Soils in house construction

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Soils in house construction Crawford, C. B.

Ser TH1 392 no. 20 c . 2

mTc

H u ~ ~ ~ E D

SOILS IN HOUSE

CONSTRUCTION

SOILS_

IN

HOUSE CONSTRUCTION

COB.

Crawford

Often s o i l s

are n o t c o n s i d e r e d seriously in house c o n s t r u c t i o n e x c e p t to n o t e t h a t soil at a construction site

means mud

when it

r s i n s ,

and

mud

m e a n s t r o u b l e . Some y e a r s apo, p r a c t i c a l b u i l d e r s , mainly

In

t h e r o a d building industry, a s s o c i a t e d

various

problems w i t h c e r t a i n

peculiarities o f

the soll. They d e c i d e d

to

t r y to i d e n t i f y the

material,

t o study

i t s

properties, and t o relate t h e s e p r o p e r t i e s

to

the f i e l d

performance of

the

s o i l . This w a s _{the b e g i n n i n g} of _{t h e s c i e n c e}

now

known as soil mechanics.

MIAT IS

SOIL?

Soil

i s a m a t e r i a l consisting

of

three components:

mineral p a r t i c l e s , nster, :and a i r . S o i l s are arbitrarily divided i n t o c l a s s e s on t h e b a s i s

or

p a i n

size. These classes a r e

g r a v e l ,

sand, s i l t , a n d c l a y .

I n

n e t w e ,

s o i l s

exist

in many

combinations of t h e s e c l a s s types.

Before a t t e m p t i n g to C e a l with s o i l s w e must be

s u r e _{of t h e i r i d e n t i f i c a t i o n . The coarse-grained s o i l s}

(we-

v e l and s a n d ) are e a s y to identify. G r n v e l i s _{a material} w i t h particles l e s s t h a n

3

i n c h e s i n diameter and r e s t e r

₈

t h a n + inch in diameter. Sand has p a l t i c l s s

from

r

_{i n c h}

_in

diameter _{d o w n to particles t h a t are just visible w i t h t h e} naked eye, Both g s a v e l s and sands a r e f u r t h e r subdivided

i n t o

c o a r s e , m e d i m , a n d

Tine.

The i d e n t i f i c a t i o n sf f i n e - g r a i n e d soils (

silt

and c l a y ) is much more difficult and many c o s t l y e r r o r s in con-

s t r u c t i o n w o r k have r e s u l t e d from an incorrect i d e n t i f i c a t i o n ,

These

s o t l s may a l s c be classified a c c o r d i n g to g r a i n size.

In

o r d e r of decreasing g r a i n

s i z e ,

silt f o l l o w s f i n e sand; and

c l a y f o l l o w s slit.

There

are s e v e r a l simple

ways

t o

f i n d

out whether

silt

or _{c l a y forms p a r t of a s o i l .} Take

a

w e t p a t o f s o i l , rnoule it i n t o a ball, and shake it vigorously

in

t h e cupped hand, The s u r f a c e w i l l become g l o s s y and

show

free w a t e r ,

If

t h e

soil i s a silt, t h e f r e e

water

w l l l d i s a p p e a r and

t h e

surface

will

become

dull

when

the

p a t o f s o l 1 is squeezed between the f i n g e r s . This will not happen w i t h c l a y soils.

Another method i s t o

s t r o k e

a Lump of w e t s o i l w i t h t h e f l a t of a pen

k n i f e

h l a d e or finger n a i l using c a n s i d e r a b k e p r e s s u r e .

IT

t h t s r e s u l t s

In

a shiny surface, clay i s p r e s e n t

A

t h i r d method

i s

t o t e s t t h e

r e l a t i v e

b r e a k i n g s t r e n g t h

of

a pfece of d r y

fine-grained soTl

when broken

or

crushed between

the

ftncerz,

T h i s g2ves

an

indication

o f

_{t h e}

r e l s t i v e amounts of silt and c l a y . Very low

dry

s t r e n g t h i n d i c a t e s the presence o f silt. Very h i g h d r y s t r e n g t h i n d i - c a t e s a p u r e c l a y and

medium

d r y strength indicates a combin- tio on o f t h ~ s e s a i l s .

