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Delarue, J.; Mariotti, M.; Meyer, J. P.
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Title:
NATIOI?AL RESEARCH COUNCIL OF CANADA Technical Translation TT-600
Some special foundation problems in Morocco. ( ~ u e l ues probl6mes sp6ciaux de fondations au Maroc
f
.
Authors : J. Delarue, M. Mariotti and J.P. Meyer-
Reference: Travaux, 39 (238): 225-232, 1955.
Translator: D. A. Sinclair, Translations Section, N.R. C. Library,
One of the major problems engaging the attention of the Soil Mechanics Section of the Division of Building Research is the study of foundations with special refer-
ence to the swelling and shrinking of the major clay types encountered as foundation material in Canada. This problem is shared with a number of other countries, Only recently, however, was it found that even such a distant land as Morocco has somewhat similar difficulties with foundations.
This developed from a study of the paper of which this is a translation. Its value and its relevance to Canadian work will be clear when it is studied. The
Division is therefore pleased to include it in its series of special Technical Translations. The Division is
grateful to Mr. D.A. Sinclair for preparing the translation.
Ottawa,
SOME SPECIAL FOUPSDATION PROBLEMS IN MOROCCO
The Public Testing and Study Laboratory (~aboratoire Public dfEssais et d'gtudes) founded in hTorocco seven years ago has been granted an ever increasing amount of space and means to enable it to render the kind of services expected from such a re- search organization in a modern state,
Placed under the technical control of the Directorate of Public Works, under the direction of the Building and Public Works Laboratories of Paris (~aborntoires du B8timent et des
Travaux Publics de paris), and enjoying the financial support of the Chambre Syndicale des Entrepreneurs Franqais in Morocco, the laboratory has the character of a public service with certain liabilities.
Its essential function is to carry out systematic tests required for the control of workings, but thanks to the generosity of the public services it has been permitted to participate in original studies occasioned by the opening up of large public projects (auscultation of rocks, systematic studies of injection grouting, a study of concrete block deformations).
It is not our purpose here to conduct a general survey of the activity of this organization on matters which will be the subject of separate papers, Rather we wish to draw the
attention of French engineers to some special foundation problems which find no equivalent in Europe and whose underlying mechanisms are gradually coming to light.
In general there are only minor differences between the problems encountered in Morocco and those of Metropolitan France,
These differences are mainly due to the nature of the materials or to the traditional labour patterns. However, the particular climate of l,?orocco, which year after year has a short rainy season followed by a very long dry one, has produced certain soils which
show a very unusual behaviour,
1. The Swelling of Clays and Their Effects
The rocky, sandy or silty formations which cover a large part of the territory are evidently not affected by the special climatic conditions which prevail in Morocco. Special foundation problems arise only when the following conditions are combined:
(a) A low water table;
(b) Deep clay fornlations, possibly covered by a thin coating of pulverulent and permeable soils. These formations often have a liquid limit above 50%, which seems to indicate a composition of the montmorillonite type. They always have a very low shrinkage limit of the order of 12 to
14%
below their natural water content. This means that every variation in their natural moisture content causes a considerable variation of volume, i.e.,swelling or shrinkage,
Due to capillarity there is always a moisture transfer through these clays from the water table towards the surface,
where it evaporates, But owing to the low permeability the amount of water lost in this way is much less than that lost by evapor- ation from an open water surface.
A clay absorbs water like a sponge, each grain of soil surrounding itself with concentric layers of water, These layers are strongly attracted near the centre, but the force of attrac- tion gradually diminishes towards the outer periphery so that less and less energy is required to separate the outer water layers from the material. The clay at the upper levels is deprived of
its layers of surface water until, as the vapour pressure at the surface of these layers decreases with the degree of dessication, an equilibrium is achieved between the water which continues to rise from the water table and the mean anl~ual evaporation,
Since these soils vsry greatly in volume they shrink as a result of the dessication by an amount that is equal to the drying which would result from the application of a hydrostatic pressure in addition to the external forces to which it is already
subject.
These soils are called t'superconsolidates". The super- consolidation sometimes affects considerable thicknesses, commonly of the order of 8 to 10 m. or more.
