GEOTECHNICAL
INVESTIGATION REPORT
Bridge Crossing At Station 1+593
FINAL
PROJECT
: Foundation Investigation for Five Bridges at
Mehal Meda – Gishe Rabel – Mekoy – Mila Mille
Road Project, Lot 1: Mehal Meda – Tor Mesaya
(Rebid)
: Zhongmei Engineering Group Limited
CLIENT
LOCATION : Amara Region Mehal Meda
BY
DABEK ENGINEERING PLC
JACROS Square to GORU Square Road, Left of YERRER Main Bridge
Phone: 251-116-639262, 251- 930106027, 251-911943453,
Fax: 251-116-633213, E-mail: info@dabekengineering.com,
Web: http://www.dabekengineering.com, P.O.Box:239/1022
Addis Ababa, Ethiopia
Date: 26/02/2024
PROJECT No.: GI-113 /2024
PROJECT
:
Mehal meda – Gishe rabel – mekoy
Mila Mille Road Project
CLIENT
:
Zhongmei Engineering Group Limited
LOCATION
:
Amahra Region Mehal Meda
NOTIFICATION OF TEST RESULT
For the proposed bridge located at Mehal Meda Gishe Rabel Road
Project, bridge crossing located at station 1+593 We have conducted the
necessary studies pertaining to foundation of the bridge including but
not limited to literature review, field reconnaissance, field exploration
and sampling, laboratory testing and finally analyzed the findings and
produced and submitted a draft report.
This is report is the final version of the draft made after incorporating
comments from the supervision consultant.
Best Regards,
Table of Contents
1
INTRODUCTION ...................................................................................................................... 1
1.1
GENERAL ........................................................................................................................ 1
1.2
Scope of the work .......................................................................................................... 2
1.3
Objective ........................................................................................................................ 3
1.4
Location of the Project Site ............................................................................................ 4
1.5
Climate ........................................................................................................................... 5
1.5.1
Rain fall .................................................................................................................. 5
1.5.2
Temperature .......................................................................................................... 6
1.6
Terrain of the Project Site .............................................................................................. 6
1.7
GEOLOGY ....................................................................................................................... 6
1.7.1
REGIONAL GEOLOGY ............................................................................................. 6
1.7.2
Local Geology, Molale i-gnimbrite (Tmi) ............................................................... 8
1.8
2
INVESTIGATION PROCEDURE AND STANDARD CODE PROVISIONS ...................................... 10
2.1.1
PLANNING ............................................................................................................ 10
2.1.2
DRILLING .............................................................................................................. 10
2.1.3
SAMPLING ............................................................................................................ 13
2.1.4
Ground Water ...................................................................................................... 14
2.1.5
Logging Procedures .............................................................................................. 14
2.2
3
Soil of the Project Site .................................................................................................... 9
LABORATORY TESTING ................................................................................................. 14
GEOTECHNICAL LAYERS ....................................................................................................... 16
3.1
ESTABLISHMENT OF DERIVED VALUES ........................................................................ 17
3.2
Bearing capacity of rocks ............................................................................................. 18
3.3
Determination Using Hoek-Brown Failure Criterion.................................................... 24
3.4
Allowable Bearing Capacity Determination ................................................................. 25
3.5
Bearing layer and Bearing capacity recommendation ................................................. 26
3.6
BEARING CAPACITY DETERMINATION Using Ethiopian Building Code Standard (EBCS7, 1995) .................................................................................................................................... 27
4
Seismicity of the project area .............................................................................................. 28
5
CONCLUSION AND RECOMMENDATION ............................................................................. 29
ANNEX-1: BORELOGS...............................................................................................................i
ANNEX-2: CORE BOX PICTURES ......................................................................................... iii
ANNEX-3: LABORATORY TEST RESULTS .......................................................................................... v
Geotechnical Investigation Report for Zhongmei Engineering Group limited
1 INTRODUCTION
This report is the findings of the Geotechnical investigation and laboratory testing for a
Bridges crossing to be constructed on Mehal Meda – Gishe Rabel – Mekoy – Mila Mille
Road Project, Lot 1: Mehal Meda – Tor Mesaya, which has a total estimated length of
65.5 Km.
1.1 GENERAL
This site investigation report, as part of the geotechnical reports for intended bridge
crossings in the project listed in the table hereunder and this report covers the bridge
crossing located at station 4+040.
Table 1.1 List of bridge crossings in this project
Bridge Name
1
1
1
Design Bridge
Length (m)
21.2
12.9
15
No. of
Borehole
2
s
2
2
6+920
1
22.13
2
37+120
1
16
87.23m
2
10
Station (km)
No. of Spans
1. River Bridge
2. River Bridge
3. River Bridge
1+260
4+040
5+720
4. River Bridge
5. River Bridge
Total
The geotechnical investigation is prepared to the requirements set forth in Ethiopian
Building Cod., on the proposed site. The geotechnical investigation is made to
determine the stratification and engineering properties of the soil/rocks underlying
the site as per the requirements outlined in the EBCS-7, 21997, and to recommend a
safe and economical foundation system for the proposed structure.
