APPLYING THE SEISMIC WAVE VELOCITY SURVEY IN THE ASSESSMENT OF PERMEABILITY COEFFICIENT (K), AT WADI AL-ASLAA AREA, JEDDAH, KINGDOM OF SAUDI ARABIA

J. Appl. Geophys., Vol. 8, No. 1, pp. 63-82, March, 2009.

APPLYING THE SEISMIC WAVE VELOCITY SURVEY IN THE ASSESSMENT OF PERMEABILITY COEFFICIENT (K),

AT WADI AL-ASLAA AREA, JEDDAH, KINGDOM OF SAUDI ARABIA

By

Shokry M. M. F.

Department of Geophysics, Faculty of Science, Ain Shams University, Cairo, Egypt

Email: Mshokry_geoph @ yahoo.com

ABSTRACT

Seismic wave velocity, geological and geotechnical properties of the shallow

ground layers at Wadi Al-Aslaa area were used integratiuely to study the permeability

coefficient (K) and to evaluate the foundation layers occurred beneath the dam and lake

construction at this area. The area is located at the northeastern part of Jeddah city,

Kingdom of Saudi Arabia and undergoes construction phase. The stratigraphy of the

site consists mainly of Precambrian Basement rocks in most parts of the area and by

Miocene succession of conglomerates and sandstones at the northeastern parts of the

area. The permeability coefficient (K) was estimated across the different sections of the

study area. The obtained results of the permeability coefficient help in establishing the

suitable percautions, which may taken into consideration during the dam and lake

construction phases.

INTRODUCTION

The construction of a dam to retain

water causes more interference with

natural geplogic conditions than does

any other civil engineering operation.

Equally striking is the critically

important function, that dams perform

in storing water for domestic supply,

for the generation of power, for flood

control, for recreation, and for

irrigation. Although the failures of civil

engineering works are always of

serious consequence, the failure of

dams is possibly more serious than

others, since they generally occur

during periods of abnormal weather,

often without warning, and almost

always with disastrous results. Defects

in foundation beds are an unfortunate

factor in many dam failures, and

another telling argument is thus

presented to support the necessity of

neglecting no single feature of

foundation beds, that may possibly

affect the dam resting upon them

(Legget and Karrow, 1982).

The record of failures of dams in

succeeding years provides a useful if a

somewhat melancholy study. Analysis

of the causes of failures indicates fairly

definitely that, the main reasons have

been (1) failure to provide adequate

spillway capacity, and (2) defective

foundation bed conditions; these two

factors have accounted for the majority

of all recorded failures. The second

2

cause of failures noted is dependent

essentially on geologic features,

although the specific reason for failure

may vary from one case to another

(Legget and Karrow, 1982).

A dam is an artificial structure

erected to support a waterproof

membrane designed to retain water

above the level, that it normally

occupies at the site of the dam; suitable

provision is made for passing a certain

calculated flow of water past the dam,

through it, over it, or around it,

depending on the local circumstances.

The membrane may be, and generally

is, an integral part of the dam structure.

The type of dam to be constructed at

any location should be determined

mainly from geological considerations;

and the actual kind of dam to be

constructed. Once its general type has

been decided, will also be dependent to

some extent on the geologic conditions

affecting the supply of structural

materials (Legget and Karrow, 1982).

Since all dams retain water to a

certain predetermined level, the flow of

water in the watercourse being

regulated is seriously affected below

the dam site; the flow is generally

regulated to a more uniform discharge

than that given by the stream itself.

Some of the geological problems

affecting the reservoirs formed by

dams may be encountered also during

construction (Legget and Karrow,

1982).

Dams founded on pervious

foundations present their own special

problems, these problems are mainly

associated with the controlled flow of

water beneath the structure. It may not

be possible to make any dam

foundation absolutely tight, but the

slight leakage occuring with dams on

so called "impermeable" foundation

beds, although associated with uplift

pressure, is generally of no other

consequence. For those dams on

admittedly permeable foundation beds,

the exact nature of the flow of water

through the underlying strata is a vital

part of the design; thus, accurate

knowledge of the water carrying

properties of the unconsolidated

materials encountered becomes a

matter of importance (Legget and

Karrow, 1982).

Since the location of a proposed

dam will generally be restricted by

topographical, economic, and social

considerations, the areas to be

examined as possible sites will be fairly

well defined. Accurate geological

scetions along possible lines for the

dam will be a major requirement.

Geophysical methods can be of great

assistance in this work, when utilized

in connection with strategically placed

boreholes (Legget and Karrow, 1982).

Wadi Al-Aslaa area at Jeddah,

Kingdom of Saudi Arabia was selected

to was construct a dam with its lake to

solve many problems of the area, in

relation to the drainage system (Fig. 1).

3

Figure 1. Study area location using Google Earth Program.

