TRANSPORT and ROAD

RESEARCH LABORATORY

Department of the Environment

TRRL REPORT LR 523

ARABIAN SALT-BEARING 'SOIL (SABKHA)

AS AN ENGINEERING MATERIAL

by

C.I. Ellis, B.Sc., C.Eng., M.I.C.E., A.M. Inst., H.E.

Overseas Unit

Transport and. Road Research Laboratory

Crowthorne, Berkshire

1973

Abstract

1. Introduction

2. Definition of sabkha

3. Geclo~'

4. Engineering properties

4.1 Summary of test data

4.2 Unsurfaced sabkha roads

4.3 Effect ofsabkha on route location

4.4 Sabkha as fill or sub-base

4.5 Sabkha as road base

5. Use of similar material from other areas

5.1 South Africa

5.2 North Africa

5.3 Australia

5.4 Sudan

6. Experiment on the Abu Dhabi-Dubai road

7. Conclusions

8. Acknowledgements

9. References

CONTENTS

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© CROWN COPYRIGHT 1973

Extracts from the text may be reproduced

provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

ARABIAN SALT-BEARING SOIL (SABKHA)

AS AN ENGINEERING MATERIAL

ABSTRACT

Sabkha is essentially a carbonate sand containing varying proportions of evaporitic minerals and is found in the salt fiats of the Arabian peninsula. This Report discusses the highway engineering properties and uses of the material and compares its performance with that of similar saline materials in other arid areas of the world. The manner in which the evaporitic minerals contribute to the performance of the material in road construction is not fully understood and in order to find out more about their influence on the performance of sabkha road bases, a full-scale experiment has been constructed to study the relative performance under traffic of three different sabkhas used as road bases under a thin bituminous surfacing.

1. INTRODUCTION

Sabkha is a term used in Arab countries to describe an inland or coastal salt fiat. It is also used to describe

the materials which form these flats.

The purpose of this Report is to provide a summary of available information on sabkha which may be

of value to highway engineers working in hot arid environments. Whilst the information in this report is mainly

concerned with the materials from the coastal fiats of Abu Dhabi and Dubai, two states within the recently

formed United Arab Emirates (formerly known as the Trucial States) much of it will be equally applicable to

similar regions elsewhere in the world.

The variable and unusual characteristics of sabkha (especially after rain) have long been experienced by

travellers in this region even if they have chosen camels rather than motor vehicles as their mode of transport.

Both Thesiger 1 and Innes 2 in their popular books about the Arabian peninsula have described some of the

difficulties encountered in traversing sabkha.

Under normal dry conditions the sabkha provides an excellent running surface for wheeled vehicles but

under high water table conditions vehicles can break through the surface crust and find themselves up to the

axles in a liquid mud.

L

2. D E F I N I T I O N OF SABKHA

The most common spelling of the word 'sabkha' has been used in this report but alternative spellings are as

follows:

sebkha, subkha, sabkah, sabhkat

A glossary of desert terms produced by the Military Engineering Experimental Establishment 3 gives the

following general definition : -

sebkha, sabkha Bottom of a closed depression, zone of evaporation for accumulated run-off

from a shallow subterranean water table, characterized by the presence of

salt deposits and absence of vegetation. Usually consists of fine textured

materials and is soft when wet. Can also be used for salt marshes or marine

lagoons.

In the particular case of coastal sabkha the groundwater is largely marine derived and Shearman 5 points

out t h a t . . . 'as geomorphological entities these coastal sabkhas are the desert zone analogues of the salt

marshes of temperate coastlines and they may be built up of carbonate or non-carbonate sediments or ad-

mixtures of the two'. Sabkha may be geologically termed a playa and under the title of 'Playa variation' J.T. Neal q has

discussed the characteristics of sabkha and other similar landforms. In order to illustrate the variations in

regional nomenclature Table 1 below has been reproduced from his paper in Arid Lands in Perspective,

edited by William G. McGinnies and B.J. Goldman, by kind permission of the University of Arizona Press,

copyright 1969.

