3.
INSTALLATION
METHODS
D. Boels,
I n s t i t u t e for Land and Water Management Research,
Wageningen, The Netherlands.
Contents
3.1
3.2
3.3
Trenchless drainage method
Trenching drainage meth0d.s
Recommendations ,
Trenchless drainage method
3.1
3.1.1
'
Introduction
The plough-in method of drain installation has been termed "trenchless",.
in contrast to the "trenching" method which..involves both soil excavation
and back-fill operations. The trenchless method.places the tubing at a prescribed depth in an open space beneath a temporari'ly displaced column of
soil. The plough blade is designed to lift and split this soil column as it
moves forward. The lifting action causes a deformation and a disruption of
the soil upwards and towards both sides. The tubing is fed in behind the
plough blade where the soil falls back around the pipe.
The two major questions associated with the-trenchlesstechnique, as indeed
with other methods too, are
3.1.2
(I)
can the tubing be laid on grade at the desired depth
within acceptable limits, and
(2)
does the condition of the soil after installation allow
unrestricted flow of water into the drain? (Reeve 3.09).
S o i l disturbance with narrow tines
The basic shape of the plough blade used for drainage operations is an inclined narrow tine. Two types of soil disturbance can occur with narrow
tines. At shallow working depths the soil is displaced forwards, sideways,
and upwards throughout the whole working depth. This type of disturbance
causes f issur ng and loosening and is termed crescent failure.
At greater working depths the tine causes crescent failure in the upper soil
layers, but in deeper layers the soil moves forwards and sideways only (lateral failure) and this causes deformation (movement with no change in density)
or compaction. The depth at which the transition from crescent to lateral fai
lure occurs is termed the critical depth (Spoor 3.08). Below the critical
depth, where only deformation occurs, the soil movement can be described as
a sequence of steady state situations of soil flow. The streamlines.coincide
with the sliding lines as have been described by Prandtl ( 1 9 2 1 ) . The flux
of soil during a steady-state situation is chosen in some arbitrary way. The
deformation of the soil related to the friction angle and the sideward distance to the plough blade can be derived from the known streamlines and assumed displacement (Boels 3 . 1 1 ) . The leading section of the tine at working
depth controls the type of soil disturbance. The tine aspect ratio (working
depth divided by tine width) and tine inclination to the horizontal in the
direction of movement (rake angle) are implement factors that influence the
critical depth. The smaller the aspect ratio and rake angle, the deeper the
critical depth. Increasing moisture content tends to decrease the critical
depth. A hard topsoil on a soft subsoil means a shallower critical depth than
in a uniform soil. Pure deformation is most likely to occur only under wet
soil conditions. In the zone with crescent failure the soil will be loosened
without deformation. On soils with high clay content, smearing is observed
on the soil-implement interface (Winger 4 . 0 4 ) . No data are available on the
long-term influence of smearing on the hydraulic conductivity.
3.1.3
Hydraulic conductivity related
to soil disturbance
As pointed out above, three types of soil disturbance can be distinguished:
loosening, compaction, and deformation without compaction or loosening. Loosening and compaction change the pore-size distribution and will influence
the original hydraulic conductivity of the soil. Loosening increases conductivity; compaction reduces it. These relations can easily be determined.
If soils have very few large pores per unit area, soil deformation can result
in the blocking of these pores, thereby reducing the conductivity. On sandy
soils where pore distribution largely depends upon the particle size distribution, deformation does not change the initial hydraulic conductivity. The
40
TABLE
I.
S o i l type
R a t i o between t h e h y d r a u l i c c o n d u c t i v i t y i n v i c i n i t y of t h e d r a i n p i p e w i t h t r e n c h l e s s
i n s t a l l a t i o n (K ) and t r e n c h i n g i n s t a l l a t i o n ( K I ) based on measurements of d r a i n
2
d i s c h a r g e and h e i g h t of groundwater t a b l e
Drain d e p t h
(m below
s u r f ace)
Country
.
