hydro-behaviour of middle eastern desert soils

Journal o f Hydrology, 138 (1992) 41-52 41

Elsevier Science Publishers B.V., Amste rdam

[2]

Hydro-behaviour of Middle Eastern desert soils

Fathi M. Shaqour" and Hasan A. Al-Sanad b

~*Department of Earth and Environmental Sciences, Yarmouk University, lrbid, Jordan bCivil Engineering Department, Kuwait University, P.O. Box 5969. Kuwait

(Received 5 October 1991; revised and accepted 16 February 1992)

ABSTRACT

Shaqour, F.M. and Al-Sanad, H.A., 1992. Hydro-behaviour of Middle Eastern desert soils. J. Hydrol., 138: 41-52.

Surface settlement in desert areas occurs in response to fluctuating groundwater levels. Model experiments have indicated a minor settlement as a result of rise and fall of water level and this is attributed to the readjustment of soil particles to a better packing condition as a result of suction forces. Deposition of salts within the pore spaces has been found to result in swelling of 0.5-1% in medium-dense model samples, whereas dense samples showed a swelling of 4-8%. The method of drying affected the results; oven drying gave greater swelling than drying by a hot air current. The type of precipitated salt was found to control the degree of swelling; calcium sulphate results in greater swelling than that caused by calcium carbonate, and the latter exceeds that induced by sodium chloride. The results of tests indicate that changing groundwater levels can cause surface settlement and/or heave related to the associated processes of salt precipitation and/or dissolution.

INTRODUCTION A N D GENERAL BACKGROUND

Desert soils of the Middle East are mainly medium-dense to dense sands, which may be of aeolian or fluvial origin. Often these soils are cemented by carbonates and sulphates as a result of evaporation of saline ground water (A1-Sanad and Shaqour, 1987; Shaqour, 1990). Drying rather than wetting is the most notable phenomenon of desert areas. However, rapid urbanization has resulted in the disturbance of the hydrogeological system in many parts of the Middle East, and rising ground water has become a major problem in several cities including Kuwait, Doha, Jeddah, Cairo and many others.

Changes in ground water levels in the form of a general or localized rise or as a result of dewatering activities may lead to settlement or heave problems,

Correspondence to: F.M. Shaqour, Depar tment of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan.

42 F.M. SHAQOUR AND HA. AL-SANAD

owing to the change in packing of soil particles, hydration-dehydration processes, salt deposition and salt dissolution. Rising ground water tends to increase the compressibility of granular soils and may lead to a slight settlement (Paal, 1984). From a series of tests, Rethati (1983) concluded that fluctuation (rise and fall) of the groundwater table always results in consolida- tion, which can occur because of capillary action. Jenning and Knight (1957) and Sandra et al. (1988) discussed settlement resulting from the collapse of sandy subsoil on wetting, based on oedometer tests on samples at both natural moisture content and under inundated conditions.

Ground heave or settlement may also occur because of the deposition or dissolution of soluble minerals by the action of ground water. Deposition of soluble minerals within the capillary fringe is likely to cause heave as a result of crystal growth pressures. Many cases of affected roads and other surfaced areas, particularly where traffic is light, are well documented (Blight et al., 1974; Tomlinson, 1978; Taylor and Cripps, 1984). Sabkha areas are charac- terized by evaporite mineral formation, accompanied by blistering in dry seasons and dissolution in wet seasons (Shearman, 1963; Bush, 1973). Processes of hydration and dehydration controlled by vegetation were described in the A1-Khiran Sabkha, south of Kuwait (Gunatilaka et al., 1980). This paper presents the results of tests on volumetric changes of desert sands, resulting from ground water level fluctuations.

EXPERIMENTAL PROCEDURE AND TEST RESULTS

Tests were carried out on uniform sand samples from Kuwait, known locally as Jahra sand (Fig. 1). Model samples of medium-dense and dense sand were prepared in stainless steel boxes (300mm x 300mm x 300ram). Small samples of dimensions 60 mm x 60 mm x 20 mm were also prepared. The model medium-dense samples were prepared to a unit weight of 16.5 kN m 3, which corresponds to a relative density of 65%, using a raining technique. The rainer consists of a flame with the sample box fixed at the bot tom and a feed box at the top. Two sieves are placed between the boxes (Fig. 2). The base of the feed box is perforated and shutter controlled, to allow sand raining. The hole diameter of the plate and the spacings between holes control the relative density of the prepared samples (Eid, 1987). By trial and error, hole diameters and spacings were varied until the required relative density of 65% was attained, at a hole diameter of 18mm and a spacing of 60 mm. The model dense samples (18.0 kN m 3) were prepared by compacting the sand in stainless steel boxes. Small-scale samples were prepared by pul- verising the sand in brass shear boxes. The amount of sand was weighed to obtain medium-dense samples. Tests were carried out to study the following conditions.

