Trenching

Trenching is the oldest sewer construction method. It was used to build most of Los Angeles’ sewers before 1990 and is still used today in new subdivisions and other areas when:

• the sewer pipe can be placed at a shallow depth – deeper sewers that are trenched require complicated digging and forms to prevent collapse of the trench
• the pipe is small diameter – larger pipes require very large trenches which can be expensive and get in the way of neighborhood activities
• there is room above ground for equipment and operations

Method: The contractor digs a trench, places the pipe in the trench, backfills the trench, and then repaves the street and repairs any other surfaces that were disturbed when digging the trench. Trenched sewer lines are typically placed no deeper than about __ feet below the ground surface. Sewers are always placed deeper than water lines as a safety factor in the event that the sewer line breaks.

Deteriorated pipes are often lined with new, smaller pipe segments through selectively placed, short trenches.

Microtunneling

Microtunneling has become a popular alternative to trenching, especially in high-density neighborhoods with busy streets and shallow utilities that often conflict with surface digging.

Method: The contractor excavates two pits, about 30 feet by 30 feet for the jacking pit. The jacking pit is where the microtunnel boring machine (MTBM) is lowered into the ground and creates a tunnel opening while passing excavated earth through the back of the machine. The MTBM is remote controlled and no workers enter the tunnel. The receiving pit is smaller, only large enough to retrieve the MTBM. Pits have to be shored up to prevent cave-ins. The excavated soil is temporarily stored until it can be hauled offsite. MTBMs are laser guided for accuracy. Cutting teeth bore through soil until the MTBM reaches the receiving pit. The pipe is then pushed into the tunnel. The tunnel and pits are then backfilled and the surfaced is repaired.

Large bore tunneling

This method of construction is for very large-diameter pipelines placed under busy urban areas and through challenging geological conditions. In Los Angeles, the largest interceptor sewer pipelines (eight to 10 foot diameter) are being constructed using major tunneling technology. Tunneling gives the City the ability to align sewers under hills and other areas that would not normally be accessible using trenching or microtunneling techniques.

Method: As in microtunneling, the contractor excavates a shaft and shores up the walls. These shafts are deeper and wider than microtunneled shafts. Workers and the TBMs are lowered to the bottom the shafts. Excavation is similar to microtunneling, but the machine is much larger with digging heads capable of boring a 25-foot diameter tunnel. A worker rides inside the TBM from where it is controlled. Excavated dirt is conveyed behind the TBM to small, open rail cars that haul the dirt back to the shaft where it is lifted out and hauled away. Concrete liner pipes are used to support the tunnel and prevent collapse. The sewer pipes are then brought into the tunnel and joined together by work crews. The machines are removed, shafts are filled, and ground surfaces are resurfaced.

Repairing Sewers

The City’s sewer repair teams commonly use three repair methods, depending on the severity and type of damage to the old sewer.

• Replacement – damaged pipes are uncovered and removed, sewage is temporarily piped around the area to be replaced and then new pipe is placed in the ground and connected to the system. This method is used when the sewer is very bad condition.
• Pipe Jacking – new sections of pipe are pushed inside the older pipeline. Crews clean debris from the old sewer, and then insert the new pipe sections between the two street openings. The sections are then grouted (cemented) together so the repaired section is watertight as well as sturdy.
• Pipe Liners – the liner is made from a plastic reinforced with glass fibers. It is inserted into the existing pipe in sections that are cured in-place. These materials are most often placed in sewers that are in adequate condition to provide support for the liner.