Ref e r ence
Remarks
K2
-
1 .o
average 1975-1977
Denmark
Denmark
O. 5
0.6
average 1972-1978 (6 p i p e s )
average 1972-1978 ( I O p i p e s )
Olesen
Olesen
I-.2
Denmark
1 .O5
average 1973-1978 ( 3
Olesen
S i l t loam
1.4
Neth.
O. 23
average 6 s i t e s 1976-1977
Naarding
S i l t y clay
loam
1.3
Neth.
o.
15 o b s e r v a t i o n s 1973-1974
Naarding
S i l t y clay
1.1
W. Germany
0.39
1
Clay
1.1
Neth.
0.8
average 1973-1975 ( I O p i p e s )
Sand
1.1
Ne t h
Sandy loam
Sandy loam
1.2
1.2
Loam
,,
19
observation
(14 pipes)
pipes)
1977-1978
Naarding
Eggelsman
Naarding
r e l a t i o n between deformation and c o n d u c t i v i t y can be d e r i v e d from c o r e s
p l a c e d i n a triTaxial a p p a r a t u s (Boels 3.11).
Clay s o i l s w i t h s w e l l i n g + a n d
s h r i n k i n g p r o p e r t i e s may r e c o v e r some of t h e l o s t pore space a f t e r deformation o r compaction (Naarding 3.04; Johansen 3.02).
The hydrau1i.c c o n d u c t i v i t y i n t h e d i s t u r b e d zone i n t h e v i c i n i t y of t h e
d r a i n p i p e can a l s o . b e d e r i v e d from measured d r a i n d i s c h a r g e and w a t e r t a b l e
h e i g h t . T h i s has been done i n f i e l d s whose d r a i n a g e systems were i n s t a l l e d
e i t h e r i n . t r e n c h e s o r w i t h the' trenchlcess method. The - r a t i o between t h e hyd r a u l i c c o n d u c t i v i t y i n t h e d i s t u r b e d zone and t h a t i n t h e u n d i s t u r b e d s o i l
a r e c a l c u l a t e d w i t h d r a i n a g e formulas developed by E r n s t ( 1 9 6 2 ) . The d a t a
p r e s e n t e d d u r i n g t h e Workshop'have been r e v i s e d and are summarized i n Tab.-I.
T a b l e 1 shows t h a t s o i l d i s t u r b a n c e by d r a i n ploughs r e s u l t e d i n a decreased
c o n d u c t i v i t y of t h e s o i l . ' T h i s
i s t r u e f o r most t y p e s of s o i l , except f o r
sandy s o i l s . There seems t o .be a tendency t h a t t h e deeper t h e d r a i n s are ins t a l l e d , t h e lower t h e r a t i o K 2 / K l
is.
Using t h e d r a i n a g e formulas. of E r n s t , i t i s p o s s i b l e t o compute t h e a d d i t i o n .al, r a d i a l - r e s i s t a n c e caused by t h e decr.ease i n h y d r a u l i c c o n d u c t i v i t y of t h e
s o i l around t h e p i p e . Owing t o t h i s e x t r a r e s i s t a n c e , t h e d r a i n s p a c i n g of
t r e n c h l e s s drainage, L
must g e n e r a l l y be narrower t h a n t h e s p a c i n g , L , of
P'
a trenched system. The r a t i o between L and L can be c a l c u l a t e d , assuming
P
t h e same d r a i n a g e e f f e c t i v i n e s s i n b o t h cases. From t h e d a t a i n Table 1 , t h e
r a t i o s p r e s e n t e d i n Table 2 are o b t a i n e d .
TABLE 2 . R a t i o between t h e d r a i n s p a c i n g f o r t r e n c h l e s s d r a i n a g e (L )
and t h a t f o r t h e t r e n c h i n g method ( L ) , assuming equal d r a i n a g e
effectiveness
S o i l type
D r a i n depth
'(m below s u r f a c e )
Lp/L
~
Sand
Sandy loam
Lo am
S i - l t loam
, S i l t y c l a y loam
S i l t y clay
Clay
42
1.1
I .O
1.2
1.2
I .4
1.3
1.1
o. 8
1.1
1 .o
O. 56
O . 50
O. 73
0.94
T h i s t a b l e shows t h a t i n most c a s e s t h e d e c r e a s e of c o n d u c t i v i t y i n t h e d i s t u r b e d zone has o n l y a s l i g h t i n f l u e n c e on t h e d r a i n s p a c i n g , a l t h o u g h under
c e r t a i n c o n d i t i o n s i t can mean a r e d u c t i o n i n s p a c i n g of 25 t o 50 p e r c e n t .