PREVIOUS PRELIMINARY

WORKS

Since the location of a proposed

dam will generally be restricted by

topographical, economic and social

considerations, the area to be examined

as possible site will be fairly well

defined. Accurate geologic sections

and mapping along possible lines for

the dam area were accomplished to

interpret and define the geologic rock

units, and to detect the main faults and

lineaments (EGEC, 2006).

Geophysical methods have great

assistance when utilized in connection

with strategical placed boreholes. A

total of 13 compressional seismic

refraction profiles were conducted at

the study area to investigate a depth of

about 40 to 50 meters with a total

length of geophone spread is 120.0 m.

The profiles layout covered the

proposed dam and lake area (Fig. 2).

Also, the magnetic survey layout was

applied and designed to cover most of

the study area, in which the magnetic

survying is controlled by the

outcropping basement at the study area

(EGEC, 2006).

The seismic refraction profiles at

the study area illustrated (EGEC, 2006)

that, the shallow layers of the site

consist of a surface layer with P-wave

velocity ranging between 240 and 430

m/s and thickness ranging between 1.0

and 8.0 m. This layer corresponds to

Dam Location

Lake

Location

Study area

N 21o 44` 8``

N 21o 46` 30``

E 39o 21` 18`` E 39o 26` 6``

4

wadi sediments (Wadi fill) of silt-sand-

conglomerate. The second layer has a

P-wave velocity ranging between 790

and 1300 m/s and thickness ranging

between 2.0 and 30.0 m. This layer

corresponds to compacted and partially

saturated sands.

The third layer has a P-wave

velocity ranging between 1400 and

3300 m/s and thickness ranging

between 10.0 and 25.0 m. This layer

corresponds to a sandstone layer (low

velocity range between 1400 and 1800

m/s) at the dam area part, and

compacted sandstone and conglomerate

layer (high velocity ranges between

2300 and 3300) at the lake area part.

The bottom layer has a P-wave velocity

ranging between 3850 and 8700 m/s

and thickness of more than 10.0 m.

This layer corresponds to fractured

basements (low velocity range 3850

and 4000 m/s) and fresh basement

rocks (high velocity range 4800 and

8700 m/s) at different locations of the

study area.

Figure 2. A base map showing the locations of the conducted P-wave seismic refraction

profiles and the comparable cross sections.

B,

C,

C B

A A,

Dam

Location

Lake

Location

P#: Seismic profile

A-A`, B-B`, and C-C`: Cross Sections

B,

J. Appl. Geophys., Vol. 8, No. 1, pp. 63-82, March, 2009.

Thirty two boreholes were drilled at

carefully selected locations at the area

of the dam and lake (the study area)

with depths ranged between 6.00 m and

35.00 m deep. The deep boreholes at

the location of the dam and lake but the

shallow boreholes at the location of the

other utilities. Seven were 35.00 m

deep, twenty one were 18.00 m deep,

one borehole was 10.00 m deep, one

borehole was 9.00 m deep, one

borehole was 7.00 m deep and one

borehole was 6.00 m deep (EGEC,

2006). The soil test borings were

drilled in accordance with the ASTM

D1586-84 (ASTM, 1989). The Unified

Soil Classification System (USCS)

(sited from Liu and Evett, 1992) and

other laboratory tests were carried out

on some of the extracted soil samples

to determine the physical and

mechanical properties of the various

encountered soil layers, according to

the ASTM standards (ASTM, 1989).

The drilled boreholes indicate that,

the near-surface and subsurface

sections consist of the following layers:

A surface layer composed of brown

silty sand. The fines recorded at this

layer are ranged between 9% and 28%.

The SPT results recorded through this

layer are ranged between 12 and 100.

This layer is encountered in all

boreholes starting from the ground

surface and extending down to a depth

varies from 2.5 to 7.0 m below the

ground surface i.e. with a thickness

ranged between 2.5 and 7.0 m.

A layer of highly fractured

sandstone with a mixture of gravel or

clay is occured at some depths and

locations. The measured Rock Quality

Designation (RQD) values for the

extracted samples of the sandstone

cores ranged between 16% and 70%,

indicating a very poor to fair quality

rock nature. This layer is recorded

below the surface layer at depths

ranged between 2.5 and 7.0 m, and

extended to the end of the borehole

depths.

The records made available from

preliminary site studies enable those in

charge of the engineering work to

decide the general type of dam to be

used and to begin their economic and

design studies (Legget and Karrow,

1982).

The geological, geophysical and

geotechnical data are used together to

correlate and integrate each other to

estimate the changing and

heterogeneity of permeability

coefficient (K) through the different

rock units appeared and exposed at the

study area, according to the above

mentioned studies.

GEOLOGICAL MAPPING OF

THE STUDY AREA

The study area is bounded by

latitudes 21º 42

' 29.30

" and 21

º 47

', and

longitudes 39º 20

' and 39

º 28

', at the

northeastern part of Jeddah city,

Kingdom of Saudi Arabia. The mapped

area is covered mainly by Precambrian

Basement rocks in most parts and by

Miocene sedimentary succession of

conglomerates and sandstones in the

6

northeastern part. Detailed field studies

indicate the presence of eight rock units

together with the wadi sediments.