Clays which have been subject to such a process
generally have a characteristic appearance. They are interlaced by a network of planes or surfaces of conchoidal appearance with very smooth faces, which usually adhere tightly but tend to open up after very slight drying, reducing the soil to a large number of tiny polyhedrons (~ig. 1)" This reticulated system is the result of the relative movements in different zones during shrinkage which has brought about the superconsolidation,
When a building is constructed on soils of this kind the existing millenary equilibrium is disturbed. Evaporation is checked while the base continues to supply moisture slowly. The clay tends to take on a new state of equilibrium which is a
function only of the applied forces (weight of soils, pressures due to applied loads). If the pressures thus transmitted are less than the forces of dessication the clay tends to swell and may lift the construction. (~ig. 2 and
3).
The resulting damage has the following characteristics: (a) The damage chiefly affects light, single-storey buildings with masonry bearing walls.
( b ) The damage r e q u i r e s r a t h e r a long time t o appear and o f t e n does not begin u n t i l s e v e r a l y e a r s a f t e r c o n s t r u c t i o n .
( c ) The c l i a r ~ ~ c t e r i s t i c cracks a r e on an angle s l o p i n g
4 5 O ( ~ i g , 2 ) towards the corner 2nd g e n e r a l l y s t a r t a t a weak p o i n t ( a door o r window frame). They a r e wide a t the top and
t a p e r i n to7uards the bottom. Everything t a k e s place a s though the corner of t h e b u i l d i n g were s e t t l i n g . IIowever, these cracks
continue t o open up when t h e corner p r a c t i c a l l y detached from t h e c o n s t r u c t i o n i s r e l i e v e d of i t s normal loads, The l o a d s a r e now t r a n s m i t t e d towards the c e n t r a l p o r t i o n of the s t r u c t u r e . The e x t e r i o r w a l l s , e s p e c i a l l y the narrow f a c a d e s , b e g i n t o l e a n out- wards,
( d ) When some l i g h t c o n s t r u c t i o n i s a t t a c h e d t o the b u i l d i n g ( a veranda o r porch) the p o s t s which support i t o f t e n break under the t e n s i l e s t r e s s and the s i d e gap may reach s e v e r a l millime t r e s .
These phenomena a r e s i m i l a r t o those which a r e observed i n Europe i n shallow foundations on s o i l s s u b j e c t t o seasonal movements. However, the d i f f e r e n c e i s n o t due t o a s i n g l e change
of s c a l e . The cracks r a r e l y open and c l o s e with t h e seasons, This i s observed i n the following two cases:
(1) When the water t a b l e i s high, a s i n t h e case of the v a s t Rharb a l l u v i a l p l a i n ; the remedy i s then simple and c o n s i s t s i n b u i l d i n g the foundations on the permanent c a p i l l a r y f r i n g e .
( 2 ) When foundations a r e very shallow and r e s t d i r e c t l y on c l a y ( a s i n the c a s e of c e r t a i n b u i l d i n g s a t Souk-El-Arba of the Rharb).
I n g e n e r a l , however, t h e c l a y s a r e covered with a t h i n l a y e r of c a l c a r e o u s , p u l v e r u l e n t formations c u r r e n t l y known a s "tuft' t h r o w h o u t Morocco, This m a t e r i a l a p p a r e n t l y r e s u l t s - from
the soluble fractions of the soil on its way up. It appears probable that in this case the surface deposit is sufficient to
disperse the rain water. In general, then, the cracks open up gradually and the seasonal fluctuations are no more than secondary phenomena which are superimposed, without greatly modifying the
appearance, on a general movement resulting finally in the raising of the construction about its centre.
This description, of course, is only a first approxi- mation. While the broad lines are clearly observable, it may be rather extensively modified owing to the heterogeneity of the
soils and to accidental causes, such as the escape of water from pipe systems, drains or cesspools, or the preferential passage
of water through fillings that are too loose.
