METHOD AND APPARATUS FOR BACKFILLING PADDED UNDERGROUND STRUCTURES

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus to stabilize the

backfill of ditches containing a pipeline, cable or other structure surrounded by sand or

other padding material to prevent rocks or other larger particles from encroaching upon the

padding material which surrounds the pipe.

2. Description of the Prior Art

Steel pipelines are used for transporting fluids, such as gas or oil, over a

long distance, and it is common practice to bury the pipelines underground. Typically, top

soil is stripped away and reserved for reapplication after the trench is backfilled. A trench

is dug to a depth which allows 3 feet of cover over the pipe and 6 inches of space under the

pipe. Subsoil removed from the trench is piled near the trench for later use as backfill

material.

The pipe is laid on supports which hold the pipe generally six (6) inches

above the trench floor. These supports can be sand bag piles or foam blocks spaced every

12 feet or so, or on a continuous bed of rock free soil. The pipe is then covered with rock

free dirt called padding so that the padding material surrounds the pipe a minimum of 6

inches under, over and beside the pipe. Typically, the padding material is sand or subsoil

with a particle size having a diameter not larger than 1 1/2 inches. Some specifications call

for a maximum diameter of 1 inch or 1/2 inch. The industry term for this construction step

is "Pipeline Padding."

In fairly low rock content areas pipeline padding can be accomplished with

a backhoe operator virtually selecting dirt without rocks and gradually pulling it into the

trench. In more rocky areas sand or screened dirt must be purchased and hauled to the

right-of-way or produced on-site by pipeline padding machines. Padding machines are

specifically designed portable screening machines which process the excavated spoil or

right-of-way subsoil to produce and place padding material over the newly laid pipe. After

the padding has been applied the remaining rock and subsoil spoil are then reintroduced

into the trench.

Figure 1 shows a cross-section of a trench 1 backfilled according to one

prior method. The pipeline 2 is surrounded by padding material 4. Typically this padding

material will have been screened from soil dug from the trench. The remaining soil 6 is

deposited over the padding material to completely backfill the trench.

A second prior art backfill method is illustrated by the trench cross-section

of Figure 2. In this method the soil removed from the trench is run through multiple

screens. The final screening creates the padding material 4 which typically contains

subsoil having particles sized up to 5/8 inches nominal in diameter. Intermediate

screenings provide intermediate layers 5, 7 and 9. Layer 5 contains 5/8" to 1 1/2" stone.

Layer 7 contains 1 1/2" to 5" stone. Layer 9 contains 5" plus rocks produced by the

multiple screened device and the remaining rock and subsoil spoil backfilled into the

trench. Layers 5 and 7 form a non-compactable barrier between the padding material 4 and

the larger rock and subsoil mixture 9. In many areas a layer of topsoil 1 1 shown in

Figures 5 and 6 is placed over the backfill to completely fill the trench.

The screening and backfilling of soil into the trench are often done by

pipeline padding machines. This backfilling device usually includes a storage hopper or

similar compartment for receiving the backfill material, a continuous conveyor and a

system of vibrating screens for sorting the backfill material. The components within the

backfilling device are supported by a mainframe which is carried above the ditch to be

backfilled. A support arm rigidly locks the backfilling device with a tractor or other means

of locomotion. The backfilling device is configured to backfill the ditch with various

layers of different sized backfill material in a single pass. One such backfilling device is

described in United States Patent No. 4,955,756.

The major problem facing owners and users of underground pipelines is

corrosion. Corrosion is an electrochemical process, requiring an anode and a cathode

electrically connected in an electrolyte. Corrosion is a self-poisoning process, and in order

for it to continue over a long period of time, a mechanical or chemical means for

depolarization or depassivation must exist. Interference with any one of these

requirements will reduce the corrosion rate.

A widely used method of protecting pipes is the use of coating materials

including tape, plastic, enamel, epoxy and urethane paint. The coatings separate the anode

and cathode or isolate the electrolyte from the metal as long as the coating is free of defects.