Geotechnical investigation work was envisaged to assess the subsoil strata from safe
bearing capacity point of view and to establish subsoil profile at the project location.
Site investigation in one form or the other is required for every engineering project.
Information about the surface and subsurface features is essential for the design of
structures and for planning construction techniques.
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As a statutory requirement, client decided to carry out geotechnical investigation.
Geotechnical investigation work consists of drilling and sampling in two boreholes at
the project location.
DABEK Engineering carried out fieldwork during the period 26th August, 2023 to 05th
of September, 2023. Selected soil and rock samples were tested in laboratory of
DABEK Engineering. Laboratory test results were obtained on 29th of September,
2023.
This geotechnical investigation report is prepared by DABEK Engineering based on
the data collected from two boreholes and from laboratory results and judgment of
undersigned based on its experience.
1.2 Scope of the work
In accordance with the requirements of the contract, scope of work is to conduct
Geotechnical investigation and laboratory testing for five (5) Bridge crossings to be
constructed on the Road Project. The geotechnical investigation is through core
drilling, in-situ tests, groundwater monitoring, sampling, and subsequent laboratory
tests on representative samples to determine the engineering properties of the subsurface formations underlying the proposed bridges. Finally, to provide safe and
economic foundations based on combined field data and laboratory test results.
Accordingly, the contract intends boreholes to be drilled to a minimum depth of 20m
in soil and decomposed rock formation. However, if sound rock formation is
encountered the drilling may be halted after 3m depth based on the quality of the rock.
That is if the RQD of the rock stratum is greater than 75%, the drilling shall be stopped
after 3m or else the drilling shall extend up to 6.0m having rock stratum is greater
than 50% in the rock formation. The Employer’s Representative shall decide to extend
the depth to 30m if required based on the actual ground condition observed during
investigation.
The service is to execute the geotechnical investigations shall be made to the best
satisfaction of the client/ERA/Supervising Consultant and Main Contractor in strict
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adherence to ERA Standard Technical Specification-2013 and widely accepted and
practiced procedures such as, ASTM04.06, ASTM04.09, BS 5930: 1981, and others.
The work performed by the Foundation Investigation shall consist of making a
complete foundation investigation for the adequate design and construction of the
proposed bridge structures. i.e, adequate program of field sampling, laboratory testing,
and engineering analysis and evaluation, with the results presented in report form.
The investigation and analysis shall be performed in compliance with in procedures
outlined in the TOR for this project and generally accepted principles of sound
engineering practices
1.3 Objective
The objective of the geotechnical investigation is to carry out sub-surface investigation
through rotary drilling, SPT, sampling and laboratory testing, investigation for each
crossings. The geotechnical investigation shall include maintaining of complete
records of all the actual findings in a report from prepared to the highest standard of
the profession. The investigation shall be carried out in accordance to the ERA 2013
Site Investigation Manual, relevant BS Codes and /or ASTM/AASHTO Standards.
The objective of the investigation service is to carryout detailed geotechnical
investigation specified above in order to characterize subsurface condition,
determined the bearing capacity of the underneath soil strata and recommended
appropriate types, sizes and depths of foundations. The service comprised of field
investigations and laboratory testing on representative samples, and finally to provide
safe and economic foundation recommendations, as outlined below:
•
Conduct in situ tests to determine the relative strength/consistency
•
Collect representative samples
•
Conduct laboratory tests to determine the engineering properties
•
Characterize the sub-surface geology into various geotechnical layers based on
combined field data and laboratory test results, and finally
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•
Determine the allowable bearing pressure for alternative options of depth and
foundation width of the proposed bridge sites.
Output of the service shall provide;
•
Excellent understanding on the nature, occurrence and set up of surface and
sub-surface geological materials and geotechnical of proposed works.
•
Engineering
characterization,
classification
and
determination
of
representatives geotechnical parameters of ground materials that form
proposed project foundation.
•
Evaluating geotechnical performance parameters with respect to intended
purpose and then to come up with identification of related geotechnical
problems and challenges, along with appraisal of reliable remedial measures
•
Forward suggestion to amend or accommodate identified geotechnical
problems
1.4 Location of the Project Site
The bridge investigation project is on the Mehal Meda -Gishe Rabel Road Project,
which is found in Menz Gera Midir Woreda, North Shewa Zone of The Amhara Regional
State. Menz Gera Midir is part of a vast area of land in the eastern escarpment of the
northern mountain, called Menz. Menz is now divided in to five Woredas: Gshe Rabel,
Menz Keya, Menz Gera, Menz Lalo and Menz Mama woredas.