The c r i t i c a l d e p t h of t h e ploughs used i n t h e experiments from which t h e
d a t a i n T a b l e 1 s t e m would be about 1.0-1.1
m o r less under t h e s o i l condi-
t i o n s m e t d u r i n g t h e d r a i n a g e o p e r a t i o n s . The s m a l l e r K / K - r a t i o s and sub2
1
s e q u e n t l y t h e small r a t i o s L /L c o u l d be caused by deformation o r compaction
P
i n t h e d i s t u r b e d zone below t h e c r i t i c a l d e p t h . I f t h e d r a i n depth w a s e q u a l
t o t h e c r i t i c a l d e p t h , i t i s most l i k e l y t h a t t h e r a t i o L / L would be one.
P
T r e n c h l e s s ploughs a r e n o t t h e o n l y cause of low c o n d u c t i v i t i e s around t h e
1
1
p i p e , extremely low c o n d u c t i v i t i e s can r e s u l t w i t h t h e t r e n c h i n g method when
s e v e r e s o i l s t r u c t u r e damage o c c u r s o r when t h e t r e n c h i s b a c k f i l l e d w i t h
t h e s o i l i n a s l u r r y condition.
I
3.1.4
Depth control and draught requirement
Depth c o n t r o l based on t h e " f l o a t i n g " plough p r i n c i p l e i s s t a n d a r d i n indust r y . The term " f l o a t i n g " plough comes from t h e f a c t t h a t as t h e plough is
p u l l e d through t h e s o i l , t h e s o i l - d r a g f o r c e s on t h e plough b l a d e a r e i n
b a l a n c e w i t h t h e t r a c t o r - d r a u g h t and plough-gravity
f o r c e s so t h a t t h e plough
seems t o " f l o a t " through t h e s o i l . Ploughing d e p t h i s c o n t r o l l e d by changing
t h e a t t i t u d e o r a n g l e of t h e plough. T h i s can be done by e i t h e r r a i s i n g o r
l o w e r i n g t h e r e a l o r v i r t u a l h i-t c h p o i n t o r r o t a t i n g t h e plough b l a d e .
Because of v a r i a t i o n s i n d r a g f o r c e s on t h e plough b l a d e , g r a d e c o n t r o l req u i r e s more t h a n simply keeping t h e h i t c h p o i n t h e i g h t on a l i n e p a r a l l e l
t o t h e d e s i r e d d r a i n g r a d i e n t . For p r e c i s e c o n t r o l , however, i n f o r m a t i o n on
b o t h t h e e l e v a t i o n of t h e plough p o i n t and plough a t t i t u d e i s needed. A l though t h i s would r e q u i r e two d e t e c t o r s , both f a c t o r s can be monitored by
p o s i t i o n i n g one d e t e c t o r a t 0 . 8 3 3 times t h e long beam l e n g t h towards t h e
r e a r of t h e plough from t h e h i t c h p o i n t .
Field-checks
of t h e d e p t h and g r a d e achieved have h i g h l i g h t e d a l a r g e number
o f e r r o r s . F a i l u r e s are d e f i n e d as d e v i a t i o n s from t h e designed depth of
more t h a n 0.05 m o r when n e g a t i v e s l o p e s o c c u r . Based on a f a c t o r a n a l y s i s ,
i t c a n be concluded t h a t e r r o r s are due l e s s t o t h e t y p e of machine ( t r e n c h l e s s o r t r e n c h i n g machine) and c o n t r o l system, b u t more t o t h e s o i l p h y s i c a l
43
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c o n d i t i o n s ( s l o p e , m o i s t u r e c o n t e n t , t e x t u r e , occurrence of s t o n e s , topography) and human parameters ( p a r t i c u l a r t h e machine o p e r a t o r ) . The b e s t
g r a d i n g r e s u l t s are o b t a i n e d i n uniform s o i l s (loamy s o i l s ) , followed by
c l a y s o i l s and sandy s o i l s (Cros e t a l . 3.10). However, i t should be noted
t h a t l i t t l e i n f o r m a t i o n i s a v a i l a b l e on a l l o w a b l e grading e r r o r s i n f i e l d
practice.