In addition to the coatings it is known practice to use cathodic protection for

the protection of metals buried underground. Present pipeline designs frequently include

cathodic protection in accordance with the known technology which is effected by

connecting the surface ofthe steel pipe either to a buried sacrificial anode or to an

impressed current source. Cathodic protection is useful in preventing localized erosion of

scratched and chipped pipe. However, cathodic protection is unable to protect from

corrosion caused by water migration under the protective coating where the protective

coating acts as an insulator. If the coating separates from the pipe, water may migrate

under the coating and set up a corrosion cell using the coating as a shield against the

cathodic protection. Furthermore, cathodic protection is expensive and typically is performed throughout the useful life ofthe pipeline.

Pipeline owners have developed techniques to observe underground

pipelines in use. Some methods enable pipeline owners to see changes in pipeline wall

thickness caused by localized corrosion. If the pipe is sufficiently corroded so that the wall

thickness becomes too thin, the pipeline must be replaced. Because replacement of

underground pipelines is expensive, pipeline owners have continued to search for better

ways to prevent corrosion. Most of this effort has been directed to developing improved

alloys, coatings and cathodic protection systems. Little attention has been paid to

preventing damage to coatings during pipeline installations. Prior to the present invention

the art has ignored the chipping and scratching of pipeline coatings which occurs after

installation.

Obviously scratching and chipping ofthe pipeline can occur during pipeline

handling on installation. A second less obvious method of scratching and chipping

involves migration of rocks into the padding layer.

Rocks can and do break into the padding and damage the pipe coating

during backfill using the method which results in the backfilled trench of Figure 1. After

construction the padding layer provides only minimal support for point loads allowing

rocks to move down into the padding layer and reach the pipe. This movement can occur

during periods when standing water accumulates in the bottom ofthe trench. The water

can mix with the padding material to an extent where the padding layer completely loses its

ability to support the overhead rocks. In this condition the padding material and the

overhead rocks change positions and the pipe becomes surrounded by rocks. Further

movement of the rocks against the pipe chips or cracks the protective coatings resulting in

corrosion. Thus, there is a need for a backfill method which creates a backfill trench in

which rocks are prevented from entering the padding layer. Preferably this method can be

practiced using existing pipeline padding machines as well as in less rocky areas where

padding can be accomplished by selective backfill.

Rocks can not break into the padding layer 4 during backfill using the

method which results in the backfilled trench of Figure 2. Layers 5 and 7, which form a

non-compactable barrier of sized stone prohibit point loads from acting on the top surface

of the padding layer 4. After construction, however, excess water in the backfilled trench

will allow the padding material 4 and the non-compactable layers 5 and 7 to gradually mix

until at some point rocks could enter the padding layer 4.

There are a class of materials known as geotextiles which are used primarily

in roadway construction for erosion control. These materials allow subsurface water to

pass while preventing adjacent soil from migrating past the geotextile layer. Light and

medium weight nonwoven geotextiles placed over or around a drainage layer of large stone

and rocks allow subsurface water to flow into the drainage core while preventing adjacent

soil from clogging the drainage system. Geotextiles are placed over railroad beds to

prevent fouling of ballast beneath railway track. Geotextile layers have also been placed

around layers of soil to create retaining walls and to allow slopes to be safely steeped.

Geotextiles as well as other types of sheet materials such as polyethylene sheets and

bentonite clay liners are also used in landfills. The art, however, has failed to recognize

that geotextiles could be used in pipeline trenches to prevent rock migration into the

padding layer.

In United States Patent No. 5,176,025 Nicholas Butts discloses a pipeline

system for preventing, detecting and containing leakage from subterranean pipeline

transporting hydrocarbon liquids and gases. This system is designed to both prevent

corrosion and contain any liquids or gases that leak from the pipeline. In Butts' system steel

pipe is wrapped with a geotextile material impregnated with hydrated lime. That wrapper

is completely covered by a protective, water impermeable layer of high density

polyethylene (HDPE) sheet. Each sheet of HDPE is welded to the adjacent sheet forming a

continuous impermeable sleeve around the pipe. Should the sleeve break and water

permeate the sleeve the lime impregnated geotextile layer dissolves creating a highly

alkaline solution providing the electric potential for cathodic protection. Since water

causes the geotextile used by Butts to dissolve that geotextile offers no protection against

rock migration resulting from water in the trench.