The geographical location of Menz Gera-Midir Wereda lies between 10°00´ N to
10°34´N and 39°17´E to 39°43´E. There are, four agro- ecological zones in the wereda;
these are Alphine (wurch), temprate (dega), sub-tropical (weina dega) and kolla
(tropical). It has a bimodal rainfall model with unpredictable patterns. The District
comprises 22 Kebelle administrations, having 2 urban Kebelles and 20 rural Kebelle.
The total area of the District is 1105.55 km2.
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Figure 1.1: Administrative and Spatial Location of the Project Area
1.5 Climate
The climate of the area varies with altitudes gradient and seasonal changes. At higher
altitude, wet season is characterized by a combination of high rain fall, frequent hail
storms and occasional snow. But during dry season, frosts are common. There are
sharp temperature fluctuations between night and day time. Generally night times are
colder than the day times.
1.5.1 Rain fall
Rainfall data recorded from the nearest metrological station at, Mehal-Meda, shows
that, the Equatorial Westerly’s and the Indian Ocean air streams are the sources of rain
for Guassa at different times of the year. Though showers of light rain can occur in any
month of the year, but informal there are two main rainy seasons (Kiremt or Meher)
between June to September and minor rainy season (Belg) in February, March and
April. The annual rainfall ranges from 1200-1600mm.
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1.5.2 Temperature
Temperature data recorded from the nearest meteorological station, Mehal-Meda,
shows that, the area is characterized by mild day temperatures and cold night
temperatures. During the dry season (December to January), the temperature would
rise up to 21˚C at day time, but it falls to -7˚C at night. In the wet season, at the day time
temperature is 12˚C while a night temperature is 3˚C.
1.6 Terrain of the Project Site
The plateau and mountain ranges are dissected by deep valley and gorges. Generally,
elevation is varying between 3000 and 4200 m.a.s.l.; the specific bridge project site has
an average elevation of 3109 m.a.s.l.
The drainage network in the plateau is formed part of the Abay drainage basin where
the streams follow the general northeast-southwest direction. Major streams in this
physiographic region include Wenchit, Jema and Beto. Parallel, sub-parallel and
dendritic drainage pattern characterize this physiographic region.
1.7 GEOLOGY
1.7.1 REGIONAL GEOLOGY
The Were-Ilu area referred to as Were-Ilu Map-sheet (NC-37-7) is referred for this
topic. The Map sheet encompasses parts of Northern Shewa, Southern Wello and
Oromia Zones of the Amhara and Heri-Resu Zone of the Afar Regional States. The
Were-Ilu area occupies the southwestern block of the Afar Depression, where the Main
Ethiopian Rift (MER) gradually funnels towards into the wide Afar depression. The
southwestern Afar Depression merges southward with the northeast striking
Northern MER and eastward with the east-northeast striking Gulf of Aden. Northern
MER, the Southern Red Sea and western Gulf of Aden lie within the Afar Depression
forming a rift-rift-rift triple junction between the Nubian, Somalian and Arabian plates
(Wolfenden et al., 2004). abrupt, which Wolfenden et al. (2004) interpreted as the
termination of the Oligocene Red Sea rift.
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The lithologic units of the Afar Depression and marginal areas can be divided in to four
broad groups:
1) Pre-rift sequences consist of Neoproterozoic basement, Mesozoic sedimentary
rocks, and Eocene-Miocene basalts;
2) Miocene igneous rocks;
3) Pliocene volcanic rocks and
4) Quaternary volcanic rocks and sedimentary rocks.
The southwestern block where the Were-Ilu map sheet, is covered by Mesozoic
sedimentary rocks, Eocene-Miocene volcanics, Miocene volcanics of Dalha series and
Afar stratoid rocks, and Quaternary volcanic rocks and sediments. The Mesozoic
sandstone is exposed in the western part in the deeply dissected river gorges of the
plateau. In the Were-Ilu area Adigrat sandstone and marine sediments (Antalo
Formation) are absent and the continental clastic sediment exposed in the area is
referred to as Upper sandstone/Amba Aradom formation. They are comprised of
sandstone, shale and marl of probably Late Cretaceous age, which represents
regressive facies of Cretaceous Sea (Tefera et al., 1996 and the reference therein).
The Cenozoic volcanic rocks in the Were-Ilu area are represented by varying
proportions of basalts and bimodal basalts and silicic rocks spanning from Tertiary to
Quaternary.
On the basis of variations in texture, mode of occurrence and stratigraphic sequence,
the Tertiary volcanic rocks in the mapped area can be divided into four lithostratigraphic units, separated from each other by unconformity. These are:• Pre- Oligocene volcanics;
• Oligocene to Miocene volcanics;
• Early- to Mid-Miocene volcanics; and
• Upper Miocene volcanics
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1.7.2 Local Geology, Molale i-gnimbrite (Tmi)
This map unit has previously been mapped as Alajae-Molale sequence comprised of
basaltic flows and silicic volcanics made up of alkaline rhyolites and comenditic
ignimbrites (Zanettin et.al, 1974). In the Were-Ilu Map sheet the unit is separately map
the silicic volcanics in the mapped area, which are dominantly ignimbrite with
subordinate trachyte from the basaltic flows. The name Molale ignimbrite is given to
this lithological unit after the town of Molale, where the rocks are well exposed. In the
type area the ignimbrite is 34 m thick and consists of yellowish gray to gray, fine to
medium grained, slightly weathered and fractured ignimbrite with minor rhyolitic tuff,
and rhyolitic obsidian. It is exposed in the western and central southern part of the
Were-Ilu Map sheet, forming ridges and hills, river and road sections and hill slopes.
