Installing large diameter pipe in underground utility projects presents unique challenges that standard trenching equipment cannot always address. When the Central Weber Sewer Improvement District in Utah awarded a critical treated water discharge line project to Whitaker Construction Company of Brigham City, the contractor faced a problem familiar to professionals in the underground utility industry: standard trench shields simply could not accommodate the pipe dimensions required. This article examines how a custom shoring solution using a pin-in-place spreader arch and stacked trench boxes enabled safe and efficient installation of 108-inch reinforced concrete pipe. For additional background on specialized equipment used in these operations, see Pipe Laying and Underground Utility Installation Equipment Specialized.
Project Overview and Site Conditions
The Central Weber Sewer Improvement District Project
Whitaker Construction Company was tasked with installing 4,500 linear feet of new treated water discharge line running from the treatment plant to the Weber River. The pipeline required 108-inch RCT (reinforced concrete tube) pipe with an outside diameter of 11 feet, installed at an average depth of 18 to 20 feet below grade. The pipe was designed with extra thick walls to serve as an anti-floatation measure, a necessary precaution given the very high water table in the relatively agricultural, rural area.
Environmental and Geotechnical Challenges
The project site presented several conditions that complicated installation:
- High water table: The rural agricultural setting meant groundwater was close to the surface, requiring anti-floatation pipe design and careful dewatering during excavation.
- Deep trench requirements: With an average depth of 18 to 20 feet, standard shallow shoring methods were insufficient for worker protection.
- Large pipe dimensions: An 11-foot outside diameter meant the trench had to be significantly wider than typical utility installations.
- Tight schedule: The project timeline ran from late December 2008 through August 2009, with the shoring equipment needed on site before Christmas.
- Heavy pipe weight: Each 8-foot joint of pipe weighed approximately 45,000 pounds, requiring heavy-duty handling equipment.
These conditions combined to create a situation where off-the-shelf shoring products would not meet the project requirements. The contractor needed a custom-engineered solution that could provide adequate clearance for the massive pipe while maintaining OSHA-compliant trench protection.
The Challenge of Large Diameter Pipe Installation
Standard trench shields typically come in 8-foot or 10-foot heights. While these dimensions work well for most utility pipe installations, they do not provide enough vertical clearance to install an 11-foot outside diameter pipe. The trench shield must be tall enough to protect workers from cave-ins while also allowing the pipe to be lowered into position without obstruction. This clearance problem is a recurring theme in underground utility construction, and solutions often require creative engineering.
For projects where trenchless methods are more appropriate, alternatives such as horizontal directional drilling or pipe bursting may be considered. These techniques are covered in detail in the article on Trenchless Technology Horizontal Directional Drilling Pipe Bursting and, which discusses methods for installing utilities with minimal surface disruption.
Custom Shoring Engineering and Design
Collaborative Problem Solving
Whitaker Construction reached out to Tom Hartman at National Trench Safety in Fremont, California, for assistance with the shoring challenge. Hartman had worked with Whitaker on previous projects and was familiar with their operational needs. The contractor’s initial idea proved to be the best approach: using a spreader arch mounted on a tall trench box to provide the necessary pipe clearance.
Hartman contacted Mike West, Vice President of Engineering at Efficiency Production, Inc. in Michigan, to discuss custom engineering a pin-in-place spreader arch and trench shield combination. The timeline was tight, but Efficiency Production had prior experience with custom engineered trench shields and arches, making them well suited for the task.
The Pin-in-Place Spreader Arch Solution
The custom shoring system consisted of several key components working together:
- A 10-foot tall trench shield as the primary shoring structure
- A 4-foot tall stacked box added on top to increase total height
- A pin-in-place spreader arch designed to bridge across both boxes
- Sixteen-foot spreaders on the front end for lateral support
- A hi-clearance arch assembly on the back end for pipe passage
This configuration delivered 140 inches of vertical clearance, compared to the 112 inches that would have been available with just a standard 10-foot box alone. The additional 28 inches of clearance was the margin that made the installation feasible.
Engineering Innovation in Trench Shielding
According to Mike West, a pin-in-place spreader arch is not an entirely new concept. What made this particular arch unique was its dual-function design. The arch was engineered to pin over the collars of both trench shields simultaneously, effectively locking the two shields together. This locking mechanism served two purposes:
- It improved structural rigidity during trench pulling operations, making it easier to drag the entire box assembly through the trench as work progressed.
- It allowed the arch to be used with just one trench shield if conditions changed, providing operational flexibility.
All products used on this project were P.E. certified to meet OSHA and MIOSHA standards, ensuring that worker safety was not compromised despite the custom nature of the shoring solution.