Geotextiles have also been proposed for use in swamp weights as an

alternative to concrete swamp weights for resisting buoyant forces acting on pipelines

crossing water inundated terrain or organics. After the pipeline has been lowered into the

ditch, a woven geotextile fabric is placed at designed intervals along the ditch. The fabric

is draped down one ditch wall, placed across the ditch bottom, over the top ofthe pipe, and

then draped up the other ditch wall. The backfill, consisting of either native ditch spoil

material, a mixture of native and imported material, or entirely imported material, is then

placed over the pipe in the normal manner. In this system the geotextile does not run the

full length ofthe trench nor does it stabilize the backfill profile to prevent migration of

rocks.

Underground pipes have also been wrapped with polyethylene webbing to

protect the pipe from rocks and other debris. This material, called rock shield, usually

consists of extruded plastic although geotextile fabric is sometimes used. Rock shield is

utilized as a second coating layer wrapped around the pipe. In this system the geotextile

does not stabilize the backfill to prevent rocks from entering or mixing with the padding

material.

Buried fiber optic, telephone and electrical cables as well as underground

storage tanks and other buried structures are often padded with sand and covered with

subsoil in the same manner as underground pipelines. Buried electrical service lines are

commonly enclosed in a polyethylene wrapping. Television and other communications

cables are constructed with a polyethylene outer layer. Movement of rocks against the

outer surface of these cables can remove insulation, bend and even break these cables.

Indeed, some electric utilities have reported rock damage to the outer layers of buried

electrical cables. Buried storage tanks and other metal structures can be damaged by

underground rock movement in the same manner as underground pipes. Therefore, any

backfill method and device which protects underground pipelines, may also be suitable for

protecting buried cable and other structures.

SUMMARY OF THE INVENTION

I provide a method and backfilling machine for backfilling a trench

containing a pipe, cable or other structure in which a geotextile material is placed over or

around the padding materials. The geotextile prevents rocks from mixing with the padding

material.

In applications where backfill is screened to provide layers of increasingly

larger size stones, the geotextiles can be placed on top of the padding layer.

I further provide an improved padding machine which carries a roll of

geotextile material and automatically lays that material over the padding material. A

spreading arm is preferably provided to keep the fabric flat in the trench while being

covered with subsequent layers.

I also provide an apparatus for placing geotextile material over the padding

layer which can be used with selective backfill practi s or when padding material is

imported.

I further provide a padding machine which automatically lays geotextile

material over padding material, which includes a material spreader/installer to protect the

geotextile from damage during the backfill process.

Other objects and advantages of my backfill method and improved padding

machine will become apparent from a description of certain present preferred embodiments

thereof shown in the drawings.

BRIEF DESCRIPTION OF THR FIGURES

Figure 1 is a cross-sectional view of a trench containing a pipeline

backfilled according to one prior art method.

Figure 2 is a cross-sectional view of a trench containing a pipeline

backfilled in accordance with a second prior art method.

Figure 3 is a cross-sectional view of a pipeline in a trench which has been

backfilled according to a first present preferred embodiment of my backfill method.

Figure 4 is a cross-sectional view of a trench containing a pipeline

backfilled according to a second preferred embodiment of my backfill method.

Figure 5 is perspective view of a trench containing a pipeline backfilled in

accordance with the second preferred backfill method.

Figure 6 is a perspective view similar to Figure 5 illustrating a pipe

backfilled in accordance with a third present preferred backfill method.

Figure 7 is a side view of a present preferred embodiment of my improved

padding machine.

Figure 8 is a perspective view of a portion of a padding machine illustrating

a second preferred embodiment of my improved padding machine.

Figure 9 is an end view of an apparatus for applying geotextile material over

the padding material which can be carried by a tractor or other mobile carrier.

Figure 10 is a side view ofthe apparatus shown in Figure 9.

Figure 11 is a cross-sectional view similar to Figure 3 of a fiber optic cable

which has been backfilled according to the first present preferred embodiment of my

backfill method.

Figure 12 is a side view of an underground storage tank which has been

backfilled according to the second present preferred embodiment of my backfill method.

