Maximizing Trenchless Technology through Extensive Subsurface Investigations

While slurry microtunneling worked well for the first phase of a wastewater system conveyance improvements project in Raleigh, N.C., the City of Raleigh Public Utilities Department (CORPUD) and its project team have expanded the design evaluations for the second phase of the project in search of trenchless technology flexibility.

The goal is to reduce construction costs without compromising risk or quality. The solution: perform extensive subsurface investigations that provide comprehensive analyses of both soil and rock, map out subsurface conditions, then incorporate multiple trenchless installation methods into your design plan with a high level of confidence.

Background

The CORPUD provides water and sanitary sewer service to approximately 183,000 metered customers and a service population of about half a million individuals. The CORPUD’s service area encompasses 270 sq miles and extends into three major drainage basins, one of which is the Crabtree Basin.
While the entire CORPUD sanitary sewer system has relatively low inflow and infiltration (I/I), it does have significant I/I challenges in certain portions of the Crabtree Basin interceptors. To address the situation, in 2009 the City initiated the Crabtree Basin Wastewater System Conveyance Improvements project, which was the highest priority project identified in the City’s Sanitary Sewer Capacity Study completed in 2008. The Raleigh-based engineering team of McKim & Creed Inc. and Hazen & Sawyer P.C. designed a three-phase plan, with the first two phases involving the installation of approximately 40,000 lf of 54-, 60- and 72-in. diameter gravity sewer.

Phase I
Phase I of the Crabtree Basin Wastewater System Conveyance Improvements project included the design and construction of 19,000 lf of 72- and 60-in. gravity sewer. This phase crossed three major roadways, including a highway interchange ramp, and followed the route of the existing sewer lines. To accurately determine the existing conditions and select a viable alignment that would ensure constructability, serviceability and future maintenance, the team used subsurface utility engineering (SUE).

McKim & Creed SUE specialists used Quality Level B and Quality Level A services to designate and locate below-ground conditions along the proposed route. Quality Level B uses surface geophysical techniques to determine existence and horizontal position of underground utilities (known as “designating”), while Quality Level A employs non-destructive digging equipment to determine the precise horizontal and vertical position, as well as size and type, of underground utilities (known as “locating”). The SUE horizontal designation included electrical power, natural gas, fiber-optic, cable television and telephone duct, potable water lines, sewer force mains, water and sewer services and some gravity sewer.  Several of the existing utility locates occurred at the proposed tunnel crossings and were critical to the vertical and horizontal designs of the tunnels. Vacuum excavation exposed the existing utilities and allowed the pipe/conduit or duct to be inspected such that the diameter/outer dimensions and material could be identified and the vertical elevation and horizontal centerline verified.
Geotechnical investigations included two to three borings conducted at each crossing at depths ranging from 25 to 55 ft. The geotechnical investigations included groundwater level measurement, unconfined compressive strength testing of rock, grainsize analysis, Atterberg limits and other typical tests and analyses. The investigations indicated the presence of loose to hard residuum and alluvium, partially weathered rock (PWR), and hard crystalline rock at the proposed tunnel locations, which meant that the excavation equipment had to be capable of penetrating soft soil conditions, hard rock conditions and mixed face subsurface conditions safely and efficiently.
After the SUE and geotechnical investigations were completed, the team then began exploring crossing methods. Traditional methods such as open-cut, hand tunneling and horizontal auger boring (jack and bore). These methods were withdrawn from consideration due their limitations relative to the crossing location and site conditions, as well as equipment sizing in some cases. Open-cut remained a viable option for creek crossings and non-NCDOT roadways, however.
The team examined compressed air hand tunneling, slurry microtunneling and tunneling by an earth pressure balancing machine (EPBM). Air hand tunneling uses an open-faced shield and compressed air to balance groundwater, prevent material migration into the open excavation and provide worker safety. EPBM excavates spoil by balancing the machine face pressure with the soil pressure. Spoils from the cutter chamber are removed through the screw conveyors, and operators, sitting in a control console located within the EPBM, guide the machine and monitor data. Microtunneling uses a remote-controlled microtunnel boring machine (MTBM) that mixes excavated material at the tunnel face with bentonite and other lubrication fluids to create a slurry. Pressure at the cutting face is balanced with earth removal, groundwater head and propulsion of the tunnel support without manned entry and excavated material captured in the slurry is pumped to the surface and separated.
Each methodology was assessed based on the criteria of required worker entry, cutting face accessibility, one-pass capabilities, line and grade accuracy, open vs. closed cutting face, groundwater conditions,and hard rock tunneling capability. Microtunneling proved to be the method with the highest level of capability and in the necessary diameter range to mitigate the risks identified for this project. It offered a safe and reliable method of crossing that minimized the risk for settlement and heave, addressed groundwater and soil conditions, and maximized worker safety.

