Geotechnical Investigation for Tawila Dam in North Darfur

بسم ه الرحمن الرحيم

University of Khartoum Faculty of Engineering Civil Engineering Department P. O. Box 321, P. Code 11111, Khartoum, Sudan

Tel. 0249-11-771577 E-mail: [email protected]

771577تلفون: سودان –خرطوم 321ص ب قسم الهندسة المدنية الهندسة كلية جامعة الخرطوم

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mailto:[email protected]

Geotechnical Investigation for By Dr. Magdi Zumrawi

Tawila Dam in North Darfur State University of Khartoum

1

بسم اه الرمن الرحيمبسم اه الرمن الرحيم

TTaabbllee ooff CCoonntteennttss

1 INTRODUCTION .......................................................................................... 2

1.1 General ..................................................................................................... 2

1.2 Study Objectives ...................................................................................... 2

1.3 Project Description ................................................................................... 3

2 FIELD INVESTIGATION ............................................................................ 5

2.1 Current Situation ...................................................................................... 5

2.2 Sampling .................................................................................................. 9

3 LABORATORY TESTING ........................................................................10

3.1 Atterberg limits ......................................................................................10

3.2 Grain Size Analysis ................................................................................10

3.3 Compaction Test ....................................................................................11

3.4 Permeability Test ...................................................................................11

3.5 Shear Strength Tests ...............................................................................11

4 RESULTS AND ANALYSIS ......................................................................12

4.1 Tests Results ...........................................................................................12

4.2 Soil Profile .............................................................................................15

4.3 Soil Bearing Capacity ............................................................................15

4.4 Stability of Spillway Section..................................................................16

4.5 Causes of the Dam Failure .....................................................................20

5 RECOMMENDATIONS .............................................................................21

5.1 Embankment Design Considerations .....................................................21

5.2 Seepage Control .....................................................................................23

5.3 Spillway Foundation ..............................................................................24

6 APPENDEX (A) ...........................................................................................25

7 APPENDEX (B)............................................................................................31



2

11.. IInnttrroodduuccttiioonn

1.1. GGeenneerraall

This is a detailed geotechnical investigation report that presents

the results of the field and laboratory investigations that carried out

for the damaged dam of Tawila in North Darfur State. After the dam

failure in July 2014, UNOPS has requested road and soil section,

university of Khartoum to undertake a technical review of the Tawila

Dam Rehabilitation Design Report. The dam was constructed in 1954

and by the early 1970s it was fully silted. It was subjected to

overtopping and a second spillway was installed to cater for presumed

increased runoff in the upper catchment. It was at this section that a

breach occurred. The dam has since not stored water and a new river

bed alignment has occurred.

The is investigation is carried out to provide an extensive

diagnosis study to point out the main structural and geotechnical

weaknesses of Tawila Dam. Clear recommendations are provided to

design a proper rehabilitation of the dam.

1.2. SSttuuddyy OObbjjeeccttiivveess

The following are detailed objectives of the study:

1) Determine the physical and mechanical properties of the

foundation soils and the embankment material at the dam site,

2) Establish a proper diagnosis of the current dam, after the

collapsing of the secondary spillway based on an extensive visual



3

inspection and the data set used to site and size the original dam,

the diagnosis had to emphasize the spillway section, embankment

and foundations structural stability.

3) To investigate the source of failures occurred in the dam

embankment and spillway section.

4) To enable an adequate and appropriate design for the new

rehabilitation work.

The investigation consisted of site reconnaissance, field

investigation and collection of soil samples. The laboratory testing

was conducted to provide the physical and mechanical properties of

the dam embankment and spillway foundation soils.

1.3. PPrroojjeecctt DDeessccrriippttiioonn

Tawila Dam is located about 70 km west of North Darfur capital

El Fasher town; it was constructed in 1954 by the State Water

Corporation. The dam is an earthen embankment (1.8 km long) across

the wadi, with two spillway structures (primary and supplementary).