T A B L E 1

Regional playa nomenclature

Locale

United States

Mexico

Chile

Australia

Russia

Mongolia

Iran

South Africa

North Africa

Arabia

Jordan

Iraq

India

Pakistan

General Terms

Playa, dry lake, alkali flat

Laguna, salina

Playa, lake

Pliazh

Gobi, nor

Daryacheh

Pan, vloer mbuga

Clay-silt Playas

Dry playa, clay playa

Laguna

Clay pan

Takir

Takyr

Dayq

Clay pan kulpfannen (lime)

Saline Playas

Salt flat, salt marsh, salina

Salina

Salina (moderate salt) salar (much salt)

Salt pan, salina

Tsidam

Tsaka, nor

Kavir

Salt pan, kalahari

Sebkha

Ghor

Hawr

Rei

Hamun

Garoet, qavat

Khabra

Oa Foydat

Sebkha, chott

Mamlahah (salt flat) sabkah (coastal salt fiat)

Sabkhat

2

The coastal sabkhas of Abu Dhabi and Dubai may be defined for engineering purposes as extensive flat

desert areas of silty sands containing a high percentage of calcium carbonate and varying smaller percentages

of gypsum, anhydrite and other salts produced by evaporation of sea water or of groundwater from the high

water table. The surface texture may vary from smooth to puffy and in some cases the salt crust forms marked polygonal pressure ridges.

The distribution of sabkha areas within the United Arab Emirates is shown in Fig. 1. and an indication of the type of terrain is shown in Plates 1 and 2.

3. GEOLOGY

The geology of the coastal sabkha plains of Abu Dhabi has been studied in great detail particularly by the

Geology Department of Imperial College London, and their findings have been well documented. 5- 18

The sabkha was formed by levelling of Pleistocene sand dunes through wind erosion and extreme storm

tides, followed by a recent period of sedimentation. Since the deposition of these sediments, there has been

extensive formation of evaporitic minerals as a result of reactions which involved the marine derived ground-

waters. The groundwaters themselves have been concentrated by evaporation, and as a result, most of them

are virtually saturated with respect to sodium chloride. The water-table is usually within 2 metres of the

sabkha surface,and salinities of the groundwater range from 70 to about 400 parts per thousand compared

with salinities of 55 to 75 parts per thousand in the adjacent lagoon waters.

Figures 2 and 3 (after Evans et al 6 )show details of a prof'fle through the sabkha on a line AA shown

on Figure 1. The sabkha is broken down into four main units but highway engineers will be mainly concerned

with units 1 and 4. The other units would usually be found in saturated conditions and it would be impractical

to excavate them for highway construction. Thus, in this area, sabkhas likely to be used for road construction

would probably be brown quartzose carbonate sands. The performance characteristics of sabkha are usually

attributed to the variable content of evaporitic minerals, and Table 2 (also from Evans et al 6) gives an

indication of the variable composition with depth.

Gypsum is most common in the sabkhas beneath the abandoned beach ridge where it occurs as scattered

crystals or as a gypsum mush. Farther inland it forms cemented masses of sediment a few mm to a few cm

thick. Butler 1 6 has stated that gypsum is the stable calcium sulphate mineral in contact with brines of

chlorine contents less than 145 parts per thousand by weight and anhydrite at chlorine contents greater than

145 parts per thousand by weight. Anhydrite occurs mainly in the brown sediments in the top o f unit 4 and

is reckoned to occur as (a) sharply defined nodules, (b) both regular and irregular contorted beds and

(c) irregularly-shaped masses which grade almost imperceptibly into the surrounding brown sediment. Except

where they are now exposed all occurrences are soft and plastic.

Low magnesium Calcite is found in the brown sabkhas together with a low dolomite content and sparse

occurrence of magnesite. Halite is present to some extent in most of the sediments above the water table.

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4. ENGINEERING PROPERTIES

In an environment where traditional road building materials are economically available the use of sabkha

would normally be excluded. However, in spite of its failure to meet conventional specifications, local

experience in the Arabian Gulf has shown that under certain circumstances its performance has been better

than expected. In general terms we may consider a good sabkha road as an example of salt stabilisation

(i.e. as a silty carbonate sand naturally stabilised with evaporitic minerals). A study of the work done on

salt stabilisation, notably in USA, provides a useful background to the mechanics of salt stabilization and

Thornburn and Mura 19 have carried out a useful review of the literature on this subject. They conclude that

salt s tabi l isat ion. . . 'has been carried out successfully in the field for many yearsbu t at tempts to utilise

laboratory tests to predict field behaviour have generally been unsuccessful. This is probably because lab-

oratory curing procedures have not usually involved wet/dry cycles which may be necessary to produce the

required strength'.

Some of the effects of salts that are most likely to influence the engineering properties of sabkha a r e : -

(a) absorption of moisture from the air by sodium chloride at Relative Humidities greater than

80 per cent.

(b) absorption of moisture from the air by calcium chloride at Relative Humidities greater than

30 per cent. (In coastal areas of Abu Dhabi the daily range of Relative Humidity is approx.

4 0 - 1 0 0 per cent.)

(c) increased surface tension is created when sodium chloride is dissolved in water thus reducing

evaporation.

