Method of Backfilling Pipeline Trenches

1. Field of the Invention

The present invention relates to a method of backfilling pipeline trenches.

2. Description of the Prior Art

According to the most conventional method of backfilling a pipeline trench, the bottom of the trench is first filled with a layer of sand or other bedding material which is then compacted to form a bed for the pipe. The pipe is then laid on this bed after which an aggregate padding material is dumped into the trench along the opposite sides of the pipe. Tampers and vibrators are use to compact the padding about the pipe to secure the pipe in its final position. However, it is difficult to obtain effective compaction of the padding in a region along the bottom of the pipe adjacent to its centerline at the interface of the bedding and padding materials. Quite often air voids remain in this region or at best it is left with only loosely compacted material, thereby creating a flow path for water that may trickle or otherwise find its way into the trench. Where the pipeline is laid on a gradient, which is common, a sufficient flow in the indicated region occurs to eventually wash away the bedding material from beneath the pipe, causing it to settle and finally rupture. Depending on the pipeline's location and the material transported, the possibility of such rupture can present a serious risk of damage to the environment, wildlife and human populations, and substantial loss of the transported resource.

Accordingly, there is a need for an improved method of backfilling pipeline trenches to avoid the aforementioned problems and risks presented by conventional backfilling methods.

The present invention is an improved method of backfilling pipeline trenches to minimize the possibility of pipeline settling and rupture, even under the severest of soil and climate conditions.

The method of the invention utilizes air under pressure to blow a granular fill material into the pipeline trench first below the pipe and then progressively upwardly about opposite sides of the pipe to at least about its spring line simultaneously to fill the trench and uniformly compact the fill material about the pipe. During the pneumatic backfilling operation, the pipeline is temporarily supported at grade in the trench on a series of removable support pads. As the pneumatic filling operation approaches each pad to a point where the pipe can be supported by previously deposited fill, the pad is removed from beneath the pipe so that the filling operation can proceed along the trench. Preferably the trench is backfilled by blowing the fill material primarily horizontally into the trench beneath the pipe and upwardly along its opposite sides using a pair of opposed vertical air ducts with 90.degree. elbows at their discharge ends projecting into the trench on opposite sides of the pipe. The removable pads are preferably gas-filled and deflatable to facilitate their removal from beneath the pipe.

A primary objective of the invention is to provide a method of backfilling pipeline trenches that will prevent the washout of bedding material from beneath the pipeline and consequential pipeline rupture.

Another primary objective is to provide a method of backfilling pipeline trenches that will provide uniform optimum compaction of the fill material beneath, about and against the pipe from the bottom of the trench upwardly to eliminate air voids and areas of loose fill and to enable the use of thin-walled pipe sections.

Another important object is to provide a method of backfilling pipeline trenches that is safer, faster, easier and more economical than prior such methods.

The foregoing and other objects, features, and advantages of the present invention will become more apparent from the following detailed description which proceeds with reference to the accompanying drawing.

In the drawing:

FIG. 1 is a cross-sectional view through a pipeline trench illustrating the prior conventional backfilling method of the prior art;

FIg. 2 is a cross-sectional view similar to FIG. 1, showing a pipeline trench backfilled in accordance with the present invention;

FIG. 3 is a cross-sectional view through a pipeline trench illustrating the method of backfilling according to the invention; and

FIG. 4 is a longitudinal sectional view through a pipeline trench illustrating the progressive steps of backfilling in accordance with the invention.

First, with reference to FIG. 1, a trench 10 contains a pipe 12 with the trench having been filled in accordance with a conventional back-filling method of the prior art. In accordance with such method, a bedding layer 14 of sand, gravel or other mineral aggregate is placed in the bottom of the trench and compacted by tampers and vibrators to the desired grade of pipe 12. The pipe 12 is then laid on bed 14, which supports the pipe at grade. Then a padding material 16, usually also a mineral aggregate, is deposited in the trench on opposite sides of pipe 12. Conventional tampers and vibrators are applied to the padding material 16 as it is fed into the trench in an effort to compact it securely about the pipe to secure it permanently in its final position. However, it is difficult to compact the padding material securely in the regions 18 at the interface of bed 14 and padding 16 adjacent to and on opposite sides of the vertical centerline 20 of the pipe because of the geometry and location of such region. As a result, frequently region 18 contains an air void or loosely compacted fill which provides a collection point for water seeping or otherwise finding its way into the trench. If the pipeline is laid on a gradient, regions 18 provide a flowpath for such water. Over a period of time water flowing along these regions will wash away bedding material from beneath the pipe, causing it to settle. If the settling is severe, a rupture of the pipeline may occur.

Referring now to FIGS. 2, 3 and 4, the backfilling method of the present invention utilizes a conventional trench 24 and pipeline 26. However, pipeline 26 is temporarily supported on removable support pads 28 at grade in its final desired position spaced from the opposite sidewalls and bottom of the trench. The pads are preferably air-filled bags equipped with air valves 28a for easy inflation and deflation to facilitate their use.

