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Engineering Journal, 45, 7, pp. 33-37, 1962-08-01
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Soil shrinkage damages shallow foundations at Ottawa, Canada
Bozozuk, M.
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National Research Council 2overnment Specifications
F
INE GRAINED SOILS are often very sensitive to changes in soil moisture content; when dried they shrink and when wetted they swell. The resulting g r o u n d movements introduce a serious problem in the design of s h a 11 o w foundations. Torchinsky14 and B a r a c o s and Bozozukl have reported building settlement problems in Eastern and Western Canada; D a w s o n h a s re- ported on the foundation movements of small houses erected or, expansive soils in the United States; Collins3 and Isaacsl(' have described founda- tion problems in South Africa and Australia, respectively; and Ward'" has shown that vertical soil move- ments depend on weather conditions in England.Damage due to soil shrinkage is a serious problem in some parts of the city of Ottawa. In many residential buildings large cracks similar to those shown in Fig. 1 may be seen, and streets and sidewalks are affected. In many cases an undulating riding sur- face develops in a street bordered by a row of large trees (Fig. 2 ) .
Site Studied
In 1954, the Division of Building Research of the National Research Council initiated a program to study the problem. An area 10 city blocks by three was selected for study. I11 general, the buildings within this area
Fig. 1. Damage caused by differential settlement.
Fig. 2. Uneven settlement of sidewalk near large map1
that it was modifiez by the inflow tures.
of fresh water ill several places. These Solar orientation of the houses ap- were then exposed by the sub- peared to have little effect on the
ent uplift which occurred after location of settlements. Most of the
laciers had disanDeared. Gener- 1'0"~es appeared to slope tobvards the
ate clamage. Cracks in ickwork up to '/s in. wide, clifferentinl settlement
4: Heavy clamage. Cracks l/4
in. or more, winclow and door frames distorted, dif- ferential settlement u p to 6 5: Severe damage. Cracks up to 2 in. wide, window and door frames badly distorted and the walls out of plumb, differential settlement es- ceeds 6 in.
Some buildings had tiltecl as com- ete units with relatively little dam- e occurring. Although the measurecl ential settlements were quite
, exceeding the ranges given in above scale, these cases were d from no damage to moderate. igure 3 shows a damage intensity based on thc house surveys. .e is no clistinct pattern to the
streets regardless of the damage zones. The worst damage occurrccl when trees grew close to the strue
tures.
The damage survey was reported in morc detail on 77 houses located along the south side of Waverly Street between Bank and Elgin; the south side of Somersct Street between Bank and Elgin; thc west side of O'Connor between MacLaren and Somerset; and along both sides of Metcalfe Street between McLeod and Gilmour Streets. To determine settlement, ele- vations were measured around the
perimeter of the house a t the first brickline. Assuming that the houses had been collstructed level, the amount of differential settlement was obtained.
Angular defolmation i n the walls
Fig. 3. Damage intensity map of central Ottawa.
/
/
o-I(/
I
N A T I O N A L MUSEUM GROUNDS$<.,I. I" r E f l
DATE OF SURVEY 2 0 - 9 - 5 5
I I
Ia
NO DAMAGE2 SLIGHT DAMAGE 4 HEAVY DAMAGE BORINGS 3
a
MODERATE DAMAGE 51
SEVERE DAMAGE*Settlements have occurred on more than one side of some houses. Therefore the total per cent distribution of settlcnlent for front, bnclr and side nlay exceed 100 per cent.
was determined by dividing the maxi- determined for various walls by di- borings were made by hand with a mum differential settlement by the viding the strain by the wall height. 1% in. spiral auger to a maximum distance over which it occurred, using Angular deformations were obtained depth of 1 4 ft., and the disturbed soil a minimum distance of 10 ft. for this for wood frame walls sheathed in samples tested in the laboratory. The purpose. Houses which had tilted and fiberboard, plywood, gypsum board water contents, maximum liquid limits were undamaged had very small and plaster, and for walls constructed and the plastic limits of the soils in angular deformations. The results of of clay tiles, brick and concrete the five zones are plotted in Fig. 4.