(d) increased solubility of limestone, dolomite and gypsum in sodium chloride solutions as compared with pure water.

instability of gypsum (CaSO 4 2H20 ) under hot and humid conditions since dehydration and

hydration occur at natural temperatures with consequent changes in volume.

Crystallization as a cementing agent.

(e)

(0

For assessing the value of sabkha to a highway engineer, it is convenient to discuss the situation under

the following headings:

(i) Summary of test data

(ii) Unsurfaced sabkha roads

(iii) Effect of sabkha on route location

(iv) Sabkha as sub-base or fill material

(v) Sabkha as road base.

For this discussion all comments relate to the types of sabkhas that are easily accessible to engineers,

i.e. the brown sabkhas rather than the grey sabkhas. The figures quoted are based on information obtained

by the TRRL and from results made available to TRRL by various organisations. It should be emphasized

that sabkha is a very .variable material and that in the present state of knowledge the selection of sabkha for

5

road works should normally be based on field trials. However, there are indications that with more experience

gained f rom controlled field trials it may be possible to select sabkhas on the basis of their chemical and

physical properties as determined in the laboratory 20.

4.1 S u m m a r y o f t e s t d a t a

Particle size distribution - results of wet sieving tests on numerous sabkha samples from Abu Dhabi and

Dubai have shown that, in general, between 55 and 70 per cent of the material lies between BS sieve sizes 300Pro

and 7 5 ~ (No.52 and No.200) and hence they are outside normal specifications for mechanically-stable

materials. Figure 4 shows how sabkha gradings compare with the grading limits normally accepted for

mechanically stable bases 20. Samples containing material from the top 150 mm (6 inches) of a sabkha area

will generally have a high proportion passing BS Sieve 75psn (No.200). Water solubility may also affect the

determination of particle size distribution if a wet sieving technique is used since all dissolved material will

be classed as passing the BS 75grn (No.200) sieve. Curtis et al 9 have recorded a sabkha 4 - 5 cm (1% - 2

inches) below the surface with a maximum 26 per cent by weight soluble in water. Samples tested in Abu

Dhabi by a T R R L team for possible use as road base materials gave water solubilities of between 3 and 12 per

cent. Where it is necessary to be precise sieving may be carried out with a non polar solvent such as kerosene

or methylene-chloride and this technique has been carried out at TRRL by Russell in his examinations of

sabkha samples and will be reported shortly.

Plasticity - the brown sabkhas are generally non plastic as indicated by zero shrinkage in the linear

shrinkage test 21 but isolated samples showed shrinkages between 0 and 4 per cent. This would indicate max.

P.I. values of about 8.

Compact ion - the brown sabkha of Abu Dhabi generally gives maximum dry densities in the BS Heavy

Compact ion Test 21 of between 1.71 and 1.91 Mg/m 3 (107 and 119 lb/cu ft) with corresponding optimum

moisture contents of 15 to 8 per cent.

CBR - for samples compacted at opt imum moisture content and 95 per cent of BS Heavy Maximum

Dry Density a CBR value between 10 per cent and 15 per cent may be expected. At 100 per cent relative

compact ion this figure may be increased to over 60 per cent but experience in the field suggests that this

higher degree of compact ion is rarely achieved in the initial construction. Laboratory samples compacted to

95 per cent relative BS Heavy compact ion and left for 20 days before testing have shown an increase in strength

to give CBR values between 25 per cent and 50 per cent. In-situ CBR's of over 80 per cent have been reported

from existing sabkha tracks and values of the same order are suggested from the results of a laboratory investi-

gation of small samples of sabkha base material taken from existing surfaced roads in Dubai.

Chemical properties - sabkha exhibits considerable variations in its chemical composition, as shown in

Table 2. Samples taken with road applications in mind have given the following range of values:

Calcium Carbonate content varies between 12 - 85 per cent

Calcium Sulphate (Gypsum and Anhydrite) between 0 - 30 per cent

and Sodium Chloride from about 0.8 - 28 per cent.

During field work in Abu Dhabi it was found convenient to make approximations to the carbonate,

gypsum and chloride contents on the respective bases of (a) the percentage of sabkha soluble in hydrochloric

6

acid, (b) loss in weight between material dried at 80 ° C and 130 ° C and (c) the percentage of material soluble in water.

The method for the more accurate determination of the chemical composition of sabkha type materials

will appear in a TRRL report under preparation by Russell, together with a more detailed discussion of their chemical properties.

4.2 Unsurfaced sabkha roads

Until 1966, except within the town of Dubai, there were no paved roads in the states which now form

the United Arab Emirates. Wherever possible the existing tracks followed the sabkha fiats since they provided

a hard flat surface for travelling over in contrast to the soft sand areas which were the only alternative.