Figure 13 is a perspective view of a portion of a second preferred

embodiment of my apparatus for applying geotextile material over the padding material.

Figure 14 is a side view of the apparatus shown in Figure 13 as padding

material is being applied.

Figure 15 is a side view similar to Figure 14 of a third preferred

embodiment of a portion of my apparatus for applying geotextile material over the padding

material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first present preferred embodiment of my method is based upon the

prior art practice of Figure 1. Referring to Figure 3, a pipeline 2 is placed in a ditch or

trench 1 and covered with padding material 4. A geotextile material 8 is placed over the

padding material and generally extends from one side of the ditch to the other. I prefer to

use woven rather than nonwoven geotextile material. Backfill containing a random

mixture of rock and subsoil 6, often called spoil, is then placed on top of the geotextile

material. If desired, top soil (not shown) can be placed over the backfill material to

complete the filling ofthe ditch.

Although geotextile material does not provide excellent load bearing

capabilities as it can be torn or ruptured nor does geotextile material have excellent load

distribution capabilities it can stabilize the padding material 4 if the backfill spoil 6

contains a low to moderate percentage of larger rocks and provided the backfill spoil 6 is

carefully placed on top ofthe geotextile material 8.

The geotextile material 8 acts as a stabilizing sheet which permits water to

pass while preventing the passing of solids, either from the padding material 4 or the

backfill spoil 6, provided that the geotextile material 8 is not torn or ruptured. There may

be other nondegradable fabrics not generally considered to be geotextiles which could be

used as this stabilizing sheet. Consequently, the padding material 4 will maintain its

integrity and not be encroached by rocks and there will be no scratching of the pipe after

the backfill process has been completed.

A second preferred embodiment of my method is illustrated in Figures 4

and 5. As in the prior art process of Figure 2, a pipe 2 is placed in trench 1 and covered

with padding material. The padding material has been created by running the spoil

removed from the trench through a series of screens. The material passing through the

finest screen is the padding material. Preferably, the padding material will contain

particles having a diameter of up to 5/8 nominal inches. Next, I place a layer of geotextile

fabric 8 on top ofthe padding layer. Then, I place a layer of stone ranging between 5/8 and

1 1/2 inches in diameter on top ofthe geotextile material to form layer 5. On top of layer 5

is placed a layer 7 containing stone having diameters from 1 1/2 inches to 5 inches. Finally,

layer 9 which contains rock having a diameter greater than 5 inches mixed with smaller

stone and soil is placed. Preferably, an automated padding machine is used to separate the

soil removed from the trench into the various layers 4, 5, 7 and 9. In most installations a

layer of top soil 1 1 will be placed on top ofthe coarse backfill layer 9. When the pipe is

initially placed in the trench it is supported by sandbags 3 or other supports. The sandbags

maintain the pipe 2 at some distance above the trench floor to allow padding material to be

placed under the pipe. In the installation shown in Figures 4 and 5, layers 5 and 7 form a

non-compactable barrier between padding and the rock and soil backfill layer 9. This

barrier distributes loads over the top surface ofthe padding material. The geotextile fabric

8 prevents mixing ofthe padding material 4 below and the sized stone 5 above.

Consequently, the integrity of both the non-compactable barrier and the padding layer 4 is

maintained and rock damage to the pipe is prevented.

A third present preferred embodiment of my backfill method is shown in

Figure 6. In this embodiment geotextile material 8 is draped down one side ofthe trench

across the bottom and up the opposite side ofthe trench. Then, the pipe 2 is placed on

sandbags 3 in the trench. Padding material 4 is deposited around the pipe. Then that

portion of the geotextile fabric 8 extending above the trench is folded over the top of the

padding layer 4 as shown. Then, backfill layers 5, 7, 9 and top soil 1 1 are placed on top of

the geotextile fabric. These layers are placed in the same manner as described before in the

embodiments of Figures 4 and 5. This embodiment of my backfill method is intended to

be used where the trench has been excavated in very unstable subsoil where rocks can

break away from the unexcavated trench walls 51 or work upward from the unexcavated

trench bottom 52.

