SANITARY SEWER IMPROVEMENT PROJECT 2003-501
Wayne Force, P.E., URS Corporation, design engineer; Shelly Pederson, City of Bloomington, project owner; Shane McFadden, P.E., Engineering and Construction and Innovations Inc. (ECI), contractor; Rob Tumbleson, Akkerman Inc., pilot tube microtunneling equipment manufacturer.
ABSTRACT: Nine jacking and reception shafts with approximately 2,074 feet of 18 inch sanitary sewer pipe, 150 feet of 36 inch steel casing, 168 feet of 18 inch restrained joint DIP sanitary sewer and 7 sanitary manhole structures bordering the Mall of America properties for the improvement of sanitary sewer flow on properties adjacent to the Mall of America in the city of Bloomington Minnesota.
A congested urban commercial location, right-of-way limitations, holiday schedules and unfavorable weather conditions present challenges regarding the installation method for the new sewer. A geological investigation was conducted, followed by a baseline geotechnical report, to help identify the existing ground conditions and any risks the geology may present. The primary risk identified in the baseline geotechnical report was the existence of a relatively high water table. Based on the findings in the baseline geotechnical report, coupled with the limited and congested working area, microtunneling or other acceptable trenchless methods were chosen by the design engineer and owner as the specified method of pipeline installation. ECI submitted pilot tube microtunneling as an alternate method of construction to the owner prior to the bid date. Pilot tube microtunneling and microtunneling were identified, by addenda, as the only acceptable methods of installation. ECI was the successful low bidder and was chosen as the installation contractor.
Shaft construction began on October 9, 2003. The shafts were installed using caisson-drilling methods. The shafts were lined with concrete encased corrugated metal pipe (CMP). Pilot tube microtunneling commenced on October 20, 2003 and was completed on April 8, 2004.
1. INTRODUCTION
The City of Bloomington 2003-501 Sanitary Improvement Project is located along the Western and Northern borders of the Mall of America property and border Highways 77 & Lindau Lane (parallel to I-494). Right-of-way limitations greatly restricted the available area for shaft and equipment placement. This dictated construction of the smallest possible launch and reception shafts. Additionally, equipment and on-site material storage had to be kept to a bare minimum. Holiday shopping schedules and unfavorable weather conditions also created challenges to the installation contractor. Figure 1 shows the project layout.
Figure 1. Site Layout
Engineering & Construction Innovations, Inc. (ECI) was chosen as the general contractor for this project. ECI self performed the pilot tube microtunneling and general sewer work with Tri State Drilling constructing the ten jacking and receiving shafts as a subcontractor.
2. BACKGROUND
When constructed, the Mall of America, located in the city of Bloomington, Minnesota became the worlds’ largest indoor shopping mall. The Mall of America’s close proximity to the Minneapolis – St Paul International Airport has made retail business in the mall’s surrounding area a high priority. The 2003-501 Sanitary Sewer Project will provide the required sanitary sewer infrastructure to allow large-scale commercial expansion in areas adjacent to the Mall of America property. The Bloomington Engineering Division submitted a specifications and special provisions package, along with invitation for bids, on July 28th of 2003. These documents were reviewed and approved by the Bloomington City Council.
3. DESIGN CONSIDERATIONS
URS Corporation, with assistance from City of Bloomington Engineering Department evaluated viable options for the installation of the 2,224 feet of sanitary sewer line. Primary consideration was given to:
- The limited space available at the site.
- The existing geology.
- Public and commercial disruption during the installation process.
- The anticipated weather during the construction season.
- Available right of ways and easements.
- Existing utilities and locations of available sanitary sewer tie-ins.
Upon evaluating these challenges, the owner and engineer chose microtunneling or an approved alternate trenchless method as the preferred installation method. Shane McFadden, president of ECI, Inc. contacted Akkerman sales staff to discuss the project challenges along with the potential soil conditions, and chose to submit pilot tube microtunneling as an alternate installation method, Addendum #2 was issued on the 29th of August to include pilot tube microtunneling, along with microtunneling, as the only approved methods of installation.
