bahir libo small scale diversion irrigation project

33
Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

Upload: others

Post on 18-Mar-2022

15 views

Category:

Documents


0 download

TRANSCRIPT

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

Table of Contents 1.Introduction ............................................................................................................................................... 4

1.2 General ................................................................................................................................................ 6

1.2 Objectives ........................................................................................................................................... 8

1.3Methodologies and Approaches .......................................................................................................... 9

1.4 Materials Used .................................................................................................................................... 9

2. Geology of the Study Area ...................................................................................................................... 13

2.1 Regional Geology .............................................................................................................................. 13

2.3 Local Geology .................................................................................................................................... 15

2.3.1 Basaltic Agglomerate ................................................................................................................. 15

2.3.2 Unconsolidated Deposit ............................................................................................................. 15

3. Seismicity Condition of the Area ............................................................................................................. 16

4. Engineering Geological and Geo-technical Investigation ....................................................................... 18

4.1 Headwork Site ............................................................................................................................... 18

4.1.2 Left Bank and Right Bank ........................................................................................................... 18

4.1.3 Stream Bed ................................................................................................................................. 19

4.2 Main Canal Route .............................................................................................................................. 21

4.3 Natural Construction Materials ........................................................................................................ 25

4.3.1 Rock Quarry ................................................................................................................................ 25

4.3.3 Fine Aggregates .......................................................................................................................... 26

4.3.4 Canal Fill Material ...................................................................................................................... 28

ANNEXTURE - 1 TEST PIT LOGS ................................................................................................................... 30

ANNEXTURE-2 LABORATORY TEST RESULTS .............................................................................................. 33

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

List of Figures

Figure 1, Seismic Risk Map of Ethiopia, 99 Years of Return Period. ..................................................... 17

Figure 2, Right Bank Figure 3, Left Banks ....................... 19

Figure 4,Thick unit of alluvial deposit and test pit taken on the stream bed of Bahir libo at the weir axis. .................................................................................................................................................................... 20

Figure 5, Geological X_ Section of Bahir Lib headwork axis ................................................................... 21

Figure 6, Test pit(B.libo M.cT.p_1) taken along the prposed main canal route .................................... 22

Figure 7, Nature of G_1 Figure 8, Nature of G_2 ..................... 23

Figure 9, Clearly exposed linearly joined rock along the proposed main canal route ......................... 24

Figure 10, Test Pit taken(B.libo M.cT.p_2) along proposed main canal route ...................................... 25

Figure 11, Rock Quarry Site for Masonry Stone and Crushed Aggregates ............................................ 26

Figure 12, Sand Sources in Mena Stream ................................................................................................ 28

Table 1, Short summary of Mena sand laboratory result ....................................................................... 27

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

BAHIR LIBO SMALL SCALE IRRIGATION PROJECT

Volume II: ENGINEERING GEOLOGY

Sept, 2016

Bahir Dar

Client: Amhara National Regional State Water, Irrigation And Energy Development Bureau

(BoWIED)

Address:

P. O. Box: 88

Telephone: 0528-200853/855

Fax: 251-08-20-65-68/204676/202040

Consultant: Amhara Design & Supervision Works Enterprise

(ADSWE)

Address:

P. O. Box: 1921

Telephone: +251-582-181023/ 180638/181201/181254

Fax :( 058) 2180550/ (058) 2180560

E-mail: amhara [email protected]

Bahir Dar, Ethiopia

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560

List of Reports

Volume I: Watershed Management

Volume II: Engineering Geology

Volume III: Irrigation Agronomy

Volume IV: Engineering Design

Volume V: Socio Economy Study

Volume VI: Environmental Impact Assessment

Volume VII: Financial and Economic Analysis

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 6

1.Introduction

1.2 General

The study and design work of Bahir Libo Small Scale Irrigation Project is being carried out

by Amhara Design and supervision works Enterprise (ADSWE). The owner of the project is

Amhara Region Bureau of Water, Irrigation and Energy Resource development.

It is known that agriculture is the back bone of our economy. Hence to maximize

agricultural production for the domestic use and to supplement industry lead economy,

irrigation is the non-alternate option. So, it will assist rain fed agriculture and alleviate the

impact of recurrent draught in the country. This can be achieved by working together with

the community, local and international NGOs and the government organizations so as to

use all the available resources efficiently and bring significant change.

