What is the difference between demolition and excavation?July 28, 2023
Planning and designing a sewer pipe or manhole replacement project requires several considerations to ensure cost-effectiveness and reliability.
Sewer Pipes & Manholes
Sewer pump stations and manholes are located at low points in the system. Manholes and sewer pipe are typically made of concrete, HDPE or reinforced concrete. The only difference in pipe construction is the material and diameter. Material standards such as ASTM or AWWA provide requirements for manufacturing and material. The manholes can be used to change the direction of flow or to merge it.
Manholes are usually made of pre-cast concrete. They come in sizes ranging from 4 to 8 feet.
Pre-cast manhole bases or cast in place can be used depending on the requirements of the project. Cast-in place bases ensure that the base is a perfect fit and are smooth to avoid any interruptions in flow. In addition to concrete, manhole barrels can also be built with HDPE, fiberglass-reinforced plastic, and polymer concrete. The most common type of concrete used is pre-cast, but chemical resistant concrete coatings should be applied due to the corrosive nature of wastewater gases. Also, soil investigations should include corrosion testing to determine if an exterior coating is required. Our projects use three types of coatings: epoxy, polyurethane and PVC liners.
Pipe connections must be gasketed and smooth to prevent interruptions in flow and ensure a watertight seal. A drop inlet on the inside can be used if the pipe meets the manhole at an elevated height. This will prevent turbulence and the release of gases that can cause odors or corrosion.
Pipe failure and the need for more capacity are two of the main reasons to replace or renovate sewer pipes or manholes. Pipe failure can be detected during a condition evaluation by noticing cracks or roots in the pipe. If they have capacity problems due to an increase in population or development, our clients may be required to upgrade or replace their systems. Some projects are started due to public policy replacement cycles that aim to prevent failures in the future.
In order to create a project that is sustainable and will benefit the community for a long time, information is essential during the planning stage. Our general checklist includes:
- Wastewater System Master Plans – Previous and Current
- Information on general plans and/or planned development
- Atlas of the collection system
- Historical wastewater flow rates
- Historical water demand (used to sometimes determine wastewater flows).
- Existing lift station details
- Inspection reports are available
- There are known issues with the collection system
We prefer to make field visits in addition to using the available information and plans, as we believe that in-person observations play a major role in ensuring successful repairs and replacements. The following items are included in our field visit checklist:
- The number of laterals and approaches
- Traffic and large commercial, residential or industrial facilities
- Water, electricity, fiber optics, storm drains and other utilities are also available.
- Physical features (trees, streams, etc.)
- Condition of lift stations (if possible) and manholes
When designing projects to repair or replace sewer pipes and manholes, it is important to consider several factors:
- Manhole inversions and rim heights
- Inversions of storm drains and elevations at the rim
- USA – Utility Paint Marks
- Waterline valves – Depth to top
- Water meters, Gas meters and other Above Grade Evidence of Utility
- Right of way, middle line of the road
- Trench lines are visible.
- Overhead electrical
- Trees (trunk diameter, drip line)
Geotechnical Engineers are also a partner in obtaining geotechnical reports and investigations to better understand the groundwater levels, native materials and soil corrosion potential. The high groundwater level can affect dewatering and the buoyancy of manholes. The Geotechnical Report includes recommendations for pipe bedding and trench backfill. This will include whether native material can be used or if imported materials are needed.
The topographic survey, together with the geotechnical report, are important factors in designing a sewer replacement. The planning information combined with the historical sewer flow allows for development and estimation of future anticipated sewer pipeline flows. After determining the required flow capacity, the design of the pipeline takes into account the ground elevations and connecting sewer elevations. Minimum and maximum speeds, as well as the maximum fullness of the pipeline (depth to diameter ratio), are also considered. These factors are combined to determine the pipeline slope and diameter.
Pipeline alignment is determined by a variety of factors including the property boundaries and right-of-way, connection points and changes in direction.
In situ Rehabilitation or Replacement
Sometimes, it is worthwhile to consider in-place pipe replacement or rehabilitation. In high traffic areas, trenchless replacement can be a more efficient option than conventional replacement. Three main options are available: pipe bursting, slip-lining and cured in-place pipe.
- The slip-lining technique pulls the new pipe through the old one, reducing its effective diameter. It can affect the capacity. The flow capacity of an older pipe can be dramatically reduced if the surface has been scoured by solids or corroded. Gravity flow is slowed by roughness in a pipeline, which lowers its overall capacity.
- Pipe burst works similarly to slip lining. However, the old pipe will be destroyed by a rotating header while the new pipe pulls in. Pipe-bursting requires consideration of geotechnical conditions, and typically, the new pipe can only be one or two standard diameter sizes bigger.
- Cured in Place Pipe (CIPP) consists of a resin that looks like a sock. It is blown or pushed through the pipe using air and then hardened with steam or hot water before being cut off at the ends. CIPP reduces the inner diameter of pipe but is used to improve capacity in old pipelines by smoothing out their interior.
Bell Excavating/Bill Wolfe Excavating have completed many collection system projects with success and reliability for both public and private agencies. We provide our communities with water, wastewater and reuse projects as we expand our team.