The project area is located in the El ku basin of the North Darfur. The

river is one of the main flow contributors to wadi El ku. The project

area as shown in Figure 1 lies approximately between 269204 E,

1493145 N and 268047 E and 1494352 N.



4

Figure 1: The site location of Tawila Dam in North Darfur State

This dam is one of the interventions of Drought Risk Reduction

Project in North Darfur State. The project is to contribute in

increasing the products of rain-fed agriculture schemes through

reduction of water storage period and use of complementary

irrigation. As per the State Water Corporation sources, the design

capacity of the reservoir is 300,000 m3, however it has lost storage

volume due to silt accumulation. The primary objectives of Tawila

dam were:

Provide surface water storage for the surrounding farmers for their

living and agriculture.

Infiltration for ground water recharge.

Protection of the town from natural flood.

The dam suffered from water erosion and damages occurred in

Tawila Dam



5

the dam body. It is required to investigate the prevailing situation and

to make proper design for rehabilitation of the dam.

22.. FFiieelldd IInnvveessttiiggaattiioonn

The field investigation was comprised of a site visit to the dam

location at Twaila and collecting soil samples. The investigation

started with the site reconnaissance in order to collect information

data about the dam construction and how the failure occurred from the

local and the contractor to assess in investigating the source and

reasons of failures. The field work included also collection of soil

samples from the embankment and the spillways foundation soils.

2.1. CCuurrrreenntt SSiittuuaattiioonn

The author together with a hydrology expert and the contractor

had conducted a visual inspection of the dam in order to detect any

obvious structural defects in the spillways and the dam embankment.

It was observed that the secondary spillway structure and the adjacent

embankment of the dam have collapsed due to excessive seepage that

caused voids creation underneath the structure. The new constructed

section of the secondary spillway has severed failure due to shear

stress and washed away during rainy season. The reservoir is no more

holding water. As per the information for the locals, the dam failure

has occurred same year where the nearby secondary spillway was

constructed and this has suspected of seepage underneath the

embankment close to the spillway as shown in Fig. 2.



6

Figure 2: The total collapsed of the secondary spillway with severed

cracks due to shear forces

The partial breaching/weakening of embankment section is also

observed at the far north part of the embankment which could lead to

total collapse. This embankment weakening was happening mainly

due to scouring effect of the incoming waddy which flows adjacent to

the embankment section parallel to the dam axis. This scouring has

caused the embankment slope to get steeper and has increased the

waddy depth which has increased the embankment height which can

lead to failure. During rainy season, this steeper section was falling in

to the waddy when it get saturated and washed downstream which

further weakens the embankment section (see Figure 3).



7

Figure 3: Weak and steep slope of the embankment due to scouring.

In general, the embankment has changed it original section

dimension and shape throughout the entire length where the crest of

the embankment is narrowed and the side slopes are flattened some

sections and steeped at others.

In addition to breaching and weakening of the embankment, the

other concern observed in Tawila dam is sediment deposition in the

reservoir area. The sediment deposition has reduced the storage

capacity of the reservoir by significant volume. This is mainly due to

huge sediment load form the catchment is caused by degradation and

less vegetation cover. There is no any sediment releasing sluice gate

provision to release flush floods carrying high sediment load are

experienced in other dams in Darfur. The sediment deposition lead to

reduction in reservoir capacity as well as reduction in aquifer

recharging to augments the ground water (see figure 4).



8

Figure 4: The sediment deposition in the reservoir area.

It was observed as shown in Figure 5 that the secondary

spillway which was recently rehabilitated is in good condition

physically.

Figure 5: The secondary spillways in good condition



9

It can be concluded that the current situation of the dam

embankment and primary spillway structure were completely

damaged and washed away and the reservoir is no more holding

water. As a result, surrounding communities have hard living

conditions due to shortage of water and absence of agricultural

activities where farming was taking place in the reservoir area during

recession period and downstream of the dam while the reservoir is

full. Additionally, depletion of groundwater which resulted in drying

up many of hand dug wells in the surrounding area. Thus the main

objective of the study is to investigate the reasons for the dam failure

in order to make a proper rehabilitation.

2.2. SSaammpplliinngg

Soil samples were taken from the foundation depth of the

damaged spillways and from the embankment materials of the dam.

Disturbed soil samples (S1 and S2) were taken from the foundation of

the spillway at 4m and 7.5m depths and the sample (S3) was taken

from the embankment material adjacent to the damaged spillway. The

soil samples were kept in plastic bags, labeled and transported to the

soil mechanics Laboratory in Khartoum University for visual

inspection and testing.



