Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~3~t~70
This invention relates to a method of hydraulic
compaction of soil via water jetting and to apparatus to
accomplish the same.
For years some people associated with the
installation of sewers for municipalities within Essex
County in Ontario, Canada, have been proposing the use of
water as an innovative way of compacting native clay
backfill of utility trenches. This method has become known
as "water jetting".
Although water jetting is generally acknowledged
to be a cheaper method of compacting trench backfill when
compared to mechanical equipment, its use has only been
allowed on a very limited scale, and usually at the
contractor's risk due to the fact that much is unknown about
the mechani~m at work.
This invention deals with both an apparatus for
and a method to carry out water jetting. It uses a
pressurized water jet to apply compactive energy to soil.
Basically, water i9 injected into the backfilled trench
beneath the upper surface of the soil until the soil is
completely saturated. Then compaction is achieved through
the action of the water (soil vibration; breakdown of clay
lumps; reduction of cohesive forces; seepage forces) that
encourages the consolidation of the backfill under its own
weight and that of water.
When water jetting is carried out properly, the
results obtained with this method are at least equivalent to
those obtained with only mechanical compaction, while the
cost is significantly reduced. Unfortunately, municipal-
ities have been reluctant to allow it to be used exten-
sively, mainly because no standards or specifications
are in existence, and until now no one seemed to be aware of
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a satisfactory water jetting apparatus, or method of
application thereof, e.g. technique.
A paper published in 1978 by Dr. J.T. Laba and Dr.
M.A.F. Sheta, both of the University of Windsor, Windsor,
Canada, reported on an in depth laboratory study dealing
with the effect of selected parameters on the behaviour of a
soil sample. The study concluded that water jetting is more
effective in backfills with mixed lump size: in deeper
trenches, with higher jetting pressure; and, with increased
seepage forces and a smaller jetted area per jet. The
effectiveness is decreased by jetting in layers and by
increasing the amount of granular material in the backfill.
Although the investigation seems to be extensive, its
authors cautioned that the laboratory results could only be
referred to as "trends" and could not be used as a direct
representation of real field behaviour. This was a
laboratory study and no attempt was made to develop a full
scale jetting apparatus or a method to employ commercially.
A survey of water jetted trenches carried out in
1979 reached the following conclusions:
Water jetting would be adequate in preventing long
term settlement: that trenches, which have been water jetted
in the last five years, have not undergone any appreciable
settlement as of yet; and, that water jetting is ineffective
in compacting granular soils. Although the results of the
study are very encouraging, it is most unfortunate that only
general descriptions and no details are available reyarding
the water jetting method and appropriate apparatus in each
case.
In fact, only rudimentary methods and equipment
were used.
The prior art presently consists of the following:
f)
MECHANICAL COMPACTION OF CLAY BACKFILL
During excavation the native clay is broken into
lumps with individual density equivalent to the original
undisturbed soil. When these lumps are replaced back into
the trench a~ backfill, a large volume of voids is created.
In order to minimize the voids, mechanically, a heavy weight
and kneading action is applied to relatively thin layers or
strata of soil (0.30 to 0.60 m). This process requires a
shear failure to each lump and its remoulding into a new
shape which fills an adjacent void.
Immediately following placement and compaction,
each lump is stable and bears on th~ underlying lumps with
several areas of contact. When seepage of rain water and
runoff reach these clay lumps, the small contact areas
are softened, and they now are no longer able to bear the
overburden since they are destabilized. Through a series of
shear and creep failures, the soil lump undergoes
deformation, and downward movement, as do adjacent lumps,
until the softened areas of contact are large enough to
support the overburden. At this stage, no further
significant settlement will occur. Unfortunately, by this
time large settlements have already occurred which express
themselves in great surface depressions and resulting in a
very uneven pavement and even possibly roadway structure
failure; or, when this approach is used for new sewages,
possible failure of the new sewer lines.
It is an object of the invention to improve back-
fill compaction, improve safety, while at the same time to
reduce costs.
In order to better understand the functions of the
backfill mass at various depths, it is desirable to divide
it into three zones.
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Zone A: This backfill is required for the
bedding and cover of the buried service.
Normally a granular ma~erial is used, with
its degree of compaction controlled by the
support requirements of the service. If
water jetting is to be used this zone should
consist of a uniform granular material, such
as clear stone, so that a free-draining bed
is provided.