These tracks performed satisfactorily with little or no maintenance for most of the year and tracks in regular

use often exhibit a characteristic black surface, which at first glance may even be confused with a bituminous

surfacing. Only in the rare event of rain or when there was an unusual combination of wind and tide, did the

tracks become impassable. Until very recently many important town roads in Abu Dhabi were sabkha tracks.

Maintenance takes the form of soaking the surface with water and then reshaping with a grader.

The good performance of unsurfaced sabkha roads would appear to depend on the formation of a thin

salt crust which becomes very tightly compacted under traffic. In a humid area the salt's affinity for water

enables the sabkha to hold enough water to keep itself bound together, but at dawn and dusk when the

humidity is at its highest the sabkha surface may become very slippery.

Deterioration of unsurfaced sabkha roads takes the form of potholing rather than rutting.

In this type of environment unsurfaced sabkha roads may still play an important role for lightly trafficked

routes and there is a good case for the construction of properly engineered unsurfaced sabkha roads to be used

as the first stage of a stage construction process.

4.3 Effect of sabkha on route location

It has already been stated that most existing unsurfaced tracks followed the sabkha plains where possible.

A decision that may need to be made is whether future new roads in coastal areas should follow a route across

the sabkha plains or whether to move the proposed routes farther inland, usually into soft sand.

The following factors will affect the decision reached:

(1) An embankment will probably be required over the sabkha plains in order to overcome the

adverse effects of:

(a) inundation by the sea at times of high tide and onshore winds.

(b) possible flooding due to freak rainstorms.

(c) high water table.

Costs will be dependent upon the height of embankment which is considered necessary and an

assessment of the risks of subsequent settlement.

(2) If economic factors are not of prime importance an inland route may overcome possible objections

to driving large distances on featureless coastal salt flats.

4 . 4 Sabkha as fi l l or sub-base

Normal specifications for sub-base require that the material should have a CBR of over 25 per cent.

At the time of compaction few sabkhas will exhibit this strength but many will attain the required strength

after a few days. Trafficking will generally encourage the formation of a sound well knit surface and improve

the degree of compaction. Sabkha has been used as fill and in effect as sub-base on the Abu Dhabi - Dubai

road which is at present under construction.

There is no evidence to suggest that selected sabkha should not be used as sub-base material in conditions

where the ground water table is likely to be more than 1 metre beneath the final road surface and the pavement

is suitably protected from the ingress of moisture.

4 . 5 Sabkha as road base

If sabkha could be used satisfactorily for road bases in this particular environment where other base

materials are scarce then road building costs could be significantly reduced.

Not unnaturally consultants with a reputation to preserve have been reluctant to use sabkha bases for

surfaced roads which are likely to carry heavy traffic for long periods. However, sabkha bases have been used

with some success on town roads in Dubai and Abu Dhabi.

During a visit to the Dubai area in 1967, over 30 small samples of sabkha were collected from borrow

pits or from bases of existing roads. These were analysed chemically and the results compared with a local

assessment of the suitability of each material given by local road engineers. At that time it was tentatively

concluded that the following chemical criteria may prove a useful guide in selecting suitable sabkha for road

use;

(a)

or (b)

(c)

A carbonate content of approx. 50 per cent by weight or greater

A lower carbonate content in conjunction with a sulphate content of approx. 5 per cent by

weight or greater.

High sodium chloride content may be tolerated provided that correspondingly high proportions

of sulphate and/or carbonate are present.

As a result of suction tests and the consideration of equilibrium moisture content in road bases it was

also concluded that adequate performance may be expected where the water table is more than 1 metre below

the surface. However in areas with large fluctuations in the depth of the watertable there may be volume

changes due to variation in moisture content, and recrystallisation of mobilised salts.

Another factor affecting the suitability of sabkha as a base is the way it behaves when covered with a

layer o f bitumen or bituminous material. In this respect also the performance of sabkha has been the subject

of conflicting reports.

The application of cut back bitumen as a priming coat on a sabkha base has met with mixed success.

Some engineers report that no problems were encountered but others have found that the bitumen layer was

completely 'rejected'. Experience from Abu Dhabi and Dubai suggests that such failures could not be attributed

to bad construction practice.

8

5. USE OF S IMILAR M A T E R I A L FROM OTHER AREAS

Materials with some similarities to sabkha have been used with varying degrees of success by engineers in other

countries and a brief summary of their experiences is given below, together with comments about likely

sabkha performance. The author has not visited any of the countries mentioned below and the information

which has been extracted from official correspondence and published reports is quoted without comment .

5.1 South Africa 22

Sabkha type materials have not been used for road bases under bituminous surfacings in South Africa

or South West Africa. However, the coastal salt deposits of the Swakopmund area of South West Africa make

good unsurfaced roads, though slippery when wet. Further inland gypseous calcretes also provide good

materials for unsurfaced roads.