Amhara design and supervision Works Enterprise (ADSWE) is established to do its part in

the study, design and supervision of water structures, in the area of land use and water

shed management, irrigation and water supply of the region as well as the country. ADSWE

has assigned a multidisciplinary qualified professional crew to study and design on the

Irrigation projects.

The project Woreda, East Belesa Woreda, is one of the draught prone Woreda in the

Amhara Region. Bahir Libo Small Scale Irrigation Project is designed based on the client’s

requirement to address the beneficiaries demand. It is believed that this irrigation project

will change the living standard of that society in supplementary irrigation as well as in full

irrigation system. Technical, environmental and socio economic issues are well addressed

by the respective disciplinary. The potential beneficiaries’ show their interest in the

execution of the project and the scheme after execution is supposed to be handed over to

the farmers cooperatives for operation and maintenance of canal system and water

regulation.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 7

For the design of various stable and cost effective engineering structures in different parts

of the project, the areas were investigated to gather engineering geological and

geotechnical input data. This engineering geological feasibility study was carried out from

April 7 to 15, 2008EC.

Geotechnical investigations are performed to evaluate those geologic, and soil conditions

that affect the safety, cost effectiveness, design and excavation of the proposed project. The

fact that insufficient, faulty interpretation of results, or failure to portray results in a clearly

understandable manner may contribute to inappropriate designs, delay in construction

schedules, costly construction modifications, use of sub-standard borrow material,

environmental damage to the site, post construction remedial work and even failure of a

structure and subsequent litigation. Hence, geotechnical investigation is usually thought of

as the backbone of engineering structure design.

Investigations performed to determine the geologic setting of the project include the

geology (nature and behavior of rock materials), soil and rock behaviors, and seismic

conditions. This parameters influence the construction site selection through the control of

the characteristics of the foundation soil and rocks, geotechnical conditions, project safety,

design, and construction, critical geomorphic processes and source of natural construction

materials.

Based on the above facts, the site selection criteria and factors that should be taken in to

account in studying the diversion site are as listed below:

Nature of subsurface material at the diversion axis and potential cannel route with the

fact that they should be sound and resistant to bear the load imposed by the weight of

the overlying materials, including the structures to be built.

Suitability (shape and depth) and stability of the valley slopes at the abutment and

capability to bear the lateral pressure imposed by the nearby soil material.

Nature of the abutment ground material in reaction with water (solution resistant rock,

soil, or nature of clay material).

Uniformity of geological formations, geological structures and their continuity.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 8

Topographic conditions of the river both upstream and downstream of the anticipated

weir axis.

Relation of the anticipated diversion axis with the potential canal route and the

available command area.

Availability and quality of naturally occurring construction materials within an

economically justified distance from the project site.

1.2 Objectives

The study is focused to give sufficient pertinent information on the geologic formations and

their suitability for proposed construction. Consequently, the specific objectives to be

addressed within this study are:

Developing overall approach on the geological parameters through investigation of

surface and subsurface geological conditions of the site to provide geotechnical inputs

needed for the design of the diversion structure and main canal, so that the project

becomes cost effective, durable and socially acceptable.

Explaining the local geology and developing findings to the rock units and structural

patterns on the context of the local area of interest.

Locating the specific sites for the construction of the headwork and potential canal

routes, which are resolute on the basis of:

Determining the geotechnical conditions of the headwork axis and potential canal

routes.

Defining the distribution of formations in space.

Suitability of the land and stability of the foundation material for the construction

and establishment of stable water tight zone.

Strength, workability and permeability of the existing formations.

Providing pertinent and reasonable information for post survey data analysis in the

design and construction of the proposed structures.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 9

Determining the qualities and identifying the sources of naturally available

construction materials within an economically feasible distance from the construction

site.

Defining ground water condition and its influence on the structures.

1.3Methodologies and Approaches

A technical and systematic procedure of geotechnical investigation includes:-

Taking Test pits and augur holes along the potential canal routes, headwork axis, and

taking representative samples from them for laboratory testing,

Observing surfaces, river cuts and gully exposures to identify the nature of lithological

units and their depositional history by making geological traverses.

Making several options both upstream and downstream of the headwork axis to select

the best diversion site and include maximum irrigable command area, so that the

project will be cost effective, durable and sanctioned by the local communities.

1.4 Materials Used

To facilitate the geotechnical feasibility study assigned for the project work and to address

the objectives set for it, the following materials were used during field investigation.

