Innovative Engineering: NTH’s Floating Mat Pipe Bedding Solution Saves Costs on Sewer Installations over Peat and Marl Deposits

Infrastructure projects often present complex geotechnical challenges, requiring innovative engineering solutions to balance cost, performance, and constructability. One such challenge arose when NTH was engaged to provide geotechnical recommendations for a sewer rehabilitation project. The original pipe was constructed in the 1960’s. Approximately 370 feet of the pipe alignment went through an area of organic soils and peat deposits. This portion was supported on an above ground trestle bearing on drilled piers in order to bridge the organic soils. Later development in the area included adding substantial amounts of fill soils and burying the pipe. The overburden fill soils added additional stress to the subsoils, causing the pipe to severely sag and settle.

Traditionally, repairs of such problems included the use of costly pile foundations due to the presence of weak marl soil, significantly increasing construction expenses.

NTH explored the subsurface soil and groundwater conditions in the area, and provided the client with several options for the pipe replacement. Leveraging its deep expertise in geotechnical engineering, NTH proposed an alternative solution: a floating mat pipe bedding system. This approach provided the necessary structural support while drastically reducing costs and improving constructability.

Understanding the Problem: Organic Peat and Marl Soil Challenges

Peat and Marl are natural deposits usually found near old lakes or other water bodies, are loose or soft and often saturated, and present significant engineering challenges especially when these deposits are thick and deep. It has low bearing capacity and is highly compressible when subjected to increased stress, leading to potential settlement and structural instability for buried infrastructure.

One of the common solutions used by many designers is to support the pipe on pile foundations that are designed to bear within deeper strata below all of the organic deposits. While technically sound, this approach comes with substantial financial and logistical drawbacks, including:

• High equipment, material and labor costs associated with pile installation
• Specialty trade requiring specialty subcontractors to construct the piles
• Extended construction timelines due to the complexity of deep foundation work
• Difficulties in accessing and working within constrained developed areas

Faced with these challenges, NTH sought a more cost-effective and technically sound alternatives that would still ensure the long-term performance of the sewer system.

The Floating Mat Pipe Bedding Concept

Instead of rigid deep foundations, NTH introduced a floating mat bedding system, a geotechnical solution designed to distribute the load across a wider area rather than transferring it deep into the subgrade. Realizing that the area has been developed for quite a long time and that there will not be any loading added to the overburden, NTH concluded that most of the settlement had already taken place, and only minor additional settlement may take place in the future.

To overcome any additional settlement (from the organic deposits), NTH worked with the client to specify pipe materials that can tolerate such settlement.
This system consists of a reinforced granular or geosynthetic mat placed beneath the sewer pipes. The mat provides uniform support under the pipe, and reduces differential settlement by spreading loads more evenly, thereby preventing localized failures. The key advantages of this system include:

• Eliminating the need for deep foundations, significantly cutting costs
• Allowing for some settlement while maintaining pipe integrity
• Faster and more flexible construction, reducing delays and specialty labor requirements

This innovative engineering solution proved to be a game-changer in balancing structural performance with economic feasibility.

Cost and Performance Benefits

The financial and technical benefits of the floating mat pipe bedding system were significant:

By shifting to a floating mat bedding system, the project saw a dramatic reduction in costs while still maintaining structural integrity and longevity.

Case Study: Implementation and Results

During the study, NTH worked closely with the client by providing clear recommendations for the proposed floating mat system to allow for seamless implementation during construction. The process involved:

1. Site Evaluation – Confirming soil conditions and identifying the optimal design for load distribution and excavation alternatives to reach the failing pipe.
2. Material Selection – Choosing appropriate geosynthetic reinforcements, granular bedding materials and pipe material.
3. Construction Execution – Providing step by step construction procedures and sketches for installing the floating mat.

This solution proved to be very useful. The same technique was later implemented by the client on another project where the designer had specified the use of battered piles to replace a failing sewer with a new one. When the new project came to light, the client recalled that 9 years earlier, NTH had provided the floating mat solution in a similar soil conditions. The client consulted with NTH, and as a result, the new pipe support design was changed from piles to the floating mat bedding.

The results were overwhelmingly positive:

• Construction costs were reduced by $850,000 when compared to the original pile foundation design.
• Project timelines were shortened, allowing for faster completion and minimized disruption.
• The sewer system remains structurally sound, with settlement well within acceptable limits.

Industry Implications and Future Applications

NTH’s successful implementation of the floating mat bedding system highlights the potential for more cost-efficient and adaptable geotechnical solutions in infrastructure projects. This approach can be applied to a wide range of projects, including:

• Roadway stabilization
• Utility installations in weak or variable soil conditions
• Pipeline and drainage system support in coastal and wetland areas

By prioritizing value engineering, NTH continues to drive innovation in geotechnical design, offering clients solutions that optimize both performance and cost efficiency.

Conclusion

The use of a floating mat pipe bedding system instead of a traditional pile foundation demonstrates how innovative engineering can transform project feasibility. NTH’s approach not only saved the client a significant amount of money but also ensured a durable and stable sewer installation in challenging peat and marl soil conditions.

As infrastructure projects face increasing budgetary and environmental pressures, solutions like this will become more essential in delivering sustainable, cost-effective engineering outcomes. By embracing innovative geotechnical strategies, NTH continues to provide practical, high-performance solutions for complex engineering challenges.