10

33.. LLaabboorraattoorryy TTeessttiinngg

The Laboratory testing program was conducted to evaluate the

physical and mechanical properties of the soils encountered during the

site exploration. The testing procedures followed were in general

conformance with those recommended in the British Standard BS

1377 (1990) and the soils were classified according to the Unified

System for Classifying Soils (USCS). The laboratory tests performed

included the following:-

Atterberg Limits

Grain size distribution

Compaction test

Permeability test

Shear strength test

33..11.. AAtttteerrbbeerrgg LLiimmiittss

The tests performed were the liquid limit, LL and the plastic

limit, PL tests. These tests were carried out on fine soils passing sieve

no. 40. The soil samples tested, about three samples and the data

analysis are given in Appendix (A).

33..22.. GGrraaiinn SSiizzee DDiissttrriibbuuttiioonn

The wet sieve analysis tests were carried out on soil samples

taken from the site of the damaged spillway section. The tests

performed are three and the data analysis is given in the tables of

Appendix (A).



11

33..33.. CCoommppaaccttiioonn TTeesstt

This test was carried out using disturbed soil sample taken from

the embankment material. The test was carried out to determine the

maximum dry density and optimum moisture content for the soil

sample tested. Representative sample was taken from the

embankment soil adjacent to the damaged spillway was subjected to

standard compaction test. The test data analysis is given in Appendix

(A).

33..44.. PPeerrmmeeaabbiilliittyy TTeesstt

The “Falling Head” and "Constant Head" method were used to

measure the soil permeability. The permeability test was carried out

on the three soil samples. The sample taken from the embankment

material was compacted at optimum water content and maximum dry

density of the standard compaction test. The tests results are given

below in Table 2.

33..55.. SShheeaarr SSttrreennggtthh TTeessttss

The triaxial compression test was conducted on the clayey soil

samples taken from the embankment at the damaged spillway

location. While the sandy soils taken from the foundation depths of

the damaged spillway were subjected to shear box test. The tests were

carried out to determine the cohesion "C" and angle of internal

friction "ф" of the soils. The tests results are given in Table 2.



12

44.. RReessuullttss aanndd AAnnaallyyssiiss

4.1. TTeessttss RReessuullttss

The tests results of the soils samples (S1 and S2) taken from the

foundation of the spillway at depths 4m and 7.5m respectively and

also the sample (S3) from embankment material are summarized in

Tables 1&2.

It is important to note that the contractor was requested to

submit any tests results for the quality control during dam

construction. He submitted only some field density tests for the

compaction of the embankment layers (Appendix (B)).

Table 1: Classification Tests Results for the Dam Site.

Test Measured

Property

Spillway Structure (S1 & S2) Embankment

(S3) Depth 4m Depth 7.5m

Sieve

Analysis

Gravel 0 5 0

Sand 94 93 1

Clay 6 2 99

Atterberg's

Limits

LL

NP NP

57

PL 27

PI 27

Classification USCS SP SW CH



13

Table 2: Physical and mechanical tests results.

Property

Spillway Structure (S1 & S2) Embankment

(S3) Depth 4.0m Depth 7.5m

Friction angle (°) 0 4.5 12

Cohesion (Kpa) 40 40 62

Permeability (mm/sec) 9.5x10-2

1.5x10-1

3.8x10-6

Density (KN/m3) 15.0 16.1 14.2

Moisture Content (%) 3.4 2.7 24.5

The grain size distribution tests results for the three soils samples are

plotted in Figure 6.