Grading accuracy d e c r e a s e s r a p i d l y w i t h i n c r e a s i n g forward speed p a r t i c u l a r
a t speeds i n excess of 0.5-0.75 m / s .
The h i t c h p o i n t , r e a l o r imaginary, should be l o c a t e d n e a r t h e f r o n t end of
t h e t r a c t o r t o b r i n g t h e r e s u l t a n t f o r c e on t h e prime mover n e a r t h e c e n t r e
l i n e of t h e t r a c k s a t t h e ground s u r f a c e s o as t o p r o v i d e uniform l o a d d i s t r i b u t i o n on t h e t r a c k s . The t r a c k l o a d can t h u s i n c r e a s e by as much as 30
p e r c e n t , thereby increasing the traction-efficiency.
The r e q u i r e d drawbar-pull depends upon t h e width of t h e plough b l a d e , t h e
working d e p t h , and t h e kind of s o i l . Table 3 p r e s e n t s some d a t a .
When t h e working depth i s below t h e c r i t i c a l depth, t h e draught i n c r e a s e s
more r a p i d l y w i t h i n c r e a s i n g width of t h e plough b l a d e t h a n when i t i s above
t h e c r i t i c a l depth.
The draught i n c r e a s e s when t h e i n c l i n a t i o n of t h e t i n e t i p t o t h e h o r i z o n t a l
i n c r e a s e s . T h i s i n c r e a s e , however, i s n o t l a r g e i f t h e a n g l e ranges from
20 t o 50'.
3.1.5
Possibilities of deepening the critical depth
The c r i t i c a l depth may be d e f i n e d as t h e depth a t which t h e energy r e q u i r e d
t o c a u s e c r e s c e n t f a i l u r e e q u a l s t h e energy t o cause l a t e r a l f a i l u r e . The
energy t o cause f a i l u r e i n t h e s o i l i s determined by t h e s h e a r s t r e n g t h of
t h e s o i l . The s h e a r s t r e n g t h i s n e g a t i v e l y c o r r e l a t e d w i t h s o i l m o i s t u r e
c o n t e n t and p o s i t i v e l y c o r r e l a t e d w i t h t h e d r y bulk d e n s i t y of t h e s o i l . The
s h e a r s t r e n g t h of a loosened s o i l i s s m a l l compared w i t h t h e i n i t i a l s h e a r
strength.
Loosening t h e s o i l s u r f a c e l a y e r b e f o r e t h e deep t i n i n g o p e r a t i o n r e s u l t s
i n a deeper c r i t i c a l depth than t h a t o b t a i n e d i n a s i n g l e - s t a g e o p e r a t i o n .
I f t h e l o o s e n i n g i s dode a t t h e same t i m e as t h e deep t i n i n g , t h e shallow
45
working t i n e s have t o be p o s i t i o n e d i n m e d i a t e l y ahead of t h e deep t i n e a t
a d i s t a n c e approximately 1-1.25
times t h e working d e p t h of t h e deep t i n e .
Draught on t h e deep t i n e a l o n e i s reduced by l o o s e n i n g t h e t o p l a y e r . The
t o t a l d r a u g h t on a combined shallow/deep t i n e arrangement i s almost t h e same
as t h a t r e q u i r e d f o r a s i n g l e s t a g e o p e r a t i o n w i t h t h e deep t i n e a l o n e (Spoor
3.08).
The ways of i n c r e a s i n g t h e c r i t i c a l d e p t h ( p r e v i o u s l y d i s c u s s e d 1 . 2 ) are
through i n c r e a s i n g t h e plough-width and r e d u c i n g i t s i n c l i n a t i o n t o t h e h o r i z o n t a l a t t h e l e a d i n g edge.
