Trenchless Sewer Repair: To Trench or Not to Trench?

The U.S. and Canada saw a rise in the construction of underground infrastructure during a surge in its economic growth in the 1950s. Now these pipelines are aging beyond standard sewer repair, having lost their structural integrity.

It is estimated that in the U.S. there is approximately 1.1 million miles of water supply mains and about the same length of sewer pipes. However, the maze beneath our feet is not limited to water and sewer pipes. There are also miles and miles of underground utilities: gas pipelines, electricity lines, telephone cables, and other tunnel systems. Undertaking repairs of these aged sewer and water pipelines is a clear and present need. (Read: The Complex World of Sewer Networks.)

Options for Sewer Repair and Rehabilitation of Pipelines

There are two options available for undertaking a repair and rehabilitation project — open trenching and trenchless rehabilitation. The method that projects the lowest cost with the greatest benefits is preferably selected unless site conditions call for the other alternative.

It is necessary to undertake a sewer system evaluation survey (SSES) to have an idea of the initial cost of construction and benefits that each method provides. SSES includes an inventory of collection system components, condition of the system, sewer system evaluation, system mapping, historical data and much more.

The Cost Factor

The cost of a typical rehabilitation project varies based on where the rehabilitation is undertaken. Highly populated and urban cities will have a higher project cost than that in a rural area or for individual homes.

However, small jobs like those required for homes will have a higher cost per linear foot compared to those undertaken for municipalities, because of the scale of the project. Populated cities also have to bear the socio-economic cost at a higher rate than that required for rural areas, especially when open trenching method is used.

The cost of a typical pipeline repair project depends on various factors such as type of utility, size, material, length and depth of installation, and subsurface conditions. In the open trench method of pipe rehabilitation, about 70% of the construction cost is spent in reinstatement of the disrupted surface, not in the pipeline. This problem is avoided in trenchless pipeline rehabilitation techniques such as cured-in-place pipe (CIPP), pipe bursting and sliplining.

Of the different trenchless methods, CIPP is considered to be the most cost effective, safe, and efficient alternative. According to research, CIPP also reduces the carbon footprint by 85% compared to open trenching, the associated cost saving estimated to be as much as 50%. (Read: Emerging Trenchless Rehabilitation Technologies: Kevlar Reinforced Composite Lining Systems.)

Costs Associated with Sewer Repair

Urban cities have more population per square foot when compared to rural cities. This means that the cost per square foot for conveniences and infrastructure is also more.

The cost factor is comprised of three categories namely:

• Direct costs.

• Indirect costs.

• Socio-economic inconvenience costs.

Often municipalities base their estimate only on the direct cost component. However it is necessary for municipalities to base their cost considering all factors to benefit everyone involved directly and indirectly in the project. (Read: The True Cost of Trenchless vs Open Trench Sewer Repair.)

Direct Costs

This is paid by the municipality and includes planning and bidding costs, contractor cost, project management and consultant cost.

Indirect Costs

This is also borne by the municipality and includes compensation for customer claims and damage to property such as roads and driveways.

Social Inconvenience Cost

This cost is borne by the public and is a result of the execution of a project. It is estimated to be twice the sum of the direct and indirect cost that the municipality pays. These costs include traffic delay, trade loss, vehicle operating cost and accidental costs, damage caused to vehicles due to uneven road surface, damage to environment and pollution arising from dust and noise.

Open Trenching Vs. Trenchless Sewer Repair

About 70% of the cost associated with open cut replacement is taken up by excavation and replacement of the dug up ground. Trenchless rehabilitation compared to open cut in populated cities and urban cities is definitely a better option because the cost of rehabilitation is higher per foot, especially with greater depths requiring shoring and extensive excavation resources.

A study was carried out by Jung and Sinha (2007) considering 8 sewer renewal projects for open trench and trenchless pipe bursting. The traffic delay costs for a renewal project for buried pipe in a high traffic volume area over a 55 day period was about 80 times the direct cost when open trench construction was used.

The same was 10 times the direct cost when pipe bursting trenchless repair method was used. Another study estimated that it was possible to achieve a 300% reduction in traffic delay costs and greenhouse gas emissions when trenchless methods were used.

Cost for Trenchless Rehabilitation in Suburban Towns

For a relining project of 10000 feet or more in a typical suburban town, depending on the condition of the sewer pipe and pipe diameter:

• Estimated cost of pipe rehabilitation:$25 per foot

• For coating a manhole 4 feet in diameter with:

  • Depth of 4 feet: $1500

  • Depth of 6 feet: $2500

• Sealing lateral connections at main: about $1400

• Sealing laterals from main sewer to home: about $2000

A suburban town of approximately 20 000 people may have about 500,000 feet of main sanitary sewers. For 1000 feet of main sewer, there may be 10 to 20 services and 4 manholes. On an average, 2 manholes and 3 to 5 services may need sealing and lining, giving an average cost of $35 per foot.

Cost of Trenchless Repair for Homeowners

The open cut method requires digging up a long and deep trench to dig up and replace old pipes, the cost can vary from $92 to $238 per foot depending on the depth of the existing pipe and the length of installation, ease of access and the current rates at the locality. (Read: Is Trenchless Rehabilitation Right for Your Home?)

• Traditional open trenching: average cost $7500.

• Trenchless methods: range between $6000 to $12000.

Trenchless Efficiency

Below is an approximate table of comparison for pipe replacement in a city per linear meter using open cut and trenchless methods:

*The above cost estimates are approximates and do not indicate the final price.

Conclusion

It is clear that trenchless technology offers a cost effective alternative for rehabilitating buried sewer infrastructure. There is an estimated 20% reduction in direct and indirect costs when appropriate trenchless methods specific to prevalent site conditions are used in place of open trenching.

Considering the value of time, significant reduction in social inconvenience cost and overall cost when trenchless rehabilitation techniques are used, trenchless rehabilitation makes more sense.