CNA Engineers

Utility Tunnel & Pipeline Design

  • Elm Creek Interceptor
  • MCES Empire Outfall
  • Minneapolis East Interceptor Phases A & B
  • Minneapolis East Interceptor Phase C
  • MCES Victoria Interceptor
  • University of Minnesota Medical Biosciences Building Utility Infrastructure
  • Utility Tunnel Design
  • South St Paul Forcemain

Elm Creek Interceptor


Our firm was the tunnel designer for the new interceptor tunnel under Highway 94 in Maple Grove, Minnesota. The tunnel was 836 feet long, 42 inches in diameter, 50 feet deep and placed in soil. The contractor was EJM Pipe Services, Inc. The tunnel was constructed using an Akkerman microtunneling machine and Hobas FRP pipe.








Back to top

MCES Empire Outfall

Selected as the tunnel designer of ten tunnels varying in size from 54” to 78” diameter with a total length of 13,400 feet and also 3,800 feet of river pipeline design on a 12-mile sewer system in Rosemount, Minnesota.
Conceptual designs were completed for the river pipeline route for construction by horizontal directional drilling, microtunneling and underwater trenching techniques, before selecting a barge underwater trenching technique. The tunnels were generally in glacial deposits of outwash and tills with a large portion of the length under the groundwater table. The tunnels pass under Highway 52, Highway 55, County Road 42 and numerous railroad tracks.


Back to top

Minneapolis East Interceptor Phases A & B

The Minneapolis East Interceptor is a 21,500-foot long sanitary sewer constructed under urban areas in the Twin Cities. The cast-in-place tunnel lining is 10 feet inside diameter for most of the alignment. Numerous structures such as access shafts, drop shafts, meter chambers, connection structures, deaeration chambers, junction structures, diversion structures and overflow structures were constructed in conjunction with the project.
By utilizing the concrete liner in conjunction with surrounding rock, the tunnel was designed to withstand 52-psi internal surge pressures, created during peak periods. The tunnel for the most part lies in easily-excavated St. Peter sandstone and partly in a three foot layer of Glenwood shale. Up to 30-foot of Platteville limestone lies within a few feet of the tunnel crown for most of the alignment.
The mainline sandstone tunnel was excavated at 12.5-foot diameter tunnel using a boring machine. Shaft and structure excavations in the limestone were done by blasting. Structures and smaller connection tunnels in the sandstone were excavated by roadheader or backhoe. Sodium silicate spraygrout, a procedure developed by CNA Engineers, protected against deterioration of sandstone walls in the tunnel and structures.
Back to top

Minneapolis East Interceptor Phase C

The Minneapolis East Interceptor, Phase C, is an 8,500 linear foot sanitary sewer constructed under urban areas in the Twin Cities. The cast-in-place tunnel lining has an 8-foot inside diameter. Numerous structures such as access shafts, drop shafts, connection structures, junction structures, diversion structures and overflow structures were constructed in conjunction with the project.
The reinforced concrete tunnel liner was designed to withstand 39-psi internal surge pressures, created during peak periods. 6,000 feet of the tunnel length was in the St. Peter sandstone and approximately 50 feet below the ground surface and 10 feet below the water table. The sandstone tunnel was excavated with a tunnel boring machine. Initial support consisted of ring beams and wood lagging.
2,500 feet of the tunnel length was under the water table in soft ground consisting of glacial tills and lacustrine deposits that had unconfined compression as low as 425 psf. The soft ground tunnel was excavated with a tunnel boring machine. Initial support consisted of ring beams, wood lagging and filter fabric. The soft ground tunnel also required the use of compressed air and compaction grouting.

Back to top

MCES Victoria Interceptor

CNA Engineers was the tunnel designer for the Victoria Interceptor Tunnel which is a 1.5 mile long sanitary sewer constructed in the Victoria, Minnesota. The fiberglass final tunnel lining has a 6-foot inside diameter. Numerous access shafts and connection structures were constructed in conjunction with the project.
7,000 feet of the tunnel length was in glacial outwash sands and up to 120 feet below the ground surface. This tunnel section was excavated with a tunnel boring machine using sand shelves. Initial support consisted of ring beams and wood lagging. 500 feet of the tunnel length was under the groundwater table in soft ground consisting of glacial tills. The soft ground tunnel was excavated with a tunnel boring machine using doors and dewatering. Initial support consisted of ring beams, wood lagging and filter fabric.


Back to top

University of Minnesota Medical Biosciences Building Utility Infrastructure

We provided the engineering services for the tunnels and shafts on this project. The tunnel provides steam to the UM Biomedical Discovery District and the TCF Bank Stadium in Minneapolis.
Three tunnel sections and two shafts were designed for the project. The main shaft was equipped with an elevator, utility lines, and stairs. The other shaft was for construction access only.
The deep tunnel was constructed in the easily excavated St. Peter Sandstone and Glenwood Shale. The tunnel roof is located within the shale, which is rockbolted and shotcreted prior to placing the concrete final liner. The 7-ft diameter tunnel was constructed in soil by pipe jacking under University Avenue. The 6-ft by 6-ft square tunnel was placed by open-cut under the re-aligned Oak Street.

The design of the main shaft included rockbolting, waterproofing, and two concrete linings. Excavation through the Platteville limestone was done using a combination of perimeter drilling and backhoe ram hammering. Our firm provided the following engineering services for the tunnel and shafts: feasibility study, preliminary and final design, construction engineering and administration.

Back to top

Utility Tunnel Design

Our firm specializes in utility tunnel inspection, analysis, repair designs and new designs. Clients for utility tunnel systems include CenturyLInk, Excel Energy, St. Paul Water Department, University of Minnesota, Minneapolis District Energy, St. Paul District Energy and Mn/DOT. Our engineers have inspected and designed repairs for over ten miles of tunnels for utility owner’s facility management programs.
CNA Engineers designs lined and unlined utility tunnels with racks for varying utilities utilizing concrete, shotcrete, masonry and spraygrout for tunnels containing water, electric, and telephone and steam lines. We have designed rectangular, arch and round tunnels, through soil, limestone and sandstone for utilities throughout Minnesota.

Back to top

South St Paul Forcemain

This project included three soft ground tunnels and two cut-and-cover river crossings. Two tunnels were constructed by pipe jacking methods. The other was constructed through a Mississippi River levee and was supported by liner plate. Our firm was hired to design, evaluate tunneling methods, and to prepare specifications.
CNA Engineers specified compressed air tunneling for ground and groundwater control. One cut-and-cover river crossing was 1300-feet-long and crossed the main channel of the Mississippi River. Test trenching was conducted during design to evaluate achievable trench side slopes.








Back to top