3.1.6
Considerations concerning the application
of the technique
Provided t h e working depth i s less than t h e c r i t i c a l d e p t h , t h e t r e n c h l e s s
t e c h n i q u e can be a p p l i e d over a wide range of s o i l s w i t h o u t t h e r i s k of s i g n i f i c a n t s o i l damage. Working below t h e c r i t i c a l d e p t h i n c o a r s e - t e x t u r e d
s o i l s and perhaps i n w e l l - s t r u c t u r e d p e a t s o i l s , t h e s e machines w i l l n o t normal.ly impair t h e e f f e c t i v e n e s s of d r a i n a g e . An a d d i t i o n a l r a d i a l f l o w resist a n c e may o c c u r on f i n e and medium-textured s o i l s when working below t h e
c r i t i c a l d e p t h , a l t h o u g h t h i s r e s i s t a n c e may d i s a p p e a r a f t e r some y e a r s i f
t h e s o i l s t r u c t u r e i n t h e deformed o r smeared zone can r e c o v e r .
Compaction o r d e f o r m a t i o n a t t h e s i d e of t h e d r a i n t u b e i s l e s s i m p o r t a n t
when most of t h e water e n t e r s t h e p i p e from above, e . g .
i n mole d r a i n a g e
schemes under perched w a t e r t a b l e s . Working below t h e c r i t i c a l d e p t h i s t h e r e f o r e n o t s o c r i t i c a l i n such s i t u a t i o n s .
The t r e n c h l e s s method r e q u i r e s a much h i g h e r d r a u g h t f o r c e t h a n t h e t r e n c h i n g
method. Under c o n d i t i o n s w i t h a w e t t o p s o i l o r low b e a r i n g c a p a c i t y , t h e
ploughs c a n h a r d l y o p e r a t e w i t h o u t winching spans. Under good t o p s o i l condit i o n s , t h e p o t e n t i a l f o r high-speed p i p e l a y i n g i s g r e a t e r w i t h t h e t r e n c h -
l e s s method. On s t o n y s o i l s t h e s e machines perform more s a t i s f a c t o r i l y t h a n
trenchers.
I f t h e l a r g e ploughs are u s e d . i n d r a i n a g e schemes l a r g e r t h a n about 10 h a ,
t h e i r p e r metre c o s t s are lower t h a n those' of t r e n c h e r s because of t h e r e l a t i v e l y low wear and tear c o s t s (about 1/3 t o 115 of t h o s e of t r e n c h i n g machines)
46
Gravel s a v i n g s are p o s s i b l e where narrower bands of envelopes are a c c e p t a b l e ,
w h i l e i n c e r t a i n c a s e s t h e reduced m i l l i n g and p u l v e r i s a t i o n of t h e s o i l may
a l l o w t u b e s t o be i n s t a l l e d w i t h o u t a n envelope, whereas w i t h t r e n c h i n g a n
envelope would be r e q u i r e d .
The t r e n c h l e s s method has some o t h e r d i s a d v a n t a g e s . E x i s t i n g ( o l d ) d r a i n s
cannot e a s i l y be connected t o t h e new-laid system. When composite d r a i n a g e
systems a r e being i n s t a l l e d , a backhoe i s always needed t o d i g h o l e s a t t h e
j u n c t i o n between t h e l a t e r a l s and t h e mains.
S e v e r e s t r e t c h i n g of p l a s t i c p i p e can o c c u r a t h i g h p i p e l a y i n g speeds
( g r e a t e r t h a n 0.5-0.75
m p e r s e c . ) u n l e s s p o s i t i v e p i p e f e e d mechanisms
a r e used.
3.2
3.2.1
Trenching drainage methods
Some experience
The t r e n c h i n g d r a i n a g e method i n v o l v e s b o t h s o i l e x c a v a t i o n and b a c k - f i l l
o p e r a t i o n s . Machines can be c l a s s i f i e d i n t o t h r e e main groups: t h e l a d d e r
t y p e ( c h a i n - d r i v e n b u c k e t s ) , t h e wheel-digging t y p e , and t h e digging-chain
t y p e . The machines may be equipped w i t h o r w i t h o u t a g r a v e l hopper. The d i g g i n g d e p t h i s c o n t r o l l e d manually o r by some automatic d e v i c e (e.g.
laser
equipment).