These rock units can be summarized

and arranged form older to younger as

follows (Table 1) and (Fig. 3) (EGEC,

2006):

Table 1. Rock units in the study area, from older to younger.

Rock Unit Location Lithology

The Basic and

Intermediate

Volcanics Unit

Northwestern Part

Highly fractured slightly metamorphosed

basalts, less abundant andesites, with

subordinate andesitic volcaniclastics.

The Gabbro Unit Northwestern and

Southern Parts

Massive medium to coarse-grained gabbros.

Some exposures are slightly metamorphosed

and others are composed of layered gabbros.

The Quartz Diorite

Unit

Eastern and

Central Parts

Massive to weakly foliated medium- to

coarse-grained quartz diorite with

subordinate amounts of diorite.

The Tonalite Unit Southeastern Part Hornblende tonalite.

The Granodiorite Unit Central Part Hornblende granodiorite to biotite

monzogranite.

The Pink Granite Unit Eastern Central

Part

Coarse grained, equigranular and massive

granite of pink color.

The Younger Dykes Basaltic composition

The Conglomerates

and Sandstone Unit

Northern and

Northeastern Parts

Thick layers of slightly lithified, highly

friable and poorly sorted conglomerates,

intercalated with thin layers of slightly to

moderately lithified fine grained sandstones

and shales, attaining an exposed thickness of

about 40 m.

The Wadi Sediments

Unit

Cover the Wadi

floors.

Fine to medium grained unconsolidated

sands.

The Quartz Diorite Unit represents

the main rock unit in the area under

consideration and constitutes about

60% of the surface area of the total

rocks.

The Conglomerates and Sandstone

Unit show great heterogeneity. The

constituent pebbles are greatly variable

in grain size, reaching up to 80 cm in

diameter. The conglomerate pebbles

are mostly angular and consist of

different rock varieties; all of them are

from the Precambrian Basement rocks

(plutonic and volcanic varieties) and

mainly consist of quartz diorite and

granodiorite together with basalts and

andesites. The conglomerate matrix

consists of fine to medium-grained

sands. The sandstone layers are usually

thin and not exceeding 30 cm in

thickness. In general, the rocks are

highly weathered and almost all

7

exposures are covered with the

weathering products, where the fine-

grained materials are weathered out

and the conglomerate boulders are left

behind covering the outcrop surfaces.

Figure 3. Geologic map of the study area (after EGEC, 2006).

STRUCTURAL FEATURES OF

THE STUDY AREA

The Conglomerates and Sandstone

Unit is in structural contact against the

Precambrian Basement rocks along a

master normal fault, that has a NW-SE

trend and downthrown towards the NE.

This unit is affected by a probable fault

along Wadi Shaib Al Burayqah of N-S

trend, as evidenced by the linear

occurrence of the exposures of the

rocks along the wadi.

A large number of faults, mainly

along the NE-SW trend, affect the

Precambrian Basement rocks. Few

faults are trending NW-SE. The most

important of these faults (EGEC, 2006)

(Fig. 1) are:

A major thrust fault running

parallel to Wadi El Maray, along

the NE-SW trend and cutting

through the quartz diorites

throughout along most of its

length. This thrust can be

considered as a master fault in the

region northeast of Jeddah city as a

whole, as it extends southwest of

the study area for a distance of

about 30 km.

A strike slip fault located in the

southern part of the mapped area

cutting through the quartz diorite

and the pink granite. The fault has

Dam Location

Lake

Location

8

a NE-SW direction and a small left

lateral displacement.

A normal fault controls the eastern

side of Wadi El Maray in the

central part of the mapped area,

cutting through the quartz diorite

along the NE-SW direction.

PERMEABILITY OF BEDROCK

One of the essential requirements,

of a rock foundation bed for a dam is

that, the entire geological structure

underlying the site of the dam, in

addition to being strong enough to

carry the designed loads, will be sound

enough to provide a watertight barrier

to the water impounded by the dam.

The first reason for having rock strata

underlying the dam, as watertight as

possible, is obviously to make sure

that, no water escapes. Not only is this

necessary from the point of view of

water conservation, but it is also

essential because any steady flow

through a solid rock formation is

certain to have some erosive action,

which in all the probabilities will

gradually but steady intensify the

defective conditions causing the

original leakage (Legget and Karrow,

1982).

Naturally, the limestone and all soft

rocks are suspect until proved to be

sound, limestone formations, especially

are unusually soluble and so

characterized by underground caverns

or open fissures. Although limestone

causes most of the geological troubles

in foundations, it was a shale and

sandstone combination, that presented

one of the most unusual examples

known to the writers of a permeable

foundation bed for a dam (Legget and

Karrow, 1982).

It will be clear that, the presence of

water in rock foundation beds for

dams, and the possibility of flow

through such bedrock, is a complex

matter, just as complex as is the

variation of geological conditions that

may be encountered. Accordingly, in

addition to study of all relevant

geological information, including the

detailed results of all boring, tests

should be carried out whenever

possible by means of the boreholes

used for obtaining core samples

(Legget and Karrow, 1982).

