 50 AF5 2.50 40 AF13 1.0 70 TP2 0.1 -0.5
Clayey /silty sand soils
20 TP10 1.0 -1.5 55 TP12 0.2 -1.5 40 TP12 1.5 -4.0 50 TP13 1.2 -2.0 25 AF24 2.5 40 AF29 2.5 30 TP5 1.0 -2.0 Clayey/ silty sand 0 TP6 0.2 -1.4 65 TP13 0.2 -1.2 20 TP14 0.5 - 2.0 20
4.2.9. Free Swell
Free swell tests consists of placing a known volume of dry soil in water and noting the swelled
volume after the material settles, without any surcharge, to the bottom of a graduated cylinder.
The difference between the final and initial volume, expressed as a percentage of initial volume,
is the free swell value (Chen, 1975). Results of free swell tests of the soils of the study area are
presented in Table 5.
According to Holtz (1956) cited in Bell (1983), soils having free swell value as high as 100% can
cause considerable damage to lightly loaded structures, and soils having free swell value below
50% seldom exhibit appreciable volume change even under very light loadings. The free swell
values of the soils of the study area vary from 0 to 70% (Table 5). There are many samples with
free swell of greater or equal to 50%. Hence, considerable attention should be given in
foundation design even for light structures on such soils (TP-3, 6, 9, 10, 11, 13 and 12) because
their value shows expansiveness property.
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4.2.10. Classification of soils
Approximate assessment of the engineering properties of soils can be obtained from the index
properties after appropriate classification is made. From geotechnical or engineering geological
point of view, the classification of soil may be done with the objective of finding the suitability
of the soil for construction of structures or foundations. Such a classification should provide
some guide to the engineering performance of the soil type and should provide a means by which
soils can be identified quickly (ISRM, 1981).
In this research the Unified Soil Classification System (USCS) and classification system
proposed by IAEG (1981) which is a modified form of the Unified Soil Classification (USC) and
the British Soil Classification for engineering purposes (BSCS) were employed. The USCS is
based on both grain size (after excluding boulders and cobbles) and plasticity properties of the
soil and is applicable to many uses. The soils are broadly classified into two categories, coarse-
grained, if more than 50% of the soil is retained on No. 200 (0.075mm) sieve and fine-grained, if
more than 50 % passes No. 200 sieve. The coarse-grained soils are further subdivided based on
PI and LL (Fig. 5 and Table 6).
The classification system proposed by IAEG (1981) is also based on grading and plasticity of
soils. Grading and plasticity are divided into a number of clearly defined ranges, each of which
may be referred to by a descriptive name and letter (Fig. 6 and Table 7).
4.3. Description and Classification of Rocks
Description is the initial step in an engineering geological investigation of rock masses. In view
of this, description of rocks of Mekelle town area was carried out according to descriptive
schemes proposed by IAEG (1981). In describing rocks of the town, logs and some in-situ tests
conducted on more than 40 site investigation boreholes and Schmidt hammer rebound test on 20
test points were evaluated in addition to visual description and the use of geological hammer and
weathering grade to estimate the engineering properties of the rock masses.
4.3.1. Dolerite
Outcrop of dolerite in the town is mainly dominant in the central and eastern part. This rock unit
has variable properties that resulted from degree of discontinuity and weathering. This rock unit
is classified into highly weathered weak dolerite (low mass strength), moderately weathered
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strong dolerite (medium mass strength) and fresh to slightly weathered strong dolerite (high mass
strength).
Figure 5. Plot of soils of Mekelle town on Casagrade plasticity chart.
Table 6. Classification of soils of Mekelle town area based on USCS.