The machines a r e s u i t a b l e f o r u s e i n most s o i l t y p e s . The c h a i n t y p e machines
may work f a s t e r t h a n t h e bucket and wheel t y p e s i n f r i a b l e uncemented materi a l , f r e e of rocks and l a r g e s t o n e s . On cemented s o i l s t h e bucket t y p e mac h i n e s and wheel-diggers
perform t h e b e s t .
Under v e r y w e t c o n d i t i o n s , some chain-type t r e n c h e r s b r i n g t h e s o i l i n t o a
s t a t e o f s u p e r s a t u r a t i o n , r e s u l t i n g i n such a s o i l c o n s i s t e n c y t h a t t h e digg i n g t e e t h a r e unable t o remove t h e material from t h e t r e n k h . F i l l i n g up t h e
open s p a c e between c h a i n and t i l e box seems t o be a s o l u t i o n t o t h i s problem.
I n s a t u r a t e d u n s t a b l e s o i l s , t h e h y d r o s t a t i c p r e s s u r e of t h e s o i l i m p a i r s a
p r o p e r p l a c i n g of t h e g r a v e l envelope. P l a c i n g a power auger i n t h e opening
around t h e tube t o f o r c e t h e g r a v e l i n t o t h e open space provided f o r i t seems
t o overcome t h e h y d r o s t a t i c p r e s s u r e .
’
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3.2.2
Stabilizing unstable trenches
A trench is unstable if yielding in the soil occurs. In principle there are
two methods of stabilizing the soil. The first is to reduce the water pressure and the second is to compensate for the overburden pressure.
'
The first method requires the water table to be drawn down. This can be done
by installing first a temporary drain tube of small diameter below the intended depth of the permanent larger drain pipe. Installing an extensive system
of well points to drain the immediate vicinity of the drain by pumping will
I
I
also lower the watertable.
Compensating the overburden pressure can be achieved by over-excavating the
bottom and sides of the trench and backfilling it with coarse gravel (Winger
4 . 0 4 ) . The trenchless plough technique may also offer advantages in these
situations.
3.3
Recommendations
Trenchless drainage installation has some disadvantages. It requires a high
draught force and may cause soil deformation or compaction when drains are
installed at great depths. Further work is needed on the soil-mechanical
aspects.of drain ploughs to minimize draught and to determine ways of ensuring that the critical depth is below the required working depth over as wide
a range of drainage situations as possible.
Smearing and compaction occur not only with trenchless methods but also with
trenching. Further investigations are needed to measure the influence of deformation, compaction, and smearing on the functioning of the drainage system.
A s has been reported, some deformed or compacted soils may recover from the
ill effects whereas some initially loose soils may consolidate through settling showing a decrease in conductivity. The soil behaviour after drainage
has been installed should be monitored to determine the extent of soil recovery or consolidation.
In some papers the need for installing drain tubes at design depth and on design grade has been stressed. However, there is no clear understanding of the
influence of installation errors on the functioning of the drainage system.
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Investigations are needed to define the importance of these errors and to
define working limits.
There is also a need to investigate the factors causing poor grade control
with automatic grading systems. These factors inc1u.de the control system,
implement hydraulics and, soil/implement interaction.
Because of the difficulty of checking depth and grade during trenchless installation, a device is required to record these parameters.
The differences in operation costs between trenchless and trenching machines
are mainly due to the relatively high wear and tear on the cutting elemen-ts
of the trenching machines. Improvements should therefore be made to the digging chain and cutters to reduce wear and tear and improve digging efficiency.
Re f e r e n c es
ERNST, L.F.
1962. Grondwaterstroming in de verzadigde zone en hun berekening bij aanwezigheid van horizontale evenwijdige leidingen.
PUDOC, Wageningen.
1921. Uber die Eindringungsfestigkeit (Harte) plastischer
Baustoffe und die Festigkeit von Schneiden. Z.fÜr angew.Mathematik
u. Mechanik 1 , 1:15-21.
PRANDTL, L.
49