Micropiles Reduce Costs and Schedule for Merchant RR Bridge Rehabilitation Jeff R. Hill, P.E. Hayward Baker Inc. 111 W. Port Plaza Drive Suite 600 St. Louis, MO 63146 314-542-3040 JRHill@HaywardBaker.com Aaron C. Kober, P.E. Modjeski & Masters, Inc. 804 North First Street St. Louis, MO 63102 314-588-8115 ACKober@Modjeski.com Background & Geotechnical Profile The Merchant Railroad Bridge crosses the Mississippi River in St. Louis, Missouri and has been in continuous service since its construction in the 1890s. In order to extend its life for another century, the Owner has implemented plans for rehabilitation and reconstruction of the structure. This work includes immediate replacement of the East and West Approach deck truss spans with new deck girder spans supported atop concrete piers and founded on deep foundations with rock sockets. Foundation and substructure work was required to be completed without removing the existing spans in order to reduce the amount of track down time. The initial design called for concrete drilled shaft foundations, extending to bedrock. However, since the new foundation work had to be performed beneath the existing structure with available headroom of less than 30 feet, drilled shafts presented an expensive foundation option. The geotechnical profile, typical of soil conditions along the Mississippi River at the North side of St. Louis, consists of: 0 to 40 ft: Silty alluvial-deposited sands/clays (very loose) 40 to 80 ft: Sands (denser with depth) 80 to 85 ft: Weathered Limestone 85: Solid Limestone bedrock Micropile Value Engineering Proposal The micropile contractor, working closely with the Owner and their structural engineer, proposed the use of micropiles as an economic alternative to drilled shaft foundations for the replacement structures. Micropiles offered several advantages over drilled shafts at this site. Micropile drill rigs are specifically designed for working in low headroom and tight access conditions. Consequently, the ability to work with low headroom proved to be very beneficial for this project. Micropiles can economically drill through debris such as urban fill more easily than drilled shafts. Micropiles can also be installed with rock cutting bits, which will drill the rock sockets more efficiently than coring with a 36-inch diameter core barrel. One final advantage offered by the micropiles is that the smaller diameter enables installation in closer proximity to existing utilities.
At certain micropile locations on this site, installation of a drilled shaft would have required either the relocation of additional utilities or enlargement of the pile cap. Micropile construction is not affected by high groundwater tables or running soil conditions, which were possible at this location, thus resulting in improved constructability and an accelerated schedule. Drilled shaft installation would have required the use of low-overhead drilling equipment, requiring the use of casing and laborious slurry drilling methods at this site. The micropile casing is the drill string and is often left in place, eliminating concern for final quality of the foundation element related to installation or removal of a temporary casing. Leaving the casing in place also reduces the risk of subsidence of the existing structure or adjacent utilities. Scope of Work The lengths of the structural replacement zones are 462 feet for the East approach, and 507 for the West approach. Six pile caps were constructed on each side of the river. Micropiles were installed under five consecutive of the six bents for each approach, incorporating an overall total of 146 micropiles supporting ten bents. The initial design provided by the Micropile Contractor used approximately one micropile for each drilled shaft. At each pile cap located immediately adjacent to the Mississippi River, low headroom was not an issue and drilled shafts were installed as originally proposed. Figure 1. Extent of rehabilitation work at East and West approach spans.