HYDRO-BEHAVIOUR OF MIDDLE EASTERN DESERT SOILS 43

I 0 0 I ' ' ' ' ' ' ' ' i , v , , , , w , I ~....,,~" . , I , , , • f

/ / ~ 80 - !

I

! t "~ 60 !

¢1 I ¢L • ~-, 40 t

~D

® 20 - I o . •

/

0 I , , , , , . . ~ 4 " ' " , , , , . . . . I . . . . . . .

LO01 0.01 O. I 1.0 tO Grain size (mm)

i C l a y I S i l t I S a n d ', I e r a v e l

Fig. 1. Gra in size d i s t r ibu t ion curve of J ah ra sand.

Effect of rise and fall of water levels

Four uniform and homogeneous samples of medium-dense Jahra sand were prepared by the raining technique to a relative density of 65%. Glass plates were laid on the surfaces of the samples, with dial gauges fixed at the top. Fresh water was introduced into the boxes from their bases to initiate a water table condition in the soil samples. The water level was incrementally raised in four steps to 28.0cm above the bot tom of the box. Volumetric changes were monitored during this rise and thereafter during cycles of rise and fall of water levels. The results of these tests are shown in Fig. 3.

The volumetric change of dense Jahra sand as a result of saturation was also studied by introducing fresh water from the base of a prepared dense sample. Such samples showed no settlement in response to wetting.

Effect of salt deposition and dissolution

Halite To study the volumetric changes caused by deposition of halite, seawater

was evaporated from model and small samples by hot air and oven drying, respectively. Model samples of medium-dense Jehra sand, previously prepared by the raining technique, were connected to a supply of seawater. Hot air (at 40-50°C) was passed across the upper surfaces of the samples to promote evaporation.

In early tests the upper surfaces were capped with glass plates as references for measuring volume changes. It was found, however, that no salts were

• J~ui~J ~ql JOj uJ~j~ 'e!p ~ ! l ~ u ~ q ~ S -g -3ff!j

,,,o O'g 0"0

~ ' J J J f J f J J J J J J J J J J J f J J J J J J J J J J J f ~

• ° . . . . . . . . .

xo~ oldm . . . . . . . . . . .

n n u d p. t lOS ~

o~.es

g A ~

~ l q ( T lxqt . to~e([ " ~ | ~ I D ~ I G I I ~ I a G D r m G B I ~ I D G D , ' r : ~ , . . . . . . . . . . .

xo E h.m~.~[ . . . . . . . . . . . . .

O V N V S - ' ] V V ' H ( ] N V ~f10(~)VHS 1,~1-I ~17


TIME ( D A Y S )

5 10 20 30 C0 50 i

~ 3. . ~ . . ~

~ . . . . . . . . . . ~.~ ~--: . ~ ,

• i I I 2o04 , , , , , ,, h -- ~ . . . . . . . . . J , t II i I~ l

OOJ I l l t I I J i i ~, , L I ' I ' Ill, i ,, o - - ' . . . . . . . . . J i t >I 1 r , , , , , ,, ,','

o~ ....... :- . , ~ ............ ~;V~L_ .................

bottom I0 2 0 30 t-.O 50

o f I c lnk . T I M E ( D A Y S )

Fig. 3. Set t lement o f J a h r a sand as a result o f chang ing wa te r level.

precipitated at the surfaces below the plates and there were no volume changes. Salts were precipitated only at the edges of the plates. The solid plates were replaced by perforated plates with about 30% open area, but even then, no salt was precipitated over a period of 1 week. Finally, the plates and dial gauges were totally removed and the samples were exposed to hot air. Thereafter salts started to precipitate on the surfaces of the samples and 1 month later salt crusts were well developed to thicknesses of 0.5-2.0mm. Blisters of a few millimetres across and about 5.0 mm in height with open cavities underneath were observed.

