Multiple Large Diameter Microtunnels with Limited Ground Cover

Transcription

1 North American Society for Trenchless Technology (NASTT) NASTT s 2016 No-Dig Show Dallas, Texas March 20-24, 2016 Paper No. TM2-T5-04 Multiple Large Diameter Microtunnels with Limited Ground Cover Steve Caneen, Huxted Tunneling LLC, Palmetto, FL H. Steve Pollack, Huxted Tunneling LLC, Palmetto, FL 1. ABSTRACT Huxted Tunneling LLC was awarded a subcontract to install 350 LF of 60 RCP and 1,000 LF of 84 RCP, in seven drives, crossing US Highway 41 in Port Charlotte, Florida. Huxted has the largest fleet of Iseki Unclemole MTBM s in North America, capable of installing various pipe materials from 26 OD to 103 OD. Huxted used their newly acquired 103 Unclemole to install the five drives of 84 RCP and a 73 Unclemole to install the two drives of 60 RCP. All microtunnel drives required large diameter microtunneling with less than one diameter of earth cover in poorly graded sand and silt. Some of the most challenging aspects of the project were the limited ground cover, unusual work shaft layout, required ground improvement, settlement monitoring and MTBM recovery beneath overhead high-voltage power lines. 2. INTRODUCTION Limited Ground Cover The project called for the modification of three weir structures and adding additional capacity to the existing box culverts crossing US Hwy. 41. ZEP Construction Inc. was the general contractor and Charlotte County is the Owner. The project design was done by GWE of Florida and Construction Engineering Consultants. Ground conditions were poorly graded sands and silts with ground water to spring line. The most challenging aspect of the work was the limited ground cover; generally less than six feet under the pavement and as little as two feet between the launch shaft and the pavement (Fig. 1). The front walls in the launch shafts were roughly 20 feet from edge of pavement while the receiving walls were less than ten feet from edge of pavement. The only soil improvement used was hydrophilic chemical grout at the microtunnel portals. Since the design did not require ground improvement, Huxted had a contingency in its bid for some pavement restoration due to the risk of settlement. Fig. (1) 3. Cofferdam Design Paper TM2-T5-04-1

2 Another major challenge on this project was the fact that the launch and retrieval shaft walls for all tunnel drives were at 70 to the tunnel centerlines. This required a significant amount of custom built steel fabrications. Fig. (2) Cofferdam Layout A steel fabricated backstop was placed between the shaft backwall and the jacking frame since the backwall needed to be perpindicular to the tunnel alingment and the distances from front wall to back wall on some drives were excessive. Ready-mix concrete could have been used for this purpose but the material cost was prohibitive. Steel was available from Huxted inventory and could be salvaged on completion of the work. The entrance and exit walls also required unique entrance/exit ring fabrications to compensate for the accute angle of the front wall. Again the same result could have been accomplished with ready-mix concrete at a significantly increasd cost (Figs. 2-3) Fig. (3) Lowering 103 MTBM Paper TM2-T5-04-2

3 Due to the proximity of the edge of pavement and the tunnel portals, it was decided that the break-in break-out technique, where sheet pile in the tunnel zone are pulled up so that the pile tips are above top of pipe, was too risky. Huxted fabricated steel cans with the flanged, gasketed entrance ring attached. The can was trimmed to fit the piles, match the angle of the front wall and welded to the sheet pile. Just prior to MTBM launch, the sheet pile inside the ring assembly was cut out. The MTBM was pushed forward to engage the rubber gasket prior to the start of excavation. Fig. (4) Steel fabricated backstop and limited ground cover 4. Ground Improvement Huxted used a two part polyurethane hydrophilic chemical grout to stabilize the soil at the tunnel portals prior to launch and recovery. The chemical grout and limited well-point dewatering controlled ground water movement during the break-in break-out process. The grout was installed on a grid pattern through the cofferdam front walls. One half inch diameter holes were cut in the sheet pile and the grout was pumped into the soil using a small diameter injection rod (Fig. 5). Huxted has been using this method for years in cohesion less soils below the water table. Paper TM2-T5-04-3