4. GEOTECHNICAL CONSIDERATIONS
A geotechnical evaluation report was conducted at the site in mid August of 2003 using a 3.25” H.S.A. auto-hammer to drill eight test bore holes. Test boring locations, as indicated on figure 2, generally coincided with the locations of the proposed manholes. Poorly graded sand bearing water to a depth of as little as 6.5 feet was commonly indicated in the reports as shown on figure 3.
Figure 3. Soil Boring Report Log Indicates Poorly Graded Sand And A
High Water Table
5. CONSTRUCTION
Shaft Construction
The first task to be started was the construction of work pads for the jacking and receiving shafts. Six of the work pads had to be cut into the side slope between the Mall of America West Parking Garage and Highway 77. The remaining work pads were located on relatively level but very size restricted ground. Tri State Drilling, ECI’s shaft drilling subcontractor, began installing eight-foot diameter launch shafts and six-foot diameter recovery shafts on October 9, 2003.
Size restricted sites and limited access to the construction site presented challenges to the installation crew.
Installation
ECI utilized a two-pass Akkerman Guided Boring Machine (GBM) for the installation of the 18-inch Mission No-dig clay pipe (22.5-inch outside diameter). After the jacking frame and guidance system are installed in the jacking shaft and properly aligned, the fist pass installs the pilot tube. The 4.25” outside diameter pilot tube is hollow and comes in 2.5-foot sections. On the leading edge of the pilot tube is a steering head, which is angled to allow for grade and alignment adjustments. Inside the steering head is an illuminated target with a long tail that points toward the bias of the steering head. The guidance system camera views the target in the steering head, the image is then projected on a LCD monitor in the jacking shaft. The operator keeps the guidance system aligned on the target as the pilot tube is pushed through the ground. Once the pilot tube reaches the reception pit, it is checked for proper line and grade.
After the pilot tube pass is complete, a Reaming head that fits the outside diameter of the product pipe is set into place and connected to the trailing end of the pilot tube. After the reaming head is set in place, it is advanced through the ground with jacked product pipe and auger assembly sections. The augers and temporary casings are set concurrently inside the one-meter long 18” clay pipe sections; the augers and casings carry spoils back to the launch shaft from the reaming head. The clay pipe sections are sequentially thrust into place as pilot tube sections are removed in the recovery shaft.
Production
Shaft 1 to Shaft EXMH 0+00 (298’):
ECI took delivery of the Guided Boring System on the 20th of October. Site set-up for the first drive (Shaft 1 to Shaft EXMH 0+00) began on the 21st. Startup and optics challenges in the guidance system delayed installation of the 260 feet of pilot tubes until the end of the day of October 28th. Once the pilot tube reached the reception shaft, it was checked for proper line and grade. The product pipe was installed and the drive completed on November 7th.
Shaft 7 to Shaft 6 (255’):
Setup for the second drive began on November 10, 2003. Pilot tube installation was completed on November 12th. Pipe installation began on the 13th but due to poor ground, the reaming head had to be modified. Once pipe installation re-commenced, there were failures in the seal ring of the product pipe that had to be overcome. On the 21st of November at 157.5 feet an obstruction was encountered that terminated the completion of the drive.
Shaft 7 to Shaft 8 Phase I (168’):
The drive from Shaft 7 to Shaft 8 began on November 25th, 2003. Pilot tube installation was completed on November 26th. The installation of the 24” Permalok casing pipe began after the Thanksgiving holiday on December 1st. Upon starting the installation it was noticed that the Permalok casing pipe was too short and did not match the auger casings, creating the probability of pushing the reaming head off the Permalok pipe. Operations on this drive had to be ceased until new Permalok casing pipe of the proper dimension could be manufactured and delivered.
Shaft 2A to Shaft 2 (140’)
Setup for the fourth drive began on December 3, 2003. Pilot tube installation was completed on December 4th. The installation of the 18” No-Dig clay pipe was completed on December 8th.
