STEMMERS RUN STEEL SHEET PILE COFFERDAM AND TRESTLE Jennifer Peirce Brandt, P.E., John J. Peirce, P.E., and Frank M. Vibbert, P.E., Peirce Engineering, Inc., Phoenixville, Pennsylvania, USA

Project documents and the United States Army Corps of Engineers’ permit for construction of the Stemmers Run Relief Wastewater Force Main across the Back River in Baltimore County, Maryland specified a minimal width, steel sheet pile cofferdam and a 12 feet wide, pile supported, timber decked, causeway trestle – both approximately 1700 feet in length. Due to the impractical width of the causeway, the contractor desired to combine the cofferdam and the causeway structures in order to minimize wetland disturbance and maximize access for construction equipment. The resulting support structure was a steel sheet pile cofferdam, approximately 17 feet wide, internally braced, and capped with transverse timber deck mats to serve as the access and work trestle for the trench excavation, pipe bent construction, and trench backfill. Construction of this cofferdam/causeway progressed from the north shoreline toward the middle of the river. Then, working back toward the north shoreline, the contractor excavated, installed the 54 inch diameter pipe and H-pile support bents, and backfilled the trench in increments of approximately 60 linear feet. Excavation spoils from one section were used to backfill the preceding section. As the pipe installation progressed toward the north shore, the steel sheet piling and timber deck were removed. As sufficient sheet piling and deck mats were removed, the contractor began the process anew from the opposite shoreline using the reclaimed materials. The project was complicated by the presence of deep, very soft, weight of hammer, silts and organic clays and an unbalanced, hydrostatic head of 20.5 feet. In addition, concurrent pile driving, excavating, and backfilling in successive cofferdam sections required the use of heavy, long reach, hydraulic backhoes, and a 200 ton capacity crawler crane which controlled the design of the timber causeway deck and its supporting sheet piles.

Background As part of a court injunction and subsequent consent decree between United States Environmental Protection Agency (EPA) and the State of Maryland for violation of the Clean Water Act, a design was completed by Rummel, Klepper, and Kahl, LLP (RKK) of Baltimore, Maryland for the construction of approximately 13,200 linear feet of 54 inch diameter force main running in a southwest direction from the proposed Stemmers Run Relief Wastewater Pump Station at Golden Ring Road to an existing force main junction chamber at the Back River Wastewater Treatment Facility on Eastern Avenue, all within the County of Baltimore, Maryland. This project was valued at approximately $26.4 million dollars and connected to other previously completed work. In August of 2005, the contract was awarded to American Infrastructure-Maryland (AIMD), a division of American Infrastructure which is headquartered in Worcester, Pennsylvania.

AIMD began work on this project in October of 2005. The work being discussed in this paper is the construction of a pile supported, 54 inch diameter, sanitary sewer force main extending approximately 1700 linear feet across the Back River, a sediment-filled estuary to the Chesapeake Bay (Figure 1). The construction cost for this river crossing work was approximately $7.9 million dollars.

Figure 1. Aerial view of river crossing, east of I-695 bridge

The force main pipe was constructed using cut and cover methods to a depth approximately 16 feet below existing ground elevation and about 18.5 feet below mean high water level. The pipe was installed just east of the existing I-695 bridge over the Back River and just north of the I-695 interchange with Route 150, Eastern Avenue (Figure 2).

Figure 2. Proposed Back River site The original RKK design concept for performing the project was to excavate, lay, and backfill the pipe between two parallel rows of steel sheet piling (SSP). The work would take place from a parallel, pile supported, timber deck, work trestle approximately 12 feet wide. A U.S. Army Corps of Engineers permit was issued for this plan in 2004. AIMD decided to investigate the elimination of the separate work trestle. Their proposed concept was to perform the entire river crossing work from a wood deck spanning between and being supported by the SSP. AIMD approached Peirce Engineering, Inc. (PE Inc.) of Phoenixville, Pennsylvania to investigate the AIMD concept and, if feasible, provide design calculations and drawings. This alternate work concept required a permit revision by the U.S. Army Corps of Engineers. The concept proved feasible and the PE Inc. design submission was approved as AIMD’s method of construction. One advantage of AIMD’s alternate cofferdam/trestle method was that it totally eliminated one of the structures needed to cross the Back River. Using the SSP as the work platform support allowed for a 28 feet wide work platform rather than the proposed 12 feet wide work platform. Another advantage was that AIMD’s method allowed cofferdam excavation, pipe placement, and backfill to be performed