PERMEABILITY COEFFICIENT

(K) ESTIMATION STRATEGY

FROM SEISMIC VELOCITY

DATA

The basic parameter defining the

flow of ground water, and the

distribution of water pressure, in

geologic media is permeability

coefficient (hydraulic conductivity).

This parameter relates the flow rate of

water through the material to the

pressure gradient applied across it

(Scheidegger, 1960; Morgenstern,

1971).

The different steps required to

estimate the permeability coefficient

(K) from seismic velocity (Vp) is

9

shown in the flow chart illustrated in

Figure 4.

Three geoseismic cross sections

were constructed carefully to

summarize the P-wave velocity

changes through the different lithologic

layers until reaching the bedrock and

basement complex layer. The cross

section locations are traced on the

attached topographic contour map.

These three sections are denoted as A-

A’, B-B’ and C-C’, respectively. The

AA’ cross section was chosen to

associate the proposed site of the dam.

The B-B’ cross section is parallel to the

A-A’ section. The C-C’ cross section is

a long-section parallel to the wadi axis

(Fig. 2).

The knowledge about the seismic

velocities is important in engineering

considerations, because the velocity is

controlled by the fundamental

parameters of elastic strength and

density. In most refraction works,

however, only the first arrivals are

recorded, giving information about the

degree of fracturing/jointing of a

known rock type, or an empirical

relation with the rock strength may be

sought (Deere et at., 1967; Helfrich,

1971). Such a relation between the P-

wave velocity, the rock quality

designation (RQD) and the jointing

factor (C) has been studied for

crystalline rocks, as shown in Table 2.

RQD index was developed to

provide a quantitative estimate of rock

mass quality from drill core logs. RQD

is defined as the percentage of intact

core pieces longer than 100 mm (4

inches) in the total length of core. The

core should be at least narrow width

size (54.7 mm or 2.15 inches in

diameter) and should be drilled with a

double-tube core barrel (Deere et al.,

1967).

Figure 4. Flow chart of the permeability coefficient (K) estimation steps.

In-site seismic velocity (Vp)

measurements

RQD estimation

(Helfrich, 1971)

Discontinuity spacing

estimation

(Vardakos, 2004)

1-D joint frequency estimation

(Palmström, 1996)

Joint opening (e) estimation

(Hoek and Bray, 1981)

Permeability coefficient (K)

calculation (Priest and Hudson, 1976

10

Table 2. Empirical relationship between jointing factor (C), seismic velocity (Vp) and

the rock quality designation (RQD) of crystalline rocks (after Helfrich, 1971).

Degree of separation

(after Helfrich, 1971) RQD value (%)

(after Deere et al

1967) C Vp (m/sec)

RQD

classification

Rock without joints 0.65-1.00 >4500 Excellent 90-100

Rock with few joints 0.45-0.65 4000-4500 Good 75-90

Rock with joints 0.30-0.45 3500-4000 Fair 50-75

Rock with numerous joints 0.15-0.30 3000-3500 Poor 25-50

Strongly jointed rock 0.00-0.15 <3000 Very poor 0-25

Figure 5 displays the relation

between discontinuity spacing and

RQD, to estimate the discontinuity

spacing when the data regarding

discontinuity spacing are not available

(Vardakos, 2004).

Figure 5. RQD-discontinuity spacing correlation (after Priest and Hudson, 1976).

A considerable amount of

experience has been gained from more

than 30 years of seismic refraction

surveys in Scandinavia. Sjögren et al.

Average Line

Dashed & open areas are

combined RQD and spacing

rating for different regions

11

(1979) carried out a comprehensive

investigation of field measurements

and gave correlations between seismic

velocities obtained in refraction

surveys and joints measured in drill

cores. Sjögren et al. (1979) have

equated the longitudinal seismic

velocity (V) measured with 1-D joint

frequency (Nl) in boreholes, as shown

in curve 1 of Figure 6.

Figure 6. RQD Correlations between seismic wave velocity (v) and joint density (Nl)

for various types of rocks (after Sjögren et al., 1979 and Sjögren, 1984).

The basic longitudinal velocity

(Vo) is that velocity which is

considered to represent the intact rock

(i.e. the rock with no joints) under the

same conditions of stress and ground

water regime, as in the field (see Fig.

7). Extension of the curves in Figure 6

indicates that, the basic longitudinal

velocity is different for each case,

probably due to the differences in

composition and other properties of the

rock. The parallel nature of the curves

indicates that, they possibly can be

calculated from Vo. Correlations

between seismic velocities and the

degree of jointing can be found from

two different approaches:

1. Initial correlation method for cases,

where no information are available on

the jointing versus seismic velocity.

2. Refined correlation method for

cases, where at least two correlations

between jointing and seismic velocities

are already known.