Measuring Tape meter

Handheld GPS

Digital camera and Sample Bags.

1.5 Location and Accessibility

The project area is located in East Belesa Woreda of North Gondar Zone, Amhara Region.

The head work site and command area are located in same kebele, Goga Kebele. The head

work site is located in sheha Got.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 10

The project Head work site is accessible by vehicle via 85km asphalt road from the regional

city Bahir Dar through Addis Zemen, 84km all weather road from Addis Zemen to Guhal,

East Belesa Wereda capital town and 30km all weather road from Guhala to Project Kebele.

The head work site and the Main command area is not accessible by road but on foot. The

head work is 3km far from the access road.

The geographic coordinate of the position of the head work site is 411036 Easting,

1376602m Northing, and 1647m Altitude in UTM_ADINDAN Coordinate system. The

command area lays between 11376444.837 & 1378190.97 North and between 410939.737

& 413920.18 Easting. (See fig 1.1 & 1.2). The command elevation also ranges from 1660.4

to 1598.5.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 11

Figure 1,Project Woreda Location map

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 12

Figure 2, Project Area Location map

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 13

2. Geology of the Study Area

2.1 Regional Geology

The Ethiopian volcanic province is characterized by two volcanic series: the Tertiary Trap

Series of the high plateau and the recent (Pliocene-Quaternary) Aden volcanic series.

Contemporaneous with the major uplift of the Horn of Africa, extrusion occurred from

fissures and centres of immense quantities of flood lavas (Traps). These lavas covered the

greater part of the Mesozoic rocks in Ethiopia.

The Trap Series consists of a very thick succession of lava flows, chiefly flood basalts

(Traps), but with Trachytes and Rhyolites occurring especially near the top of the Series.

The Trap Series essentially pre-dates the Rift faulting and therefore forms the great heights

of the Ethiopian plateau. The Aden Volcanic Series post-dated the Rift faulting and is largely

confined to the floor of the Rift System where it lies on the Trap Series (Mohr, 1962).

According to Blandford (1869), three volcanic units are distinguishable: the Trap Series of

the plateau subdivided into the Ashangi and the Magdala groups; and the Aden Series. The

Ashangi group includes predominantly basaltic rocks while the overlying Magdala group is

essentially Silicic. Young (Quaternary) volcanic rocks were designated as the Aden Series

(Kazmin, 1962).

There are three volcanic cycles for the north eastern part of the western plateau, Zanettin

and Justin-Visentin (1975):

First cycle: pre-Oligocene volcanic activity giving rise to the outpouring of the

Ashangi basalts that cover the upper sandstone unit.

Second cycle: after a long quiescence, the second volcanic cycle began to erupt in

the lower middle Oligocene and cover the Ashangi penplain. This cycle started first

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 14

with flood basalts (Aiba basalts) and later with large quantities of ignimbrites

(alkaline rhyolites and trachytes) with variable quantities of interbedded basalts,

collectively known as the Alaji rhyolites. This cycle culminated in the emission of

large quantities of basalt, termed Tarmaber basalts, that cover most of the Alaji

rhiolites.

The third cycle is confined more to the rift and its periphery and also to the Lake

Tana Area of Plio-Pleiostocene age.

The Trap Series, consisting of Asahngi, Aiba, Alaji and Tarmaber formations, make up the

regional rock units from bottom to top in the north eastern part of the western plateau.

The Ashangi formation is tholeiitic and/or basalts (Zenettin et al., 1978). The Aiba basalts,

which overlie the lower Ashangi basalts unconformably, are transitional between tholeiitic

and alkaline basalts. The Alagi rhyolites and basalts, which are a product of fissural

volcanism, are dominantly Silicic and are made up of alkaline rhyolites and Sodic

ignimbrites intercalated with variable quantities of transitional basalts. Tarmaber basalts

are products of central eruption and are alkaline in composition.