Teeth a r e v e r y s e n s i t i v e

to

g r i t and so t h e p r e s e n c e

or absence o r

grit

c a n be 2 . e t e r m i n e d by b i t l n g

a

piece of soil. If no erit: ... i s f e l t l r i t h t h e t e e t h , t h e m a t e r i a l is _{c l a y a i z e .}

The p l a s t f c i t y of a s o i l indicates t h e relative m o u n t s of s i l t or c l a y p r e s e n t . Pure c l a y s a r e h i g h l y p l a s t i c and can'na moulded

I n

the flngers. S o i l s

o f

law plasticity

will

crurn7Jle e a s i l y indiceking a _{silty s o i l . A}

soil o r medium plasticety i n d i c a t e s a combination o f t h e two

types. A f u r t h e r i n 3 . i c a t i o n

of

s i l t i s o b t z i n e d

if

the soil

powders easily when d r y ,

If

a

wet s i l t is manipulated in t h e hand

it

w i l l b e easy

to

c l e a n the

hand s i n c e

t h e s o i l w i l l d r y

and ? u s t off. ?lay s o i l s on

the

o t h e r hand w i l l pull the s k i n :<hen d r y i n g a n d are 6 i f f i c u l t to wash o f f ,

SPECIAL S O I L TYPES

_-.

O r p m S e Soil

Same s o i l s c o n t a i n an appreciable m o u n t of o r g a n i c

matter.

It

i s i . m p o r t a n t t o i d e n t i f y t h e s e soils because o f t h e i r c r i t i c a l p r b p e r t i e s .

They

c a n b e identified by a d a r k

brown o r

b l z c k c o l o u r and

by

t h e i r strong odour. Organic

s o i l s are o f t e n a s s o c i a t e d

with p o o r l y

d r a i n e d

areas and

t h e y a r e u x u a l l g u n d e s i r a b l e as a foun2.ation m a t e r i a l because of t h e f r h i g h compressib5lfty. Top s o i l

Th4s

i s a s p u c i a l t y p e

of o r g a n l c soil

w e l l known

f o r i t s groperties

which s u p p o r t p l a n t g70wth.

It

t o o 2s h i g h l y compressi'ble.

Fill i s

a man-made deposit of

n a t u r a l

soils

or

waste material.

It

can u s u a l l y be i d e n t i f i e d by t h e grass, twigs, bricks, and g l a s s i n it; there is u s u s l l y a layer

of

t o p s o i l b e n e a t h t h e

fill.

Th5s

Is

generally a material of questionable properties a n d should n o t be b u i l t upon w i t h o u t

investigatton.

T i l l

Till

i s an u n s t r a t i f i e d soil of _{asserted g r a i n s i z e s .}

It

may

contafn

all r a n g e s of s o i l s

from

l a r g e

b o u l d e r s

down

t o t h e f i n e s t clay.

Loam

i s a term commonly used

in

a g r i c u l t u r e ' but n o t u s u a l l y u s e 2

i n

e n g i n e e r i n g , The

term refers

t o the g r a i n

size o f t h e soil, P u r e loam c o n s i s t s o f equal parts

of

smd, s i l t and c l a y . ETarZations such as sandy loam and silty loam r e f e r t o a predominance of a

c e r t a i n p a i n size.

WATER

IN

SOTL

It

i s t h e w a t e r component

i n

soil which c a u s e s all yroblems a s s o c i a t e d w i t h const~uction. The a m o u n t o f w a t e r in a s o l 1 may v a r y 3aily and s e a s ~ n ~ l l y and this v a r i a t i o n may c o n s i d e r a b l y change t h e properties

of

_{the soil. In same heavy}

c l a y s such as _{those i n t h e O t t a w a a r e a , s i g n i f i c a n t}

_volume

changes occur seasonally due t o changing moisture c a n t e n t ,

Thls

change in moisture content 5.3 caused p a r t l y

bg

s e a s o n a l

v a r i a t i o n s

i n

p r e c i p i t a t i o n a d p a r t l y by the u s e

o f

s o i l

moisture by t r e e s and y e g e t a t i o n . T h i s results

in

l o s s of s u p p o r t , particularly to shallow f o o t f n g s ,

and f i n a l l y causes

damage to t h e

s t r u c t u r e .

At some e l e v a t i o n below the surface, free w a t e r 9 s

enccuntered I n the s a i l .

ThZs

e l e v a t i o n i s c a l l e d the

groundwater t a b l e , B e l o w the groundwater t a b l e

all

the v o i d s

in

t h e s o i l a r e f

flled

w i t h water, Above t h e groundrsrater t a b l e t h e r e i s _{u s u a l l y same a i r in t h e w i d s of the s o i l ,}

The

p o u n d w a t e r t a b l e

is the

e l e v a t i o n at which

water

is

e n c o u n t e r e d

in open wells,

In coarse-grained

s o i l s and

silty

s o i l s the w a t e r t s b 2 e can b e determined e a s i l y on the b u i l d i n g

site by m e a s l ~ i n g t h e d e p t h t o f r e e water

in

a b o r e hole or by o b s e r v i n g t h e occurrence of f r e e w a t e r

in

an excavation. S i n c e c l e g soils aye lmpe~rneable, the water l e v e l must be o b s e r v e d

f o r

a c o n s i d e r a b l e t i m e

in

o r d e r t o a c c u r a t e l y 6 a t e r - m i n e t h e g r o u n d w a t e r l e v e l .

