Manholes & Catchbasins

Installation
Precast concrete manholes have become the construction industry’s product of choice over other products for many reasons, not the least of which is ease of installation. Precast concrete manholes can be easily installed on demand and immediately backfilled. The degree of soil compaction around the manhole and remaining trenches is never a problem, making installation faster and easier. Contractors are familiar with how to handle precast concrete manholes and can easily install them. Standard sealants and flexible joints are readily available and are vital to watertightness.
Quality
Because precast concrete products typically are made in a controlled environment, they exhibit high quality and uniformity. Variables affecting quality typically found on a job site – temperature, curing conditions, material quality and craftsmanship – are nearly eliminated in a precast plant.
Long-Lasting
Precast concrete strengthens with time, while other materials can deteriorate, experience creep and stress relaxation, lose strength and/or deflect as they age. The load-carrying capacity of precast concrete is derived from its own structural qualities and does not rely on the strength or quality of the surrounding backfill materials. Studies have shown that precast concrete products can provide a service life in excess of 100 years. For severe service conditions, additional design options are available that can extend the life of the precast concrete product. This is extremely important when calculating life-cycle costs for a project.
Watertight
ASTM C478, “Standard Specification for Precast Concrete Manhole Sections,” specifies the proper manufacture of quality, watertight precast concrete manholes. Standard watertight sealants and gaskets are available that are specially designed for use with precast concrete, making multiple seam precast concrete manholes very easy to construct. Vacuum testing is an easy means of verifying water tightness, either in the plant or in the field, as detailed in ASTM C1244, “Test Method for Concrete Sewer Manholes by Negative Air Pressure (Vacuum) Test Prior to Backfill.”

INSTALLATION PROCEDURES

Lifting Apparatus
Use approved lifting slings that will adequately lift the weight of the units and that conform to OSHA standard 29 CFR 1910.184. The use of an approved or rated spreader bar is preferred. When lifting manhole bases and risers, make sure the chain or cable lengths are long enough to prevent contact with tongue-and-groove area, and are kept at appropriate lifting angles. Use wooden blocks between sling and manhole wall if necessary.
Recommended Manhole Bedding
Use a minimum of 6 inches of approved bedding material compacted to 90% proctor in an area not less than the base area, and preferably 6 inches beyond the outside radius of the manhole base. The area under incoming and outgoing pipes should be treated the same way to prevent shearing of pipes. Local ground conditions may require additional bedding thickness, according to the engineer’s recommendations.
Setting Manhole Base
Set the manhole base on graded bedding according to job specifications, making sure boots or pipe openings match design elevations. Level the top of the manhole base in both directions.
Pipe Connections
Follow the manufacturer’s recommendations for connections and check with the manufacturer if precast inverts are supplied.
Joint Installation
There are three major joint gasket types; Butyl Gasket, Confined O-Ring, Offset / Pre-Lubricated Gaskets. Follow the manufacturer’s recommendations for installation.
Backfill Procedure
If you are vacuum testing, do the test before backfilling. Detecting leaks at this time is easy and repairs are simple. Distribute backfill around the manhole in equal proportions to prevent tipping. Compact the fill in lifts, the same as with a standard trench procedure. Backfill material should be clean and free of large rocks.