Zone B: This zone fills the area between the
pavement structure and the pipe cover. Its
main requirement is that it be stable under
its own weight, traffic vibrations and
downward water seepage forces.
Zone C: This zone forms part of the pavement
structure. It must be capable of
distributing part of the traffic loading and
remain stable under the weight of the over-
burden and applied pavement loads.
The zone which occupies the largest volume is Zone
B. This is the ideal plac~ to use native clay backfill and
water jetting, since it would eliminate settlement problems
and provide for a more economical and convenient backfilling
operation.
The invention thus contemplates hydraulic
compaction as a method of soil compaction. It achieves
certain advantages.
Cost: The cost reduction is one of the most
significant advantages of water jetting (hydraulic
compaction) when compared to mechanical
compaction. Actual savings based on 1980 costs
is up to 19~ of the total cost of a sewer
.~
project. Up to 70~ of mechanical con,paction cost
could be saved.
Safety: Water jetting can be safer than
mechanical compaction since men and machinery are
not required to enter the trench behind the pipe
laying operation. In addition, the trench can be
backfilled very quickly, and right to the surface,
thus eliminating the danger a deep excavation
poses to workmen, pedestrians and vehicular
traffic.
Subgrade Vniformity: When native material is
used, this method of compaction restores the
entire subgrade to its original state before
excavation, eventually eliminating any trace of
the interphase between the trench and its
surroundings. Because of the properties of the
backfill material and the undisturbed soil are
very similar, any settlement will be very small
and uniform in nature. This cannot be ~aid for
mechanical compaction, where large differential
settlements are a very real possibility.
Interference with Traffic: Since mechanical
equipment is not required to enter the excavation,
much narrower trenches can be used when water
jetting. This results in less excavation, less
disposal of excess material and less surface
restoration. This not only contributes to cost -
savings, but also makes for a much faster
operation, therefore causing less inconvenience
for the regular road user.
More Complete Compaction: In the majority of
cases mechanical compaction cannot adequately
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compact the backfill adjacent to the trench
walls. Water jetting eliminates this problem, as
the water is able to seep into all voids,
including those nearest the walls.
There may appear to be some disadvantages or
inconveniences to hydraulic compaction of soil.
Greater Time Delay: This delay occurs because
normally an entire section of sewer is completed
before the water jetting is carried out. Also
some time must be allowed for the backfill to
settle and regain sufficient bearing strength
before traffic can be allowed to travel over the
trench (this can vary from a few days to over a
week). This would not to be an unreasonable time
delay, since similar delays are common to
mechanical compaction.
High Residual Moisture Content: After water
jetting the moisture content of the backfill
decreases from the saturation value to a value
which is approximately 4% to 6% above the Proctor
optimum value. This value is constant for any
particular soil, and is known as the field
capacity or the specific retention, the point
below which the water cannot be drained. Although
those skilled in the art have generally regarded
this as a disadvantage it should be noted that a
flexible pavement will perform better on a soft
uniform grade than on a hard non-uniform ~ubgrade,
hence this may actually be an advantage. Further,
it has been suggested that the best mechancial
compaction results are obtained at a moisture
content approximately 4~ above the Proctor
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optimum. The high moisture content also precludes
any significant amount of infiltration and the
associated settlement.
Freezing Weather: Water jetting cannot be carried
out during periods of sub-freezing temperatures.
As a result, the winter months are lost for sewer
construction. (This, of course, does not apply to
areas which do not require restoration until the
following spring or summer.)
Additional Bac~filling: Even if the backfill
level before jetting is several feet above the
surrounding ground, once the water jetting process
has been carried out the trench surface settles to
a level below the ground elevation. Additional
clay or granular backfill must then be placed and
compacted mechanically.
The invention therefore contemplates, firstly, an
appara~us for injecting, systematically, into soil, water
whereby the adjacent soil is compacted with the aid of
hydraulic forces, the apparatus comprising:
(a) a hollow conical member defining a hollow
interior extending into a cylindrical member
as an extension thereof, the members defining
a plurality of apertures that communicate the
hollow interior to its exterior;
(b) a source of water supply; and,
~c) a flexible conduit interconnecting and
communicating the nozzle with the water
~upply.