Figure 2. West and East approach locations, Merchant RR Bridge. Pre-Production Probe Drilling beneath West Approach Prior to production work, the micropile contractor performed probe drilling near Bents W3, W4, and W6 of the West Approach. The size and shape of the West Approach pier caps, as well as the exact location of the micropiles at those bents was contingent upon the exact location of several existing site obstructions, which could not be easily relocated. These obstructions included 48- inch and 108-inch diameter sewer lines at an approximate depth of 20 feet, a high-pressure gas line, a fiber optic line, a water line, and an in-service rail line. The Micropile Contractor advanced the probe drill string with water flush in an effort to accurately locate several of the underground obstructions. After locating the obstructions, several of the projected micropile locations were probed in the same manner to double-check their dodging of underground obstructions during production work. Where obstructions were identified as miscellaneous urban fill, the micropile contractor was able to use core bits to core through such obstructions. Micropile Installation Following probe drilling, two drill rigs were mobilized to the East Approach to begin installation. To meet production schedule requirements, up to four drill rigs were used at certain times during the project. With headroom of 18 feet, 10 foot pieces of casing were used to make up the micropiles. Threaded casings eliminated the need for welding in the field. In some locations, the general contractor was able to perform partial excavation in order to increase the available headroom. In other locations, a drill rig requiring only 14 feet of headroom was used to install the micropiles. The micropile contractor drilled the casing (9.625 inch OD x 0.545 inch wall) to depth using rotary drilling with external water flush. Water from the Mississippi River was used for the
drilling operation. The micropiles were advanced through the soils on site, including fill, looseto-dense sands, and weathered rock. They were then socketed a minimum of five feet into solid limestone. Drill water and spoil was returned to the surface through the annular space. Depending on the individual load conditions of the pile cap and the existing site obstructions, the micropiles were either installed vertically or battered at 1.5 on 12. This battering was designed to carry the longitudinal rail loads as provided by the structural engineer. Once each micropile was socketed, a tremie pipe was lowered to the bottom of the casing and the casing was filled with neat cement grout. An on-site colloidal mixer, specifically designed for neat cement grout, was used to provide a uniform and consistent grout. The volume of grout placed in each micropile was recorded to ensure full grouting of the casing. After tremie grouting each micropile, a pressure cap was fitted to the top of the micropile, and additional grout was pumped into the pile to a minimum pressure of 150 psi. Pressure grouting of the rock socket is performed to fill any voids or fissures in the rock and provide structural contact between the micropile drill tooling and the solid dolomitic bedrock. Following installation of the micropiles at each pile cap, the general contractor excavated to the planned bottom elevation of the footing. The micropile contractor then cut the micropiles to a specified elevation. Steel bearing caps were then placed on each micropile, and the micropiles were cast into a reinforced pile cap provided by the General Contractor. Installation procedures for the East and West approaches were identical. Figure 3. Typical micropile and subsurface profile at the Merchant Railroad Bridge site.
Figure 4. Micropile locations adjusted due to utilities conflict at Bent 6 (West approach). Figure 5. Typical micropile layout for East and West approaches.
Figure 6. Micropile installation at Bents 2 and 3 beneath the East approach. Figure 7. Micropile installation in 17 ft of headroom. Figure 8. Micropile installation at Bent 3 (West approach).
Quality Control Documentation Driller s logs recorded all drilling parameters during operations. Grout Strength Testing Independent laboratory tests of 3 x 6 cylinders confirmed that the minimum 28 day unconfined compressive strength of 5,000 psi was achieved. Grout samples were taken daily. Steel Properties Independent laboratory testing of random coupons of the micropile casing material were used to confirm the structural properties of the casing. The casing material is API Grade N80, with a tensile strength of 80 ksi. Load Testing The micropiles were designed to carry 325 kips and were tested to 650 kips using ASTM D 1143 Quick Load test Procedure. One load test was completed on the west side of the river and one load test was completed on the east side of the river. Both load tests were completed on production micropiles. Production micropiles were also used as the reaction anchors for the load test. Earthquake design loads were not considered. Figure 9. Load test setup on production micropile.
Figure 10. Detail A from Figure 9. Figure 11. Load test setup.
0.00 Total Movement vs Load 0 100 200 300 400 500 600 700 800-0.50-1.00 Deflection (in) -1.50-2.00-2.50-3.00-3.50 Merchants Bridge Micropile Load Test by Hayward Baker Elastic Deflection vs. Load 9.625 OD x 0.545 Wall DL = 300 Kips Length of Pile = 90' ASTM D1143 Quick Load Test Test Date 9/22/04 E d B i i R k Load (kip) Average Total Deflection Theoretical Elastic Deflection Tip Movement Linear (Average Total Deflection) Linear (Tip Movement) Figure 12. Total Movement vs. Load. Test preformed on production micropile. Conclusion Micropiles offer an economical alternative to drilled shafts where tight access, low headroom, or urban fill adversely affects the construction schedule of a schedule critical project. The micropile alternative not only shortened the foundation rehabilitation schedule by several weeks, but realized a significant cost savings for the Owner. Acknowledgements The authors extend their appreciation to the following parties for helping make this project a success: Terminal Railroad Association (Owner), St. Louis Bridge Company (GC), Keeley & Sons, Inc. (JV), and Midwest Testing (Engineer).