open cut pipe installation
Those undertaking underground pipe installation ask themselves one basic question; what items both above ground and below ground will be in the way of my pipe or trench? Obstructions are avoided as much as they can be by the project design engineer by way of locating existing utilities and surveying above grade items, but they aren’t always discovered or considered. Aside from the obstruction factor, the other driving force behind pipe installation is whether the pipe being installed is pressure or gravity fed.
Pressure pipe such as water and gas can often be routed around other underground obstructions by way of fittings during installation. Gravity fed pipe systems such as sanitary and storm sewers on the other hand are pre-engineered to convey their contents by way of sloped pipe and the associated force of gravity, so when unplanned obstructions are encountered the obstruction itself will need to be moved out of the way of the pipe’s intended course. Pipe is sloped just enough that it will move water at the right pace, not too quickly (which can erode pipe systems and manholes/catch basins) and not too slowly (which can lead to undesirable standing water and flowability issues). When an obstruction is another gravity fed pipe, things can get much more challenging.
The two other primary things to be considered when underground pipe is being installed, more so with gravity fed pipe, is pipe ‘line’ and ‘grade’. The term ‘line’ refers to final resting place of the pipe in terms of the horizontal. The term ‘grade’ refers to the final resting place of the pipe in terms of the ‘vertical’ or pipe ‘invert’ elevation.
A pipes invert is the exact elevation of the bottom of the inside of the pipe (aka the elevation at which it’s contents flow). With pressure pipe grade is still important, but with gravity fed pipe grade is critical both because it controls slope and proper content flow. It also must be correct in order to line up with precast manholes and catch basins which often have pre-drilled holes and invert systems. A pipe’s crown in the elevation at the top of the inside of pipe. Lastly a pipes springline is the center line of the side.
Aside from the above fundamentals, there are a host of other considerations when it comes to estimating and anticipating underground pipe trenching and installation. The following all factor into the ease of installation, excavation, productivity, resources required, pipe performance/testing, impediments, site restoration, etc :
-Pipe testing (Deflection, Pressure, Infiltration, Exfiltration, Bacteria, Etc)
-Sawcutting (If road is present) for clean pavement edges
-Restoration Cleanup (Sod, Topsoil, Seeding, Etc for areas not under pavement)
-Utility Pole Holding (When obstructing intended trench or pipe pathway)
-Underground Utility Obstructions – (marked, Unmarked, Noted, and Non-Noted existing utilities can slow and obstruct the progress of pipe installation. This refers to water mains, water services, sanitary sewer mains, sanitary sewer services, storm water mains, storm water services, gas mains, gas services, cable, power, fiber, telephone, etc)
-Overhead Wire Clearances (telephone, cable, Fiber, Etc low enough to block passage of equipment)
-Side Loading or rear Loading Excavation Equipment (for removing existing spoils)
-Front End Loader Access to trench (for installing aggregate pipe bedding and backfill aggregate)
-Traffic Control/maintenance (Will the pipe laying activity impede normal traffic activity)
-Type of Pipe Being Installed (PVC SDR35, HDPE, C-900, Concrete, Steel, Etc)
-Type of Soil (Clays and Silts are cohesive/plastic in nature and more likely to hold their shape after excavator bucket passes, sandy soils are more prone to caving in if not shored/supported). ‘Stand-Up time is amount of time an unsupported trench wall will hold it’s own shape before caving in. The factors influencing stand up time are soil type, water saturation, depth, weather, and construction or traffic activity around the trench. If the soil conditions are that of clayey or silty soils, and the soil is in a semi-saturated state, the excavation process is relatively simple.
-Flow Stoppage/Bypass (most common at tie-in points of new to existing or replacement of an existing system, the existing pipe flow must be maintained/bypassed while new pipe is being installed.)
-Dewatering (After heavy rains, thaws, in perched pockets, or when natural groundwater level is at or above working elevation, water in trenches must be removed/pumped so workers can work and pipe can be installed as specified by the engineer)
-Compaction testing (Will a field testing agent need to be present to assure backfill compaction above pipe meets engineers specifications)
-Temporary Pavement (In Roadways or Vehicle Traffic Areas that must be reopened prior to the final trench paving replacement operation, temporary asphalt, asphalt cold patch, chip seal, or similar must be installed for maintenance of smooth ridability of traffic vehicles)
-Pavement marking (Striping) Replacement (If pipe being installed in roadway, take note of any pavement markings that will require replacement after permanent trench patch paving operation takes place)
AVERAGE TRENCH WIDTHS FOR OPEN CUT UNDERGROUND PIPE INSTALLATIONS BASED ON DEPTH AND 6-8″ DIAMETER PIPE
-1-2’ Deep – 12-24” width
-2-4’ Deep – 24-36” width
-4-6’ Deep – 36-48” width
-6-10’ Deep – 48-54” width
-10-14’ deep – 54-60” width
-14-17’ deep – 60-66” width
-17-22’ deep – 66-72” width
This information is general in nature and can vary considerably depending on :
-Company standards and expectations
-Soil Type (Silty clays are more suited to holding their shape – sandy/gravely soil is not)
-Size of the pipe being installed
-Size of the trench box being used
For a sizable project, a typical storm drainage trench excavation crew will consist of 2 laborers, 2 operators, and a foreman. If only installing shallow pipes or underdrains, this total crew size can often be reduced to 3 or 4 men. The excavator is for the trenching and the loader will be for dumping stone backfill into the trench or moving spoils to be loaded onto trucks for removal. For deeper pipe trenches receiving premium granular backfill, 8-22′ or more, an extra operator and excavator/backhoe may be needed, particularly for the increased amount of compaction required in lifts behind the excavation/pipe operation. The equipment utilized will be a backhoe or excavator and a front end loader.