Backed contacts separate the unit from the underlying Alajae basalt. Plagioclase
phyric basalt horizon separates the unit from the overlying Tarmaber Megezez
basalt.
Figure 1.2 Geology of the Project Area
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The Molale ignimbrite is yellowish grey to grey, fine grained and slightly to moderately
weathered, fractured and columnarlly jointed. It has light grey, greenish grey, light
green and pink to grayish purple weathering color. At places, the rock is phenocrystsrich with quartz and k-feldspar crystals and rarely contains obsidian fragments.
Phenocrysts-rich varieties at Mehal Meda and Zemero areas contained opal. In the
western part of the area around Gebya-Dar ridge and Degolo village, the ignimbrite is
coarse grained with quartz and k-feldspar crystals. The proportion and grain size of
the quartz and k-feldspar crystals are increasing at Degolo.
1.8 Soil of the Project Site
The site is covered by rock outcrop, Geological formation along the river bank
upstream and downstream of the proposed bridge site also show the same surface
formation, particularly, medium grained slightly weathered & highly fractured
ignimbrite rock with a slight color tone variation is observed.
Figure 1.3 Soil Map of the Project Area
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2 INVESTIGATION PROCEDURE AND STANDARD CODE
PROVISIONS
2.1.1 PLANNING
A subsurface exploration program depends upon the type of structure to be built and
also upon variability of the strata at proposed site. Sub-surface explorations are
generally carried out in three stages.
i. Reconnaissance: Prior to our field exploration, DABEK engineer visually
evaluated the site and surrounding areas. His observations were used in
planning exploration, in determining area of special interest, and in relating site
conditions to know geologic conditions in the proposed project area. Subsurface
exploration program includes visit to a site and study the map and other
relevant records. The information about the following features is obtained:
ii. General topography of the site.
iii. Existence of underground water mains, power conduits, etc. at the site.
iv. Existence of settlement cracks in structure already builds near site.
v. The evidence of landslides, creep of slope and shrinkage cracks.
vi. The satisfaction of soil observed from deep cuts near the site.
vii. Depth of ground water table as observed in wells and drainage pattern.
viii. Type of vegetation existing at the site.
2.1.2 DRILLING
The boring was performed to maximum depth of 12.0m below the existing ground
surface elevations for borehole namely, BH01, however due to deterioration of
security condition at the site it was not possible to conduct BH02.
One boreholes were located at the drilling positions GPS Coordinate and there on
ground positions.
Table 2.1.: Borehole GPS coordinates
ID
EASTING NORTHING
station
573416.62
1141618.93
1+593
BH01
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Drilling was on Centerline of existing
bridge and started on existing back fill
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Figure 1.4 Bore hole location map
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Figure 1.5 FIGURE SHOWING RIVER CROSSING FOUNDATION MATERIAL FORMATION
Note; [Drilling is started on the fill, river depth from drill start elevation is about 5.8m]
The drilling operation was carried out using a core drilling machine. The methods
applied in the investigation were in compliance with the standards of the
American Society for Testing Materials /ASTM D2488/ for site investigation.
A continuous rotary core drilling technique was used in order to obtain high quality
cores. During drilling, complete geological materials reflecting the boreholes sections
were collected as precisely as possible. In dense and compacted layers, a wet drilling
technique was used starting with 116mm diameter with tungsten carbide bits fitted to
a single core barrel. In sound rock formations flash water drilling method was
employed.
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As soon as soil or rock cores are removed from core barrels in each run, they were
placed in standard partitioned wooden core boxes and described immediately on site.
Finally, the boxes are properly photographed.
As the site geology turns out to be rock, conducting Standard Penetration Test (SPT)
was not necessary, collection of Undisturbed Soil Samples (UDS) and Disturbed or
wash Soil Sample (DS).
Plate 2.1: Core drilling machine at project site
2.1.3 SAMPLING
Desired rock and soil samples were collected, where favorable geological layers are
encountered at depths shown on the log sheet of each bore hole and transported to the
main soil laboratory of the company in order to perform the required test.
Soil samples and rock samples were collected from core boxes and split spoon
samplers for laboratory tests. A total of 4 disturbed soil samples and 2 rock core
samples were collected for laboratory determination of engineering properties as
summarized in the following table.
Table 2.2 List of Field Sampling for the two Boreholes
rock core samples
Activities
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Quantity
2
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2.1.3.1 FIELD TESTING
As the site geology turns out to be dark brown clay soil and rock, conducting Standard
Penetration Test (SPT) was conducted for brown clay soil section only. The following
table summarizes the field coring and testing activities.