The phenomena tvhich vre have described are not peculiar either to ~ ~ o r o c c o or North Africa. They were discovered several years ago in a11 semi-arid co~mtries. They have noL been noticed here previously owing to the very recent introduction of rnodern customs. In the appendix we give a bibliography of publications dealing with this question. The work of Wooltorton in Central Burma and Jennings in South Africa have been of great value to us, But whereas these investigators studied primarily the be- h a v i o w of buildings and the effect of constructional practices we have been concerned first and foremost with defining the physical and mechanical characteristics of these swelling clays and in studying the process of swelling itself,
2, The Characteristics of the Preconsolidated Clays
The purpose of a complete study would of course be to provide data which would permit us to predict the extent of the movements to be expected, their distribution throughout various points of a structure, their distribution over a period of time and in short to provide as accurate and Faithful a picture of the expansion phenomena as Terzaghi has given in his studies of
the consolidation of clays and the settlements of structures
erected on wet clays. Unfortunately, however, the development of a useful theory did not appear possible for a long time to come. The same problems are encountered hamper the development of
a completely satisfactory consolidation theory and which,
essentially, are connected only with elements of second order: (a) The heterogeneity of sites which are particularly susceptible within this type of soil where limestone evaporation, silty formations and clayey formations are intermingled without any definite pattern.
(b) The necessity of defining rigorously the new physical characteristics; expansion pressure, expansion curve.
(c) The necessity of assuming that the movements are vertical only, whereas experience shows, and Mr. Wooltorton has confirmed this in Burma, that the horizontal co~rrponents of the movement are of considerable magnitude.
Other difficulties are also present which are peculiar to this type of problem:
(a) The movements of the water in the soil obey the laws of capillarity and not, as in consolidation, the laws of permeability due to the effect of a force gradiant (~arcy's ~aw).
These laws are much more complex and are still in the process of being reduced into a rational form,
(b) The angle from which the building is seen from the water table is small. Evaporation may continue to take place laterally and it is impossible to evaluate its effect accurately, The following conclusion m y be drawn: only differential move- ments are detrimental to the maintenance of a structure, Settle- ments which do not exceed half the total calculated settlement are currently being tolerated. It is probable that in tlie case of swelling this amount is greatly surpassed and the differential movements reach values m a h higher than half the maximum ampli-
(c) Over deep foundations (shafts or piles) the general expansion movement results in a vertical friction which tends to raise the shaft. We are seeking to discover the laws which will enable us to calculate the forces thus developed. It is not un- common to encounter shafts which have been broken in tension. Ho~vever, experiments on this are difficult to perform.
(d) Settlement of a clay increases its density and causes increased mechanical resistance, Capillary absorption on the part of clay, on the other hand, results in a settlement of the structure accompanied by a fairly large reduction in the mechanical properties of the soil, In establishing a pattern,
strictly speaking all these decreased resistances should be tabulated.
These problems can only be approached step by step and only partial answers can be given to all the questions which arise. We must also bear in mind that a complete study must necessarily be a collective undertaking extending over several generations.
A. Swelliw Characteristics of Clays
Figure 4 gives the odometric type of curve which is
obtained from swelling clays, The method employed will be de- scribed briefly. A sample of soil enclosed in the cylinder is placed in the presence of water and is subjected to a cycle of
loading and unloading, The loading is begun rapidly in order to oppose all swelling. As soon as the first settlements appear complete stabilization is awaited at each stage.
The consolidation curve gives the voids ratio, i.e., the ratio of volume of voids to volume of solids, as a f i c t i o n of the pressure. The fraction relative to the loading is made up of semi-logaritlmic co-ordinates of two-straight-line segments
connected by a curve, a horizontal one corresponding to the range of pressures where the material would have a tendency to swell and an oblique straight line, called a pure curve, which
corresponds to the range of pressures under which the material is compressible. The more clayey the soil, the greater will be the slope of the pure curve, After unloading, the voids ratio, and hence the volwne occupied by the soil sample, are m c h
greater than the initial voids ratio and initial volume, respec- tively, Casagrande has given a graphical construction which
enables us to deduce the preconsolidation load from this pressure curve. The graphical construction indicates this pressure to be
2.7
kgm./sq, c mIt is important to determine whether this pressure is actually the pressure the soil could exert on the walls opposing its movement if all displacement were prevented, We devised a cell of thin brass permitting direct measureinent (~ie;.
