Clearing includes the removal of all brush, shrubs, trees, crops, and other obstacles.
In wooded areas bulldozers with timber blade attachments can clear brush and small trees. Power saws are used on trees too large to be pushed over by bulldozers.
Salvage of merchantable timber is often required; specific arrangements must be made and defined to the contractor for disposal of nonmerchantable timber, stumps, brush, etc.
Clearing and grading of the construction working strip provides a relatively smooth
"roadway" for the construction equipment and vehicles involved in laying the line.
Grading is usually done by bulldozers, but it may be necessary to use rippers or blasting to assist in removing boulders and rock. In level, lightly vegetated land, or after initial bulldozer grading, motor graders can be used effectively. Care is taken to prepare a path free of obstructions so that subsequent operations can move along without interruptions. Graded material—soil, rock, grass and light vegetation—is pushed to one side of the working strip. Topsoil should be segregated so far as prac-tical, so that it is available for replacing on the surface after grade restoration in order to promote revegetation. Normally clearing and grading will be several miles head of the remainder of the spread, in order not to bottleneck subsequent opera-tions.
Temporary bridges, flumes, or culverts are placed as needed so that equipment and vehicles can cross irrigation and drainage ditches. In very steep terrain, detours ("shoo-flys") may be needed for construction vehicles, while equipment for ditching and laying are assisted by bulldozers and winching.
If special measures are needed for erosion control during the construction period, these will be installed by the clearing and grading crews.
Trench Excavation
Trench excavation ("ditching") includes all work to construct the trench for the pipe. The most favorable ditching conditions are level terrain and rock-free dirt, where one or more wheel trenching machines can move down the graded working strip, excavating a neat, vertical-sided trench and giving a uniform spoil bank. In
rough terrain or in areas having boulders and cobbles in the soil, excavation is done by backhoes. At wet locations such as creekbeds, backhoes, draglines, or clam-shells do the excavating.
In rocky terrain heavy-duty rippers mounted on bulldozers are often adequate to loosen the rock for subsequent excavation by backhoes. When the rock cannot be handled by rippers, wagon drills or drills suspended from sideboom tractors are used to bore holes in the rock along the ditchline. The rock is then blasted, with the broken rock removed from the ditch by backhoes. Blasting should be done before the pipe is strung since flying rock will damage the pipe. If it becomes necessary to shoot rock in cultivated areas or in the vicinity of pipe or surface structures, the blast should be thoroughly matted to contain all fragments. Six-by-six timbers lashed side by side with wire rope typically provide an adequate mat. Also, it may be necessary to reduce the powder charge in order to blast safely. (Blasting must be conducted by certified individuals.) Blasting alongside an existing buried pipeline must be handled very cautiously to avoid damage. Normally delay-type blasting caps should be used to stagger the individual detonations in any one charge, thereby reducing the peak shock transmitted to the adjacent structure. The safe charge to use is a function of the formation (granite, shale, lava, caliche), the depth of the drill hole, and the proximity to existing structures; it must be determined locally by experienced powder men. Test holes may be dug alongside an existing line in the vicinity of blasting to determine if the pipe moves as a result of the shock.
Because a pipeline cannot follow the bottom of the ditch precisely, a little extra depth should be allowed to ensure obtaining required cover. This extra depth can be obtained at almost no extra expense during the initial ditching operation, but reditching or, worse, lowering the line after it has been welded and lowered-in is a very expensive and unsatisfactory alternative. The Company construction represen-tative should make it clear to the contractor that such rework is at his cost.
Company personnel should avoid passing judgment on the depth of ditch, as this may be interpreted by the contractor as relieving him of his responsibility to obtain the required cover and may lead to poor workmanship or additional cost to the Company. (Company’s acceptance or rejection of depth of cover should be made as the pipe is laid, before backfilling.)
At washes and gullies the trench must be cut well below the bottom of these depres-sions, with gradual approaches on either side to avoid vertical bends in the pipe.