Table 2.3 List of Field Coring and Testing
Field Testing
Quantity
For Bore hole-1 advanced to maximum depth of 12.0m 1
Activities
Standard Penetration Test (SPT)
2.1.4 Ground Water
6
The ground water level is measured with a dip meter at the end and the starting of
each drilling days. It is also measured two days later from the end of the drilling works
until the water become static or constant level. The ground water level under BH-1and
BH-2 doesn’t exist within the drilled elevation.
2.1.5 Logging Procedures
In logging the exploration boreholes, a vertical profile is be made parallel with one pit
wall or boreholes. The contact between geological units is be identified and drawn on
the profile, and the units’ samples as recommended disturbed samples and IS: 8763 –
1978 (sand), IS: 10108 – 1982 (fine grained soil) for undisturbed samples.
Characteristic and type of soil or lithologic contacts is be noted. Variation within the
geologic unit are described and identified, and indicated on the borehole log wherever
the variation occurs. The sample locations is be shown in the respective log and their
location written on a sample tag showing the station location and elevation. Ground
water should also be noted on the exploration borehole log (is not observed in this
case).
2.2 LABORATORY TESTING
Laboratory testing entails the establishment of index and engineering properties.
Laboratory testing purpose is to quantify the index properties of the soil and thereby
allowing the soils to be classified according to defined rules. Soils named according to
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standard classification system have the same meaning to engineering and construction
professionals and can be used to estimate engineering properties for the design of the
foundation.
The following table summarizes the laboratory tests planned to be carried out for the
boreholes are summarized below. However, the actual lab tests conducted are decided
by the client.
Table 2.4 List of Laboratory Tests
1
Specific Gravity
2
Uniaxial compression of rock
Table 2.5 summary of laboratory test result on rock samples
No.
No.
2
2
Borehole
BH01
Depth of
sample, m
4.00
Bulk Specific gravity
(SSD)
2.09
Water
Absorption
1.64
Compressive Strength,
Mpa
BH01
10.00
2.28
1.80
42.06
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3 GEOTECHNICAL LAYERS
A. GREYISH TO PINKISH MEDIUM GRAINED SLIGHTLY WEATHERD & HIGHLY FRACTURED
IGNIMBRITE ROCK
In borehole number 01, from NGL to a depth of 5.0m a layer of a medium grained
slightly weathered & highly fractured ignimbrite rock with a slight color tone variation
is observed. .
Figure 3.1: BH – 01 Rock material from NGL to 5.0m depth
B. GREYISH TO PINKISH MEDIUM GRAINED SLIGHTLY WEATHERD & FRACTURED IGNIMBRITE ROCK
In borehole number 01, the material profile from a depth of 5.0m to 12m from NGL a
layer of a medium grained slightly weathered & fractured ignimbrite rock with a slight
color tone variation is observed.
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Figure 3.2: BH – 01 Rock material from 5.0m to 12.0m
3.1 ESTABLISHMENT OF DERIVED VALUES
The bearing capacity analysis will be made so as to recommend safe and economical
geotechnical parameters. The parameters comprise the bearing layer, footing level,
allowable bearing pressure and type of foundation.
The choice of a particular type of foundation depends on the magnitude of the
structural loads, the nature of the subsurface strata, the type of the superstructure and
its specific requirements. In terms of their seating depths within subsurface. These are
normally categorized as shallow and deep foundations. For reasons of economy,
shallow foundation is the first logical choice of a foundation unless it is considered
inadequate.
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The allowable bearing pressure is the maximum net intensity of loading that can be
imposed on the soil with no possibility of shear failure or the possibility of excessive
settlement. It is hence the smaller of the net safe bearing capacity (shear failure
criterion) and the safe bearing pressure (settlement criterion) that has to be
considered. As a result of this, considering the materials properties of the project sites,
the foundation grounds will be analyzed by taking into account their strength and
settlement characteristics.
3.2 Bearing capacity of rocks
The calculation of the bearing capacity in intact and heavily jointed rocks based on the
failure mechanism presented in Figure 3.5 can be carried out in a similar way as for
soil formations.
Figure 3.5. Failure mode of a homogeneous rock due to the loading of a spread footing.
Figure 3.6 Scheme of the failure wedges in rock.
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This is a simplified and conservative methodology based in active and passive wedges
defined by straight lines developing in the rock beneath the foundation (Figure 3.6).
With the increase of the applied load, and as it approaches the maximum capacity of
the rock, cracks begin to appear and progressively growing forming wedges and zones
of crushed rock. This condition result in dilatancy of the rock and in the formation of
radial cracks that expand to the exterior and may reach the surface
For a footing of “infinite” length (L»B) built in a horizontal rock surface, it is assumed
that the rock beneath the foundation is in a compression state similar to a sample in a
triaxial test. The maximum principal stress in the wedge A (σ1A) is equal to the stress
transmitted by the foundation (q) if the weight of the rock is not considered. Wedge B
is also under a stress state similar to the one presented in a triaxial test with the
maximum principal stress (σ1B) acting horizontally and the minimum principal stress
(σ3B) acting vertically.