For the initial correlation method,

Palmström (1995) has shown two

different potential expressions, which

may be used to represent the

relationship between jointing and

seismic velocity, where no previous

correlation exists:

Nl = (V0)3.4

× v - 2.8

(1)

Nl = 3(V0 /v)Vo/2

(2)

KEY 1. Average results of jointed,

unweathered, igneous and metamorphic rocks of Paleozoic

age in Scandinavia

2. Jointed granite, granodiorite, and andesite from the Andes, Chile

(based on data from Helfrich, Hasselstrom and Sjogren, 1970).

3. Jointed and weathered

metamorphic rocks from the Andes (based on data from

Sjogren, 1993). The rock are quartzite, and

various schists and shales.

4. Jointed Triassic and Permian sandstones from Tanzania

(Sjogren, 1984).

12

where: V0 is the basic velocity of intact

rock under the same conditions

as in the field.

v is the measured in-situ

seismic velocity (km/s)

Both correlations rely on the

assessed magnitude of the basic

velocity (V0) which represents the site

dependent (in situ) velocity for intact

rock. Where V0 is not known, it is

recommended to use the velocity for

intact rock under the same conditions

as in the field (wet/dry, orientation of

anisotropy, stress conditions, etc.) from

laboratory testing. Joint openness and

possible joint fillings may, however,

effect the accuracy of both correlations

described above where V0 is assessed

from laboratory measurements from

tables in textbooks.

Figure 7. The principle difference between the basic seismic velocity (Vo) and the

natural or maximum velocity (Vn) (after Sjögren et al., 1979 and Sjögren, 1984).

For the refined correlation method,

Sjögren et al. (1979) have presented the

following expression to calculate the

degree of jointing from the measured

seismic velocities:

ks x Nl = 1/v - 1/Vn (3)

where: Vn is the maximum or 'natural'

velocity in crack and joint free rock

(see Fig. 7).

13

ks is a constant representing

the actual in-situ conditions.

Nl is the 1-D joint frequency

(joints/m) along a drill core or scan

line.

Palmström (1996), used a set of

data from core drilling and seismic

measurements to establish the

relationship between the degree of

jointing and the longitudinal seismic

velocities greater than 3000 m/sec. And

then from the known value of this 1-D

joint frequency (Nl), the volumetric

joint count and the block, volume can

be calculated (Fig. 8). And concluded

that, the seismic refraction

measurements provide a useful and

very attractive tool for the

characterization of the degree of

jointing.

Mathematical correlations between

seismic refraction velocities and the

degree of jointing can be applied before

information on jointing from core

drilling or surface mapping is available.

In this way, it is possible to obtain

information of the probable jointing at

an early stage during investigations. It

should, however, be noticed that in

these calculations local differences

such as the composition of rock types,

mineral content, etc. are averaged, and

that the calculations require input of an

assumed 'basic velocity' (V0) of the

intact (fresh or weathered) rock. The

accuracy of (V0) highly influences the

quality of the assessments Palmström

(1996).

Figure 8. Various correlations between

seismic velocities and 1-D joint

frequency (after Palmström, 1996).

The engineering geological

assessment of the study area must be

proposed mainly for the rock mass

effect on the different rock units

appeared and outcropped through the

study area. The rock mass

classification, which called the

Geomechanics Classification or the

Rock Mass Rating (RMR) system

(Bieniawski, 1976), was modified over

the years as more case histories became

available and to confirm with the

international standards and procedures

(Bieniawski, 1979 and 1989). RMR

system was used to study the different

engineering geological characterization

of the study area to calculate the joint

opening (e), which will use directly or

indirectly in the estimation of the

Permeability Coefficient (K).

14

Twenty stations are located in the

project area, taking in consideration the

proposed dam site and its future lake,

where possible changes in the

engineering properties of the

surrounding rock masses could be

expected. In each of these stations, the

technical investigations and

characterization of the rock masses and

rock materials were carried out (EGEC,

2006).

The engineering geological

investigations and rock mechanics

studies on the study area mainly

include discontinuity surveying,

sampling and laboratory tests. As the

joint system and its engineering

properties control the propagation, flow

and movement of the water in the rock

mass, a special attention should be

given to the study of the characteristics

of the joint systems. These

discontinuities will be primarily

responsible for the permeability of the

available rock masses, that decreases

with increasing depth. A joint analysis

was carried out at each station on the

available rock masses in the site

including orientation, spacing,

persistence, roughness, aperture and

filling, in order to make quantitative

description of the discontinuities to be

used in the geomechanics classification

(RMR) for most rock units distributed

along the Wadi (EGEC, 2006).

These five parameters are the main

factors utilized to classify a rock mass

using the RMR system (Geomechanics

classification), a. Uniaxial compressive

strength of rock material, b. Rock

quality designation, c. Spacing of

discontinuities, d. Conditions of

discontinuities and, e. Ground water

conditions. The classification

parameters, their ratings, guidelines for

classification of discontinuity

conditions and rock mass classes

determined from total ratings can be

summarized in Table 3.

