Long-Distance, Limit-Pushing

A  new distance record for hard rock auger bores was achieved in December 2009 — U.S. contractor Gonzales Boring & Tunneling of Tigard, Ore., completed a 600-ft long trenchless crossing.

“Preparation, a qualified crew and the right cutting head matched to the right auger boring machine made for a successful crossing,” said Gonzales Boring & Tunneling president Jim Gonzales. The project represented the longest distance ever bored using a Robbins Small Boring Unit (SBU-A), a type of cutterhead mounted with disc cutters for excavating hard rock and mixed ground.

The record capped a series of limit-pushing projects across the United States, including a 544-ft long auger bore using a 36-in. SBU-A in Clermont County, Ohio. “The machines really weren’t a factor limiting the project — controlling grade was the only difficulty. With the same auger boring machine (ABM) and SBU-A, we could have gone another 100 ft,” said Capitol Tunneling Inc. project superintendent Rick Blevins. A third record-breaking bore in the 54-in. diameter range was accomplished in Louisville, Ky., where contractor Turn-Key Tunneling Inc. excavated a 352-ft long highway crossing.

Longer crossings highlight rapid advancements in technology, particularly for boring attachments using disc cutters. However, achieving groundbreaking status is anything but straightforward — optimal machine performance from both the auger boring machine (ABM) and boring head is most often determined by several factors including geology, correct auger usage and contractor experience with the technology.

Setting the Standard in Ohio

Started in June 2009, the Bramblewood Trunk Sewer line has resulted in cost-savings and increased
efficiency for Clermont County, Ohio. The new 8,625-ft long line connects the Bramblewood wastewater treatment plant to the newly renovated Wards Corner wastewater treatment plant, allowing the city to upgrade from an old and outdated plant currently in use.

General contractor Smith and Brown Contracting completed the majority of the pipeline, leaving a long, 544-ft crossing beneath Interstate Highway 275. Sub-contractor Capitol Tunneling Inc. began the work in June 2009 with a 36-in. SBU-A and 60-in. diameter ABM. “The owner and consulting engineers specified that the crossing be auger bored, but the engineers were skeptical about an ABM being able to bore that long of a crossing. We were able to convince them of the capabilities of the SBU-A and ABM setup and were able to bore further than the plan required, resulting in a savings to the general contractor,” said Capitol Tunneling president Kyle Lucus.  

The landmark bore was excavated through shale and limestone, ranging from 8,000 to 19,000 psi UCS. “The bedrock consisted of thinly bedded shale with thick limestone layers, which are very different in consistency and can cause drifting of the cutting head. But, with the SBU, we couldn’t tell the difference — the cutters handled the limestone as if it were shale rock for the length of the bore,” said Lucus.

Controlling grade was a challenge throughout the project, but ultimately resulted in a bore just 3.8 in. high — well within the design limits. “We were boring uphill, with the result that toward the end of the bore the rock was softer at the top of the head and harder at the bottom. We monitored the grade with a water level and hand-mined a short section at the bottom of the casing to keep the head level,” said Blevins.  

The crossing was completed in two months at rates of 40 ft per 10-hour shift, and required no disc cutter changes.

Distance Record in Oregon

The 600-ft record crossing in Tigard, Ore., had all the right variables to achieve record-breaking status. Gonzales Boring & Tunneling needed a solution for three gravity sewer crossings — 230 ft, 600 ft and 320 ft in length — through rock and mixed ground.

The crossings formed part of the Locust Street Sanitary Improvements Project, No. 6335. Approximately 1.1 mile of gravity sewer were installed by general contractor Northwest Earthmovers Inc., but several areas below houses, neighborhood streets, a small creek and a service facility, needed to be excavated with trenchless methods. The pipeline, for owner Clean Water Services, will increase capacity in the area and stop overflows currently plaguing the system once complete.

The three crossings were initially designed as a pilot tube microtunneling project using vitrified clay pipe. However, meetings between Gonzales, other local contractors and the project owner eventually resulted in the contract being opened up to other trenchless methods, including auger boring. “The owner has saved over $1 million on the trenchless section alone over its original cost-estimates for pilot tube microtunneling. Because the owner listened to the construction community, it saved both time and money and kept the dollars local,” said Gonzales.

