Fast-growing trees can cause house damage

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FAST-GROWING

TREES

by

K.N.

Certain species of fast-growing trees, such as poplars, willows and elms, require copious quantities of water to support growth. Water is taken up by the r o o t s and transpired at the leaf surfaces; in midsummer one tree may require as much as 100 gallons of water a day. In w e t periods, the

groundwater is recharged with rain. During long dry spells, however, ground-

water lost to tree roots and directly by evaporation to t h e atmosphere i s not replenished and the moisture content of the soil is steadily reduced. Under these conditions tree roots extend in search sf untapped water supplies in the undisturbed clay that frequently l i e s just below foundation level.

The clays in the O t t a w a region are open-structured; they have h i g h moisture contents and thus high shrinkage potentials. High moisture c o n t e n t s

usually mean t h a t , even in dry years, some water will always be available to tree r o a r s . High shrinkage potentials unfortunately mean t h a t when water is withdrawn s o i l volume decreases and settlement results. If water were

extracted uniformly from a deposit of clay, settlements would a l s o be uniform but, because t h e root system is mare dense in the immediate v i c i n i t y of an

individual tree than at the o u t e r l i m i t where root extension takes place, the intensity of s h r i n k a g e is greatest near t h e tree, diminishing with d i s t a n c e u n t i l it is negligible j u s t beyond t h e furthest roots. House foundations

that happen to be in or on clay affected -in this manner s u f f e r destructive

differential settlements which, in turn, cause breaking of foundation walls

and diagonal or stepped cracks through the exterior f i n i s h . A r e l a t i v e l y

new house damaged in t h i s way, together with the s t a n d of trees responsible f o r the soil shrinkage, is s h a m in Figure 1.

Observations made by t h e National Research Council of ground settle- ments adjacent to a l i n e of elm t r e e s show how t h e proximity of t r e e s a f f e c t s

t h e magnitude of movements ( F i g u ~ e 2). In 1955, the d r i e s t year on record, settlements at t h e g-round surface exceeded 3 in. next to t h e t r e e s and more than 1 / 2 in. 40 ft away. Of greater significance were t h e large settlements

at the depths where house foundations a r e usually placed. Close to the t r e e s

these were greater than 2 in.; even 30 f t away they were in excess o f 112 in.

Damage t o b u i l d i n g s r e s u l t i n g from t h e r a c k i n g effect of such differential

settlements can be easily imagined.

Clays a r e composed of t h i n particles shaped like razor b l a d e s or sheets

of mica, What makes the clay d e p o s i t s of Ehe Qttarlra region rutusual i s t h a t the particles assume a configuration resembling a Ccard house4 (Figure 3 a ) . The arrangement of particles i s r e f e r r e d to as the " s o i l fabrict' and is a legacy

of t h e depositional h i s t o r y . Because the f a b r i c i s "openv wate-r c o n t e n t s are high as are t h e shrinkage p o t e n t i a l s , because t h e space between particles is o f t e n as much as two thirds of the t o t a l s o i l volume, Consequently, when the quantity of water h e l d in t h e spaces is greatly reduced, t h e f a b r i c y i e l d s and t h e soil "shrinks", [ F i g u r e 3b .) This shrinkage due to l o s s of volume is shown in Figure 4 . Unfortunately, t h e volume l o s t by s h r i n k a g e in these deposits can o n l y be partially recovered and only when an excess o f water becomes a v a i l - able a t some l a t e r date. Settlements caused by s h r i n k a g e due to water loss may b e large and the question of p r e v e n t i n g o r a v e r t i n g s e ~ i o u s damage t o b u i l d i n g s is of economic importance.

A l l of t h e soils in the Ottawa r e g i o n are n o t p r o n e to s e r i o u s shrinkage;

i t i s a problem only in areas where s o f t , compressible clays exist between foundation leuel and depths of 1 2 to 15 feet. A homeowner or b u i l d e r can assess any site by o b t a i n i n g samples of t h e s u b s o i l with an o r d i n a r y auger.

I f any of t h e samples can be remoulded into a slurry o r a s o f t p l a s t i c mass, t h i s i n d i c a t e s high water content. Such soils should then be formed into a w e l l - d e f i n e d shape (a cube or a cylinder using a soup can) t h a t can be observed

as it d r i e s . If it s h r i n k s substantially it is a problem s o i l .

All species of trees do n o t require t h e same large q u a n t i t i e s of moisture

f o r growth. Usual1y, faster-growing trees use t h e most water. Conifers grow comparatively slowly and may be safely placed closer to buildings than deciduous trees. The species o f trees mast f r e q u e n t l y involved in settlement problems are

poplar t r e e s because, b e s i d e s having very rapid r a t e s a f growth, they are

usually p l a n t e d in lines and close t o g e t h e r . Weeping willows and elms are a l s o troublesome. Maple, Chinese e l m and o t h e ~ broad-leafed species a r e l e s s

hazardous b u t have caused shrinkage of clay in dry years.