The Tarmaber Series consists of lenticular basalts with a large amount of tuffs, scoriaceous

lava flows and typical red paleosoils (Merla et al., 1973). The Tarmaber basalt is a thick

sequence that reaches 1,000 m and when thicker, characterizes volcanic centres such as

Guna Terara. The Tarmaber basalts, which directly overlie the Ashangi basalts (in

northwestern Ethiopia), mark the Tertiary volcanic cycle in the region. Based on a few

sections from the eastern and southern part of the plateau, the above was assumed to be

valid until recent times for the entire western plateau. However, according to the recent

works of Pik et al. (1998), this stratigraphy is not valid for the whole north western

plateau. Furthermore, field observations of the group on well exposed escarpments suggest

the existence of a continuous lava sequence from the base to the top of the plateau, rather

than the piling of two or more successive and stratigraphically distinct units as suggested

for the north eastern part of the plateau.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 15

Based on this recent study in the north western plateau, from samples collected from

bottom to top of the lava pile in a section located north eastern of Gondar (near the study

area), the lavas mostly consist of basalts, except on the top of the plateau, where ignimbrite

tuffs, a few metres thick, are exposed.

2.3 Local Geology

The geology of the study area is characterized by unconsolidated deposit and basaltic

agglomerate. Brief description of these geological units is presented in the following

section.

2.3.1 Basaltic Agglomerate

The major part of the study area is covered with basaltic agglomerate rock type. It is

predominantly exposed at the left and right banks and the surrounding ridges near the

headwork site. It is mainly found as blocky form. However, in few localities namely in the

left and right banks, closely to widely spaced joints has been observed.

2.3.2 Unconsolidated Deposit

Most part of the stream bed are covered with unconsolidated deposit. The deposit has been

classified as alluvial on the basis of its origin.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 16

3. Seismicity Condition of the Area

Earthquake is the result of a sudden release of energy in the earth’s crust that creates

seismic waves. The seismicity of an area refers to the frequency, type and size of

earthquakes experienced over a long period of time.

To construct effective and long-lasting hydraulic structures, assessments on susceptibility

to seismic hazards are fundamental issues. Therefore, the degree of seismicity of the area

should be taken in to account with great emphasis as an important design parameter by

evaluating the seismic nature of the area from history and records of previous earthquakes

and knowledge of local geology. However the seismic susceptibility study is usually made

with the application of secondary data from the national geophysics and geo-observatory

data sources. The most famous edition in the Ethiopian context is the seismic susceptibility

map of the country by Laike Mariam Asfaw, 1986. According to the seismic hazard map of

Ethiopia, the country has been subdivided in to five seismic Zones as: Zone (o), Zone (1),

Zone (2), Zone (3) and Zone (4), the no hazard, the low hazard, the moderate hazard, the

higher hazard and the highest hazard zones, respectively.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 17

Figure 3, Seismic Risk Map of Ethiopia, 99 Years of Return Period.

Bahir Libo Project Site

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 18

4. Engineering Geological and Geo-technical Investigation

This part emphasizes on the details of engineering geology and geo-technical aspects of the

project focusing on strength and permeability characteristics of the underlying formations

over which proposed structures lies, mainly at headwork and along main canal route. In

addition to this, required natural construction materials' source areas and their suitability

conditions are discussed.

4.1 Headwork Site

For this project a simple weir diversion structure has been proposed for diverting the

water to the main canal. The headwork site geological surface and subsurface conditions

have been investigated based on the nature of the proposed structure. At the site and

immediate vicinity, the stream flows along moderate slope course and its bed is mainly

covered with thick layer of all sized recently deposited alluvial sediment. Such deposit

covers from bank to bank in downstream and upstream direction. On the other hand, the

right banks of the stream at the headwork site are made up of different geologic materials;

Both banks have well defined vertical well with moderately to highly weathered and

jointed basaltic agglomerate. This nature extend both in upstream and downstream

direction. Detail geologic nature of the banks, and bed of the stream along the headwork

axis and immediate vicinity are described and their potential geotechnical influence on the

proposed structures also discerned/detected below, with remedial measures.

4.1.2 Left Bank and Right Bank

The engineering geological foundation conditions at both banks are nearly similar, except

variation in extent or dimensions of the rock units. It has steep slope nearly vertical well defined

rock banks. The surfaces of both banks are covered with moderately to highly weathered and

jointed rock. These rock has a characteristic of light yellowish color coarse grain basaltic

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 19

agglomerate. These rock unit have steep slope nature with widely spaced joints with maximum

opening of 2cm. Beside to its jointed nature it undergoes moderately to highly weathering stage.

on the right bank, it has up to 5m height while in the left bank it reaches up to 8m. At about 7m

upstream, the stream channel minder with a bank of highly jointed and weathered nature. Hence

both banks are unstable and loss forming small caves. These weak and pervious banks are need

protection work in the headwork site up to 20m in upstream and downstream direction.