5).
A sample of soil enclosed in this cell without any possibility of expansion was moistened through tlrvo porous stones. Exact measure- ment of the tensile forces developed in the walls by means ofelectrical extensometers using a simple calibration quickly gives the pressure developed by the soil,
Generally speaking this pressure is always slightly less than the preconsolidation pressure and more or less equal to the pressure at which the odometric curve departs from the horizontal. For the sample studied thls was 2.1 kg~n,/sq,cm,
Finally we must be able to calculate the swelling which may take place beneath the @onstruction owing to a decrease of pressure on the soil, the latter pressure being transformed into
the swelling force at the effective restraint borne by the soil, Since clay is a material showing hysteresis, the unloading curve is a function of its previous history, and strictly speakiw it would be necessary to load the soil to its swelling pressure and
then to unload it. We assume that the various return curves would be deduced one from the other by a translation parallel to the vertical axis, and work with the curve indicated by the dashed line in the figure, This hypothesis has produced good results in
calculating the vertical movements during on-th-spot tests where we undertook a forced moistening of the soil,
B. Depth of Superconsolidation
While studying a well-defined zone (the Safi plateau) we mere able to make a sufficient number of tests (more than
thirty) so that an attempt could be made to determine the rate of superconsolidation in depth and to estimate the thickness of the zone beyond which the effects OF evaporation are practically negligible,
The study was difficult owfng to the grezt heterogeneity of the clays, which contained very different proportions of silt and whose plasticity index varied from 20 to 60, However, this enabled us to consider a second problem, Since a11 these
materials belong to a common geological formtion they have all attained their state of equilibrium under the action of external agents (weight of soils, evaporation) which have had identical effects at the same depth; the swelling pressure, which is the physical result of this, should, under these conditions, be a function of the depth only, the slight variations due to the fineness factor affecting only the equilfbrium water content and
the extent of the srrellfng,
After a few exploratory tests we adopted the liquid limit L.L. to define the fineness of the fraction of soil under study, This factor is comparatively easy to measure accurately and for soils of a similar nature and sirnilar formation it
suf'fices to define them entirely, By definition we called the relative water content:
where a is the water content necessary for complete saturation of the pores in the c l a p Otherwise, it differs little from the natural water content, which is 1
-
2% lower in absolute value,Thus this relative v~ater content varies from 0
-
1when the true water content varies from 0
-
L. L.The small circles in Fig. 6 give the relative water contents measured at different depths. These group themselves fairly well along a mean curve which is almost vertical.
It is also possible, without going into the details of the operation, to determine the simple laws connecting the liquid limit with the compressibility index (slope of pure curve) and the voids ratio under a fixed pressure greater than the pre-
consolidation pressure, for example 10 kgm,/sq. cm., i. e., E = 10,
frorn the different pressure-void ratio curves.
It is then possible to draw the curve which, for a given depth, would represent the relative water content in the absence of all preconsolidation, or, conversely, the curve of consolidation pressures which terminate in the observed mean relative water contents, (Fig.
7).
It is seen at once that the effect of dessication is evident for a depth of about twelve metres, The consolidation pressure deduced from these theoretical variations does not vary greatly with the fineness of the clay, The curve begins to de- flect at approximately 10 mo to take in the weight of the soils above the level being studied at approximately
13
rn.On the same diagram we have plotted for comparison with the theoretical curve the points representing the swelling
pressures with the liquid limit of the sample indicated for each point, as determinedby measurement. This provided us with two
typical curves corresponding to liquid limits in the vicinity of 80 and
40
which are very nearly parallel to the theoreticalOn the experimental curves it is noted:
(a) That the measured swelling pressures are somewhat lower than the theoretical preconsolidation pressures.