Secondly, the invention also contemplates a method
of compacting soil comprising the steps of:
(a) applying to a point source, a flow of water,
rf ()
and to the point source, a downward force,
whereby, as result of the flow of water, and
the weight, the point source tends to
traverse into the depth of the ground;
(b) terminating at a prescribed depth the
downward force and hence further penetration
of the point source;
(c) applying thereto an upward force, whereby,
under the continuous flow of the water out of
the point source as it slowly migrates to the
surface causing the soil adjacent thereto to
be inundated with water whereby, the soil is
compacted by hydraulic forces.
Additionally it may include drainage of the
injected water through a sewer pipe or the pumping of the
water out from the injected region and in appropriate
circumstances restoration of the surface (Zone C) by
mechanical compaction.
The invention will now be described by way of
example and reference to the accompaning drawings in which:
Figure 1 is a perspective of a nozzle defining its
orifices which provides the hydraulic flow to adjacent soil.
Figure 2 ix a diagramatic perspective for
explanation of the nozzle of Figure 1 in soil and supplying
hydraulic fluid thereto.
Figure 3 is a typical connection of the nozzle of
Figures 1 and 2 to a source of water supply with an
additional pressure creating device for hydraulic fluid.
Figures 4, 5, 6, 7, and 8 are exemplary drawings
in elevation, of the nozzle, in Figure 4, having penetrated
to the base of a region ~Zone B) of backfill, and in Figures
5, 6 and 7 being slowly forced up under the influence of
.
g
force F while at the same time eminating water, and in
Figure 8, the purging of the subjacent saturated granular
regions A and B of water whereby compaction i5 ensured.
Figures 9 and 9a are cross section elevational
views of the steps by which the apparatus is utilized and
the method is accomplished.
Figures 10 and 11, respectively, are diagramatic
representations of soil samples with clumps 1 through 10
shown, as at Figure 10, with interstelar regions or spacial
regions prior to compaction, and at Figure 11 with
compaction where those regions are destroyed.
~ igures 12 and 13 are a diagramatic represen-
tations of adjacent soil clumps with hydraulic surface
tension forces thereon, that occur at a soil/water
interphase.
Referring now to Figure 1, a nozzle 30 is
generally shown located in region 30' and is of conical
structure having a plurality of orifices 0, and adjacent
thereto and communicating therewith, cylindrical member 30''
both which define a hollow interior 33. The hollow interior
33 communicates, as more clearly seen in Figure 3, to a
source of water supply, for instance a hydrant 40 by virtue
of a hose 50. It may be convenient, when for example the
hydrant 40 is incapable of supplying a sufficiently high
volume of water under a significant pressure, to interpose a
pump 60 and hence to force feed the nozzle 30. It will now
be apparent to those skilled in the art th~t other
appropriate water sources may be used with equal success
such as a river, reservoir, etc. provided, however, that the
volume of water and its head or pressure is satisfactory.
Preferably, to monitor the function of the
apparatus, the upper terminal of the nozzle 30, has a
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pressure gauge 35 associated therewith which reads the
hydraulic pressure of the water in the hose 50 and in the
nozzle 30 and is thereby ideal for water reading.
Figure 4 depicts, not in total detail, a section
through a manhole M, the manhole communicating to a sewer S
which has been recently imbedded in the ground and to which
an overburden, which may be backfill, generally shown as 70
has been applied. The overburden 70 generally forms three
regions, from top to bottom, Zones C, B and A as previously
identified at page 5 of this disclosure.
The manhole has, at its base, and just super-
adjacent to the sewer S, an orifice in which is placed a
removable plug P. Depending on the situation, this orifice
may or may not be plugged during the injection of water into
the backfill. Since Zone A is granular material it covers
the plug P, the removal of the plug P will purge the zone A
of any moisture or water that may subsequently be applied
thereto as will hereinafter be explained.
Referring to Figure 4, the nozzle 30 is first
placed on top of the ground and the water is allowed to
outflow the nozzle 30. A downward force F is applied to the
nozzle causing the nozzle to reverse and to penetrate into
the ground so as to eventually locate itself as shown in
Figure 4. Water jetting now commenced by the continuation
of the water flow will now be described. The nozzle 30 for
the commencement of water jetting is located in the lowest
regions of Zone A, which is composed of granular material
and immediately overlies the sewer S. Optionally, if
penetration into the granular material is too difficult, the
nozzle may be located on the upper surface of Zone A.