Firstly the pipe will typically come on a flatbed semi truck, bundled together in groups. The bundles should be lifted off the truck with a forklift, crane, or excavator strong enough to lift the weight of the bundles. If the pipes are delivered loose they will have to be removed one by one using caution not to scratch the pipe or lining. This is often done with textile slings strung through the center of the pipe, or wrapped near the center of gravity of the . The pipes should be stored on a flat surface. Leaving the pipe in bundles during truck unloading is ideal as restacking them on the ground loose can involve considerable extra work with slings, wood blocking etc. The next step is to ‘string out’ the pipe along the sides of the trench. This process expedites installation once the actual pipe installation begins. If the ground that the strung out pipes are placed is not flat, wood timbers or sandbags may be needed under the pipes to keep them from moving around.
Line and grade references are achieved, often by way of layout stakes provided by a professional surveyor. These stakes are placed a certain distance apart, often 25-100′ apart along pipe lengths, and at notable features (manholes, catch basins, valves, bends, tees, etc). The stakes are often provided at centerline of pipe (temporary) with a parallel set offset to a certain distance from centerline stakes (usually +/- 10′). These stakes will often include the following information: identity (centerline or offset), distance if offset, marked line with a dimension for vertical distance to the pipes proposed invert. These stakes are utilized throughout the course of the work by way of a grade laser and story poles in the trench bottom. If pipe installation occurs on a pavement surface where stakes are not possible or practical, a nail is instead placed in the pavement with painted information adjacent and/or a spreadsheet providing needed information.
The backhoe or excavator will start excavating the trench from the starting point (and moving backward) using a bucket that is most practical based on the contractors equipment stock and intended width of the trench. A laborer will provide the eyes needed to establish excavation/bucket depth to bottom of trench (based on layout stakes) and relay this accuracy to back to the operator with verbal and visual cues.
Because all gravity pipe and most pressure pipe comes in sticks and has joint intervals, the need to have laboreres inside the trench pushing and guiding pipe bells and gaskets ‘home’ to their final seated position, the need for laborers to enter the trench exists. Also trench bottom elevation checking and pipe bedding leveling/grading will need done as well. OSHA requires that all trenches 4’ or deeper have appropriate means of entrance/egress, i.e. steps, ramps, ladders, etc however the popularity of ladders with safety professionals and enforcers has been on the decline. Also, guard rails have become an OSHA requirement as well, which some contractors choose to install on the trench plates at the top of the trench.
Dirt caving in from the sides of the trenches is a concern for 2 reasons. Firstly if the contractor is required to backfill with premium material, cave-ins will increase the amount of premium backfill required to be hauled in, which increases the material and trucking costs on the project, not to mention the loading and trucking costs of removing the additional cave-in material. The other concern with respect to cave-ins is safety. Dirt is much heavier than the layman may assume. A large cave-in onto an unsuspecting worker can be lethal.
During any pipe trench process where the excavated material can’t be kept on site, the material is typically hauled off in trucks immediately upon excavation. This keeps the site much cleaner, something that is more important in urban and suburban areas than in rural or non-inhabited areas. The spoils removal trucks typically creep right along with the excavator operator as he goes, receiving spoils and trucking them to a predetermined location to be dumped.
If the trench depth is 5′ or more, it is common for a trench box to be utilized for trench wall shoring, which provides a safe area inside the trench for the pipe laborers to work. The trench box is pulled or dragged through the trench to needed locations by way of the excavator bucket and/or chains/hoists. It is backfilled behind as it’s moved along. Many trench boxes have interchangeable parts, plates and ‘spreaders’, which allow for them to be customized. Many contractors will set a minimum amount of space needed to work inside a trench box, which in turn governs the needed trench width. A good rule of thumb might be:
TRENCH WIDTH = outside diameter of pipe at the bell + 8″ (working space around bell perimeter side one) + 8″ (working space around bell perimeter side 2) + trench box plate depth side 1 + trench box plate depth side 2 + 2″ (overcut/waste side 1) + 2″ (overcut/waste side 2)
There are typically 4 main reasons bedding is required for most pipe types: to provide for consistent support under the pipe bending lengthwise, to increase the loading strength of the pipe, to spread loading pressure away from the joint and out more evenly among the length of each piece, and lastly to provide a platform for lining up and leveling the pipe. Pipe is most often bedded and encased with aggregate or sand, typically +/- 6-12″ around the perimeter of the pipe. If the total width of the pipe and surrounding bedding is less than the needed width of the trench, a slot or channel is typically cut at the bottom of the trench to accommodate. Above this pipe/bedding slot is a benched area which often provides a resting place for the trench box plates to rest on.