e survey are given in Table I. blocks. These results are given in There is no distinct change in
Table 11. Atterberg limits of the soil from one
Relation of Deformation to Damage The racking tests described in these zone to the other. The plastic limits
he average settlement in the "no reports17 show that wood frame walls in every case vary from 25% to 30% damage" zone was 1.3 in., producing are more flexible than solid masonry and the liquid limits from 70% t an angular deformation of 1/180 or walls by a factor of 10. Fiberboard, 80%. The mean water content in r 0.7 in. in 10 ft. Badly damaged plywood, gypsum board and plaster lation to depth, determined math houses had an allgular deformation of walls can withstand an angular de- matically by the method of lea 1/50. Skempton and MacDonaldIZ formation of from 1/60 to 1/270 in squares, shows that it is similar showed that the maximum an@ar the laboratory, whereas solid masoniy five zones. The principal differ cleformation for "no damage" condi- walls withstood values of only 1/500 that the maximum range in tions for large heavy structures was to 1/1000. Field observations in content is greater in the z
1/300, which is considerably less than Ottawa show that masonry walls have severe damage than in the zone o that measured in this survey. They withstood angular deformations of no damage. The maximum moisture concluded, however, that if settle- 1/18O without distress. Deformations content deviation from the mean in ment proceeded at a slow enough in the field occurred, however, over zone one was about 13%, in zone rate, much larger deform:ltions could a period of 60 years or more, a very three about 19% and in zone take place without damage to the slow rate of deformation compared about 27%. It appears that the structure. with the laboratory tests which were gree of damage is reflected in The National Bureau of Standards completed in a matter of hours. variation in the water content of
has issued numerous building mate- soil, and that this variation has r
rials and structures reports giving the Soil Survey sultecl -from clifferential clegrccs 0
results of racking tests on 8 ft. by 8 ft. Fifty-five soil borings were made in drying.
walls made of various building mate- the various zones at the locations Borings lnade llear a house at th rials. These tests were reviewed and shown in Fig. 3 in order to correlate corner of Nletcalfe and Frank Streets an angular deformation at first crack damage intensity with soil type. The s h o ~ e c l that the average water con-
clay tiles with cement-lime mo units with cement-lime mortar lock units with cement-lime mortar
Influence of Trees
Fig. 6. Variation in n~aximum vertical ground movements in Leda clay near elm
ig. 5. Annual ground water table trees in 1955. fluctuation from Frank St. to
Waverly St.
'
L O T L A Y O U T SHOWING W E L L P O I N T S It to much greater
er. The decrease of
d e m e n t s in streets ntercept and drain surface water cause the soils
11 not restore completely the s reasoning it is ap- uent surface watering clayey subsoils from educe shrinkage dur- summer. Small trees, or those
In addition, placing foundations at such depths where seasonal moisture changes are small and far from trees and streets will lessell the risk of foundation movement.
Conclusions
A survey of residential buildings in central Ottawa revealed a wide range of damage intensity due to settlement. The maximum angular deformation for no damage conditions in brick houses appeared to be 1/180. Greater angular deformations producecl cracks in the exterior walls. Laboratory rack-
mg
tests carried out by the U.S.National Bureau of Standards showed that masonry walls could only with- stand deformations of 1/1000, al- though wood frame walls could with- stand much greater deformations without damace. 0
An investigation of the properties of the soil in the five zones of damage showed no distinct variations. There was a greater variation in the water content of the soil, indicating greater differential shrinkage, in the zone of severe damage than in that where no damage occurred.
Paved streets and sewers tend to drain away surface water and cause a lowering of the ground water table and a reduction of the water content of the soil. The subsequent shrinkage in soil volume changes the structure of the soil and causes settlement of the ground surface. Consequently buildings on clay soils in Ottawa show greater settlements on the sides facing paved streets.
Large trees can cause severe settle- ments in buildings and damage to streets and sidewalks. By yemoving water through their root systems they cause a local drying out of the soil. The resulting non-uniform settle- ments can cause severe cracking in buildings and a ''roller coaster" sur- face i n a roadway.
Drying of undisturbed Leda clay breaks down its flocculated structure, forcing the plate-like particles into a more nearly parallel arrangement. Be- cause it also has low osmotic swelling, this clay does not completely regain its original volume on wetting.
To minimize the effects of ground movements in clays, lawns and gar-
Fig. 7. Water content variation in Leda clay near a 40 ft. elm tree at the Building Research Centre, N.R.C.
I
/ GROUND S U R F A C E 7 I I1
L I
0 2 0 40
DISTANCE FROM 4 0 ' HIGH ELM TREE. FEET
placed at a depth in th seasonal moisture change mum.
Acknowledgements
The author is indebted t Crawford, head of the Soil M Sectioil of the Division of Research, for his assistance an tion in this study. The paper tribution from the Division of Bu ing Research, National Resea Council of Canada, and is ITIF'' with the approval of the Director the Division.
References
1. Baracos, A. and M. Bozozuk. Sea
movements in some Canadian c
Proc. F o d r t h Internat~onal Confer on Soil Mechanics and Foundation aineering, Vol. 1 p. 264-268, 1957.
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