The rock masses in the studied area

vary in their engineering characteristics

from a location to the other one and

from a rock unit to the other one. The

RMR of the rock masses are ranging

between fair rock quality (class III) to

good rock quality (class II) and the

measured field data can be summarized

in Table 4. The recorded joints across

the study area displayed the

occurrences of fractures with average

aperture is 15 mm filled with soft

weathering materials (EGEC, 2006).

In most rock types flow through

intact rock is negligible (defined by

Kprimary), and essentially all flow occurs

along the discontinuities (defined by

Ksecondary). The term secondary

hydraulic conductivity refers to flow in

the rock mass and encompasses flow in

both the intact rock and any

discontinuities that are present. These

conditions result in secondary

hydraulic conductivities having a wide

range of values depending on the

persistence, width and infilling

characteristics of the discontinuities

(Wyllie, D. C. and Christopher, 2004).

According to Darcy`s law, the

permeability coefficient (hydraulic

conductivity coefficient) K is defined

as:

15

)()(2121

hh

Vl

hhA

QlK

(4)

Considering a cylindrical sample of

soil or rock beneath the water table in a

slope. The sample has a cross-sectional

area of (A) and length (l). Water levels

in boreholes at either end of this

sample are at heights (h1) and (h2)

above a reference datum and the

quantity of water flowing through the

sample in a unit of time is Q with the

discharge velocity (V). The substitution

of dimensions for the terms in equation

(4) shows that the parameter (K) has

the same dimensions as the discharge

velocity (V), that is length per unit

time. The units most commonly used in

ground water studies is centimeters per

second (Wyllie, D. C. and Christopher,

2004).

The equivalent permabilities

coefficient (K) of a parallel array of

cracks with different openings are

plotted in Figure (9) which shows that,

the permeability of a rock mass is very

sensitive to the opening of

discontinuities (Hoek and Bray, 1981).

Since these opening changes with

stress, the permeability of a rock mass

will therefore be sensitive to stress.

Table 3. Rock Mass Rating System, (after Bieniawski, 1989).

a. Classification parameters and their ratings

Parameter Range of values

1

Strength of intact

rock

material

Point-load strength index

> 10 MPa

4-10 MPa

2-4 MPa

1-2 MPa

For this low range

uniaxial compressive

test is preferred

Uniaxial comp.

strength

> 250

MPa

100-250

MPa

50-100

MPa

25-50

MPa

5-25

MPa

1-5

MPa

< 1

MPa

Rating 15 12 7 4 2 1 0

2 Drill core Quality RQD (%) 90-100 75-90 50-75 25-50 < 25

Rating 20 17 13 8 3

3 Spacing of discontinuities (mm) > 2000 600-2000 200-600 60-200 > 60

Rating 20 15 10 8 5

4

Condition of dicontinuities (See c)

I II III IV V

Rating 30 25 20 10 0

5

Ground

water

Inflow per 10 m tunnel length (l/m)

None < 10 10-25 25-125 > 125

(Joint water press)/

(Major principle ) O < 0.1 0.1-0.2 0.2-0.5 > 0.5

General conditions Dry Damp Wet Dipping Flowing

Rating 15 10 7 4 0

I: Very rough surface, not continuous, no separation, unweathered wall rock. II: Slightly rough, surfaces separation < 1 mm, slightly weathered walls.

III: Slightly rough, surfaces separation < 1 mm, highly weathered walls. IV: Slickensided surfaces or Gouge < 5 mm thick or separation 1-5 mm continous.

V: Soft gouge > 5 mm thick or separation > 5 mm continous.

16

b. Rock mass classes determined from total ratings

Rating 100 – 81 80 – 61 60 – 41 40 – 21 < 21

Class number I II III IV V

Description V. good rock Good rock Fair rock Poor rock V. poor rock

c. Guidelines for classification of discontinuity conditions

Dictontinuity length

(persistence)

Rating

< 1 m

6

1-3 m

4

3-10 m

2

10-20 m

1

> 20 m

0

Separation (aperture)

Rating

None

6

< 0.1 mm

5

0.1-1.0 mm

4

1-5 mm

1

> 5 mm

0

Roughness

Rating

Very rough

6

Rough

5

Slighty rough

3

Smooth

1

Slickensided

0

Infilling (gouge)

Rating

None

6

HF < 5 mm

4

HF > 5 mm

2

SF < 5 mm

2

SF > 5 mm

0

Weathering

Ratings

Unweatherd 6

SW 5

MW 3

HW 1

Decomposed 0

HF: Hard filling, SF: Soft filling, SW: Slightly weathered, MW: Moderately weathered, HW: Highly weathered

Table 4. Rock Mass Rating analysis of the various rock units in the study area.