The methods illustrated in Figures 4 thru 6 can be practiced using the

padding machine shown in Figures 7 and 8. Referring to Figure 7, the padding machine 18

has a frame 20. A hopper 22 having a grizzly 24 is placed on top ofthe frame. When

backfill material is placed on top the grizzly large rocks will fall off forming layer 9. Other

material will pass through hopper 22 onto conveyor 28. If the padding machine is ofthe

type described in my United States Patent No. 4,955,756, the conveyor will deliver the

material to a double screen within housing 32. Material having a diameter between 1 1/2

and 5 inches will fall through chute 34. Stones having a diameter between 5/8 inches and 1

1/2 inches will fall from the bottom edge 35 of housing 32. In an alternative machine

having a single screen in housing 32 there is a large screen 26 at the bottom of hopper 32.

Material having diameter between 1 1/2 and 5 inches will fall from the back of screen 26

onto chute 30. That material will drop to form layer 7. Material passing through screen 26

will fall onto conveyor 28. That conveyor moves the material onto screen 32. Stones

having a diameter between 5/8 inches and 1 1/2 inches will not pass through screen 32.

Rather, they will fall onto chute 34 and then down to form layer 5. In both types of

machines, the fine material having a diameter of 5/8 inches or less drops through screen 32

onto chute 33 and into the trench to form the padding material 4.

In the improved padding machine 1 provide a support 36 on which I place a

roll 38 of geotextile material 8. Geotextile material is routed around roller 40 held by

bracket 39 and then laid on top of the padding layer 4. Roller 40 also levels the padding

layer during placement ofthe geotextile fabric. Although I prefer to place the geotextile

fabric on the padding layer, I have shown an optional support 36a. Using this support the

geotextile fabric is placed between layers 5 and 7 as shown in chainline.

In Figure 8 I show an alternative configuration ofthe padding machine

wherein the roll 38 of geotextile fabric 8 is mounted directly on the padding machine

frame. A pair of leveling arms 42 having a roller 44 between them extends from chute 33.

This arrangement maintains the geotextile fabric on the padding layer as the remaining

layers 5, 7 and 9 are deposited.

When selective backfilling is done or padding material is imported the

geotextile material may be placed in the trench using the apparatus 60 shown in Figures 9

and 10. A rectangular support frame 61 has a mast 62. A cable 63 and clevis 64 are

provided to attach the frame 61 to a tractor or other mobile carrier indicated by chainline

70. A roll of geotextile material 66 is carried on rack 65 attached to the top of the frame. I

prefer to provide a motor 75 for unrolling the roll of geotextile material. I prefer to provide

a padding leveler 69 which is connected to the bottom of the frame by telescoping steel

tubing 68. A hydraulic cylinder 67 is provided to adjust the height of the leveler 69. 1

prefer to provide brackets 71 on the rear ofthe leveler 69 which holds roller 72. A

spreader 73 preferably having a pair of arms with a roller connected between the arms such

as is used in the embodiment of Figure 8 extends from the bottom ofthe frame 61.

Geotextile material is fed from roll 66 under roller 72 and spreader 73 onto padding

material 4. The leveler 69 assures that at least a portion of the padding material will be at

the same level as the spreader to enable even placement ofthe geotextile material.

As shown in Figure 1 1 buried fiber optic cable 80 having a transmissive

core 81 , reflective layer 82 and protective coating 84 is padded with sand or other padding

material 4. A geotextile material 8 is placed over or around the padding material. Then

backfill containing soil 6 covers the geotextile materials. If desired, top soil (not shown)

can be placed over the backfill material to complete filling ofthe ditch 1. The geotextile

material prevents rock migration from the backfill soil into the padding layer.

Consequently, migrating rocks cannot strike the cable 80 and damage the protective

coating 82 and reflective layer 81.

An underground storage tank 90 with access pipe 92 shown in Figure 12 is

surrounded by a padding material 4. Geotextile material 8 is placed on or around the

padding layer. Then the spoil is selectively returned to the trench to create layers 5, 7 and

9 containing stone of progressively larger nominal diameters. Topsoil or concrete (not

shown) may be placed over the top layer. As in the embodiment of Figures 4 and 5 the

geotextile material prevents mixing ofthe padding material and the stone above the

geotextile material. Furthermore, layers 5, 7 and 9 form a non-compactable barrier.