0

10

20

30

40

50

60

70

80

90

100

0.001 0.010 0.100 1.000 10.000

%P

AS

SIN

G

PARTICLE SIZE(mm)

UNIVERSITY OF KHARTOUM FACULTY OF ENGINEERING

CIVIL ENG. DEPT. - ROAD & SOIL SECTIONProject: Tawila dam in North Darfur State Sample: Spillway Foundation Soil at 4m depth

Grain Size Distribution {Gravel = 0%, Sand = 94%, Silt / Clay = 6

SILTFINE MEDIUM COARSE

SAND

FINE COARSE

GRAVELCO

CLAY



14

0

10

20

30

40

50

60

70

80

90

100

0.001 0.010 0.100 1.000 10.000

%P

AS

SIN

G

PARTICLE SIZE(mm)


CIVIL ENG. DEPT. - ROAD & SOIL SECTIONProject: Tawila dam in North Darfur State Sample: Spillway Foundation Soil at 7.5m depth

Grain Size Distribution {Gravel = 5%, Sand = 93%, Silt / Clay =


SAND

FINE COARSE

GRAVELCO

CLAY

0

10

20

30

40

50

60

70

80

90

100

0.001 0.010 0.100 1.000 10.000

%P

AS

SIN

G

PARTICLE SIZE(mm)


CIVIL ENG. DEPT. - ROAD & SOIL SECTIONProject: Tawila dam in North Darfur State Sample: Embankment Material

Grain Size Distribution {Gravel = 0%, Sand = 1%, Silt / Clay = 99%}


SAND

FINE COARSE

GRAVELCO

CLAY

Figure 6: Grain size distribution for the three soils samples.



15

4.2. SSooiill PPrrooffiillee

The subsoil profile for the dam is determined based on the

visual inspection at the site and the tests results of the samples as

shown on plots of Figure 6. The profile generally reflected similar soil

stratifications with slight variations. The soil profile at the location of

the spillway structure consists of a thick layer of sand. At the top of

the layer of 4 m thick is fine Sand was encountered. This is followed

by fine to medium Sand was encountered at 7.5 m. Generally the

subsoil in the dam site is sand of non plasticity. This soil was

encountered mostly mixed with silt. The permeability of this soil is

high leading to water seepage under the spillway.

The whole dam section is made of homogeneous embankment

material. Investigation was conducted to assess the type of material

and its properties used on embankment construction. The construction

material for the embankment construction is clay soil of high

plasticity and very low permeability.

4.3. SSooiill BBeeaarriinngg CCaappaacciittyy

The bearing capacity of the spillway foundation soil at depth

7.5m can be determined using Terzaghi bearing capacity equation:

qu = CNc Sc + q Nq + 0.5 γ B N γ S γ

The foundation of the spillway structure was placed at 7.5m

depth on fine to medium sand. The average foundation soil properties

can be obtained from the tests results as given in Table 2:

Average bulk density (γ) = 16.1 KN/m3



16

Soil cohesion (C) = 4.5 KN/ m2

Angle of internal friction (ф) = 40◦

Foundation depth = 7.5m

Foundation width = 3m

The bearing capacity coeff. : Nc = 34.9, Nq = 20.5, N γ= 18.8

Vertical surcharge pressure q = 16.1* 7.5 = 120.8 KN/ m2

Foundation shape coeff. Sc = 1.3, S γ = 0.80

By substituting the above properties in Terzaghi’s equation, the

ultimate bearing capacity qu = 3285 KN/m2. By using factor of safety

equal 5, the allowable bearing capacity qa= 657 KN/ m2.

4.4. SSttaabbiilliittyy ooff SSppiillllwwaayy SSeeccttiioonn

The main and secondary spillways of Tawila dam are checked

for stability against overturning and sliding. The main spillway cross

section is shown in Figure 7 below.

Figure 7: Cross section of main spillway



17

The weight of materials

Concrete Masonry Sand ( sand ) Sand (sat ) Clay (C)

2250 Kg/m3

2640 Kg/m3 19.6KN/m

3 16.0KN/m

3 40KN/m

2

Centroid of spillway section 321

332211 ***

AAA

rArArAr

Area of spillway section

264.32.1*)5.021.1(21.1*2

2.09.0*5.05.0*)2.09.0(5.0*75.0 mAtotal

mr 00.164.3

05.2*2

5.021.167.0*21.1*

3

255.0*

2

5.021.138.0*)