It is suggested that the main problems in using such materials may be crystallization of soluble salt and

hydration causing blisters and lifting and cracking of bituminous surfacings over unstabilised bases. The above

factors may additionally cause disintegration of the top inch of cement-stabilised bases made f rom this material.

The South West Africa Roads Department specify a maximum of 1 or 2 per cent gypsum in bases under

bitumen, but this figure is not based on a study of failed and unfailed bases; a higher gypsum content than

this would probably be allowed for unstabilised bases. Weinert and Clauss 23 suggest maximum chloride

contents of 0.5 per cent and sulphate contents not exceeding 0.05 per cent.

The presence of water may create hazards such as:

(a) Swelling due to the transition of anhydrite to gypsum

(b) 'sulphate' attack on cement stabiliser due to gypsum content

(c) retardation and possible prevention of the hardening process of the cement stabiliser due to high quantity of gypsum

(d) collapse due to solution of halite crystals.

Acid formation is not considered as a hazard when carbonates are available for neutralization.

From these comments it may be concluded that the South Africans would consider most sabkha as

unsuitable for use as a base under a bituminous surfacing but acceptable for unsurfaced roads.

5.2 North Africa 24~ 25, 26, 27

Material from the "chotts" of the Sahara has not been used for road construction because the simulta-

neous presence of chloride and clay in a gypseous sand led to large differential swelling in the base. The

gypseous sands that were used for road construction contained ~ 30 per cent CaSO 4 and only about 5 p e r cent

CaCO 3. The compressive strength of the gypseous sand is a function of the gypsum content and of the

percentage of fines. In general roads were not built on "chot ts" sfnce it was believed that deformations

would occur sooner or later. Material containing smaller percentages of gypsum (CaSO 4 2 H 2 0 ) were used

as sub-base.

Acceptable construction practice requires that the material must be wetted homogeneously using some

form of pulvimixer, to a moisture content which takes into account the eventual evaporation caused by the

9

high temperatures. Compaction can then be carried out at opt imum moisture content. Pavement layers must

be extremely even and grader operators must produce perfect surfacings first time. Reworking results in

laminations due to the non-adhesion of layers brought back to the surface and therefore defects must be

remedied by complete re-mixing.

The completed base is sealed with a cutback bi tumen and a two or three layer surface dressing was

found to be the most suitable surfacing

(a) because the layer is thin, deformations can occur without cracking

(b) because of the fairly high proportion of bituminous binder added, ageing is slow and the binder

retains its flexibility.

Occasionally a "coffee grounds" effect is caused by the use of certain binders as a priming coat. Surface

cohesion is lost due to the selective absorption of the binder flux oils 24 and it is reported that it may be

cured by using a more viscous cutback.

5.3 Australia 28' 29

It is believed that due to its high calcium carbonate content sabkha should behave better as a road

material than material f rom the clay pans in arid Australia. Suggested selection criteria would be maximal

calcium carbonate content and minimal proportions of halite anhydrite and gypsum.

Cole and Lewis 29 have shown that sodium chloride contents in excess of 0.5 per cent are sufficient to

cause disruptions of compacted sandy clay soils beneath untrafficked areas. The bitumen surface over such

areas exhibited a complete lack of bond between the bitumen and the underlying soil' or gravel. The soil

immediately beneath the bitumen had become ' f luffy ' and unstable.

5 . 4 Sudan 30

A material used for unsurfaced roads and known locally as salt marsh is available along the Red Sea

coastal plain and has been described as basically a silty substance containing varying amounts of sand and

vegetable matter. I t is found in all low lying areas that are either subject to periodic inundation by sea water

or have a very high water table. In the latter case it is thought that salt pan that is excavated from more than

2 metres above standing water level will not possess the hygroscopicity necessary to retain the moisture

required for compaction. At the other end of the scale there is salt-marsh that lies just above and below

standing water level that is highly hygroscopic containing 40 per cent - 50 per cent salt by volume. This

material is grey in colour and when rolled out and dry will form a hard durable water resistant surface.

Two features are responsible for the durability of salt pan, both fundamentally depending on the retained

moisture content of the surfacing; hygroscopicity of the material and, the height of the water table. Grey

salt pan will last well on ground over a low water table, but surface (brown) salt pan relies on a high water

table, (i.e. within 1.5 metres of surface). Thus the use of this material resolves into two groups depending

on the standing water level of the country being traversed.

10

6. E X P E R I M E N T ON THE ABU D H A B I -- D U B A I R O A D

In order to gain further information on the use of sabkha as a base material, a full-scale experiment was made

on the Abu Dhabi-Dubai road by a team from the Overseas Unit, TRRL, during August-September 1971.