(b) That the fineness affects the results more than would be expected from the theoretical considerations, This, however, may be a result of experimental scatter due to the fact
that the swelling pressure becomes more and more difficult to measure as the material becomes more silty and less subject to
swelling,
(c) That no great error will be incurred by substituting a mean vertical straight line corresponding roughly to a constant pressure of 2 kg~n,/sq,cm, for the family of curves representing
the swelling pressures,
Regularities of this sort cannot be found for all sites and there are regions where the superconsolidation evidently
reaches incalculable depths,
C. On-the-spot Swelliw Measurements
We were able to make direct measurements of the
swelling pressures and amplitudes of a soil under forced moisten- ing at two different regions, We shall not go into great detail in describing the apparatus used, as this has already been done in a paper given at the Zurich Congress of Soil Mechanics,
A steel disk (Fig, 8) was lowered into a timbered excavation and was loaded with a weight equal to the weight of the soil which had been removed, The clay was artificially satu- rated by means of perforated pipes terminating in a bed of sand placed around the perimeter of the disk and covered with filling materials, The deformations of the disk were recorded by means
The swelling pressure was measured by mounting the
disk on a very rigid planking and interposing a ring dynamometer, The observations were extended over periods of three to five months,
Expansions were observed which increased proportionally with the square root of the time. In one case the swelling
reached 2.1 cn. and in the other case
4
cm. The swelling rates and their distribution over the test period corresponded very closely to those deduced from the mean consolidation curves,applying Terzaghi's theories on settlement, Generalizing, it m y be assumed that after a very long time a swelling of 5 , 0 crn. would have been reached in the first case and 12 cm. in the second.
Only in one case was it possible to measure the swelling pressure accurately on the spot, It developed slowly with the degree of moistening of the soil and its increase was again a linear
function of the square root of the time, reaching a maximum at the end of two months with an approximate fixed value of 1.9 kgm./ sq, cm, The laboratory tests gave a mean pressure of 2 kg.rn,/sq,cm,
These tests were clearly carried out under conditions which differed rather considerably from the natural ones. Never- theless they at least show that there is some hope of reproducing in situ phenomena in laboratory tests and that a rational theory may be developed when it becomes possible to determine the move- ments of the capillary water with accuracy
D. Mechanical Properties of Swellinp Clays The cracking of clays often makes it difficult to interpret the laboratory tests. It seemed preferEble to us to carry out systemtic loading tests carried to the point of free ramming. A disk 20 to 40 cm. in diameter is placed on the soil
in a timbered excavation at a given depth and is loaded. The curve representing the deformations as a function of the pressure
is practically rectilinear up to a value which we shall call the linear deformation limit, then it deviates. From this point on,
the deformations increase much more rapidly than the loads and tend towards an asymptote for a pressure which corresponds to total compaction,
For a material showing internal friction these values depend on the lateral pressure exerted about the perimeter. To prevent the breaking curve from extending laterally in the soil in place above the loaded surface the soil was excavated widely and
replaced by lightly packed filling materials which act well as a simple overload, By varying the overload it is possible to employ a system of equations by which the cohesion and angle of internal friction can be calculated.
Since the expansion causes a softening of the foundation soil, the same tests repeated after lateral soaking of the soil by means of perforated pii2es. But in order to simulate the natural phenomena, where this -:/eakening takes place after con-
struction on a soil which is already loaded, the movements of the disk were limited during the wetting process by clamping it to a planking with a wefght of
15
metric tons.If
we seek to interpretthe test results on the a s s ~ t f o n that the linear deformation limit corresponds to the critical pressure at which the first
irreversible slidings take place
-
a pressure defined by Ferrandon for circular foundations-
and assume that the ramming pressure is defined b y Caquotts or Terzaghi's classical forrnulae-
we find that all the values fit together harmoniously when the follo~ving values are applied, e,g. to the Safi clays:An internal friction angle of 10 to 12';
A natural cohesion of
700
gm,/sq,cm. reduced to 400gm./sq. cm. due to prolonged wetting,
Now, the laboratory tests, carried out on natural con- solidated samples under a standard fixed load and then rapidly
sheared, are translated, both in the course of classical shearing tests and in the course of triaxial tests, into a curve of in- creasing shearing strength with an angle of apparent internal friction of 23O and a cohesion of
350
to 600 gm./sq, cm., depend- ing on the strata,The apparent contradiction of these figures can be
resolved by assuming that the shearing strength is related to the standard pressure by the expression
T =
(3.5
+
po tan 12')+
N tan lo0,where po is the pressure exerted on the soil
-
swelling pressure in the case of soil in the natural state, weight of the earth on top of the foundation plan in the case of artificially moistened soil,This expression brings out the extent to which the pressure due to the capillary forces contributes to the physical strength of the soil. It enables us to determine the admissible working load in all systems of foundation, at all stages of con-
s truc t ion,
3.