Thereupon Zone A is allowed to be flooded and when full
flooding occurs the pressure gauge 35 will indicate a slight
113~ rt0
variation in water pressure in the conduit 50 indicating to
the operator (not shown) the effect of water inundation in
Zone A upon the flow thereof from the nozzle and hence
increasing resistance thereto. At this time the operator
knows that Zone A has been flooded and causes to be imparted
onto the nozzle 30 or stem an upward force which is depicted
as F in Figures 6 and 7. This upward force F may be caused
by mechanical device such as the lift, crane or other
device, or also as conveniently by a human being, the nozzle
30 is slowly lifted up through the overburden 70, through
Zone B, while at the same time water 80 eminates from the
various pores in the nozzle as depicted in Figures 6 and 7.
When the overburden 70 in Zone B is totally saturated, and
the pores of the nozzle reach the surface of Zone C as seen
in Figure 7, the supply of water from the hydrant 40 may be
terminated and the nozzle lifted out of contact with the
ground.
At this point in time the region beneath the
nozzle and to a substantial degree, up and down the trench
length are inundated with water. When, for instance, the
trench has a width of approximately 2 metres, saturation
caused by such nozzle will extend up and down the trench
length for about 2 to 4 metres on either side o~ the
location of the nozzle. The actual extent of saturation,
and therefore settlement depends upon the porosity of the
soil, the type of soil, and the lump size of the soil. Such
event is depicted in Figure 9, by the phantorn nozzle 30R.-
At that location, 30R, the entire underlying overburden 70
is saturated. The nozzle may then be moved to an adjacent
'0 location as for example 30C and the entire process is
repeated until the entire length of trench has been
completely water jetted. ~ne nozzle location 30C in Figure
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9 depicts an intermediate interhole of water jetting at
location 30C where complete saturation has taken place of
Zone A and Zone B only the lower half thereof has been
saturated.
The phantom location 30L is the next jetting
location. When that location has been water jetted, the
full extent between the two manholes M will have been water
jetted. Referrins now to figure 8 the plug P may then be
removed and Zone A purged of water. Since Zone A is
granular the total Zone A will be purged. This will cause a
drainage from the superadjacent Zone B and also from Zone
C and will encourage compaction. After this has occurred
and referring to Figure 8 now, the upper crust 45 begins to
fall since compaction is occurring.
Thus the water flows into the sewer S through the
orifice in the bottom of the manhole when the plug P in the
manhole is removed, this is depicted in Figure 8. Thi~
induces further compaction of t~e soil whereby the upper
crust of the backfill, takes on that new profile shown in
Figure 8. Normally the extent of the depression thereby
created in the upper surface when the trench depth is more
or less then 4 metres, is to an extent of 0.60 to 0.90
metres, more or less in depth.
Now again referring to Figures 1 and 2 and to an
explanation of the hydraulic reactions caused by the nozzle
30 as indicated in Figure 2, the arrow indicates the flow of
water into the nozzle. That flow of water gushes out of the
plurality of orifices in the nozzle indicated as 0. That
water flow traverses, for example, according to the arrows
and dash lines indicated in Figure 2, eroding certain
regions 43 of the various lumps of soil 45. Erosion from
regions 43 traverses into the spaces 55 and floods the same
with a mixture of soil and water. In this way, the
hydraulic action causes the mi~ration of soil into the inner
spacial regions between the adjacent lumps of soil and hence
fills the voids defined by those lumps. This causes
compaction. Each jet of water gushins from each orifice has
a force which vibrates, breaks down and forces the soil
lumps closer together. This again causes compaction. Once
the water is in contact with the soil, it causes a reduction
in the soil's cohesion, making the lumps soft and not able
to resist the compactive forces applied by its own weight
and by the seepage forces as the water is drained out of the
backfill. Figures 10 and 11 depict lumps 10 of clay prior
to water jetting; and after jetting.
It has been found, that ideally, and to deliver
the maximum energy the total area defined by the plurality
of orifices O in the nozzle 30 varies according to the
length and type of conduit 50. For example, for a fire
hose, with inner diameter of 60 mm, this total orifice area
is in graphæ 7 and 3 which will be discussed hereafter, 30%
and 60~ of the conduit's cross-sectional area, as shown, for
hose lengths of 100 metres and 8 metres respectively.