Salt concentration below the suface crust seemed to drop drastically, indicating little or no salt deposition, as shown in Fig. 4. However, saturation by flesh water of one of the samples crusted with chloride salt caused dissolution of the salt crust and led to settlement. One sample was soaked from the bot tom by introducing fresh water and another was inundated con- tinuously from the top while volumetric changes were monitored. The results are presented in Figs. 5 and 6.

Calcium sulphate and carbonate A series of tests were carried out in which sulphates and carbonates were

precipitated chemically to simulate the formation of such salts in naturally occurring uncemented soils. A chemical process was necessary because it is difficult to precipitate considerable amounts of these salts directly by evaporation of seawater, as their solubilities are very low (Brownlow, 1979). Small and model samples of medium-dense sand and model samples of dense sand

46 v.M S.AQOUR AND ..A. AL-SANAD

S a l t C o n t e n t (%}

I 2 3 4 5 6 " / * I I I I I I

~. . . - -o

75-

a a ,.= 150-

2 2 5 -

300

Fig. 4. Average salt content for two boxed Jahra sand samples, subsequent to salt deposition by evaporating sea water.

were prepared with 2% by weight of calcium sulphate and calcium carbonate separately. The samples were initially mixed, first with sodium sulphate and then with sodium carbonate, and then an adequate quantity of calcium chloride solution was introduced to allow chemical reaction to take place as follows:

Na2SO, + CaC12 + 2H20 --* CaSOa-2H20 + 2H20 + 2NaC1

Na2CO3 + CaC12 ~ CaCO3 + 2NaC1

Hot air was passed across the surfaces of the model boxed samples to enhance drying, whereas oven drying was used for the small samples. Volumetric changes were measured in the same way as for the chloride samples, by recording the height of the sample before and after salt deposition. Swelling of 0.5-1% was recorded in the medium-dense model samples, whereas the dense model samples showed a swelling of 4-8% as a result of deposition of carbonate and sulphate salts respectively. Acicular crystals of calcium sulphate were observed, whereas the carbonate salt showed lath-like crystals. Detailed discussion of these results is given below.

Effect of hydration

To simulate hydration processes which can occur in desert areas as a result of extreme temperatures, other model samples of Jahra sand were prepared. The sand was mixed with 0.5% by weight of the semi-hydrous mineral basanite

HYDRO-BEHAVIOUR OF MIDDLE EASTERN DESER]-SOILS 47

o.o~.

E E ~ , 0 . 2 5

E

0 . 5 0 ° \

0.75

i [ i t

~ 141-

24 48 72 96

T i m e i n H o u r s

Fig. 5. Settlement o f artificially chloride cemented medium-dense Jahra sand due to gradual rise o f water from the bot tom of the sample.

(CaSO4" ½H20) and reconstituted to a medium-dense state of 16.5 k N m -3 unit weight. Fresh water was introduced from the base while volumetric changes were monitored. The use of only 0.5% basanite was to illustrate the effect of even minor amounts of such a mineral in changing the soil volume by hydration. The result of this experiment is shown in Fig. 7.

D I S C U S S I O N

Tests to measure volumetric changes of Jahra sand as a result of changing water levels within the model boxed samples have shown a settlement of, on average, 4.0 mm. The first stages of the tests showed continuous settlement during the rise of water level. Additional settlement was recorded as a result of a rapid drop of the water level to the bottom of the box, after which no further movement was recorded despite repeated cycles of rise and fall of water level. These results are in agreement with those of Rethati (1983), who recorded settlement during rising groundwater levels as discussed above.

48 F.M. SHAQOUR AND H.A. AL-SANAD

0.0,

A

E E - 0 . 5

E

E

~ -1.0

7- ° .g e-.

.S _ I.~

0"~ c,

~=.

0 o (3o

_2.0 i I I I I I I I I I

0 20 40 60 80 I00

T i m e ( H o u r s )

Fig. 6. Settlement due to innundating previous artificially chloride cemented Jahra sand.