directly in front of the equipment and in line with the cofferdam and pipeline rather than by reaching from the side, thus improving visibility, safety, and productivity. This alternate cofferdam/trestle method also minimized the amount of wetland disturbance along the estuary by eliminating the parallel trestle structure. Design The design for the cofferdam used the Shoring Suite Plus, Version 7.3, retaining wall computer program from CivilTech Software. The lateral earth pressures were calculated using CivilTech’s EPres software based on input of the expected soil properties. These calculated earth pressures were then imported to CivilTech’s Shoring software to compute the required SSP section and the wale and brace loads. The design was based on a single tier, braced system with a triangular earth pressure distribution due to the very soft, upper, retained soils (Figure 3). The required depth of SSP toe embedment was calculated by the CivilTech software. The upper level brace at Elevation +4.0 was not considered in the SSP design but was incorporated into the deck construction in order to maintain proper deck alignment and provide additional deck stiffness.

Figure 3. Generalized earth and water pressure diagrams

The ground conditions through which the pipeline was to be constructed consisted of a layer of dark gray, very soft, silt/clay mixture. The silt/clay layer was overlain in some areas by a mass of organic material, commonly referred to as meadow mat. This weight-of-hammer (WOH) material extended to as much as 30 feet deep. Below the very soft silt/clay material was a 5 to 10 feet thick layer of medium dense to dense sand and gravel (N = 32 to 50 blows per foot) over very stiff silty clay (N = 50 blows per foot to 100 blows per 5 inches). During its installation, the SSP would typically sink under its own weight for 15 to 25 feet and would then be vibrated an additional 10 to 15 feet with an ICE Model 22-30 vibratory pile hammer having a 2,200 inch-pound eccentric moment. The SSP was then driven to its design depth with an ICE Model I-19 diesel hammer with a maximum rated energy of 43,225 foot-pounds. The normal depth of the water across the Back River varied from about 0 to 3 feet. The water level was tidal with Mean High Water (MHW) at Elevation +2.17 and Mean Low Water (MLW) at Elevation +1.02. The top of the SSP was at Elevation +6.0 with regularly spaced openings at Elevation +4.0 to allow water to flow over the SSP in the event of a flood and to limit the design hydrostatic pressure on the SSP during a high water event. Construction Phasing Prior to the start of construction, AIMD decided to construct and install the cofferdam and trestle in thirds. Starting from the north shore, AIMD initially installed two-thirds of the proposed cofferdam. As construction proceeded and the middle third of the pipe installation was completed, its SSP was removed for use at the opposite shoreline. An initial starter cofferdam was left-in-place for making the final connection between the first and third pipe runs. As work was being completed in the second-third of the cofferdam at the north shore, the reclaimed sheets were then installed starting at the south shore and proceeding toward the starter cofferdam. This sheet pile sequence allowed AIMD to work from both sides of the estuary simultaneously. The schedule benefits of working in thirds far outweighed the cost of purchasing only half of the SSP and doing the work in halves.

Trestle and Cofferdam Construction The cofferdam utilized hot-rolled, ASTM A572, Grade 50 (Fy = 50 kips per square inch), Z-section, steel sheet piling, approximately 45 feet in length, with a minimum required section modulus of 42.6 cubic inches per linear foot of wall. The two rows of parallel SSP were spaced at approximately 15.5 feet center to center. This wall spacing was chosen to correspond with an even number of sheet piles which formed the bulkheads between individual cofferdam segments. As the SSP was being installed, W21X101 cap beams were installed with their webs placed horizontally onto the top of the SSP walls. A W21X101 has an inside flange to flange dimension of 19.8 inches which allowed it to fit over and cap the 16.77 inch SSP height. The cap beams were then welded to the SSP. Centered over the SSP and sitting on the cap beam webs were HP14X89 deck beams (Figures 4 and 5). These deck beams served as the supports for the 5 feet wide by 1 foot thick by 28 feet long timber deck mats which provided the equipment access, or trestle, along the top of the cofferdam.