Pit/ BH
Depth (m)
Lithologic type
G* Sand Silt Clay LL PI USCS classification (%) (%) S** Soil name
TP1 02 -1.5 Clay soils
0 13 58 29 29 14 CL Lean clay TP4 0.15 –1.0 0 14 66 20 40 19 CL Lean clay TP8 0.3 -2.0 0 13 60 27 55 29 CH Fat clay TP9 0.3 -3.0 0 10 46 44 59 37 CH Fat clay TP11 0.2 -1.5 0 14 36 50 41 26 CL Lean clay AF5 1.4 0 8 42 50 51 26 CH Fat clay MS 3.8 0 6 39 55 81 40 CH Fat clay MS2 1.9 0 12 46 42 59 31 CH Fat clay TP3 0.2 -0.5
Silt soils
0 11 45 44 67 33 MH Elastic silt TP3 1.0 -3.0 0 12 56 22 50 12 MH Elastic silt TP6 1.4 -2.4 0 25 33 42 52 22 MH Elastic silt with sand MS4 5.6 0 8 44 48 71 35 MH Elastic silt MS2 5.3 0 9 52 39 67 31 MH Elastic silt TP2 0.1 -0.5
Sandy clay/ silt soils
0 28 40 32 56 32 CH Clay with sand TP10 1.0 -1.5 0 38 16 30 38 14 CL Sandy lean clay TP12 0.2 -1.5 0 34 40 26 45 23 CL Sandy lean clay TP12 1.5 -4.0 0 18 39 43 48 26 CL Lean clay with sand TP13 1.2 -2.0 0 34 38 28 71 37 CH Sandy fat clay AF5 2.5 0 21 47 32 43 18 CL Lean clay with sand AF13 2.5 0 28 52 18 41 18 CL Lean clay with sand AF24 2.5 0 30 48 22 40 13 ML Sandy silt TP5 1.0 -2.0 Clayey /
silty sand soils
2 73 16 8 21 5 SC-SM
Silty, clayey sand
TP6 0.2 -1.4 0 51 13 36 66 39 SC Clayey sand TP13 0.2 -1.2 3 62 23 12 29 8 SC Clayey sand AF29 2.5 0 67 24 9 36 15 SC Clayey sand
* Gravel, ** symbol.
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L O W ( L )
C L - M L
C L
C I
C H
C VC E
M L M IM H
M V
M E
U ( U P P E R P L A S T IC IT Y R A N G E )
IN T E R M E -D IA T E
( I ) H IG H V E R Y H IG H
(V )
E X T R E M E L Y H IG HP L A S T IC IT Y
(E )
A - L IN E
PLA
STIC
ITY
IND
EX (%
)
L IQ U ID L IM IT ( % )
0
1 0
2 0 3 0 4 0
5 0
6 0
7 0
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0
C = C L A Y M = S IL T L = L O W H = H IG H I = IN T E R M E D IA T EV = V E R Y H IG H E = E X T R E M E L Y H IG H
(H )
M E K E L E T O W N S O IL S
Figure 6. Plots of soils of Mekelle town on plasticity chart based on IAEG.
Table 7. Classification of soils of Mekelle town area based on IAEG. Pit/ BH
Depth (m)
Lithologic type
G* Sand Silt Clay LL PI IAEG (1981) (%) (%) S** Soil name
TP1 0.2 -1.5 Clay soils
0 13 58 29 29 14 CL CLAY of low plasticity TP4 0.15-1.0 0 14 66 20 40 19 CI CLAY, of intermediate plasticity TP8 0.3 -2.0 0 13 60 27 55 29 CH CLAY of high plasticity TP9 0.3 -3.0 0 10 46 44 59 37 CH CLAY, of high plasticity TP11 0.2 -1.5 0 14 36 50 41 26 CI CLAY of intermediate plasticity AF5 1.4 0 8 42 50 51 26 CH CLAY of high plasticity MS 3.8 0 6 39 55 81 40 CV CLAY of very high plasticity MS2 1.9 0 12 46 42 59 31 CH CLAY of high plasticity TP3 0.2 -0.5
Silt soils
0 11 45 44 67 33 MH SILT of high plasticity TP3 1.0 -3.0 0 12 56 22 50 12 MH SILT of high plasticity TP6 1.4 -2.4 0 25 33 42 52 22 MH SILT of high plasticity MS4 5.6 0 8 44 48 71 35 MV SILT of very high plasticity MS2 5.3 0 9 52 39 67 31 MH SILT of high plasticity TP2 0.1 -0.5
Sandy clay/ silt soils
0 28 40 32 56 32 CH CLAY of high plasticity TP10 1.0 -1.5 0 38 16 30 38 14 CS
(CLS) CLAY, sandy, of low plasticity
TP12 0.2 -1.5 0 34 40 26 45 23 CS (CLS)
CLAY, sandy, of low plasticity
TP12 1.5 -4.0 0 18 39 43 48 26 C (CI) CLAY, sandy, of intermediate plasticity
TP13 1.2 -2.0 0 34 38 28 71 37 CS (CLS)
CLAY, sandy, of very high plasticity
AF5 2.5 0 21 47 32 43 18 C (CI) CLAY of intermediate plasticity AF13 2.5 0 28 52 18 41 18 C (CI) CLAY of intermediate plasticity AF24 2.5 0 30 48 22 40 13 M (ML) SILT of low plasticity TP5 1.0 -2.0 Clayey /
silty sand soils
2 73 16 8 21 5 SM (SML)
SAND, silty, of low plasticity
TP6 0.2 –1.4 0 51 13 36 66 39 SC (SCH)
SAND, clay, of high plasticity
TP13 0.2 -1.2 3 62 23 12 29 8 SC (SCL)
SAND, clayey, of low plasticity
AF29 2.5 0 67 24 9 36 15 CS (CLS)
CLAY, sandy, of intermediate plasticity
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4.3.1.1. Highly weathered, Weak Dolerite (low mass strength)
This rock is found along the eastern side of the town and the central part of the town. It is dark
greenish, medium to coarse grained, and moderately to highly weathered. The material strength
was estimated in the field and varies from 2 to 20 MPa. Joints of WNW (2750, 2850 and 2900)
strike are common, in places NNE strike joints are observed. The joints are in general vertical
and the spacing varies from 4 to 20cm. Joint surfaces are planar and rough, and very narrow to
moderately narrow aperture. Clay, calcite and silica are common infill materials. Joints were dry
during the time of investigation (February 2002 and January 2004).