2

5.021.1(

Normal condition

Vertical forces forceUpliftWF masonryV

KNFV

09.5605.3*71.1*81.9*2

19.25*64.3

Horizontal forces SDSUHForceessureForceessureF // PrPr

Resultant force KNFFF HVtesul 85.6455.3209.56 2222

tanRe

Net Moment ckWcountercloclockWo MMM

KN

hhhhF SDsatSDmasonrySUWSUsatH

55.325.0*16*2

15.0*9.25*

2

195.0*81.9*

2

11.2*16*

2

1

*2

1*

2

1*

2

1*

2

1

2222

2

/

2

/

2

/

2

/



18

KNm

M CClockW

45.106

3

7.0*7.0*6.19*

2

15.0*5.0*9.25*7.0

3

1*5.00.1*9.25*64.3 22

KNm

M ClockW

09.68

2

1*95.0*95.0*81.9*

2

1

2

1*1.2*1.2*16*

2

15.*71.1*05.3*71.1*81.9*

2

1 22

KNmMMM ckWcountercloclockWo 36.3809.6845.106

mFR

MxFRxM net

net 59.085.64

36.38*

The centroid of the resultant force passes through one third of

the bottom thickness of the spillway which insures that the dam

section is safe.

Forces acting on a subsurface dam

Moment resisting over turning=

KNmM r 46.106.

Moment causing overturning= rupliftrPFM SUo **/

KNmM o 09.68

Factor of safety against overturning

!56.109.68

45.106ReSafe

momentgOverturnin

MomentsistingFSO

SDSsandDSUSsandUmasonrymasonryr rWrWrWM //// ***



19

Factor of safety found against overturning is much bigger than

the minimum value of factor of safety required. This however ensures

that the height of the structure can be extended (rise up) to increase

the spillway height of the dam. The increased pressure force will be

resisted by the spillway section.

Factor of Safety against Sliding

Sum of sliding forces = upstream pressure force - downstream

pressure force

KN

hhhh SDsatSDmasonrySUWSUsat

55.325.0*16*2

15.0*9.25*

2

195.0*81.9*

2

11.2*16*

2

1

*2

1*

2

1*

2

1*

2

1

2222

2

/

2

/

2

/

2

/

Sum of forces resisting sliding = cohesion of rock material* area

of contact

= KNAC foundation 41.6871.1*40*

Factor of safety against Sliding

!10.255.32

41.68ReSafe

ForceShearing

forcessistingFS S

The same is applied for secondary spillway and found safe.



20

4.5. CCaauusseess ooff tthhee DDaamm FFaaiilluurree

From visual inspection the main cause of Tawila dam could be

due to seepage and construction mistakes.

Seepage of water through the body and foundation of the earth

dam may lead to piping and excessive erosion from concentrated

leaks, causing sever failures. Seepage of water through the foundation

of the earth dam may lead to piping and excessive erosion from

concentrated leaks, causing sever failures. Water seeping through the

earth dam may have four bad effects:

1) Seeping water generates erosive force which dislodges particles

from the soil structure and cause rearrangement or migration of the

fine to voids between larger grains.

2) Internal erosion of the soil mass finally progress backwards from

the exit points to form open pipes.

3) The internal pressure in the soil water can reduced that part of the

soil strength that is developed by internal friction thereby, lead to

weakening of the soil mass and even failure by shear.

The most probable cause of spillway failure may be due to poor

construction. The new constructed part of the spillway has no link

with the old part of the spillway. As settlement occurred in the new

one, the two parts were completely separated due to shear forces.

Thus leading to collapsing and wash away with water flow of the new

constructed portion of the secondary spillway.

The embankment close to the spillway (right wing) of Tawila



21

dam got saturated due to excessive seepage. It might have been

eroded, producing small slumps/slides presenting relatively steep

faces which once again became saturated by seepage from the

reservoir and slumped again, forming slightly higher and unstable

faces. This raveling process kept on going until the remaining portion

of the embankment became too thin to withstand the water pressure,

thus leading to the collapse of embankment and right wing-wall.

55.. RReeccoommmmeennddaattiioonnss

Based on the investigation results, it is clear that the failure of

the spillway structure and the adjacent embankment of the dam is

mainly due to seepage and improper design and construction.

55--11 EEmmbbaannkkmmeenntt DDeessiiggnn CCoonnssiiddeerraattiioonnss

For the new rehabilitation, the proposed earth fill dam will have

a section with outer upstream slope of 1V:3H and downstream slope

of 1V:3H as shown in Figure 8.