Special FoundationsTwo types of foundation have been regularly employed to oppose the movements of the soils, namely deep foundations and shallow ones,
(a) Deep foundations
This is the system most generally employed wherever it is possible to determine the preconsolidation depth fairly accurately, It consists in anchoring the foundation by means of deep piles around or inside the normally consolidated zone, The safe working load is obtained by extrapolating the results of ramming tests. At present, it is from 3 to 4 ~cgm./sq,cm,
their base, on the one hand to increase the useful load and on the other hand to ensure anchorage against vertical motions. Under the effect of swelling the pile is in fact subjected by friction to tensile forces, the effect of which may be consider- able, This type of foundation has recently been employed in Texas and in South Africa, where it is usual to reinforce the piles with a rather high percentage of steel. In Morocco heavy, non-reinf'orced concrete is used. However, tests are being carried
on in the laboratory to determine the magnitude and distribution of the forces due to swelling of the soil which is traversed.
A n essential precaution consists in separating the
longitudinal beams joining the tops of the piles and supporting the walls from the soil. If this is not done, especially if the ground is used as a sunken form, the longitudinal beam quickly assumes the role of a ground-sill and Finally has to withstand soil reactions for which it has not been designed. The piles
are then ineffective and it is not uncommon to find them fractured in tension.
This solution is obviously impossible when the depth of normal consolidation cannot be determined in advance. Also it is too expensive for light constructions and it is then necessary to have recourse to shallow foundatfons,
(b) Shallow foundations
It is not sufficient, obviously, just to exert a
pressure on the soil equal to the swelling pressure in order to combat all movement. For light buildings the pressure quickly becomes diffused in depth while the swelling forces are still
considerable. The source of the movements may thus be situated far below foundation level, To be convinced of this it is only necessary to subject a ram resting on a dry clay to a cornpara-
tfvely heavy load, 'Slhen the clay is moistened at its base the upper face will undergo a general movement affecting the ram,
which may then execute a ramming action when the moisture has sufficiently weakened the upper strata.
Since the difficulty is due mainly to the fact that classical foundations, being too flexible, may undergo deforrn- ations incompatible with the maintenance of the superstructure masonries, only two solutions are possible:
(1) To increase the rigidity of the foundation so as to limit its deformations to values compatible with the maintenance of the superstructure,
(2) To increase the flexibility of the walls so as to enable them to share the movements without breaking,
With regard to the first ty-pe of solution, there are still no data available for calculating the required foundations, It has therefore been necessary to introduce arbitrary hy-potheses which must, of course, err on the side of caution, It should be borne in mind, however, that it is necessary to protect the weak points, i,e,, generally speaking the corners and the door and window openings,
In various countries constructions of an empirical nature have been studied and similar ones have been tried in Morocco. A construction was studied in Texas for light, villa-
ty-pe buildings and some applications have already been commenced in Morocco. The author of the study remarks that the classical floor, apart from its cost, would not furnish a solution to the problem as the distribution of soil pressures would differ too greatly from the calculated values, It seemed to him that the damage was due less to the actual bending of the walls than to
the twisting resultfw therefrom at the corners, to which
traditional construction is poorly adapted, This will best be combatted by adopting for the seating of the construction a shallow slab having a flat upper surface and a lower pyramidal one. This gives a foundation profile for which the inertia is at
a maximum in the centre where the concentration of pressures is greatest. Flat reinforced slabs have also been suggested with two rows of reiMorcements, an upper and a lower, paralleling the two diagonals of the slab,
We, in turn, have recommended tall, narrow foundation footings of great vertical inertia, The extreme form would be a ladder beam comprising two ties joined above and below by a wind bracing and the intervening spaced filled with heavy concrete, Since the mathematical hypotheses are still difficult to define,
it is assumed that the weight of the wall is transmitted entirely to the central third, but conversely in case of an accidental inflow of water into a corner, it is assumed that the long side may rest on the outer thirds, leaving the centre free,
Another solution, attempted in South Africa, has not been tried out in Morocco, It consists in considering each wall
as a beam capable of resisting stresses by its overall inertia. The mathematical hypotheses are substantially the same as in the preceding case, The tensile stresses are taken by reinforcements corresponding to the traditional ties above and below. Additional reinforcements are simply added to take into account the concen-
tration of stresses near the window and door openings and to resist the shearing stresses in the parts weakened by these
openings, This mode of construction, which is easy to execute in brick, where thick joints can be used economically for the accormo- dation of the reinforcements, is difficult to carry out in masonry, It would be well adapted to monolithic concrete.