ECI crews prepare for the installation of 18" No-Dig Clay pipe within the limited space available.
Shaft 5 to Shaft 4 (214’):
Setup for the fifth drive began on December 10, 2003. Difficulty was encountered with high ground water pressure effects on the shaft seal. Pilot tube installation was completed on December 12th. The installation of the 18” No-Dig clay pipe was completed on December 17th.
Shaft 5 to Shaft 6 (247.5’)
The drive from Shaft 5 to Shaft 6 began after a scheduled Holiday season shut down on January 5, 2004. Pilot tube installation was completed on January 6th and pipe installation began on the 7th. On the 9th of January at 235 feet an obstruction was encountered that terminated the completion of the drive.
Shaft 7 to Shaft 8 Phase II (168’)
Pilot tube microtunneling operations re-commenced on January 26th for the completion of the Permalok casing run (Shaft 7 to Shaft 8) under Landau Lane. Operations were hindered by extremely cold weather. Temperatures to -30° Fahrenheit were experienced during the week, making equipment operations difficult. On the second of February one of the conveyance auger sections sheared, halting operations. The casing was approximately 12’ from the reception shaft at the time of failure. ECI elected to jack a shield and hand mine from the receiving shaft to recover the reaming head and complete the casing installation. The run from Shaft 7 to Shaft 8 was completed on February 6, 2004.
Shaft 3 to Shaft 2A Phase I (260’)
A second shift was added to the construction crew and a second jacking frame was supplied by Akkerman Inc. to expedite operations at this time. Setup for the eighth drive began on February 9, 2004. Pilot tube installation was completed on February 11th. On the 12th of February during night shift, the drive was abruptly stopped and the auger shaft sheared. It was determined later that the reaming head intersected the underside of a thrust block of an 18” water main. This water main was incorrectly located on the project plans and was also incorrectly located during the utility locate. The pilot tube passed under the thrust block, however the thrust block deflected the reaming head down creating enough force to shear the auger flight.
Recovery Shaft 1 to Recovery Shaft 2 (120’)
Due to the proximity of Recovery Shaft 2 to the designed Manhole #6, it was decided to move Manhole 6 to the Recovery Shaft 2 location. Prior to pilot tube microtunneling this alignment, the existing Shaft 6 had to be backfilled, as the pipe alignment intersects the shaft location. Setup for this drive began on February 16, 2004. Pilot tube installation was completed on February 18th. The installation of the 18” No-Dig clay pipe was completed on February 20th.
Shaft 3 to Shaft 4, Two Shift, (328’)
Setup for the tenth drive began on February 23, 2004. Pilot tube installation was completed during the night shift on the 23rd. The installation of the 18” No-Dig clay pipe was started on February 24th and completed on February 26th during the night shift.
Shaft 1 to Shaft 2, Two Shift, (325’)
Setup for the drive from Shaft 1 to Shaft 2 began on February 27, 2004. Pilot tube installation began that night. High thrust pressures and excessive pilot tube drift was encountered at approximately 200’. Pilot tube installation was successfully completed on the 3rd of March, the installation of the 18” No-Dig clay pipe was started the same day. Jacking pressures began elevating dramatically near the same area that the pilot tube operations experienced problems. A pull assist unit was placed in the receiving shaft (Shaft 2) to help get through the bad area. Thrust forces overcame the capacity of the jacking frame at approximately 195’ at which point the drive was stopped.
Recovery Shaft 3 to Shaft 2
Setup for the twelfth drive began on March 22nd. Pilot tube installation was completed on the 23rd. The installation of the 18” No-Dig clay pipe was started on same day and was completed on the 24th.
Shaft 2A to Water Main Recovery Shaft
Setup for the thirteenth and final drive began on April 5th. Pilot tube installation was completed on the 6th. The installation of the 18” No-Dig clay pipe was started on the 7th and completed on the 8th during night shift.
Final Project Completion
The sewer system was commissioned on the April 14, 2004. The project was entirely completed on May 14th, 2004.
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