Parameters

Basic and

intermediate

unit

Gabbro

unit

Quartz

diorite

unit

Granodiorite

unit

Conglomerate

& sandstone

unit

PLSI (MPa) Value 31-32 6-22 6-23 6 3

Rating 15 12-15 12-15 12 7

RQD (%) Value 33-40 23-66 22-77 36-43 81

Rating 8 3-13 3-17 8 17

SD (mm) Value 10-60 10-60 10-60 10-20 60-150

Rating 8-10 8-10 8-10 8 15

CD Rating 0-10 3-17 10-17 5-10 15

Ground

water

Value Completely dry

Rating 15 15 15 15 15

RMR 46-58 41-70 48-74 48-53 69

Class number III III-II III-II III II

Description Fair Rock

Good-

Fair

rock

Good-

Fair rock Fair Rock Good Rock

PLSI: Point Load Strength Index, SD: Spacing of discontinuity, CD: Condition of discontinuity

17

Figure 9. Influence of joint opening (e) and joint spacing (b) on the permeability

coefficient (K) in the direction of a set of smooth parallel points in a rock mass (after

Hoek and Bray, 1981).

By applying the above mentioned

steps, the Permeability Coefficient (k)

is estimated for the different rock units

appeared through the seismic velocity

sections (Table 5) and plotted on these

sections (Figs 10, 11, and 12).

The cross section A-A` shows a

clear difference in the basement depth

from NW and SE. The eastern part of

the dam site characterized by a shallow

depth of a basement rocks with a

seismic velocity of 4800 m/s and

permeability coefficient of 910 cm/sec

which reflects a relatively fracture

intensity and weathering action. The

contact between the basement rocks

and compact sandstone can be

explained by the erosion paleo-surface.

10-6

0.001 0.005 0.01 0.05 0.1

Joint opening e - cms

10-8

10-7

10-5

10-4

10-3

10-2

10-1

1.0

10

Perm

eab

ility co

efficient K

– cm

/sec

A

B

C

A: 100 joints/meter

B: 10 joints/meter

C: 1 joint/meter

18

The depth to basement is about 25-30

meters to southeast. Meanwhile, the

depth exceeds more than 60 meters to

the west. This explained by the

direction of dip to the SW direction.

The compacted sandstone layer is

missed to the southeast direction due

to erosion. This layer has a seismic

velocity of 3300 m/sec and the

estimated permeability was 3250

cm/sec. The upper two layers are

partially saturated and saturated

sandstone. The permeability and

porosity of the first three layers as high

and contain ground water potential as

indicated from the seismic wave

velocities.

Figure 10. Permeability coefficient (K) heterogeneity through cross section A-A'.

19

Figure 11. Permeability coefficient (K) heterogeneity through cross section B-B'.

Figure 12. Permeability coefficient (K) heterogenity through cross section C-C'.

20

Table 5. The Permeability Coefficient (k) estimation for the various rock units.

Profile Lithology 1Vp

(m/sec)

2RQDest.

(%)

3RQDmea

(%)

4Mean

discontinuity

spacing (mm)

51-D joint

frequency

(joint/m)

6e

(mm)

K

(cm/sec)

P4 cS.S & Cong 3100 30 16 – 70 79 13 15 3700

P3 cS.S & Cong 3300 40 16 – 70 93 10 15 3250

P1 fBa 3850 70 NA 158 9 15 2800

P10 fBa 4000 75 NA 163 7 15 1000

P12 fBa 4000 75 NA 163 7 15 1000

P2 Ba 4800 90 NA 337 4 15 910

P10 Ba 8100 95 NA 558 1 15 505

P5 Ba 7700 95 NA 558 1 15 505

P7 Ba 8700 95 NA 558 1 15 505

cS.S: Compacted Sandstone, Cong: Conglomerate, Ba: Basement, fBa: Fractured Basement, NA: Not

available. 1Vp: Compressional wave velocity measured in the field. 2RQDest.: Estimated RQD value (Table 2) (Deere et at., 1967; Helfrich, 1971). 3RQDmea: Measured RQD value from the borehole extracted samples. 4Mean discontinuity spacing: Estimated value (Fig. 5) (Priest and Hudson, 1976). 51-D joint frequency: Estimated value (Fig. 8) (Palmström, 1996). 6e: Joint opening value calculated from RMR classification system of the study area. Note: The drilled boreholes not reached the basement rock layer.

The cross section B-B` shows a

clear dipping of the layers to NW

direction and missing of saturated sand

and sandstone layers in SE direction

but replaced by compacted sandstone

and conglomerate layers to be

shallower to the SE direction. The

recorded seismic velocity of this layer

is 3300 m/sec and decreased to 2400

m/sec to the SE direction. The

estimated permeability coefficient is

3250 cm/sec and expected to decrease

in SE direction due to decreasing in the

seismic velocity in relation with the

less compaction of that layer and it

became at a shallower part of the

section. The shallower basement depth

is about 60 meters at this Section.

According to the depth of the basement

at this section the recorded seismic

velocity increased to be 7700 m/sec

and also due to the absence of joints

and fractures the estimated

permeability coefficient was 505

cm/sec but increased to 2800 cm/sec at

the fractured parts of the section with

seismic velocity value is 3850 m/sec.