4.3.1.2. Moderately Weathered, Medium Strong Dolerite (medium mass strength)
This rock is slightly stronger than the above subunit, mainly found along the eastern part and
along the central part of the town (Fig. 7). It is dark-greenish in color; medium to coarse grained
and moderately weathered. The strength of the rock varies between 13 to 45 MPa. Two sets
(WNW and NNE and random joints are dominant which are vertical and medium to widely
spaced. The joint surfaces for the systematic once, is planar and rough. The separation or
aperture is variable and ranges from very narrow to moderately narrow. Clay and calcite are the
common infill materials, occasionally silica veins are observed.
4.3.1.3. Fresh to slightly weathered, Strong Dolerite (high mass strength)
This engineering geological subunit forms steep cliff in the eastern side of the town. The rock
material strength varies from 45 to 150 MPa. Two sets of joints are dominant; WNW and NNE.
In places columnar joints are observed. The joint wall or surface is planar and smooth, and the
aperture varies from tight to narrow.
4.3.2. Sandstone (moderately weathered and weak)
It covers limited area in the southern side of the town. It is friable and less cemented. It is whitish
to light yellowish in color, medium to coarse grained and slightly to moderately weathered. Its
rock material strength is weak, estimated about 105 MPa. Three sets of joints (2850, 0200 and
0850 strike) are dominant. The WNW strike joints are closely spaced while the other two sets are
widely spaced, but all the joints are vertical. The joint wall or surface is planar and rough and the
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aperture varies from tight to moderately wide (0 to 2.5 cm). Calcite and silica are observed as
infill material. It is horizontally bedded with bed thickness up to 0.5m.
4.3.3. Limestone-Marl-Shale Intercalation (moderate-highly weathered and weak)
Though the strength of each layer varies from weak (shale) to strong (limestone), it is considered
as weak engineering geological unit or rock with low mass strength. It is variegated (gray,
yellowish, dark brown, etc), crystalline and slightly to highly weathered. The intact rock strength
is variable, from very weak for shale to strong for limestone. Three vertical joint sets (3200, 0450
and 3600) and horizontal joint are observed in this intercalation unit. The joint surfaces or wall is
generally planar and rough, and tight to moderately narrow aperture (0-3 cm). The spacing of the
systematic joint set is on average 0.5 to 1.5 m. This intercalation unit is horizontally to sub-
horizontally bedded with bed thickness of 0.2 to 1.5 m. In places the beds are inclined in
different direction due to the intrusion of dolerite.
4.3.4. Limestone (slightly weathered, strong with high mass strength)
This rock covers limited area, mainly outcrop along streams. It is black and in places light
yellowish in color, finely crystalline and fresh to slightly weathered. The rock material strength
ranges from 62 to 160 MPa. Three vertical joint sets (3200, 0450 and 3600) and horizontal joint
parallel to bedding planes are the dominant. The joint surface or wall is planar and rough to
smooth, and the aperture varies from tight (for horizontal joints) to 5 cm (for vertical joints). The
vertical joints are widely spaced (0.5 to 2.5 m). This limestone unit is bedded; in most places
horizontal and bed thickness reaches up to 1 m.