1.0

El El - 0.75

m

0.5C

e l

"" 0.25 o

¢: a

0.0 I i i i 20 40 60

T i m e ( H o u r s )

Fig. 7. Change in height of the medium dense Jahra sand due to the occurrence of 0.5% basanite mineral.


Tests on Jahra sand compacted to maximum dry density and optimum moisture contents showed no settlement as a result of rising water levels. These results are probably best explained by readjustment of soil particles to a more stable packing condition, caused by suction forces induced during the partially saturated state between dry to soaked conditions. Such forces were unable to affect the sand which was already compacted to the optimum level.

Precipitation of chlorides as a result of evaporating seawater by hot air drying caused a very slight increase in vertical height (of less than 0.5%) in both medium-dense and dense Jahra sands. The precipitated salts were observed to concentrate close to the surface as salt crusts; below this, the salt concentration decreased rapidly, as indicated by the salt content depth for two boxed samples. Oven-dried small samples showed similar small volumetric changes to a maximum of 1%, as a result of precipitation of very small amounts of halite. Using samples containing previously precipitated chlorides, gradual settlement to a maximum of 0.6 mm was brought about by a gradual and stepped rise of the water level. Inundation of a second sample showed a rapid initial settlement of about 1.2 mm, as a result of dissolution of the salt crust, and gradual settlement thereafter.

Tests involving the chemical precipitation of sulphates resulted in a range of volumetric effects which were apparently controlled by particular factors in the various tests: type of salt; method of drying (whether by oven or hot air), which seemed to influence the rate of precipitation of the salts; pore sizes of the host sand particles and the form of crystallization. The influence of these factors is discussed in the following paragraphs.

Precipitation of calcium sulphate and calcium carbonate salts resulted in a maximum increase of up to 8%. Samples in which sulphate salts were precipitated showed a greater increase in height than those containing carbonate salts. This might be explained in part by the form of crystallization; acicular crystals were observed in the case of sulphates, whereas carbonate and halite crystals had lath-like habits. Sperling and Cooke (1980) reported that acicular crystal forms exert more pressure than prismatic or tabular types. Prismatic and tabular crystals exert more pressure than lath-like forms (Winkler and Singer, 1972). Similarly, the different swelling characteristics may relate to the hydrated nature of the calcium sulphate (gypsum) compared with anhydrous calcium carbonate and sodium chloride.

Results of tests on the large model samples showed less increase in height than for the small samples. This difference in swelling between hot air and oven drying might be caused by variation in the rate of drying, which is supposedly higher in oven-dried samples. This might lead to higher super- saturation ratios, which would result in higher crystallization pressures (Winkler and Singer, 1972) and greater volumetric changes.


Dense Jahra sand experienced greater increase in height as a result of precipitation of calcium salt than did the medium-dense sand, although the amount of salt was equal in both soils (2% by weight). The difference could be explained by variation in pore sizes; these are presumably smaller in dense samples (better packing) and consequently crystallization pressures are greater in these samples as discussed earlier. Comparing the total porosity of both medium-dense and dense sands (37% and 30% respectively) with the amount of precipitated salts (2% by weight) shows that the pore spaces in both sands can easily accommodate the precipitated salt without necessarily any change in volume. To illustrate this, let us assume a unit volume of 1 m S for each of the medium-dense and dense sands.

For the medium-dense sample: Bulk density of soil is 1.65 Mg m 3 Porosity is 37%, and hence volume of voids is 0.37m 3 Mass of salt is 0.2 x 1.65 = 0.033Mg Volume of precipitated salts is 0.033/2.3 = 0.0143 m S For dense sand: Bulk density is 1.82 Mg m 3 Porosity is 30%, and hence volume of voids is 0.3 m 3 Mass of salt is 0.02 × 1.82 = 0.0364Mg Volume of salt is 0.0364/2.3 = 0.0158m 3 It can be seen that the volume of voids in both medium-dense and dense

Jahra sand samples is much greater than the volume of the precipitated salts. This might lead to expectations of no consequent volumetric changes, but actually both soils expanded. This expansion can best be explained by the mechanism of salt deposition, which is in part controlled by the pore sizes and interconnections. Soils with large pores connected by networks of narrow pores are relatively susceptible to expansion by crystal pressures (Taylor and Cripps, 1984).