Figure 4. Cap beam and deck beam welded to top of sheet pile (welding and timber deck mat not shown)

Figure 5. Cap beam and deck beam with 28 feet long timber deck mat trestle

HP10X42 braces were installed across the excavation at Elevation +4.0 between the W21X101 cap beams to maintain deck beam alignment and provide additional top support for the SSP (Figure 6). These HP10X42 braces were installed at 17 to 22 feet on center to allow placement, between the braces, of the 20 feet long pipe sections.

Figure 6. Typical deck system detail As the SSP installation extended southward across the Back River, the timber deck mats were installed and attached to the HP14X89 deck beams with 5/8 inch diameter, threaded studs welded to the deck beams. Plates and nuts, countersunk below the top of the mats, held the deck mats in place. The 5 feet wide by 1 foot thick by 28 feet long timber deck mats served as the working platform for the entire project. The deck mat connection allowed for easy removal, as required, for pipe installation and allowed equipment to work both ends of a segment concurrently. The timber mat work deck was designed for AASHTO HS20-44 truck loading which is equivalent to a three-axle truck having a maximum 24,000 pound axle design load for timber structures. Vehicles were restricted from traveling on the portions of the timber deck which cantilevered beyond the SSP deck beams. The 6.25 feet long, cantilevered portions of the timber deck were designed for a miscellaneous construction load of 300 pounds per square foot. The crane load was supported directly over the SSP walls. The SSP was designed to resist this vertical load in friction only. Additional, vertical load resistance due to end bearing was conservatively ignored. The ability of the SSP to support the crane and deck loads was verified by counting driving blows per foot of SSP penetration and by use of dynamic pile driving formulae. Continuous, 8 inch by 8 inch, timber curbs were installed longitudinally along the

deck as guides to maintain the cranes, trucks, and other equipment in their proper position (Figures 7 and 8).

Figure 7. Manitowoc Model 777 crane

Figure 8. Phase 1 construction photo A 150 ton capacity, Manitowoc Model 555 crawler crane sitting on the timber deck was used to drive the SSP, set braces, and place deck mats. The SSP was set into position and partially driven with the ICE Model 22-30 vibratory hammer. The ICE Model I-19 diesel hammer was then used to drive to the sheets to their final length (Figure 9).

Figure 9. Driving steel sheet piling with the vibratory pile hammer Roughly at the two-thirds point of the 1700 feet long cofferdam, additional steel sheet piling was driven in the EW direction to form a cofferdam bulkhead which served as the starting point for the pipe installation (Figure 10). Twenty linear feet away, north toward the shoreline, a second SSP bulkhead was installed and, at an additional 60 linear feet, the first segment bulkhead was installed. Excavation in the starter cofferdam, between the first two bulkheads, was initiated using a CAT 345 hydraulic excavator, again working from the timber deck mat system. These initial excavation spoils where trucked from the site.

Figure 10. North side, 20 feet long, initial cofferdam (at right end) with pipe installation taking place in a 60 feet cofferdam segment When the excavation reached approximately 10 feet deep, a horizontal W24X162 wale was installed longitudinally against each SSP wall at Elevation -5.0. Next, W10X68 cross braces were installed at approximately 17 to 22 feet on center to allow for pipe installation (Figure 11).

Excavation continued within the bulkhead to Elevation -16.5. To vertically support each 20 feet pipe section, one double HP12X53 pile bent was driven to refusal.