The object of the experiment was to study the performance of three different sabkha materials. The essential

differences between the three sabkhas are tabulated in Table 3. One experimental section of each type of

sabkha was sealed above and below in order to isolate the base from moisture movements. For comparison

a gravel and a crushed rock base were also included in the experiment (see Figure 5).

T A B L E 3

Chemical characteristics of sabkhas used

for full scale experiment

Silica

Sodium Chloride

Calcium Chloride

Magnesium Chloride

Calcium Carbonate

Magnesium Carbonate

Calcium Sulphate

Sabkha Sabkha Sabkha A B C

26.6

5.4

4.3

0.3

11.7

7.6

33.2

28.6

5.7

2.1

0.3

56.0

1.7

NIL

45.3

0.1

3.1

0.2

37.2

1.7

4.2

The experiment was constructed on a sabkha embankment crossing a sabkha plain about 5 km from

the junction with the Abu Dhabi-A1 Ain Road. Up to formation level the construction was the same as that

for the non-experimental road on either side. During construction of the experiment, traffic was diverted

onto the floor of the sabkha plain.

The compaction of the formation was checked by means of dry density tests determined in accordance

with the 4 in sand replacement method specified in BS 1377 21. Sections 4, 5 and 8 were then sprayed with

two applications of crude oil in order to provide a waterproof surface. The base material was spread by a

motor grader, sprayed with water to raise the moisture content for compaction and then mixed and levelled

with a grader. Compaction was checked again using the sand replacement method of determining dry density

and the surface was then sprayed with MC2 followed by a double surface dressing. A detailed report will be

made when evidence is available on the relative performance of the different sections.

It is intended that the sections should be covered with a 50 mm thickness of asphaltic concrete surfacing

before the contractor finishes the whole road at the end of 1972. However, if the surface dressed sabkha

sections perform satisfactorily it is hoped to persuade the authorities to delay overlaying until an indication

of their long-term service under traffic is established. This would give an indication of whether, for lightly-

trafficked roads, surface-dressed sabkha, ultimately strengthened with premixed overlays when traffic

increases, would be satisfactory and economical for this area.

Whilst it is too early to suggest which of the three sabkhas gives the best performance as a base material

it may be of value to note some of the construction problems associated with laying these materials. Sabkha

'A' laid on Sections 3 and 4 was by far the easiest material to use. Its natural moisture content was such that

11

it could be compacted without the addition of further water and rolling very quickly produced a firm well

knit surface. In contrast Sabkha 'C' laid on Sections 7 and 8 required the addition of a considerable quantity

of water and all attempts at compaction produced a loose and overstressed surface. The surface finally

achieved was only barely acceptable and was produced by successive watering with saline water and rolling.

Some of the above problems but less severe were experienced with Sabkha 'B'. All sections were opened to

traffic for two days to improve the surface condition before MC2 prime was sprayed.

7. CONCLUSIONS

1. Sabkha is a very variable material and thus its performance in road bases is likewise bound to be very

variable.

2.

.

On present evidence the most suitable sabkha for road building is likely to be one which satisfies the

following characteristics:

(a) an~ability to compact to produce a sound well-knit surface

(b) a carbonate content greater than 50 per cent by weight o r lower carbonate content with a

sulphate content approximately 5 per cent by weight or greater.

(c) high sodium chloride content provided that correspondingly high proportions of sulphate

and/or carbonate are present.

Standard laboratory tests do not necessarily'provide adequate prediction of field behaviour and field

trials remain the most reliable method of assessing the performance of sabkha as a road building

material. However, with further experience from full-scale trials it may be possible to select sabkhas

for use as bases from the results of laboratory tests.

4. I f sabkha is used as a base material the road surface should be at least 1 metre above the highest water-

table.

5. Different selection criteria are needed for surfaced and unsurfaced sabkha roads. For unsurfaced roads,

the hygroscopic properties of sodium and/or calcium chlorides assist in maintaining the necessary moisture

content required to give a sound well-knit surface but these constituents are not necessarily beneficial under

a bituminous surfacing.

8. ACKNOWLEDGEMENTS

The author wishes to acknowledge the help and assistance received from many people and organisations.

In particular:

H.H. Sheikh Zaid bin Sultan A1 Nahyan - Ruler of Abu Dhabi for his cooperation and support for the

T R R L team based in Abu Dhabi.

H.E. Sheikh Hamden bin Mohammed, A1 Nahyan, Minister of Public Works,

and Engineers and Officials f r o m : -

Ministry of Public Works, Abu Dhabi

Dubai State Engineer

Trucial States Development Council

12

British Petroleum

Consolidated Contractors Co. Darwish Engineering Co.