If the footing rests at the surface then σ3B is null, otherwise it would be equal to the
vertical stress correspondent to the dead load of the rock weight above the base level
of the foundation. When the wedges fail, the minimum principal stress in wedge A
(σ3A) is the uniaxial compressive strength of wedge B which corresponds to the
uniaxial compressive strength of the rock mass. Using the Hoek-Brown (HB) (Hoek &
Brown, 1980) failure criterion the uniaxial compressive strength of the rock mass can
be determined by the following expression:
where mb, s and a are the parameters for the HB criterion; σc is the uniaxial
compressive strength of the intact rock and σ1 and σ3 are, respectively, the maximum
and minimum principal stresses. The following equation allows computing the maximum principal stress acting in wedge A (σ1A). The minimum principal stress in the
same wedge (σ3A) is the strength of the rock in wedge B and it is equal to the uniaxial
compressive strength of the intact rock when σ3b is null. In this case, the uniaxial
strength of a jointed rock mass can be approximated by the following expression:
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Table 3.1. Values of Cf1
Shape of the foundation
Cf1
Rectangular (L/B > 6)
1
Rectangular (L/B = 5)
1.05
Rectangular (L/B = 2)
1.12
Square
1.25
Circular
1.2
and the bearing capacity is equal to the maximum principal stress in wedge A, which is
given by:
This equation can be rewritten in the following form:
Where,
The formulation presented by equation above represents the bearing capacity as a
fraction of the uniaxial compressive strength of the intact rock. The values of Nσ,
defined as the bearing capacity factor, can be computed using equation shown below.
The allowable bearing capacity (qa) is related with the strength of the rock mass by
means of a safety factor (SF) and a correction factor related with the foundation shape
(Cf1) (Table 3.1):
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In most of the cases, values between 2 and 3 are considered for the SF. These values
assure a small risk of achieving high settlements; a safety factor of 4.0 is adopted in
this project as we have based our recommendation based on one borehole and
geological interpretation of the area.
In the case of foundations built at a certain depth, equation 6 must be rewritten so that
it considers the increase on σ1 as a result of the confining stress qs applied at the
surface. The minimum principal stress (σ3B) is equal to qs and the allowable bearing
capacity is then given by the following equation:
Where,
Hoek et al. Suggested the following equations for calculating rock mass constants (i.e.
mb, s and a); Once the Geological Strength Index has been estimated, the parameters
which describe the rock mass strength characteristics, are calculated as follows:
For GSI > 25, i.e. rock masses of good to reasonable quality, the original Hoek-Brown
criterion is a pplicable with
and
a=0.5
For GSI < 25, i.e. rock masses of very poor quality, the modified Hoek-Brown criterion [14]
applies with
s=0
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The Geological Strength Index (GSI), introduced by Hoek and Hoek, Kaiser and Bawden
provides a system for estimating the reduction in rock mass strength for different geological
conditions. This system is presented in Figure below.
Figure 3.7; Estimate of Geological Strength Index GSI based on geological descriptions
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Table 3.2. Values of the constant mi for intact rock, by rock group.
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3.3 Determination Using Hoek-Brown Failure Criterion
A drilling operation was carried out at one of the abutments (Tor Mesay Side), BH01, to a maximum
depth of 12.0m below the existing ground surface elevation of the borehole. However, the proposed
drilling at the opposite (Mehale Meda side) side abutment, BH-02, was not possible due to security
concerns in the area. Since the geological formation along the river bank both upstream and downstream
of the proposed bridge site has shown consistency with the drilling output at BH-01, the borehole data
analysis of BH01 applied to both abutments. It is important to note that the site is homogeneous in both
abutments as shown in figure below.