On

m a n y b u i l d i n g s l t e s "perched" water t a b l e s occum, t h a t is, t h e w a t e r t a b l e 5,s at a h i g h

elevgtion locall::

due t o

an impermezhle layer at a r e l a t i v e l y s h a l l o w d e p t h .

I C A T DOES S O I L

DO?

Sot1 s u p p o r t s b u i l d i n g structures. Some s o l l s c a n p r o v l d e t h i s support n a t u r a l l y b u t o t h e r s o i l s r e q u i r e s p e c i a l t r e a t m e n t s t o p e r f o r m t h e i r requfreme

n t

s p r o p e r l y , S o i l acts as an i n s u l a t o r r e t a r d i n g frost p e n e t r a t i o n and r e d u c i n g heat flow f r o m subsurface structures such as basements,

So11 is e porous m a t e r i a l which moulds e a s i l y

i n t o

any shape.

I t s

p r o p e r t i e s a r e most appreciated when it is

n o t

available for u s e m d must be imported in o r d e r t o shape t h e building l o t znd t o s u p p o r t g r o w t h o f v e g e t a t i o n , I t is a r e l a t i v e l y simple m a t t e r to e x c a v a t e s o i l a n d

t o

b a c l r X l l

e x c a v a t i o n s w i t h s o i l .

It

19 a l s o a simple mstter t o _shape

terxaces, d i t c h e s , and b a r r l e r s with sol'l, S o i l

i s

a 12ving m a t e r i a l .

It

b r e a t h e s d a i l y a t s h a l l o w d e p t h s

and

seasonally

at

d e e p e r d e p t h s

due to temperature changes, This f e a t u r e

permits it to support v e g e t a t i o n and

to

purify sewage. T t

a c t s a s a s t o r a g e reservcrir f a y precip%tatian,

Some

t y p e s

of'

s o i l . c a n b e used as impervious mats t o _{p r e v e n t w a t e r}

_entering

t m d e s l r e d l o c a t i o n s . A good b u i l d e r u s e s

his

s o i l w i s e l y .

B e

makes

an

extra e f f o r t t o c o n s e r v e t o p s o i l .

n o t only in

the

_{i n t e r e s t s}

_{o f}

_{c o n s e r a a t i o n but also to p r e v e n t t h e expense} of importing

this

necessary m a t e ~ i a l ,

Properties of s o i l s which can be u s e d t o I d e n t i f y

s o i l

types have been n e r t i a n e d , Each o f t h e s e s o i l types has properties which c a n be p u t t o _{good u s e in construction}

work. The f o l l o w i n g ~ r o y e r t i e s a r e

or p a r t i c u l a r g e n e r a l

interest,

f

erme abili

ty

G r a v e l s and sands a r e porous and t h e r e f o r e permeable t o w a t e r flow. S i l t s are muck l e s s permeable and c l a y s a r e p r a c t i c a 1 7 ~ ~ imperrne25le. This p r o p e r t y i s o r much s i g n i f i c a n c e

in

t 5 e d e s l m o f t i l e beds f o r the disposal of

s e p t i c

t a n k

e f f l u e n t . it is u s e l e s s

in

either of t h e s e a p p l i c a t i o n s to n t t s ~ p t t o 2 i s p o a e of water

in

a pure clay or in an a r e a

where

t h e

roundw water

t a b l e i s v e r y high,

Any

attempt t o d o s o w i l l

b e unsuccessful. A knowledge o f the p e r m e a b i l i t y a l s o g i v e s

sone idea of t h e d r a i n a g e charncteristics of' v a r i o u s s o i l s .

For most m i n e r a l s o i l s c o m ~ s e s s i b i l i t g i s n o t a problem Sn the c o n s t r u c t f a n of s m a l l buildings such as

s i n g 1 3 family dwellings. The presence o f o r g a n i c m a t t e r however indicates h i z h conpressibility. B e f o r e any s truc- t u r e i s b u i l t o n t h e s a soils a t h o r o u g h s o i l s Investigation s h o u l d be c a r r i e d out.