Frequently, loose soil will be bulldozed into a sharp wash during grading. "False ditch" or ditch constructed in loose soil overlaying the natural bottom will be eroded with the first rain and leave the pipe exposed or inadequately covered. The depth of ditch must be measured from the original ground elevation.
Many right-of-way and permit agreements for cultivated or grazing land require topsoil to be removed, preserved, and replaced on top of the backfill. Normally, this requires ditching twice, removing the topsoil first and throwing it out farthest from the ditch. The remainder of the ditch can be completed with the spoil bank adjacent to the ditch. This will permit the backfilling operation to replace the spoil in the proper sequence.
Although there are exceptions, sometimes as a normal practice (many production field lines), sometimes owing to special circumstances (unusual environmental conditions) trench excavation generally precedes pipe stringing and welding. The amount of open ditch ahead of the pipe laying should be limited as influenced by local conditions. In terrain where the ditch will hold for several days, it may be desirable to ditch several miles ahead. In terrain where the open ditch impedes farming or irrigating operations, the ditch must be followed closely by the entire spread in order to minimize damages. Entire crops may be lost if pipeline ditching cuts off irrigation water for too long a period. In some types of soil, an open ditch will not stand more than a few hours. This condition must be anticipated and the spread must be closed up to follow immediately behind ditching.
Occasionally ditching progress will be slowed by difficult excavation, and the pipe lineup crew will catch up to the excavation operations. A contractor will then want pipe lineup and welding operations to overtake ditching because loss of laying progress is costly. When this happens, it is necessary to bend the pipe by guess-work, which frequently results in short cover or misplaced bends when the ditch is finally completed. This practice should be stopped as soon as it is discovered. This will give the contractor an economic incentive to improve ditching progress by adding more equipment or working longer hours.
At locations or areas where there are existing crossing pipelines and cables, these underground facilities should be located using line locators and exposed by hand, then carefully excavated with mechanical equipment. Before uncovering the existing facilities, the owner or authority should be notified so he can take whatever action is considered appropriate. Sufficient depth of ditch should be excavated so that a 12-inch minimum clearance will be obtained between the new line and the crossed facility. Normally new lines pass below existing facilities.
Wherever men will be working in trenches over 5 feet deep, the trench sides must be sloped or shored to conform with OSHA regulations. Similar requirements apply in Canada. When soil conditions are unstable, or become unstable, as after rains or because of heavy equipment working too close to the edge, excavations shallower than 5 feet must be sloped or shored. Figures 600-1 and 600-2 (from the Company Safety In Designs Manual) indicate recommendations on sloping for various soils.
Spoil banks should be kept at least 2 feet from the edge of any trench. See the Company Safety in Designs Manual and OSHA Safe Work Practices 2226, Exca-vating and Trenching Operations. (Company field personnel should be aware that at least one court ruling determined that an engineering firm could be held liable for injury and death of a contractor’s employee for failure to take appropriate action after discovering that safe trenching methods were not being followed. In Canada also, Company personnel can be held responsible.)
Padding
Where rock and rocky soils could damage the pipe coating during laying and covering of the line, suitable bedding and backfill material must be provided. As the first step in achieving rock-free material around the pipe, normally a minimum of 6 inches of dirt or sand is brought in from another source and placed in the ditch
bottom. Preferably, this padding material is preferably spread uniformly along the trench, but it is often acceptable to place padding in about three-foot long piles at 15- to 20-foot intervals along the ditch. Alternatively, sandbags filled with dirt or sand may be used to support the pipe off the ditch bottom, but care should be taken that spacing is close enough so that bearing loads at the sandbags do not damage the coating. Arrangements for acquiring and hauling padding dirt are normally the contractor’s responsibility, but should be monitored by Company field personnel to ensure the contractor is not getting material without proper arrangements.
Where there is extensive rock and a scarcity of bedding and backfill soil, costs to obtain and haul suitable material will be great and other alternatives to protect the coating such as a tough "rockshield" wrapping or a urethane foam should be consid-ered. The rockshield should be perforated so that it does not shield the pipe from cathodic protection current. The construction specification must be clear on the shielding method to be used because of the cost significance in bidding and construction.