BH-1, ZONE-1; Depth 0.0m to 5.0m
Parameters
Values
Intact uniaxial compressive strength (σc, MPa)
35
Geological strength index (GSI)
25
Hoek–Brown constant of intact rock (mi )
17
Disturbance factor (D )
0
Hoek–Brown constant of rock mass (m b)
1.16723992
Hoek–Brown constant of rock mass (s )
0.00024037
Hoek–Brown constant of rock mass (a )
0.5
Strength of rock mass (σcmass, MPa)
5.28
BH-1, ZONE-2; Depth 5.0m to 12.0m
Parameters
Values
Intact uniaxial compressive strength (σc, MPa)
42
Geological strength index (GSI)
36
Hoek–Brown constant of intact rock (mi )
17
Disturbance factor (D )
0
Hoek–Brown constant of rock mass (m b)
1.72892367
Hoek–Brown constant of rock mass (s )
0.00081599
Hoek–Brown constant of rock mass (a )
0.5
Strength of rock mass (σcmass, MPa)
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3.4 Allowable Bearing Capacity Determination
Taking in to account of confining pressure, for the various depths allowable bearing capacity at
various depths and foundation proportion is provided here under
BH-1, ZONE-1; Depth 0.0m to 5.0m
density of
the rock (γ)
qs
σ'3
Hoek–Brown Hoek–Brown
Geological Hoek–Brown
Hoek–Brown
constant of constant of
(σc,
strength
constant of
constant of
rock mass
rock mass
MPa) index (GSI) intact rock (mi) rock mass (mb)
(s)
(a)
foundation
depth, m
L/B shf
20.00
0.01
1.27
3.00
20.00
0.01
1.44
4.00
20.00
0.01
1.59
5.00
2
5
6
1
1.1
1.1
35
35
35
25
25
25
17
17
17
1.167239916
1.167239916
1.167239916
0.00024037
0.00024037
0.00024037
qa
0.5
0.5
0.5
2.13
2.40
2.71
Hoek–Brown Hoek–Brown
Geological Hoek–Brown
Hoek–Brown
constant of constant of
(σc,
strength
constant of
constant of
rock mass
rock mass
shf MPa) index (GSI) intact rock (mi) rock mass (mb)
(s)
(a)
1
42
36
17
1.728923669 0.00081599
0.5
1.1
42
36
17
1.728923669 0.00081599
0.5
1.1
42
36
17
1.728923669 0.00081599
0.5
qa
3.95
4.28
4.74
BH-1, ZONE-2; Depth 5.0m to 12.0m
density of
the rock (γ)
qs
σ'3
foundation
depth, m
21.00
0.01
2.44
5.20
21.00
0.01
2.58
6.00
21.00
0.01
2.75
7.00
L/B
2
5
6
Considering excavation or removal of overburden, conservatively removing confining pressure for the
various depths allowable bearing capacity at various depths and foundation proportion is provided here
under
BH-1, ZONE-1; Depth 0.0m to 5.0m
density of the
rock (γ)
qs
σ'3
foundation
depth, m
L/B
Geological
strength index
(σc, MPa)
(GSI)
shf
Hoek–Brown
constant of
intact rock
(mi)
Hoek–Brown
constant of
rock mass
(mb)
Hoek–Brown
constant of
rock mass (s)
Hoek–Brown
constant of
rock mass (a)
qa
20.00
0.01
0.54
-
2
1
35
25
17
1.167239916
0.000240369
0.5
1.32
20.00
0.01
0.54
-
5
1.05
35
25
17
1.167239916
0.000240369
0.5
1.39
-
6
1.12
35
25
17
1.167239916
0.000240369
0.5
1.48
Geological
strength index
(σc, MPa)
(GSI)
Hoek–Brown
constant of
intact rock
(mi)
Hoek–Brown
constant of
rock mass
(mb)
Hoek–Brown
constant of
rock mass (s)
Hoek–Brown
constant of
rock mass (a)
20.00
0.01
0.54
BH-1, ZONE-2; Depth 5.0m to 12.0m
density of the
rock (γ)
qs
σ'3
foundation
depth, m
L/B
shf
qa
21.00
0.01
1.20
-
2
21.00
0.01
1.20
-
5
1.05
42
36
17
1.728923669
0.000815988
0.5
2.79
21.00
0.01
1.20
-
6
1.12
42
36
17
1.728923669
0.000815988
0.5
2.97
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17
1.728923669
0.000815988
0.5
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3.5 Bearing layer and Bearing capacity recommendation
A bearing layer for abutments can be considered to be at a depth of 3.0m for Abutment at BH-01, in the
medium grained slightly weathered & highly fractured ignimbrite rock. Since drilling at the opposite side
of abutment cannot be made due to deterioration of security condition at the area, our recommendation
is based on borehole made at one side of the bridge and geological observation and interpretation.
The proposed bridge site and the surrounding areas are mainly constituted by rock outcrops. The
geological formation along the river bank upstream and downstream of the proposed bridge site has also
displayed consistency with the drilling output at BH-01. Therefore, it is assumed that both abutments
have similar subsurface conditions. To account for uncertainties, a safety factor of 4 has been adopted to
provide a more conservative allowable bearing capacity. Based on this, an allowable bearing capacity of
1.3 MPa can be assumed at a depth of 3.0m below NGL. As a final recommendation, the geologist/
geotechnical engineer on-site is advised to compare the surface formation of the two abutments during
excavation for any inconsistencies during construction..
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3.6 BEARING CAPACITY DETERMINATION Using Ethiopian Building
Code Standard (EBCS-7, 1995)
The information gathered from the borehole profile along with field test result,
Ethiopian Building Code Standard EBCS-7 provides presumptive values for rocks. For
the project rock type the group falls in group -2 with closely spaced jointing and
applying a 50% reduction as per the recommendation for open jointing the value is in
the range of 2Mpa to 5Mpa.