A goad rule t o follow I s to assume that thc s o i l may be a f f e c t e d as

f a r away fsom the t r e e t r u n k as the t r e e is h i g h although t h e d i s t r i b u t i o n of roots within t h e soil f o r t r e e s of v a r i o u s species is n o t the same. A rapidly growing t r e e , t h e r e f o r e , should not be p l a n t e d closer to a b u i l d i n g than a

distance equal to its h e i g h t a t maturity. A fast-growing tree already on a

b u i l d i n g s i t e should he removed if t h e distance between i t and t h e b u i l d i n g would be less than t h i s guideline requires. Groups or lines of t r e e s e x t r a c t

more moisture than individual trees and t h e e f f e c t s of drying and shrinkage may occur even f u r t h e r away. It is t h e f i n e rootlets of t r e e s t h a t absorb

water from the s o i l . The larger r o o t s give some indication of the a c t u a l

reach of t h e tree r o o t s , but they serve basically only to convey the moisture

and nutrients from t h e r o o t l e t s t a the t m k .

Difficulties encountered d u r i n g d r y periods where trees have been

planted too close to a building should be attended to as quickly as possible. Cracking o f walls n e x t to the trees is the first indicator of d i f f e r e n t i a l settlement; t h e only s u r e method of preventing any f u r t h e r damage is to remove these trees.

I f thc weather is consistently w e t , settlements caused by trees may n o t be encountered f o r years. Most problems arise when summer rainfall drops below normal. Water t h a t is usually available near the surface is quickly used up, t h e water table is lowered and the rootlets search out t h e water supply in t h e pores of t h e clay. The relative dryness or wetness of a summer

can be estimated from weather records. A d a i l y balance o f moisture used by

trees (a quantity related to mean daily temperatures and l e n g t h of day] against water supplied by precipitatton can be plotted and compared with records from other years. F i g u r e 5, a p l o t of such calculatfons, shows that it i s normal in Ottawa f o r soil moisture d e p l e t i o n to reach a maximum of about 7 i n c h e s by September. Studies and experience have shown t h a t it is at about this value that shrinkage begins ta occur below f o o t i n g level. Lower temperatures reduce the need f o r moisture and f a l l rains gradually recharge the ground with water. Figure 5 also shows plots f o r the driest year on record, 1955 [when s o i l moisture depletion exceeded 15 inches), f o r one o f the wettest (19521 (when t h e maximum value was j u s t over 3 inches], and for

1974 (when s o i l moisture depletion was almost 10 inches, a value well beyond

the t h r e s h o l d of shrinkage damage)

.

Although Ottawa clays have little potential to swell, where serious s h r i n k a g e and house cracking have accurred very quickly, some reversibility can sometimes be effected by keeping trenches around the foundation wall full of water f o r some weeks.

The most salient aspects of the problem are:

(13 The larges t h e t r e e the greater the need for moisture and the higher the

chances that a drier zhan normal year will cause s o i l shrinkage.

( 2 ) Fast-growing t r e e s require larger quantities a£ moisture. Chances are t h e r e f o r e h i g h e r that they will cause s o i l shrinkage,

(3) Studies in Ottawa and elsewhere i n d i c a t e that soil f u r t h e r away from the trunk of the t r e e than t h e Cree i s h i g h is n o t likely t o be affected. Tt should be kept in mind however t h a t as a tree grows in h e i g h t , zones o f soil t h a t were unaffected when it was young nay be subjected to root activity when it matures.

(4) Patterns of settlement damage caused by trees is directly r e l a t e d t o the proximity o f the trees. Settlement and damage are greatest closest to the tree, diminishing as the distance increases.

( 5 ) The most troublesome species of trees are willows, poplars, and elms. (Poplars, because they are frequently planted in rows, have been found to be the most damaging.) Large trees a£ other species have a l s ~ been the cause of clay shrinkage and settlements.

House damage caused by trees can be avoided if proper measures are taken at the start. Such measures need not prevent the achievement of

L E T S ( a ) M I C R O S C O P I C V I E W O F C L A Y FABRIC B E F O R E S H R I N K A G E L O S S O F V O L U M E ( b ) M l C R O S C O P l C V I E W OF C L A Y F A B R I C A F T E R S H R I N K A G E F I G U R E 3 R E D U C T I O N OF V O L U M E O F C L A Y R E S U L T I N G FROM L O S S OF W A T F R

Figure 4. Three cylinders of Ottawa c l a y showing effect o f change o f water content.