Sir William Halcrow & Partners

Pauling (Middle East) Ltd.

Sauti Overseas Anstalt

9. REFERENCES

1. THESIGER, W. Arabian Sands, Penguin Books Ltd. Harmondsworth, 1964.

2. INNES, M. The Doomed Oasis, Fontana Books, 1963.

3. MILITARY ENGINEERING EXPERIMENTAL ESTABLISHMENT CHRISTCHURCH. Report No. 1124

Glossary of local physiographic and hydrogeological terms about World hot deserts. (Military Engineering Experimental Establishment, May 1969).

4. NEAL, J.T. Playa variation. Arid Lands in perspective. Edited by W.G. McGinnies and B.J. Goldman. The American Association for the Advancement of Science and the University of Arizona Press.

5. SHEARMAN, D.J. Origin of marine evaporites by diagenesis. Transactions of Institution of Mining and Metallurgy. Vol 75 1966 Bulletin No. 717 pp B208-B215.

6. EVANS, G., V. SCHMIDT, P. BUSH and H. NELSON. Stratigraphy and Geologic,history of the sabkha, Abu Dhabi, Persian Gulf Sedimentology, 12 (I 969) 145-159.

7, EVANS, G. The recent sedimentary facies of the Persian Gulf region Philosophical Transactions of the Royal Society of London 1966, 259 No. 1099 Series A pp 291-298.

8. SHEARMAN, D.J. Recent anhydrite, gypsum, dolomite and halite from the coastal fiats of the Arabian

shore of the Persian Gulf. Proceedings of the Geological Society of London No. 1607 (Session 1962-3) Published 2 July 1963.

9. CURTIS, R., G. EVANS, D.J.J. KINSMAN, and D.J. SHEARMAN. Association of dolomite and anhy- drite in therecent sediments of the Persian Gulf. Nature, London. 197 pp 679--680. 1963.

10. EVANS, G., C.G.St.C. KENDALL, and Sir P.A.d'E SKIPWITH. Origin of the coastal fiats, the sabkha of the Trucial coast, Persian Gulf. Nature, London Vol 202 pp 759-761 1964.

I I. WELLS, A.J. Recent dolomite in the Persian Gulf. Nature, London Vol. 194 pp 274-275 1962.

12. SHEARMAN, D.J. and P.A. d'E. SKIPWlTH. Organic matter in recent and ancient limestones and its role in their diagenesis. Nature, London Vol 208 pp 1310-1311 1965.

13. EVANS, G., D.J.J. KINSMAN and D.J. SHEARMAN. A reconnaissance survey of the environment of recent carbonate sedimentation along the Trucial coast, Persian Gull Developments in Sedimentology Vol. 1. Proceedings of 6th. Int. Sed. Congress 1963. Elsevier Publishing Co. 1964.

13

14.

15.

16.

17.

18.

19.

20.

21.

EVANS, G., and D.J. SHEARMAN. Recent celestine from the sediments of the Trucial coast of the

Persian Gulf. Nature, London Vol. 202 pp 3 8 5 - 3 8 6 1964.

KINSMAN, D.J.J. The recent carbonate sediments near Halat el Bahrain, Trucial coast, Persian Gulf.

Developments in sedimentology Vol. 1. Proceedings 6th Int. Sed. Congress 1963. Elsevier Publishing

Co, 1964.

BUTLER, G.P. Modern evaporite deposition and geochemistry of coexisting brines the sabkha, Trucial

. . . . . ' -- "-- '~ voi. ,~ pp tvia,~,~t ~96:,. ~ o a s t , A~abian ~ u , . Journal o f Sedimentary Petrology" ~ ' " " No. I 7 0 - 8 9 . . . . 1. • ,,

WELLS, A.J. and L.V. ILLING. Present day precipitation of calcium carbonate in the Persian Gulf.

Developments in Sedimentology Vol. 1. Proceedings 6th Int. Sed. Congress 1963. Elsevier Publishing

Co. 1964.

KINSMAN, D.J.J., S. GOLUBIC, R.K. PARK, and R.J. PATTERSON. Early diagenesis of carbonate

sediments in a supratidal evaporitic setting. Second Annual progress report and third semi-annual

progress report (1970) (Preliminary) Department of Geological and Geophysical Sciences, Princeton

University USA.

THORNBURN, T.H., and R. MURA. Stabilization of soils with inorganic salts and bases: A review of

the literature. Highway Research Record No. 294 pp 1 -22 1969.