Figure 4, Right Bank Figure 5, Left Banks

4.1.3 Stream Bed

At the proposed headwork site the stream bed or course is well defined, nearly straight,

and minder at about 7m upstream from headwork site. Along the weir axis, the bed is

made up of one major geologic materials, as seen from surface observation. These are all

sized recently deposited alluvial deposit.

On the proposed weir axis and some kilometer upstream and downstream test the stream

had covered with thick unit of transported sediment. At the foundation test pit was taken

(B.Libo Fo.T.p_1), but due to in flow of water through these thick sediment test was

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 20

interrupted at shallow depth (0.5m). Due to these generally the structure are founded on

these thick alluvial sediment. Hence from test pit taken on the foundation area, sample are

taken to ADSWE soil laboratory for gradation test.

Figure 6,Thick unit of alluvial deposit and test pit taken on the stream bed of Bahir libo at the weir axis.

Since the structure are founded on these thick alluvial deposit, effective grain size (Silt

Factor) for such deposit are taken 3mm from gradation test of foundation sample.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 21

Figure 7, Geological X_ Section of Bahir Lib headwork axis

4.2 Main Canal Route

For this particular project, the main canal route has length of about 3.5km. Within this

length, the canal passes different geologic materials, which varies both surfacially and to

depth. The distribution of the geologic materials with respect to the canal route, and their

characteristics, especially permeability, stability and workability have been investigated

during the field work using both visual inspection and subsurface by taking test pit. About

2 test pit (B.libo M.cT.p_1 and B.libo M.cT.p_2) and Augur hole were bored at places where

there is geologic variation and no surface natural exposures/cuts are not found.

From simple visual examination of the route, two geological materials have been identified

at different reaches. The materials are:-

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 22

a. Thin layer of silty Clayey

b. Moderately to highly weathered rock

The distribution of these materials along the main canal and their geotechnical properties,

especially permeability, and workability are described below from the outlet point to end.

Hence for this project, a main canal has been proposed in the left side of the diversion structure.

From about out let to about chainage 1+480m, the ground is characterized by

relatively thin layer of Silty clayey soil (which ranges from 0.4 to 0.6m), as it can be

seen from test pit taken(B.libo M.cT.p_1) . The soil has light brown color. It is dry in

all depth. The soil possesses medium plasticity, with stiff consistency. Under this

unit highly friable weathered ash had encounter (see in test pit log ANXXTURE 1). It

had whitish color with pervious nature. This portion of the main canal route is a

farmland having moderate slope. Since the top soil is removed during excavation

and the rout lies on the underlying pervious rock, the canal should be designed as

lined.

Figure 8, Test pit(B.libo M.cT.p_1) taken along the proposed main canal route

On the above chainage (From out let up to a chainage of 1+480) there are two

gullies (G_1 and G_2) found on 417m and 805m from headwork site respectively.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 23

They have similar geological nature at the stream bed, with recently deposited

alluvial sediment dominantly fine sand and silt. The width of G_1 are wide and very

short bank relatively to G2. Banks of both gullies are light brown color, weak silty

clayey soil. The thickness of these soil in G-2 reach up to maximum of 1m as easily

see from natural cut. under that weathered and fragmented volcanic rock are

expected.

Figure 9, Nature of G_1 Figure 10, Nature of G_2

From chainage 1+480 up to 1+592m, the route pass on clearly exposed dark gray

color highly jointed rock. These rock have very narrowly spaced linearly aligned

joints with very tight opening. Hence these jointed rock have pervious nature

proposed main canal route along the chainage should be lined probably in masonry.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 24

Figure 11, Clearly exposed linearly joined rock along the proposed main canal route

From chainage 1+592m up to End, the ground is characterized by relatively Thin

layer of dark brown color Clay soil (average thickness of 0.9m), as it can be seen

from test pit taken(B.libo M.cT.p_2) . The soil has dark brown color. It is dry in all

depth. The soil possesses medium to highly plasticity. Under this unit highly

weathered and jointed rock had encounter (see in test pit log ANXXTURE 1). It had

dark gray color with pervious nature. This portion of the main canal route have

moderate slope and Since the top soil is removed during excavation and the rout

lies on the underlying pervious rock, the canal should be designed as lined.