Figure 8: Embankment section crest width and slope

1

3 Homogeneous

Embankment

1

3

3m



22

An impervious blanket immediately upstream of a dam can be

used to seal the reservoir bottom and sides and thereby reduce

seepage quantities and pressures beneath a dam (Figure 9). If the dam

contains a permeable shell, the impervious blanket must be extended

up the embankment slope to effectively control the seepage problem.

Figure 9: Upstream impervious blanket

In order to control the escaping of fine particles, a rock toe drain

will be provide which consists of filter and rock. The purpose of the

filter and drain is to provide a way for seepage to exit the dam without

causing excessive erosion of the dam material.

The two basic dimensions for this dam are the height (H) and

the crest width (B). The height chosen will depend on the depth of

water at full supply level (D) and the freeboard required. Crest width

will depend on the height.

Upstream impervious blanket

Homogeneous

Embankment



23

Figure 10: Cross-Section of Homogenous Dam with Toe Drain

Hand-placed riprap is recommended to protect the slopes of the

dam embankment against destructive wave action in the upstream and

from weathering effects for downstream slopes.

55--22 SSeeeeppaaggee CCoonnttrrooll

To avoid and prevent any seepage of water below the spillway

structure that may occur, some technical solutions and precautions

should be considered in design and construction of foundation.

Because of water percolates through the foundation soil of the

spillway, use vertical moisture barriers to a depth not less than 10m

at three different locations below the concrete slab.

Because water percolates through earth dam; when however it

stagnates in the downstream portion of the dam and over saturates

it; the slips of the material of this portion may occur, resulting in

failure of dam. The taking out of this water from the downstream

portion so that there may be no slips, it is necessary since the

material in the downstream portion of dam has no good natural

drainage properties. Drainage may be in the form of toe drain.

D

H

D+2m

H/3



24

55--33 SSppiillllwwaayy FFoouunnddaattiioonn

The choice of foundation type for any site depends on many

factors such as the superstructure imposed load, subsoil profile,

placement condition and the foundation soil bearing capacity.

Generally the spillway structure of the dam will imposed

significant surcharge pressure on the foundation soil. The spillway is

structurally required to be stable and the foundation settlement should

be within the permissible limit.

Taking these factors into account with the foundation soil

bearing capacity, 657 KPa, raft foundation can support the spillway

structure. But the foundation soil of the spillway structure of sand soil

that has high permeability will increase water seepage and erosion at

upstream and downstream aprons. Therefore the best foundation

option is pile foundation of end bearing support.

Vertical moisture barriers are also suggested to reduce variation

of moisture content in the foundation soil and prevent any seepage of

water through this sandy soil. These barriers are to be erected to a

depth not less than 10m at three different locations, below the edges

and center of the concrete slab (i.e. at upstream, downstream and

center of the dam). The purpose of these moisture barriers is to reduce

or prevent water flow in the zone of sand soil below the dam.



25

AAppppeennddiixx ((AA))

Laboratory Tests

Data Analysis



26

Project:

Sample Description:

100

(mm) Wt. (g) (%)

44.500 0.0 0.0 100

37.500 0.0 0.0 100

25.000 0.0 0.0 100

19.000 0.0 0.0 100

12.500 0.0 0.0 100

9.500 0.0 0.0 100

6.350 0.0 0.0 100

4.760 0.0 0.0 100

3.180 1.2 1.2 99

2.360 1.4 1.4 97

2.000 0.8 0.8 97

1.180 2.8 2.8 94

0.600 6.9 6.9 87

0.425 8.7 8.7 78

0.211 38.1 38.1 40

0.075 34.2 34.2 6

Gravel 0% D10 0.085 mm

Sand 94% D30 0.170 mm

Silt & Clay 6% D60 0.310 mm

USCS SP Cu 4

AASHTO A-1-a Cc 1.1

B.S seive

Tawila Dam in North Darfur

Sandy Soil (S1)

Retained %age Passing

م ه الرحمن الرحيم س بUNIVERSITY OF KHARTOUM

Faculty of Engineering - Civil Eng. Dept.