Equipment Specifications and Installation Methods
Trench Shield Configuration
The stacked trench shields used on this project were Efficiency Production HT-6 models, consisting of a 10-foot by 28-foot box and a 4-foot by 28-foot box. The table below summarizes the key specifications of the shoring system.
| Component | Specification | Purpose |
|---|---|---|
| Primary box | HT-6 10 ft x 28 ft | Main trench shoring structure |
| Stacked box | HT-6 4 ft x 28 ft | Additional vertical clearance |
| Front spreaders | 16 ft length | Lateral support at front end |
| Rear arch | Hi-clearance spreader arch | Pipe clearance at back end |
| Total clearance | 140 inches | Allows 11 ft O.D. pipe passage |
| Standard clearance | 112 inches (10 ft box only) | Insufficient for this project |
Pipe Handling and Placement
The project required laying approximately 550 joints of pipe, each 8 feet long. At roughly 45,000 pounds per joint, the pipe handling requirements demanded heavy machinery. A Caterpillar 385 excavator was used to lift joints directly from delivery trucks and lower them into the trench. The excavator’s capacity and reach were essential for managing the weight and dimensions of the reinforced concrete pipe.
Installation productivity averaged 8 to 10 pipe joints per day. This production rate reflected the careful work required to align, seat, and connect each massive section while maintaining proper shoring protection and trench safety protocols.
Additional Infrastructure Work
In addition to the main pipeline installation, the project included construction of three cast-in-place structures. Two of these were ground pipe to box culvert transition structures, which required additional excavation and forming work. These transitions connect the round pipe sections to rectangular culvert structures at the discharge points.
Proper pipe installation techniques are also critical in other contexts, such as residential and commercial plumbing. For information on below-grade pipe work, see the article on Plumbing Under a Concrete Slab a Complete Guide, which covers layout and installation methods for pipes placed beneath concrete slabs.
Safety, Compliance, and Project Outcomes
OSHA Compliance and Trench Safety Standards
Trench safety regulations under OSHA require protective systems for any excavation deeper than 5 feet. The 18-to-20-foot depths on this project placed it firmly within the category requiring engineered shoring. Key compliance considerations for large diameter pipe installations include:
- Soil classification: The type of soil determines the appropriate shoring design and angle of repose for sloped excavations.
- Water table management: High groundwater requires dewatering systems and shoring rated for hydrostatic pressure.
- Surrogate loads: The weight of excavated material stored near the trench edge must be factored into shoring design.
- Access and egress: Trench depths over 4 feet require ladders, ramps, or other safe means of entry and exit at 25-foot intervals.
- Atmospheric testing: Deep trenches in agricultural areas may require testing for hazardous gases or oxygen deficiency.
Equipment Certification and Quality Assurance
All custom shoring equipment used on this project was P.E. certified, meaning a licensed professional engineer reviewed and approved the designs. This certification is essential for custom engineered solutions because standard tabulated data cannot cover non-standard configurations. The certification process verifies that the shoring system can withstand the expected loads, including soil pressure, surcharge loads, and the dynamic forces of pipe placement and trench box pulling.
Project Timeline and Contractor Background
The project began on December 26, 2008, and was scheduled for completion in August 2009. The shielding equipment was ordered around December 1, 2008, and Whitaker needed the shields on site before Christmas. Efficiency Production turned the order around in approximately three weeks, meeting the tight deadline.
Whitaker Construction Company has been in the underground utility business for 56 years and remains a family-owned company based in Brigham City, Utah, approximately 60 miles north of Salt Lake City. Their long history in the industry and experience with large diameter pipe installations made them well qualified to handle the challenges of this project.
Proper shoring and shielding are also essential in other construction contexts involving below-grade installations. For information on related safety practices, see the article on Safe Use of Ipc Chimney Pipes Code Compliance.
Key Lessons for Large Diameter Pipe Projects
The Whitaker Construction project demonstrates several lessons that apply to any large diameter underground utility installation.
- Assess clearance requirements early: Pipe outside diameter must be measured against available shoring clearance before equipment is ordered. The 28-inch difference between standard and custom clearance was the margin that made this project possible.
- Involve shoring specialists during planning: Early consultation with trench safety experts like National Trench Safety can identify custom solutions before project delays occur.
- Verify manufacturer experience with custom work: Efficiency Production’s prior experience with custom engineered shields and arches was critical to delivering a working solution on a tight timeline.
- Design for operational flexibility: The dual-function arch that could work with either one or two boxes gave the contractor options as ground conditions changed.
- Plan equipment mobilization carefully: With pipe joints weighing 45,000 pounds each, crane and excavator specifications must be confirmed before site work begins.
These principles apply across the full range of underground utility work, from small diameter residential connections to the massive 108-inch pipe installed on this project. Careful planning, custom engineering when needed, and adherence to safety standards remain the foundation of successful underground pipe installation.
Whether using open-cut trenching methods with custom shoring or trenchless alternatives, each approach requires its own set of specialized knowledge and equipment. Understanding the full range of available techniques helps contractors select the most appropriate and cost-effective method for each unique project condition.