In addition to the above-mentioned joints or discontinuities in the most engineering geological
units, large fractures or faults and intrusions of dolerite also affects the rock mass strength (Figs.
3 and 7) and the presence of most of joints are reflections of these large structures. The strike of
most of the joints is almost the same to the strike of large structures or faults. Based on the
description of the soils and rocks from field and laboratory data, delineation of engineering
geological units was made to produce an engineering geological map for the town with
appropriate legend which signifies the engineering behavior of each unit (Fig. 7).
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Figure 7. Engineering geological map of Mekelle town.
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5. CONCLUSIONS AND RECOMMENDATIONS
5.1. Conclusions
Geological and Engineering geological map of the town was produced at a scale of 1:10,000.
Four lithological /soil types were identified: clay, silt, sandy clay/silt and clayey/silty sand soils.
The moisture content varies from 15.8-40.9% for clay; 21.7-34.7% for silt; 6.6-20.5% for sandy
silt/clay and 14.2-23% for clayey/silty sand soils. Grain size analyses of the soils indicate that
most of the soils of the town are composed of fine fractions.
Liquid limit of the soils varies from 29-59% for clay; 50-67.4% for silt; 37.5-70.8% for sandy
clay/silt and non-plastic to 66% for clayey/silty sand soils. Similarly the plasticity index of the
soils varies from 14-36.6% for clay; 12-32.6% for silt; 13.3-37.4% for sandy clay/silt and non-
plastic to 38.6% for clayey/silty sand soils. Hence, the soils are generally moderately plastic to
extremely plastic. Based on liquid limit the plasticity of the soils varies from low to extremely
high. The consistency index of most soils is above unity and their liquidity index is below zero,
indicating the swelling behavior of most of the soils in the area. The free swell of the soils of the
area is highly variable from 0 to as high as 70%, indicating the potentially expansive behavior
and needs considerable attention in design of even light engineering structures.
Classification of soils and rocks of the town was also carried out. Based on USC system the clay
soils fall in CL (inorganic clays of low to medium plasticity, silty clays and lean clays) and CH
(fat clays) type; silt soils in MH (inorganic silts, elastic silts); sandy clay/silt soils fall in CH, CL
and ML (inorganic silts, very fine sand, rock flour, silty or clayey fine sands) and the clayey/silty
sand soils fall in SC (clayey sand, sandy clay mixtures) and SC-SM (silty clayey sand soils) type.
The lithological field description and classifications made are generally in agreement with
laboratory results. The potential expansion behavior or swelling nature of the soils is found to be
low to very high and about 50% of the soil samples show medium to high degree of expansion.
Classification of rocks was made by considering lithologic units as main groups and by
considering factors or parameters, which affect the engineering property of the rock; some of the
main units are classified into engineering geological subunits. These units and subunits are
dolerite (with subunits: highly weathered weak dolerite, moderately weathered medium strong
dolerite and fresh to slightly weathered strong dolerite), moderately weathered weak sandstone,
moderately to highly weathered weak limestone-marl- shale intercalation and slightly weathered
strong limestone.
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5.2. Recommendations
Potentially expansive soils were identified in the town in many places. Hence, care should be
taken in constructing civil engineering structures on such type of soils. Moreover, expansive
soils in the town were found to cause failure of roads, buildings, drainage pipes and culverts. In
such areas proper investigation before design and close quality control during construction is
very crucial to minimize unnecessary cost and failure of infrastructures. Expansive soils are
difficult to use in the construction of highway, lightweight structures, construction of subsurface
drainage by concrete pipes, etc due to their swelling nature. Hence, removing part of the
expansive soil (moisture variation ranges 3 to 4 m) or lowering the foundation deeper than
normally used for stable soils is recommended. Further, detailed study on expansive soils (areal
distributions and mineralogical composition) and engineering properties, like swelling potential,
consolidation characteristics, rock mass properties, are also recommended.
6. ACKNOWLEDGMENT
The author acknowledges the Ethiopian Ministry of Science and Technology for providing the
partial grant for the research and the Department of Earth Sciences of Mekelle University for
providing logistic support. The technical support extended by CO-SAERT geotechnical
laboratory for analyzing soil samples is dully acknowledged. Drs. Nata Tadesse, Kifle
Weldearegay and K. Bheemalingeswara; and two anonymous reviewers deserve special thank for
their critical comments and constructive suggestions during the preparation of this manuscript.
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