4 Fig. (5) Chemical grout injection Unfortunately the project design did not require pre-treating the tunnel zone under the pavement to reduce the risk of settlement during the tunneling operation. Typically with this diameter and with ground cover less than one tunnel diameter, jet grouting or some other permeation grouting technique would be required. In this case it was not required so in order for Huxted to reduce risk, a contingency sum was included in the bid price to cover possible repairs due to settlement. No settlement was measured at the two 60 crossings while just over an inch of settlement was measured at the first 84 crossing. No other settlements were observed. Paper TM2-T5-04-4

5 5. MTBM Operation All Huxted MTBM operators have extensive experience in microtunneling with limited ground cover in various soil conditions. Huxted MTBM operators each have over 20 years operating experience with Iseki equipment. Our operators have worked all over the world and were all trained by the MTBM manufacturer. This operating experience on a wide variety of projects is essential to limiting unwanted ground movements. The only measured settlement occurred on the first 84 drive of the Port Charlotte project while the operator was developing his operating techniques using a larger MTBM in tough conditions. An experienced MTBM operator is absolutely key to limiting damage due to settlement and a successful project. The operator observes cutterhead torque, earth pressure at the face, speed and slurry pressure at the face. These parameters help him adjust his excavation rate in order to prevent surface settlement or heave. Operators that are most successful do develop a feel for how the MTBM is behaving in the ground and this feel comes from a wide variety of experiences. Fig. (6) MTBM Operator prior to recovery of 88 MTBM Paper TM2-T5-04-5

6 6. Settlement Monitoring All of the microtunnel drives were crossings of US Highway 41, an FDOT roadway. FDOT permits require a settlement monitoring program when crossing vital infrastructure. Zep Construction developed and performed the monitoring plan which included numerous measurement points on tunnel centerlines and on either side of the tunnels. The points were PK nails driven into the pavements prior to the microtunnel operation. These points were established by a surveyor who measured and recorded the baseline conditions. During microtunneling, the monitoring points closest to the MTBM position were measured daily to give the MTBM operator feedback on any ground movement. Once tunnel excavation was completed the monitoring points for the drive were measured weekly for the next four weeks. Fig. (7) Crew with Mortimer recovering 103 MTBM Paper TM2-T5-04-6

7 7. Overhead Conflicts All of the 84 tunnel drives had high voltage powerlines directly overhead of the tunnel portals on the MTBM recovery side of the highway. Mortimer is shown monitoring the recovery operation (Fig.7). The ground elevation at the portal was maintained just below the bottom elevation of the MTBM. Steel rails were placed below the MTBM as it was pushed out of the portal. Once the MTBM was fully exposed and the leading edge of pipe one was engaged in the rubber gasket, a cable and snatch-block system was rigged to the MTBM and the crane The crane pulled the cable horizontally until the MTBM was clear of the overhead powerline conflict so that the lattice boom crane could maintain the required safety distance from the powerlines. The total pull length was about 60 feet. The snatch-block rigging was removed and the MTBM was rigged conventionally for a vertical lift. The MTBM was loaded on a truck and repositioned across the highway for the next drive (Fig. 8). Fig. 8 Limited ground cover and overhead conflicts. Paper TM2-T5-04-7

8 8. Summary Microtunneling with less than one tunnel diameter of cover, in poorly graded cohesion-less soil, crossing under critical infrastructure, comes at increased risk for the microtunnel contractor and to a lesser degree the other stakeholders. Huxted Tunneling LLC has learned to mitigate these risks by using MTBM operators with many thousands of feet of pipe installed. Our operators training and experience are all with a common make and model of MTBM. After 20+ years using this equipment all over the world, our operators have developed a good understanding of the equipment they use, in a wide variety of applications and soil conditions. Even so, ground improvement should be installed to limit the risk of settlement under these conditions. A good ground movement monitoring program can also be a useful tool for the microtunnel contractor if measurements are done routinely as the tunnel is excavated. Unwanted ground movements can be relayed back to the operator so the excavation process can be modified and unwanted ground movement can be limited. Even with these measures, completing these tunnels with limited ground movement is a major achievement. Paper TM2-T5-04-8