In

h o u s e construction t h e b u i l d e r 5 s i n t e r e s t e d i n s o i l s t z b i l i t y as it a f f e c t s e x c a v a t i o n and t h e d e p t h

to which

a particular s o i l may b e e x c a v a t e d w i t h o u t f a i l u r e of

the

b a n k s .

It

is w e l l known t h a t s a n d s a n d g r a v e l s can have only

a

very linited v e r t i c a l c u t before m a t e r i a l b e g i n s t o fall

into

t h e e x c a v a t i o n and assumes

a

n a t u r a l s l o p e , Clays, how- ever, can b e c u t in v ~ s t i c a l slopes t o a r c o n s i d e r a b l e d e p t h

and, under o r d i n a r y c o n d i t i o n s , r e m a i n s t a b l e for pephaps t w o

t o

_{three w aeks depending}

_an

_{t h e weather, S l o p e s}

_{c a n}

_{a l s o be}

l i m i t e d

time

b u t

t h i s m a t e r i a l is

potentially

treacherous. -

1ti

i s

v e r y s u s c e n t i b l e

t o

InstxbTlity resulting

f r o m pre- c i p i t a t i o n and seepage,

Frost A c t i o n

Frost action

may

be d e f l n e d as

any

detrimental

e f f e c t

on

t h e soil or t h e s t r u c t u r e a s s o c i a t e d w i t h f r e e z i n g and t h a w i n g .

In

some s o i l s , a s t h e frost l i n e penetrates below t h e surface, w a t e r T s drawn iq f r o m b e l o w f o r m i n g h o r l z o n - t a l l e n s e s of pure i c e . T h i s phenomenon r e s l r l t s i r 7 an uneven

h e a v i n g

o f

t h e s o i l s u r f a c e . hben t 5 e heaved s o i l thaws, t h e

water in t h e i c e l e n s e s i s r e l e e s e d c r e a t i n g a v e r y u n s t a b l e

s o i l

c o n d i t i o n .

In

the

w o r s t c o n d T t i o n it w i l l result

In

a liquid

slurry

o f 3051 and w a t e r , This phenomenon d o e s n o t occur in clean sznds and gravels b u t it m a g p r e s e n t a problem

in

dLrty g r a n u l a r material, The phenomenon is most

?renounced

in

silty

solls.

It

i s ~ e n e r a l l y

n o t

s e r i ~ u s in c l a y s but f i s s u r e d c l a y s c a n have c o n s i d e r a b l e h e a v i n g , The prablem

of

f r o s t a c t i o n should be

of'

much

concern t o

t h e b u i l d e r .

It

affects drj. wways and shallol-r f ootinps.

Many c a s e s have been r e p o r t e d where unheated basements xere hgaved and cracked and e v e n destroyed Sy f r o s t a c t i o n

dur5ng construction. A l s o c a s e s are known w h e r e the excava-

tion w a s done d w 1 n g cold weather, f o o t i n g s were placed

on

frozen ground r e s u l t i n g

in

s e r i o u s settlements when t h e

round

T i n a l l y t h a w e d ,

I,

F r o s t

A c t i o n

In

r e g i o n s where s e a s o n a l f r e e z i n g a i r t

emyeratures

o c c u ~ ,

frost

a c t i o n in s o i l s i s u s u a l l y one

o f t h e

major

p r o b l e m s a s s o c i a t e d w i t h b u i l d i n g . Some typical r e s u l t s of t h e e f f e c t of f r o s t

a c t i o n

have b e e n mc:ntTsned. The

solution

t o t h e s e problems is n o t usuzlly d i r r i c u l t ,

If

foundations a r e b u i l t on s o i l s u s c e p t i b l e to frost t h e f o o t i n g s s h o u l d extend to a d e p t h Lower than -the maximum f r o s t p e n e t r a t i o n

or

t h e s u s c o p t l b l e m a t e r i a l should be replaced by a n o n - h e a v i n g material such as

c l e a n

s a n d

or

g r a v e l .

For

l i g h t structures the l o a d s can b e t r a n s m i . t t e d by s h o r t p o s t s t o

an

e l e v a t e o n

below f r o s t , A l l founilatfons must n a t u r a l l y be protected f r o m r r o s t actlon d u r i n g construction and o f c o u r s e , no f o o t l n g s h o u l d ever

be

p l a c e d on f r o z e n g r o u n d .