O 'REILLY, M.P., and R.S. MILLARD. Roadmaking materials and pavement design in tropical and

sub-tropical countries. Ministry of Transport, RRL Report LR 279, Crowthorne, Berks. 1969

(Road Research Laboratory).

BRITISH STANDARDS INSTITUTION. British Standard 1377:1967. Methods of testing soils for

civil engineering purposes, London 1967 (British Standards Institution).

22. Correspondence between TRRL and National Institute for Road Research, South Africa.

23. WEINERT, H.H., and K.A. CLAUSS. Soluble salts in road foundations. Proc. 4th Reg. Conf. Soil

Mech. Fndn. Eng., Cape Town, Vol. 1 pp 213 -218 . Discussion in Vol. 2.

24. Correspondence between T R R L and Laboratoire Centrale du Ponts et Chaussees.

25.

26.

27.

28.

14

Roads in the Sahara Desert -- The primary role of gypsum in the building of desert roads. African Roads

and Transport. January/February 1960 Vol.XVII! No.1 p 27.

FUMET, P. Roadways made of gypseous sand and of chemically stabilized sand. Symposium on soil

stabilization. Nairobi 3 - 8 Oct. 1960. (Scientific Council for Africa South of the Sahara).

FENZY, E. Particularites de la technique routiere au Sahara. Revue Generale de Routes et des

Aerodromes. No. 411 de Juin 1966.

Correspondence between T R R L and Commonwealth Scientific & Industrial Research Organisation, Australia.

29. COLE, D.C.H., and J.G. LEWIS. Progress report on the effect o f soluble salts on stability of compacted

soils. Third Australia - New Zealand Conference on Soil Mechanics and Foundation Engineering. 1960. (Institution of Engineers, Australia. Sydney, 1963).

30. BIDGOOD, R. Report on the use of natural hygroscopic materials for road surfaces and bases (with

reference to the Red Sea coastal plain - A.E. Sudan.) (Unpublished).

15

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,--~,~ D ias tem HW High w a t e r level

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Dalnrn i t~

Fig. 2. SABKHA PROFILE ON AA FROM FI6.1

A A ..-.~ m) l~- Dolomite common _ I

1 BR - i Magnesi te present I

m White salt crust ~j '- on surface ~ ~ - ~ ~ "~

-o ; : . b 7 ° ' " O l(km) ~ . " ~ ~ / " , , , / }

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,.~.r -r '~- -vvv" Unconformi ty 1771 Zone of a bundant L H L i th i f ied hor izon V I A gypsum,gypsum mush ,. ; , ,~ ( d i a s t e m )

Fig.& UPPER COMLEX SABKHA FROM FIG.2.

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OISTRIBUTIOH OF SABKHAS

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Sabkha plain near Abu Dhabi

I - : . . . . . . . . . L " . . . . :~ :-~: Y::~1_"2 ]L , .S - .

Neg No CR 1148/71/1 PLATE 2

Sabkha plain with outcrop in background

(1787) Dd891796 3,500 1/73 H P L t d . , S o ' t o n G1915 PRINTED IN ENGLAND

A B S T R A C T

Arabian salt-bearing soil (Sabkha) as an engineering material: C I ELLIS, B.Sc., C.Eng., M.I.C.E., A.M. Inst., H.E.: Department of the Environment, T R R L Report LR 523: Crowthorne, 1973 (Transport and Road Research Laboratory) . Sabkha is essentially a carbonate sand containing varying proportions of evaporitic minerals and is found in the salt flats of the Arabian peninsula. This Report discusses the highway engineering properties and uses of the material and compares its performance with that o f similar saline materials in other arid areas of the world. The manner in which the evaporitic minerals contr ibute to the performance of the material in road construction is not fully understood and in order to find out more about their influence on the performance of sabkha road bases, a full- scale experiment has been constructed to study the relative performance under traffic o f three different sabkhas used as road bases under a thin bi tuminous surfacing.

A B S T R A C T

Arabian salt-bearing soil (Sabkha) as an engineering material: C I ELLIS, B.Sc., C.Eng., M.I.C.E., A.M. Inst., H.E.: Department of the Environment, T R R L Report LR 523: Crowthorne, 1973 (Transport and Road Research Laboratory) . Sabkha is essentially a carbonate sand containing varying proportions of evaporitic minerals and is found in the salt flats of the Arabian peninsula. This Report discusses the highway engineering properties and uses of the material and compares its performance with that o f similar saline materials in other arid areas of the world. The manner in which the evaporitic minerals contr ibute to the performance of the material in road construction is not fully unders tood and in order to find out more about their influence on the performance of sabkha road bases, a full- scale experiment has been constructed to study the relative performance under traffic o f three different sabkhas used as road bases under a thin bi tuminous surfacing.