The cross section C-C` passes

through the axis of the Wadi from SW

to NE direction. A major fault is

recognized at this section. The

saturated sand and sandstone layers

missed in SW direction but replaced by

compacted sandstone and conglomerate

layers to be shallower to the SW

direction. The recorded seismic

velocity of this layer is ranged between

3100 m/sec around the fault plane and

3300 m/sec at the other parts of the

section and decreased to 2300 m/sec to

the SW direction. The estimated

permeability coefficient has high value

of 3700 cm/sec around the fault plane

and decreased to 3250 cm/sec at the

21

other parts with high seismic velocity

and far from the fault plane and

expected to decrease in SW direction

due to decreasing in the seismic

velocity in relation with the less

compaction of that layer and it became

at a shallower part of the section like in

the case of section B-B`. The basement

depth attains a depth of about 15

meters to the SW direction and about

70 meters to NE direction. Also, high

seismic velocity recorded for the

basement layer at deep depths reached

about 8700 m/sec with low estimated

permeability coefficient of about 505

cm/sec, but the values of the seismic

velocity decreased at the shallow parts

of the section and around the fault

plane to be ranged between 3850 and

4000 m/sec for the fracture parts of the

basement and the permeability

coefficient increased in this case to be

ranged between 1000 and 2800 cm/sec.

The important factors influencing

ground water within a slope are

distribution of rock types, and details

of the structural geology such as fault

infillings, persistence of joint sets and

the presence of solution cavities. These

features can result in regions of low

and high hydraulic conductivity within

the slope that are termed aquitards and

aquifers, respectively (Wyllie, D. C.

and Christopher, 2004).

Typical ranges of secondary

hydraulic conductivity for the different

geological materials of a variety rock

types, as well as unconsolidated

deposits, covers13 orders of magnitude,

and for any single rock type the range

can be four orders of magnitude. This

shows the difficulty in predicting water

inflow quantities and pressures within

slopes (Wyllie, D. C. and Christopher,

2004).

SUMMARY AND CONCLUSIONS

The previous preliminary

geophysical, geological and

geotechnical works, beside the cross

sections drawn through the study area

reveal that, there are about five rock

units above the basement complex

section, which is the bedrock in the

dam and lake area case study.

These rock units are surface wadi

fill, partially saturated sand layer,

saturated sand and sandstone layer,

compacted sandstone and conglomerate

layer, and the fractured basement and

non-fractured basement complex.

Water seepage is a common

problem in most of the dam sites,

where the impounded water seeks paths

of least resistance through the dam, its

foundation and abutments. The ground

conditions and the geological features

of the dam site greatly influence the

amount of seepage and its relevant

effects. From this point of view, the

aim of our study is to estimate the

calculated permeability coefficient (K)

for the layers suggested for the

construction of the dam and for the

underlying layers, which will reserve

the water at the lake area, that includes

the most compacted and competent

layers above the basement, as well as

the compacted sandstone and

22

conglomerate and the weathered

fractured basement layer.

The basement rock section is

divided into two tears are the fractured

basement and the compacted basement

complex tear (fresh rock) according to

the comparable variation in the

compressional velocity values recorded

through this section that ranged

between 3850 and 4000 m/sec for the

fractured basement, but ranged

between 4800 and 8700 m/sec for the

fresh basement and ranged between

2300 and 3300 m/sec for the

compacted sandstone and conglomerate

unit.

The estimated permeability

coefficient (K) illustrated that, it

increases by the decrease of the

compressional wave velocity. The

estimated permeability coefficient (K)

ranged between 3250 and 3700 cm/sec

for the compacted sandstone and

conglomerate layer, ranged between

1000 and 2800 cm/sec for the fractured

basement layer and ranged from 505

and 910 cm/sec for the basement

complex bedrock.

Based on the obtained results, the

basement complex bedrock lied at a

depth from the ground surface ranged

between 50 and 60 meter, has a low

permeability coefficient. The bedrock

at this level is recommemded to be

suitable for deep constructions if

needed and for reserve water at the lake

area.

There are limitations in the use of

seismic refraction interpretations of

jointing assessments to estimate the

permeability coefficient. These mainly

stem from the fact that there are several

properties and features influencing on

the seismic velocity, and it is

impossible to avoid uncertainties when

variations in the velocity is linked

mainly to one or more of these.

Knowledge of the geological

conditions linked with comprehensive

experience in refraction seismic

measurements is important in reducing

these limitations.

Increase in stress level is known to

cause closing of the joints and hence

the potential to indicate variations in

joint density is reduced where the

measurements are performed in deep

depths. Thus, for this reason the

seismic refraction measurements

generally give better results on or near

the surface where the stress level is low

or moderate and with greater seismic

velocities than 3000 m/sec.

Acknowledgments

The author admires the very helpful

cooperation of EGEC staff for

providing the field data. Also, the

author thanks the two anonymous

reviewers for valuable comments that

helped to clarify and summarize the

content of this article.

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