Perhaps one of t h e

w o r s t

e f C ~ c t s of frost a c t i o n is on unheated s l a b f o u n d ~ t i a n s , p a F t i c u l a r l y under a t t a c h e d g m a g e s . In t h i s c a s e t h e p o r t f o n of t i slab n e a r t h e h o u s e

does not heave due

to

h e a t l o s s f r o m the house while t h e

portion of t h e s l a b f a r t h e s t away f r o m t h e house may heave

house w i t h o u t a breezewag tho f o o t i n g s must b e deepened t o

extend

below

frost

p e n e t r a t i o n or the

frost-heaving

soil must b e replaced. H e a t i n g o f

the

f l o o r s1 n:? may p r e v e n t

further

damage but t h i s

is

p r o b a b l y an

uneconamical solution,

If

t h e r e is a breezeway between t k house and ,gai-.age some s o r t

sf f l e x i b l e construction may be u s e d which w i l l p e r m i t s i g n i - f i c a n t movemenh

of

_{t h e garage.}

2, B u i l d i n g

on

Filled

Ground

If possible b u i l d i n g s s h o u l d not b e constructed on filled g ~ o u n d .

If

t h i s c annot b e a v o i d e d , the s o i l s h o u l d

be placed s o t h a t the fill d o e s n o t settle. The s u r f a c e af t h s fill s h o u l d be observed o v e r a p e r i o d ol" t i m e t o ensure t h a t the s o i l I s

n o t

s _{ettling, The best t ;ype o f s o i l to u s e}

in

b a c k f i l l w i l l d epend upon t h e desLred p r o p e r t i e s of _{t h e} back

fill.

For

maximum

compaction

a

c l e a n sandy g r a v e l w T t h

a

_{wfde v a r i a t i o n in g r a i n} s i z e is b e s t .

For

good permeability g r a v e l a

are

b e s t , s a n d s not b a d ; s i l t s and c l a y s a r e prac-

t i c a l l y impermeable and a r e n o t s u i t a b l e .

If

f r o s t h e a v i n g i s a p o s s i b i l i t y , T i n e - z r a l n e d s o?ls must

be

a _{v o i d e d .}

3.

-

Compaction

S e t t l e m e n t

can

b e p r e v e n t e d by p r o p e r l y c ompacting

the S a c k

rill. This

can

b e clone sinply,

in

m a s t c a s e s , by plat- i n g t h e s o i l

in

t h i n l a y e r s

(4

t o

₆

_{i n c h e s ) and hand}

_tamplng each l a y e r .

If

t h e soil

i s

t o o d r y

it

i s _{d i f f i c u l t t o compact} b e c a u s e o f f r i c t i o n between t h e g r a i n s .

If,

on

the o t h e r hand,

the s o i l i s t o o wet

St

c a n n o t b e c o m p a c t e d p r o p e r l y because t h e w a t e r w F l l h e p l n

t o

d i s p l a c e mineral p a r t i c l e s . d i t h

most

s a i l s m a n i p u l a t i n g

t h e s o t 1 _{between t h e f i n g e r s w i l l}

indicate whether t h e so51 is t o o d r y

o r

t o o

wet

f o r p o d con-

p a c t i o n . Water

c a n t

hen be added or _{t h e} so _{13 may b e a llowed} t o dry

in order

t o permit b e t t e r com?action.

On

large j o b s

rnechar_2:al equipment may b e _used

_but

_{t h i s} c a n n o t n o m a l l y

be j ~ s t i f i e d

for

small o p e r a t i o n s ,

krn

Failures of Foundation

Walls

F o u n d a t i o n walls

f a i l

frequently

for a v a r i e t y of' r a s o n s .

In

some c a s e s failure i s c a u s e d by w a t e r p r e s s w e s ;

~ o o d u r a i n a g e

can

p r e v e n t t h i s , Some T a i l u r e s o c c u r due t o b a c k - f i l l i n g b e f a r e adequate b r a c i n g

of

t h e foundation w a l l

has b e o n p r o v i d e d or due

to

t h e a d j a c e n t surface l o a d i n g caused b y heavy

eclnstruction equipmmt,

Tkesa f e i l u ~ e s can be p r e v e n t e d b y ensurf-ng t h a t

w

2 1 1 s a r e dequately braced e i t h e r

t e m p o r a r i l y

o r

by

f l o o r construction b e f o r e back-

filling

and b y keeping heavy c o n s t r u c t i o n equipment

away f r o m t h e w n l l f o r a d i s t a n c e at l e a s t e q u a l t o t 5 e & p t h of

the

w a l l .

This

b r i e f

account

of c h a r a c t e r i s t i c s of various soil t y p e s is

an

_{a t t ; e : ~ p t t o showthat soil}

_when

_{understood and}

p r o p e r l y used,

T s

not

a c o n s t r u c t i o n h a z a r d but i s a

v a l u a S l e

built3inp m a t e r i a l .