Pipe is most often laid with the bell end facing in the direction in which the pipe content flows. This allows for the pipe to be pushed home a bit easier. If the pipes cannot be connected by hand guidance, the spigot end of the pipe being laid can be gently pushed (with a protective barrier in between) with the excavator until the pipes are appropriately connected. Gravity pipe is usually laid from the outlet (deeper end) and laid upstream; aka uphill. This allows for better management of drainage issues during rains or groundwater issues. If wet or unsuitable subgrade exists at the trench bottom, undercutting and replacement with engineered fill may be required.
Most types of pipe have indicating marks of some sort on the spigot ends showing the installer’s when the joint has been pushed home. A gasketing system of some sort is common for many pipe types, possessing varying infiltrating/exfiltration resistances. Once the pipe is installed to proper line, grade, and joint is pushed home, the remainder of the bedding is placed in the trench, followed by the backfill material.
Backfilling pipe trenches involves using either trench spoils or premium engineered backfill material to fill in the trench from the top of the pipe to the desired grade above. There are 2 primary types of backfill conditions. The first type is for areas not underneath or near pavement or a building structure. This type of trench backfill can typically be filled with the excavated spoils or other soil types which are accepted by the project owner or the engineer. The second type of trench backfill condition is done utilizing ‘premium’ backfill to provide a stable enough product given the proximity to vehicle traffic or building structure. Premium backfill refers sand, coarse-graded aggregate, fine graded aggregate, or a mixture of the two.
No matter the backfill material being used, the correct way to backfill a trench is by doing it in lifts of material, typically between 4-12” thick, and watering/compacting each lift to it’s maximum dry density prior to placement of the next. This process helps keep the trench above the pipe from settling over time due to loading, erosion, or other means. This ‘dry density’ goal is discovered by lab testing done referred to as a ‘proctor’ test, which analyzes how much moisture a particular soil makeup can have in order to compact it to it’s densest state, with the fewest amount of air voids.
'Ho-Pac' Pipe Trench Compaction
There are several different ways to compact the trench backfill material. The most common piece of equipment used is a hydraulic plate tamp attachment (or ‘Ho-Pac’) to the end of a backhoe or excavator boom. Some of these attachments currently have a ‘quick-connect’ option now where the Ho-Pac can be attached to the boom without the need for the operator to even leave the machine. For other hard to reach and smaller areas, gas powered walk behind plate compactors or manual ‘jumping jack’ compactors may be used. Once enough material has been compacted above the pipe to safely protect it, small vibratory or static rollers may even be used.
County and state agencies have become more particular about trench backfilling operations as intermittent ‘settling’ issues can become contentious and costly operations to fix, especially when they occur long after the project was originally completed. No matter the equipment utilized, the engineer or inspector will typically want to be assured that the trench backfill lifts have been compacted sufficiently by having a professional testing agency complete testing every so often with a ‘nuclear’ gauge on the lifts to assure 90-100% of max compaction is obtained. Compaction testing will yield testing reports, but is also somewhat of a quality control procedure used to guide the contractor to which locations may need more compaction attention.
Settling Pipe Trench
Flowable or ‘Controlled Density Fill’ is often permitted and used to backfill certain trenches, and may even be required in some areas when the engineer is particularly concerned with setllement. FLowable fill is a cementitious material with a higher water/cement ratio than concrete and is typically ‘self-leveling’ which means it essentially places itself.
Another consideration when backfilling and compacting pipe trenches is the strength of the pipe installed below. For the less rigid commonly used pipes such as SDR35 PVC pipe and Corrugated HDPE pipes, etc, the backfill and compacting operations must be done with more caution as damaging the pipe below can happen quickly and can go unnoticed. For the more rigid commonly used pipes such as Ductile Iron and Concrete Pipe etc, caution should still be taken during installation, but the pipe can typically withstand more pressure from above.
Also caution must be used when backfilling and compacting pipe trenches to assure the trench walls do not cave in. It is common for a pipe trench crew to only lay as much pipe as they can backfill that same day, to avoid leaving open trenches overnight or for longer periods of time. When backfilling trenches around new and existing structures it’s common to hand tamp or tamp with a compact plate compactor to avoid damage. Because of this compaction results may be slightly less than that of heavier more efficient equipment.