Design Considerations
Once the SUE and geotechnical investigations were complete and the tunneling technology had been selected, two key design elements had to be considered: tunnel launch shaft construction and pipe material. Depending on the size and length of the tunnel, the equipment staging footprint was limited to a maximum of 40 ft x 150 ft because of the location of the existing sewer line and Crabtree Creek. The team evaluated the spatial constraints of the crossing locations to determine if the launch and receiving shafts could be installed at the intended beginning and ending points of the tunnel, and adjusted as necessary.
When evaluating piping material, the team selected fiberglass reinforced jacking pipe for a single-pass installation approach. While reinforced concrete pipe (RCP) or steel pipe were available options for 72- and 60-in. pipe, both products would have required a corrosion-resistant additive, coating or liner. The fiberglass reinforced pipe (FRP) offered a structurally sound product for tunnel pipe while also providing inherent corrosion resistance.
A single-pass approach was proposed for the design because of available MTBM equipment size limitations, construction cost associated with increased diameter and NCDOT’s requirement that the annular space between the casing and carrier pipes be grouted, which eliminated the ability to remove and replace the carrier pipe in the future.
The FRP jacking pipe is fairly uncommon for tunnel crossings under NCDOT roadways, especially in a single-pass scenario. NCDOT typically requires trenchless crossings 6 in. in diameter or larger to be encased. After meetings with the key design groups within NCDOT and the submittal of the encroachment packet, the CORPUD received approval of the single-pass installations with FRP jacking pipe at all three crossings.
Construction
In fall 2013, the contractor successfully completed all three Phase I microtunnel installations. During the installation of the microtunnel beneath the existing I-440 overpass ramp and twin 60-in. reinforced concrete pipe (RCP) interceptors, the subsurface rock tended to be more “plastic” than anticipated and resulted in excessive wear to the MTBM cutters on the front of the boring machine. Several cutters had to be replaced during the installation, but the design of the boring machine allowed for this to be done with the MTBM in place. Mixed faced conditions and even unexpected material (buried trees) were encountered at other tunnels, but the MTBM was operated with caution and at specific mining rates to minimize risk of binding the cutter head or encountering misalignment. These crossings were also completed successfully.
In all, the MTBM technology offered numerous benefits, including:
• Remote controlled/no worker entry required
• Dewatering not required along tunnel
• Balanced face pressure (i.e., minimized potential for surface settlement and heave)
• High line and grade accuracy
• Medium-size footprint/flexible
• Ability to handle mixed face conditions
• Capable of one-pass tunnel installation

Phase II
Despite the success of the microtunnel installations on Phase I, the CORPUD wanted to look for opportunities to reduce project tunneling construction costs on Phase II. This required the project team to perform more extensive geotechnical investigations and evaluate opportunities at the Phase II tunnel crossings to use other tunnel methods. The project team of McKim & Creed and Hazen & Sawyer worked closely with the geotechnical engineer, Falcon Engineering of Raleigh, to develop a comprehensive geotechnical investigation that included additional analyses and material testing. Extensive geotechnical investigations were performed to evaluate the properties of the subsurface material, with additional testing for rock, and were completed in accordance with ASCE guidelines.
The Phase II geotechnical investigations also included seismic refraction testing using multi-channel analysis of surface waves (MASW) methods where access allowed. Correlating the seismic refraction data with SPT and rock core boring data allowed the team to study areas between borings in a more comprehensive manner to search for anomalies in conditions and create a more accurate subsurface profile. This multi-modal exploration approach reduces the chance for subsurface surprises and allows for more accurate quantification of rock excavation or other difficult subsurface conditions. Ultimately, the design team gained an increased confidence in allowing alternate methods while providing prospective bidders with a highly detailed picture of the subsurface. These benefits proved valuable in mixed face conditions or crossings where conditions varied significantly from one boring to the next, as well as confirming consistency of subsurface conditions at crossings that appeared relatively homogenous based on boring data.
Using the results of the additional geotechnical evaluations, the team determined that four of the eight proposed tunneling crossings were candidates for alternate methods to MTBM. For two of these four tunnels locations, the subsurface ground conditions consist of primarily alluvial and residual soils. At these locations, pipe ramming will be permitted as an acceptable alternate crossing method to microtunneling. At the other two tunnel locations, the subsurface conditions consist almost entirely of partially weathered rock and crystalline hard rock along the proposed tunnel drives. Here, tunneling by rock tunnel boring machinery (TBM) will be permitted as an acceptable alternate crossing method to MTBM.
Phase II is currently in the permitting review process, with the tunnels portion of the project scheduled to be bid in the summer 2014. For four of the eight tunnel crossings, bidders will have the option of pipe ramming, rock TBM or microtunneling for their construction method. Due to geotechnical conditions, only microtunneling is allowed for the other four locations. Offering the flexibility of multiple construction methods for the applicable locations eliminates the need for the contractor to submit an alternate bid to the CORPUD, and removes the city from shouldering the responsibility of selecting the most appropriate method for these crossings.
This approach enables the CORPUD to make use of multiple technologies, which is expected to maximize the competition and allow the market to dictate the most appropriate and economical solution for each crossing. Regardless of the technology selected, the CORPUD wins in the end. 

Chris Windley, P.E., is a senior project manager at McKim & Creed Inc. McKim & Creed is headquartered in Raleigh, N.C. and has offices throughout the South.