After completing the initial 230-ft crossing in clay and basalt, the SBU-A was launched for its second 600-ft bore on Oct. 28, 2009. The disc cutterhead was used with a 72-in. ABM and steel casing. Rock conditions on the second crossing consisted of basalt at various rock strengths from 7,000 to 17,000 psi UCS. Crews monitored line and grade, and were able to maintain advance at about 40 ft per 10-hour shift.

A contractor-designed steering system guided the SBU-A to within one hundredth of an inch design grade. Despite the mixed ground conditions, no disc cutters required changing after 250 total m of boring.

Factors Contributing to Record-Breaking Projects

“The truth is that record-breakers are determined by several factors,” said Robbins SBU sales manager-Western U.S. Chris Sivesind. “These include improvements in auger boring machines and disc cutterhead technology, as well as geology. One of the most important aspects is often the contractor’s confidence in the equipment and crew, which eases the heightened level of risk taken on demanding projects.”     

Improvements in the Auger Boring Machine

Much of the ability for trenchless projects to excavate longer lengths is due to significant improvements in ABM technology. In the late 1970s, the average 60-in. diameter ABM operated with 500,000 lbs of thrust and 160 hp and was limited to projects in soil at lengths of less than 500 ft. Within the last 40 years, the technology has grown in both power and diameter range, operating at up to 300 hp and 1.8 million lbs of thrust using 94-in. steel casing.

Other major ABM improvements in the last 15 years include larger diameter hex drives. The drives allow for larger hex augers, 5 in. in diameter and larger, which are needed on long distance bores at high horsepower and torque. Improvements in steering accuracy have also allowed for accurate installation of small diameter casings using a pilot tube boring system or hydraulic steering system. In addition, specialized cutting heads have extended the range of geology that an ABM is capable of excavating, from soft ground and rock less than 11,000 psi UCS to hard rock greater than 29,000 psi UCS and consolidated mixed ground.  

Improvements in Cutterheads Using Disc Cutters

First developed in 1996, the Robbins Small Boring Unit is a disc cutterhead used with standard auger boring machines. The SBU-A, in diameters from 24 to 72 in, consists of a circular cutterhead mounted with single disc cutters capable of excavating rock from 4,000 to more than 25,000 psi UCS.  In the launch pit, the machine is welded to the lead steel casing, while the ABM provides both torque and forward thrust to the cutterhead. Openings in the cutterhead called bore scrapers collect spoil from the face, where they are transferred to a full-face auger for removal.

Over the last 10 years, SBU cutterhead technology has improved for specific ground conditions. Cutterhead configurations that feature a combination of carbide bits, two-row tungsten carbide cutters, and single disc cutters are being used for a variety of types of mixed ground. In addition, tool steel for the disc cutters themselves has resulted in fewer cutter changes and less downtime, particularly for the relatively short distances of most trenchless crossings.

Adaptations in Auger Diameter  

On long bores, contractors seeking the greatest efficiency use hex augers at least 6 in. smaller in diameter than the size of the bore. The smaller auger diameter prevents torque building and sudden torque unwinding. On long crossings with higher torque requirements, larger diameter augers can flex against the side of the casing, building up torque and unleashing the force suddenly — a scenario that can destroy gearboxes and damage valuable equipment.

Ground Conditions

Whatever the ground conditions may be, consistent ground seems to be a contributing factor for many record-breaking projects. All three of the record-breaking SBU crossings were excavated in uniform medium to hard rock, with no fractures and little groundwater present.

In addition, ground that is too hard or too soft can hinder progress. Very soft rock less than 3,000 psi UCS can clog the cutterhead, requiring slowed rotation and advance, particularly if groundwater makes the cutting face sticky. Very hard rock of 36,000 psi UCS or more requires higher thrust loading on the disc cutters and can also slow progress.

Contractor Experience  

Contractor experience with multiple successful bores, as well as willingness to accept risk, is critical to completing a record-breaking project. Ideally, contractors should have experience with multiple ABM and SBU projects at various diameters.

Ultimately, quality support and contractor willingness to attempt long crossings may be the highest predictor of success on the project. All three contractors had more than 25 years of experience in auger boring and felt that field service was invaluable. “The technology worked very well for both crossings. The field service support we received was unmatched, and we hope to receive similar support for future jobs in hard rock,” said Gonzales.

Desiree Willis is a technical writer for The Robbins Co., Kent, Wash.