Using a pair of air ducts 30, 32 projecting into the trench on opposite sides of the pipeline 26, a granular fill material 22, such as crushed rock or other suitable mineral aggregate, is blown into the trench under pneumatic pressure. First the space in the trench beneath the pipeline is filled with the granular material at a position unsupported by pads 28 as shown in FIGS. 3 and 4. Then filling progresses upwardly in the trench simultaneously on opposite sides of the pipeline by gradually withdrawing air ducts 30, 32 upwardly until the trench is filled with the granular fill material and firmly compacted about the pipeline to at least approximately the spring line 21 of pipe 26. The granular fill material from the bottom of the trench to the upper limit of compaction may have the same size composition. It has been found that mineral aggregate such as crushed rock or gravel, up to 4-inch minus in size, can be successfully blown into place and compacts well under pneumatic pressure to provide a suitable fill material.

As the pneumatic filling operation progresses along the trench and approaches one of support pads 28 so that the pipe is supported by the fill, such pad is deflated and removed from beneath the pipe so that the filling operation can continue uninterruptedly along the pipeline. Thus as the filling operation proceeds, pads 28 are deflated and removed one by one as necessary until the backfilling operation is completed and all of the pads have been removed.

After a section of trench has been backfilled pneumatically to approximately the spring line of the pipe, the remaining upper portion of the trench section is backfilled with additional fill material 29 in the conventional manner using a bulldozer or other dozer blade-equipped vehicle to push the backfill material into the trench. Thereafter such loose upper fill is compacted using conventional compaction vehicles and equipment.

The pair of air ducts 30, 32 used to blow the granular material into the trench under pressure are connected to an above-surface source of air pressure and supply of aggregate fill material (not shown). The aggregate can be fed from a hopper into the air ducts using a conventional impeller-type feeder with built-in air lock feature as manufactured, for example, by Rader Pneumatics, Inc., of Portland, ORE. Such ducts are preferably capable of moving not only vertically and longitudinally within the trench, but also about their generally vertical axes so that their lower outlet ends 34, which are upturned slightly, can direct the granular fill material beneath the pipeline and in other directions as required along the sides of the pipe and trench.

By pneumatically blowing the granular fill material into the trench beneath the pipeline and working gradually upwardly simultaneously about both sides of the pipe, the air voids and areas of loose fill described with respect to the conventional method of FIG. 1 are avoided. Also, the method provides a uniform all-around external aggregate contact pressure on the buried portion of the pipe to counteract internal pressures on the pipe from the material flowing therethrough. This enables the use of pipe of thinner wall section than would normally be required using conventional backfilling methods. From experimentation it has been found preferable to blow the fill material into the trench generally horizontally. This simplifies the blowing of material beneath and along the pipe and against the sides and previously filled section of the trench. It also provides optimum compaction. For this reason the upturned lower discharge ends 34 of air ducts 30, 32 are preferably formed with a 90.degree. elbow. Backfilling a section begins with the elbow at the bottom of the trench spraying material beneath the pipe and against the wall of previously backfilled material. In this sense the fill material is blown into the trench generally in a direction opposite the direction in which backfilling progresses, as shown in FIG. 4. The elbow 34 is gradually raised as the bottom of the trench is filled, blowing material from side to side and toward the rear of the trench in the process.

In FIG. 4 the steps of the method are illustrated in sequence, progressing from right to left. In the section 40 of trench 24, the subsurface pipeline 26 is temporarily supported at grade on inflatable support pads 28. In section 42 of the trench immediately to the left of section 40, the air ducts 30, 32 are shown in the process of pneumatically backfilling the lower portion of the trench where the pads have been removed. In this section pneumatic backfilling continues upwardly to a level 43 just above spring line 21 of the pipe. In a third section 44 of the trench to the left of section 42, the upper portion of the trench has been backfilled with fill material 29 and compacted to the soil surface using conventional compaction equipment, burying the pipe and completing the backfilling operation.

In one test backfilling operation, 3/4-inch minus aggregate was used to backfill a trench for a pipe 4 feet in diameter. The pipe was supported on pads at 20-foot intervals in the trench with at least 6 inches of clearance between the pipe and the sides and bottom of the trench. A low-pressure pneumatic system developing pressures of less than 20 pounds per square inch was used to blow the material horizontally into the trench at a velocity of about 100 feet per second. Filling proceeded upwardly to the spring line of the pipe. Substantially uniform compaction was achieved with Proctor densities of the resultant fill being 95 percent or more. It is believed that by using velocities of greater than 88 feet per second, Proctor densities of greater than 90 percent can be consistently achieved.

Having illustrated and described the method of the invention in what is now its preferred embodiment, variations and modifications in the details, procedures, materials and techniques of such method will become apparent to persons skilled in the art. I intend to claim as my invention all such variations and modifications as come within the true spirit and scope of the following claims.