In the present experiments, salts were precipitated by allowing the calcium chloride solution to rise by capillary action, which means that the solution is mostly concentrated at the contact between the grains, and it is there that the salt deposition most probably occurs, leading to expansion. For the medium- dense sand, slight and immediate readjustment of grains could be possible, whereas for the dense sand such an adjustment is unlikely, and consequently a greater increase in volume may be expected. This may explain the measured variation in volumetric increase from about 1% for the medium-dense to about 8% for the dense sands.


CONCLUSION

In the present study it was found that volumetric changes of soils can occur as a result of processes which are common in desert areas. Such processes are mainly groundwater level changes, precipitation and dissolution of salts. Model experiments on medium-dense sand showed that cycles of the rise and fall of water levels can cause settlement which is significant at the first stage only and becomes negligible at later stages. Very small or even no settlement was noticed in dense samples as a consequence of water-level fluctuations. Tests on artificially cemented sand indicated an increase of volume as a result of salt deposition or hydration of previously deposited anhydrous salts in the soil. Surface settlement in response to dissolution of these salts by inundation or rising water levels was evident.

ACKNOWLEDGEMENTS

The authors thank the Kuwait University Research Unit for financing this work, and the British Council for their financial support. Thanks are also due to A.C. Lumsden and S.R. Hencher, Leeds University, UK, for their con- structive comments, useful ideas and fruitful discussions.

REFERENCES

AI-Sanad, H. and Shaqour, F., 1987. Effects of groundwater level fluctuation on the engineering properties of desert sands. Kuwait University Research Unit, Final Rep. Ev028, 364 pp. (unpublished).

Blight, G.E., Stewart, T.A. and Theron, P.F., 1974. Effect of soluble salt on performance of asphalt. Proc. 2nd Conf. on Asphalt Pavement in South Africa, Durban, South Africa, Section 3, pp. 1-13.

Brownlow, A.H., 1979. Geochemistry. Prentice-Hall, Englewood Cliffs, N J, 498 pp. Bush, P., 1973. Some aspects of the diagenetic history of the Sabkha in Abu Dhabi, Persian

Gulf. In: V.H. Burser (Editor), The Persian Gulf. Springer-Verlag, Heidelberg, pp. 395-407. Eid, W.Kh., 1987. Scaling effect in cone penetration testing. Ph.D. Thesis, Virginia Polytechnic

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from Al-Khiran, Kuwait. Nature, 288: 257-260. Jenning, J.E. and Knight, K., 1957. The additional settlement of foundations due to the

collapse of structure of sandy subsoils on wetting. Proc. Fourth Int. Conf. on Soil Mech. and Found. Eng., 27 August, Cape Town, pp. 316-319.

Paal, T., 1984. Change of groundwater level - change of design parameters. Proc. 6th Conf. on Soil Mech. and Found. Eng., 1984, Budapest, pp. 229-236.

Rethati, L., 1983. Groundwater in Civil Engineering. Elsevier, Amsterdam, 451 pp. Sandra, L.H., Houston, W.N. and Spadola, D.J., 1988. Prediction of field collapse of soils due

to wetting. J. Geotech. Eng., 114 (1): 40-58. Shaqour, F.M., 1990. Effects of groundwater level changes on the engineering properties of

desert sands in Kuwait. Ph.D. Thesis, Leeds University, 306 pp. (unpublished).


Shearman, D.J., 1963. Recent anhydrite, gypsum, dolomite and halite from the coastal flats of the Arabian shore of the Persian Gulf. Proc. Geol. Soc. London, 1907: 63-64.

Sperling, C.H.B. and Cooke, R.V., 1980. Salt weathering in arid environments, Part 1. Theore- tical considerations. Geography, No. 8. Bedford College, London.

Taylor, R.K. and Cripps, J.C., 1984. In P.B. Attewell and R.K. Taylor (Editors), Mineralogical Controls on Volume Change in Ground Movements and their Effects on Structures. Surrey University Press, pp. 268-303.

Tomlinson, M.J., 1978. Airfield construction on overseas soils. Symposium on Airfield Con- struction on Overseas Soil - Section II, Paper 6239, pp. 232-246.

Winkler, E.M. and Singer, P.C., 1972. Crystallization pressure of salts in stone and concrete. Geol. Soc. Am. Bull., 83: 3509-3514.

hydro-behaviour of middle eastern desert soils

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