Consequently, rock damage to the storage tank is prevented.

In Figures 13 and 14 I show an alternative configuration of a padding

machine 94 wherein a roll of geotextile fabric 95 is mounted so as to hang below the

padding machine frame 96. The roll of geotextile fabric is held by fabric roll supports 98

which are provided at each end of a shaft 100. Supports 98 can be adjustable to accept

varying widths of geotextile fabric roll 95. Also, the shaft can be provided with bearings

(not shown) for ease of unrolling. A tensioner 102 may be provided to control the

unrolling of the roll of geotextile material so as to resist unwinding of the roll 95 as the

padding machine 94 is pulled by the tractor or other mobile carrier along the trench. This

is an alternative to providing a motor for unrolling the roll of geotextile material (Figures 9

and 10). The tensioner could be a bar, roller or flat sheet resting on the roll or a brake

acting against the sides or core of the roll. A geotextile spreader/installer 104 can be

provided to keep the geotextile fabric 8 spread as it is laid on top ofthe padding material 4

and to prevent collapsing of the fabric as it is unrolled. This also helps to lay the fabric

directly on the padding material as the layer 5 is spread on top of the geotextile fabric. The

spreader/installer 104 is preferably made of a strong flexible material such as that used for

conveyor belting and having attached on a bottom portion 105 neoprene rubber where the

geotextile fabric meets and passes under the spreader. It eliminates the need for the roller

40 shown in Figure 7 which may be damaged or restricted by rocks. As the fabric is

unrolled from the geotextile fabric roll 95, it slides underneath the bottom portion 105 of

the spreader/installer 104, while the top surface 106 of the spreader/installer prevents all or

a portion ofthe material which forms layer 5 from falling on unsupported geotextile fabric,

preventing damage to the fabric. This spreader/installer preferably is wider at the top than

at the bottom because trench walls are usually sloped. A top width of 40 inches (101.60

cm) and a bottom width of 30 inches (76.20 cm) can be used. In an alternative

embodiment, one end ofthe spreader/installer 104 may be attached on the shaft 100 so as

to provide resistance on the geotextile fabric roll 95 such that it acts as the tensioner 102.

Similar to the apparatus shown in Figure 10, a padding leveler 108 may be

connected to the frame 96, such as by telescoping steel tubing 1 10. A hydraulic cylinder

1 12 may also be provided to adjust the height of the leveler 108. In the embodiment

shown in Figure 15 I also provide a roller 1 14 for the geotextile fabric positioned well

above the padding layer 4. The carrier may be an idler. The geotextile fabric is unrolled

from the roll 95 and passes under the carrier 1 14 and then slides underneath the bottom

surface 105 ofthe spreader/installer 104.

As shown in Figures 14 and 15, the padding machine may also include

chutes 1 16, 1 18 from the padding machine or other padding material source extend through

the frame 96. The chutes deliver the different layers of spoil on top of the geotextile fabric

8 after it is placed over the padding material 4. The padding material is placed over the

pipe 2, or other buried structure such as the fiber optic cable 80 or tank 90, and the padding

machine 94 is moved in the trench while the leveler 108 levels the padding material. As

the geotextile fabric unrolls from the roll, it slides underneath the geotextile fabric

spreader. Alternatively it may also pass underneath the idler 1 14 in Figure 15 prior to

sliding underneath the spreader/installer. Material having a diameter between 5/8 inches

and 1 1/2 inches will fall from the first chute 1 16 onto the geotextile fabric 8 to form layer

5. Material having a diameter between 1 1/2 and 5 inches will fall from the second chute

1 18 on top ofthe prior stone layer to form layer 7. Large rocks from the backfill can then

be placed on top ofthe larger size stone layer 7 to form layer 9. Top soil may then be

placed on the layer 9.

Although I have shown certain present preferred embodiments of my

method and padding machine, it should be distinctly understood that the invention is not

limited thereto, but may be variously embodied within the scope ofthe following claims.