On the subject of flexible foundations little valid information is available, Some experiments have been carried out in Texas with constructions of ceramic bricks of complex shape laid without mortar, The results appear promising.
Conclusion
In this article we have preferred, instead of under- taking a statistical study of Moroccan soils, which would have been of local interest only, to draw the attention of engineers
to some problems which are still unf'amiliar.
It must be realized that under certain soil and climatic conditions phenomena of a paradoxical nature may occur, Buildings may heave rather than settle; in foundations which cover a wide area, narrow, tall ground-sills should be used; it is better to build multi-storeyed buildings than one-storey structures. The
observations and measurements made thus far lead us to believe that the laws governing these matters may be clarified somewhat in the near future. However, before safety and economy can be reconciled in these matters many more experiments will have to be conducted and the cooperation of all the trade associations
interested in construction work will be needed.
Bibliography on the Expansion of Clays General
Terzaghi and Peck. Soil mechanics in engineering practice. p.128,
Burma
1. Wooltorton, D, A preliminary investigation into the subject of black cotton and Kyatti soils of the Mandalay
District. Proc, Int. C.onf, Soil Mech., Carribridge, Massachusetts,
1936.
2, Wooltorton, D, Movements in the desiccated alkaline soils of Burma, Proc, Am, Soc, Civil Ebgineers, January
1950.
3.
Discussion on the above (same publication, November1950).
South Africa
1. Jennings, JOE, and Kenkel, D. J. The use of under-reamed-pile foundations on expansive clay soils in South Africa.
National Building Research Institute, Pretoria, Bulletin no,
3,
p.9-15, September 1949.2, Steyn, Keeve, Reinf'orcement in brick walls as a means of preventing excessive cracking of buildings. National Building Research Institute, Pretoria. Bulletin no.
5,
p.49-64, September
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Jennings, J.E. Foundations of buildings in the Orange Free State gold fields, J, South Af'rican Instn. Engineers,49
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( 8 ) , March 1951.4. Rigby, Chas, A, Crack-resistance housing. Pt. 1 and 2,
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Jennings, J. E. ~oulkvement des bfltiments sur les argiles dessgchc?es. Comp te rendu 3e Congrks International de Mgchanique des Sols, Zurich,1953.
1, Smith, C.W, Study of design criteria for floating of
structural concrete slab floors laid on grade, Housing Research Foundation, San Antonio,
2, Dawson, R,F. The design of building footings on expansive clay soils. Proc, BRAB, Housing and Building in Hot Humid and Hot Dry Climates, Noveniber 1952.
Fig* 1
A t y p i c a l sample o f expanding clay. Note f l a t o r conchoiaal
f r a c t u r e surfaces and the s i z e of the shrinkage cracks,
Fig, 2 A t y p i c a l corner crack i n a l i g h t building. Fig* 3 Cracks i n a l i g h t b u i l d i n g e r e c t e d on a swelling s o i l ,
Pig* 8
On-the-spot swelling pressure,
In front soil watering pipe. A ring dynamometer trznsmits the soil pressure to a rigid planking.
Fig*