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 25

Figure 12, Test Pit taken(B.libo M.cT.p_2) along proposed main canal route

4.3 Natural Construction Materials

During the site investigation, natural construction materials required for the construction

of the various proposed engineering structures at the headwork and within the farmland

have been assessed, and possible quarry sites and borrow areas have been identified

within the vicinity of the area as much as possible. In addition to the identification, the

quality, quantity, accessibility condition and ownership of each proposed production sites

have also been studied and described in this report; on separate sub-sections below. The

natural materials required for the construction of the proposed hydraulic structures

include rock for masonry stones, aggregates (both coarse and fine), impervious soil for fill

and/or lining, backfill soil, and water.

4.3.1 Rock Quarry

Quarry site that can be used for production of rock for masonry stone and crushed coarse

aggregates has been assessed during the field work session within the vicinity of the

project area at economic distance for hauling. One possible quarry site has been identified

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 26

within left side/bank of the headwork to about 200m and 650m downstream following a

continuous ridge bordering the left side of Bahir Libo stream. The rock is located in the

same Kebele, at Zenaboch and Sheha “Got” to the project respectively.

Figure 13, Rock Quarry Site for Masonry Stone and Crushed Aggregates

Naturally, this rock is strong as it is generally fresh except very thin cover of weathered

zones, which need to be removed during rock dressing and before crushing From this site,

rock of different sizes can be produced; ranging from large boulders to coarse gravels that

masonry stone and coarse aggregate can be produced easily.

4.3.3 Fine Aggregates

Borrow areas for fine aggregate or natural sand have been assessed starting from the

project stream itself. Natural deposits of such materials couldn’t be found when assessed

within the beds of the stream in the project area; rather very coarser sediments and rock

exposures are found covering almost the entire bed of the Bahir Libo stream. Seeing to this

nature of the stream, other distant streams have been explored to identify the best source

areas for fine aggregate or natural sand that can be used for this particular project. During

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 27

exploration of this natural sand, at a distant one stream was identified as a possible source

of fine sand. The stream is known as ‘Mena’. It is located at about 90km from the project

site, within Arbay belesa Woreda

Both visual investigation and laboratory testing of the sand deposit found in the proposed

area have been done. It has more than 60% of sand with almost nil percent of silt and clay

even if it has significant percent of Gravel. In the area good quantity, which suffices the

need of this project, is available. The deposit is dominated by sand sized transported

alluvial sediments, though some amount over sizes are also observed. The individual grains

of sand are mainly originated from basalt rock and reveals high crushing resistant that the

sand has good quality in this respect.

The samples have been analyzed at Amhara Design and Supervision material Testing

laboratory to characterize the gradation of the sand deposit. According to these laboratory

test results (See Annexture-2).

Table 1, Short summary of Mena sand laboratory result

Sand Sources Mena Pump_sample_1 Mena Pump_Sample_2

Gravel 37.69 20.05

Coarse Sand 58.32 61.14

Fine Sand 3.38 16.35

Silt and Clay 0.61 2.46

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 28

Figure 14, Sand Sources in Mena Stream

4.3.4 Canal Fill Material

Along the main canal route, there are some places where the original ground level is lower

than the designed canal bed level (CBL). In this case the OGL need to be filled till at least to

the CBL. For such fill, natural fine grained soil material having low to medium plasticity,

non-expansive and impervious is required. For this project such soil material

source/borrow area is identified at about Tire kebele at about 1.8km from the headwork

Specific Got of Bisrako

From visual examination of the borrow area, the soil has reddish brown color, and

composed of dominantly Clay with Gravel. It is residual soil having low plasticity and low

liquid limit. It is non expansive soil.

According to the Unified Soil Classification System, the soil group lies in Low plastic Clay

(CL). Such soil type possesses a maximum dry density (MDD) lies within 1.80gm/cc, and

the associated optimum moisture content (OMC) 17.53%. The construction of the fill can be

then done using these parameters

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 29

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 30

ANNEXTURE - 1 TEST PIT LOGS

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 31

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 32

Bahir Libo Small Scale Diversion Irrigation Project, Geological and Geo-technical Study Report

ADSWE, IRRIGATION & DRAINAGE P.O. BOX: 1921 TEL: 058--218--06--38/10 23 FAX: 058--218-0550/0560 Page 33

ANNEXTURE-2 LABORATORY TEST RESULTS