As an indicative comparison the result obtained from EBCS -7 is vital but the result
obtained form detail analysis is adopted for design.
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4 Seismicity of the project area
The methods of assessing likely earthquake intensity and frequency at a given site are
complex, requiring reasonable judgment and collection of geological and seismic data.
Due to this complexity for the structures with lesser magnitude, the tendency is to rely
upon seismic risk maps. The maps are often published in notional or state building
codes, which recommend the engineering precaution to be taken in each rank of
hazard in the map.
Basic information has been taken from Ethiopian Building Code standard, Ethiopia is
divided into zones of approximately equal seismic risks based on the known
distribution of past damaging earthquakes. By definition, the hazard with in each zone
can be assumed constant.
Figure 4.1. Seismic Hazard map
From the seismic hazard map of Ethiopia, the project site falls under zone 4, which
corresponds, to moderate damage of intensity VII.
This map is based on the
amplitudes to be expected during 100 years return period. Based on seismic design
code of Ethiopia (ES EN 1998:2015) design ground acceleration 0.2g (for Zones, 4).
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5 CONCLUSION AND RECOMMENDATION
The following conclusions are drawn as the result of the drilling investigation and
walk over survey.
The following conclusions are drawn as the result of the drilling investigation and
walk over survey;
✓ A single boring was carried out at the (Tor Mesay Side) abutment (BH01) with
drilling reaching a maximum depth of 12.0m below the existing ground
surface elevations. Unfortunately, drilling could not be carried out for BH02,
(Mehale Meda side) abutment due to security concerns at the site.
Based on the results of the single borehole and geological observations, we
have recommended a course of action for the project. The site is surrounded
by rock outcrops, and the geological formation upstream and downstream of
the proposed bridge site is consistent with the drilling information obtained
at BH-01. This suggests that both abutments have a similar subsurface
condition.
However, to account for any uncertainties, we have adopted a safety factor of
4 to provide a more conservative allowable bearing capacity. As a final
recommendation, we advise the geologist/geotechnical engineer at the site to
compare the surface formation of both abutments during excavation to
identify any inconsistencies during construction.
✓ The structure shall have its foundation level at a depth of 3.0m for both
Abutments, in the medium grained slightly weathered & highly fractured
ignimbrite rock., or below the depth of scour as to be determined from
hydraulic design, based on the design requirement for the highway vertical
alignment foundation could be situated at a selected level. For the design of
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the bridge foundation, allowable bearing capacities proposed as 1.3Mpa,
disregarding confining pressure shall be adopted.
✓ Other considerations like keying foundation in the bed rock shall be as per
the requirement of ERA-2013 Geotechnical design manual.
✓ The foundation rock should be as horizontal as possible
✓ The site is free from surficial visible geological structure which would affect
the proposed bridge.
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ANNEX-1: BORELOGS
Project Number:
DAB/GI/2023/136
Drilling Contractor:
1+593
YAREGAL MOLA
Drill Crew:
DABEK ENG
Dabek Ticket Number:
Started: 20-Nov-23 Bit Type:
Backfilled:
Graphic Log
SPT Blow
Counts
(blows/t)
Sample Number
N/A
Sample Type
Drill Rig Type:
Diameter: 89 mm
Completed:
30-Nov-23 Hammer Type:
Groundwater Depth:
EAST
NORTH
Hammer Weight: 65 KG
Hammer Drop:
Elevation:
Total Depth of Boring:
37 P37
12.0 M
Lithology
Soil Group Name: modifier, color, moisture,
density/consistency, grain size, other descriptors
Rock Description: modifierm color, hardness/degree of
concentration, bedding and joint characteristics, solutions,
void conditions.
0.00 m
GREYISH TO PINKISH MEDIUM GRAINED
SLIGHTLY WEATHERD & HIGHLY
FRACTURED IGNIMBRITE ROCK
3.40- 3.58 m RCS
5.0m
6.50- 6.68 mRCS
GREYISH TO PINKISH MEDIUM GRAINED
SLIGHTLY WEATHERD & FRACTURED
IGNIMBRITE ROCK
12.0CM
Dabek Engineering P. L. C.
Boring Log: Sheet __of ___
Standard Penetration Slit Spoon Sampler (SPT)
California Sampler
StabIlized Ground water
Shelby Tube
CPP Sampler
1
Boring No.
Groundwater At time of Drilling
Bulk/ Bag Sample
Additional Test
STION
Logged By:
Date 08/20/2023
Address, MEHAL MEDA
Depth (m)
ZHONGMEI
ENGINEERING
GROUP LIMITED
Client:
Moisture
Content (%)
MEHAL MEDA - GISHE
RABEL - MEKOY - MILA
MILLE ROAD PROJECT
Dry Density
(g/cc)
Project
ANNEX-2: CORE BOX PICTURES
BOREHOLE – 01
Date 30/11/2023
BOX-1
Box-2
Box-3
ANNEX-3: LABORATORY
TEST RESULTS