When the plurality of orifice diameters aggregate
to an area in excess of the cross-sectional area of cylinder
inefficiencies in water compaction are achieved.
The use of the nozzle 30 has the effect of
applying water energy in small, concentrated and powerful
jets, distributed over a large area evenly and efficiently,
so that the soil lumps are broken down, and penetration of
water into all bacXfill voids is facilitated.
Numerous modifications will now be apparent to
those skilled and practicing in the art. It is not intended
to deny a limit to the invention to the exact construction
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or operation shown, nor to the apparatus described, and
accordingly, suitable modifications to both the apparatus
and the method is contemplated within the scope of the
invention. I do, however, prefer that the inside diameter
of the nozzle be at least 40 mm; the aggregate area of the
orifices in the nozzle 30 be around 1250 square milllmetres;
and the nozzle length that disposes the orifices (30'') be
about 350 mm. The angle of the tip or cone 30' of the
orifice thereof should be inclined at approximately 45. In
such application the water pressure preferrably ranges
between 200 to 400 KPa. and the volume of water consumed
will be in the order of 30 litres per second. Pressure
ranges from 0 to 700 ~Pa. may be used.
For those skilled and practicing in the art the
attached graphs 1 through 7 will be of value in
understanding the utility of the invention.
Graph 1 depicts total settlement in centimetres
against time after jetting in days under two conditions, the
porous nozzle of the type disclosed in this invention, and a
full open probe having no aperture. In other words the full
opening probe is one whose cross-sectional area or orifice
is identical in size to that internal diameter of the hose
or conduit feeding the water to it. You will note that more
settlement is achieved with the porous nozzle of the
invention.
Graph 2 is similar to that of graph 1 except it
depicts settlement after a given amount of days as a
function of energy emitted by the nozzle. Note that in all
cases the majority of the settlement occurs within two days.
Graph 3 plots water jetting force in newtons of
the agreggate area of the openings of the orifices 0 as
against a cross-sectional area of the hose. You will see
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that the idealized condition is very close to a ratio .5:1 -
that is where maximum energy i8 emitted to the soil.
This is ~hown a second time, with more plotting points, in
graph 7.
Graph 4 is a plot of the degree of compaction as a
percentage over the energy applied over a given elapsed
period of time from 10 days to 10 months as shown.
Graph 5, which is analogous to that of graph 1 and
shows the degree of compaction over time plotting the full
probe opening and the porous nozzle of the invention. Note
that higher degree of compaction is achieved, and more
immediately, with the nozzle of the invention.
Graph 6 also plots the porous nozzle of the
invention as against the full probe opening on a graph
showing vane shear in Kilo Pascals over a lapsed period of
time after jetting. Note that a stronger backfill is
obtained with the nozzle of the invention.
TO lAL SETTLEMENT, cm
o
o ~ r~ ~-~ ~
r _ ~ Z r
-~t
-
-
17
113~70
TOTAL SETTLEMENT, cm
~ a o
` ~
, ,,
7(1
~ t l l --t ~ t~
640 ~ ~~~ ~ h- âm
560- _ /
1 E1,--~ Lh: 25m _
480- / ~
320 ¦ I ~ j \ Lh ~ 70 m
/ Lll- 100 m
~, 24 0 - ~
~: .
C~ _
80 -
Lh: LENGTH OF HOSE - _
O ~ t I I 1- t 1-
0 02 0 4 0 6 0 8 1 0
(AREA OF OPENING) / (AREA GF HOSE)
r~r~lnl
'70
DEGREE OF COMPACTION %
o ~ ) Cl) r ~ 0 c~
o I I ~ I i. _ I I l l I
s
~0
DEGREE OF CCMPACTION %
a~ o ~ ~ ~ co
O ._ . . . ~ t I . I
Q- ~ ~
I ~ I I ~ ~ I- I I
i? l
VANE SHEAR, KPa
~ o ~ O
O I I I t
~-
O
640 ~ \
/ ~;- 8m
s60! ,~
480
t ' ~ Lh: 25m
,_ ~
3 160 Lh: 100~
--~ Lh:LENGTH OF HOSE ~' )
O l l ~ l ~ l
0 0 2 0 4 06 0-8 1 0
(AREA OF OPENING)/(AREA OF HOSE)
(~rar)h 7
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