Sieve Analysis for Granular Materials

Sample Total Weight (g)

Based on BS 1377



27

Project:

Sample Description:

100

(mm) Wt. (g) (%)

44.500 0.0 0.0 100

37.500 0.0 0.0 100

25.000 0.0 0.0 100

19.000 0.0 0.0 100

12.500 0.0 0.0 100

9.500 1.5 1.5 99

6.350 2.0 2.0 97

4.760 1.6 1.6 95

3.180 0.4 0.4 95

2.360 5.2 5.2 89

2.000 1.8 1.8 88

1.180 8.1 8.1 79

0.600 21.8 21.8 58

0.425 15.6 15.6 42

0.211 27.3 27.3 15

0.075 12.6 12.6 2


Sand 93% D30 0.32 mm

Silt & Clay 2% D60 0.65 mm

USCS SW Cu 4

AASHTO A-1-a Cc 0.9





Based on BS 1377

B.S seive


Sandy Soil (S2)




28

Project:

Sample Description:

100

(mm) Wt. (g) (%)

44.500 0.0 0.0 100

37.500 0.0 0.0 100

25.000 0.0 0.0 100

19.000 0.0 0.0 100

12.500 0.0 0.0 100

9.500 0.0 0.0 100

6.350 0.0 0.0 100

4.760 0.0 0.0 100

3.180 0.0 0.0 100

2.360 0.0 0.0 100

2.000 0.0 0.0 100

1.180 0.0 0.0 100

0.600 0.0 0.0 100

0.425 0.0 0.0 100

0.211 0.0 0.0 100

0.075 0.9 0.9 99


Sand 1% D30 0.075 mm

Silt & Clay 99% D60 0.075 mm

USCS CH Cu 1

AASHTO A-7-6 Cc 1.0





Based on BS 1377

B.S seive


Clayey Soil (S3)




29

Project: Tawila Dam in North Darfur

Sample Description

1 2 3 4 5 6

5785 5948 6072 6118 6103 6061

4365 4365 4365 4365 4365 4365

1420 1583 1707 1753 1738 1696

945 945 945 945 945 945

1.503 1.675 1.806 1.855 1.839 1.795

159.6 176.1 191.8 198.9 192.0 193.5

147.0 152.8 158.3 160.0 151.7 149.5

20.84 20.61 22.77 23.31 22.74 23.47

10.0 17.7 24.7 28.5 31.3 35.0

1.367 1.424 1.448 1.444 1.401 1.330

M.D.D= 1.452 g/cm3

O.M.C = 24.5 %


Soil Compaction -- Based on BS 1377

wt.of wet soil+cont(g)

Sample

wet density(g/cc)

Clayey Soil (S3)


dry density(g/cc)

Moisture determination

Density determination

wt.of wet soil+mould(g)

wt.of dry soil+cont(g)

wt.of cont(g)

moisture content(%)

wt.of mould(g)

wt.of wet soil(g)

volume of mould(cc)

1.32

1.34

1.36

1.38

1.40

1.42

1.44

1.46

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

DR

Y D

EN

SIT

Y (

gm

/cm

3)

MOISTURE CONTENT(%)

COMPACTION CURVE



30

Project: Tawila Dam in North Darfur

Sample Description

Test

sample 1 2 3 1 2 3

No of Blows 30 26 23

Wt. of wet soil +container (g) 46.1 48.4 51.3 23.8 26.1 26.2

Wt. of dry soil +container (g) 35.3 36.9 38.7 22.3 24.2 24.6

Wt. of container (g) 14.67 15.35 15.31 16.79 17.39 18.52

M.C% 52.8 53.5 54.2 27.0 27.4 26.6

LIQUID LIMIT (%) 53.5

PLASTIC LIMIT (%) 27.0

PLASTICITY INDEX (%) 26.5

Determination of Liquid Limit & Plastic Limits of soil



LIQUID LIMIT PLASTIC LIMIT

Based on BS 1377

Clayey Soil (S3)

52.6

52.8

53.0

53.2

53.4

53.6

53.8

54.0

54.2

54.4

54.6

10 100No of blows

Mo

istu

re

Co

nte

nt

(%)



31

AAppppeennddiixx ((BB))

Field density Tests Results

During Construction

(Contractor Documents)

Geotechnical Investigation for Tawila Dam in North Darfur

Documents

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