Figure 11. Typical cofferdam section Pipe Installation The initial pipe section extended into the 20 feet long starter cofferdam from the first 60 feet long northern bulkhead. A reinforced concrete collar was poured around the pipe and against the second SSP bulkhead which then remained in place for the duration of the pipe installation. The far end of the first pipe section, within the starter bulkhead, was then plugged. The pipe plug provided a seal to keep water out of the first 1200 feet long cofferdam as the pipe was installed toward the north shore. The second SSP bulkhead with the concrete collar provided a seal to keep water out of the remaining 500 feet long cofferdam as the pipe was installed toward the south shore (Figure 12).

Figure 12. Initial cofferdam staging Three lengths of pipe were placed within the first 60 foot long cofferdam segment and 20 feet long starter segment. This placed the northernmost end of the initial pipe run approximately 8 feet from the northern bulkhead of the first 60 feet long cofferdam segment. Each 20 feet long pipe section weighed 25,000 pounds and was supported by a single, steel, pipe cradle attached to the HP12X53 bearing pile bent (Figure 13). The bearing piles were driven and pipe was placed using the Manitowoc 555 crawler crane sitting on the trestle deck mats.

Figure 13. Pipe cradle and pile bent detail Using the pipe bent bearing piles as soldier beams, the area just south of the northern bulkhead was sealed around and over the pipe

with 4 inch nominal thickness, timber lagging boards. A steel wale and two, removable pipe braces were installed to support the HP12X53 bent piles which were oriented in their weaker, y-direction (Figure 14). The pipe braces were wedged between the HP12X53 piles, which were to remain, and the SSP, which were to be removed. Thus, the pipe braces could not be welded to either the SSP or to the HP12X53 bent piles. This brace connection problem was resolved by using 4 inch diameter pipes as the braces and placing them in short pipe sleeves welded to the HP12X53 piles and the SSP.

Figure 14. Pile bent pipe brace detail This connection allowed the 4 inch pipe braces to pull free without pulling against the permanent HP12X53 piles when the temporary SSP was eventually removed. After the timber lagging and a pipe braces were installed at the bent near the 60 feet bulkhead, the installed pipe sections were backfilled using material being excavated from the next, consecutive, 60 foot, cofferdam segment (Figure 15).

Figure 15. Sixty feet long cofferdam segment with 54 inch diameter pipe installed This leapfrogging process of using the excavated material from one 60 feet long cofferdam segment to immediately backfill the previous 60 feet long segment eliminated the need to haul, stockpile, and then return soft, saturated, excavation spoils from each cofferdam segment. This procedure was repeated and progressed northward for one 60 feet long cofferdam segment and 3 pipe sections at a time until one-third of the required pipe was installed and backfilled. At that time, the first third SSP was removed and trucked to the south shore of the river. While pipe installation continued in the second third at the north shore, work for the remaining third, approximately 500 feet long, then began from the south shore and proceeded north toward the initial 20 foot bulkhead and the first-laid pipe section in the middle of the Back River (Figure 16). The SSP, upper bracing, and decking were installed using a 200 ton capacity, Manitowoc Model 777 crawler crane, again operating from the timber deck. Excavation, lower wale and brace installation, pipe placement, and backfill then progressed

southward toward the south shoreline. The SSP, deck beams, deck mats, wales, and braces were reused for the second phase cofferdam and trestle and, at completion of the river crossing, were reclaimed for use on other projects.

Figure 16. North side steel sheet pile removal for reuse on the south side Conclusion AIMD proposed and implemented an alternate method of access and construction for a 1700 linear feet crossing of the Back River estuary in Baltimore County, Maryland and for the installation of a 54 inch diameter force main as part of a 13,200 linear feet project for Stemmers Run Relief Wastewater Force Main project. This alternate eliminated a separate, parallel work trestle for the entire length of the estuary crossing and made use of the proposed cofferdam for the support of the timber work deck. This plan minimized disturbance in this important estuary of the Chesapeake Bay. As well, it improved access to the work by providing a wider work deck which improved the contractor’s visibility, productivity, and job safety. The Stemmers Run Relief Wastewater Force Main project utilized over 2,700 tons of steel sheet piling, approximately 640 tons of steel wale beams and braces, and over 280 tons of H-Piles for the pipe support pile bents. Cofferdam, trestle, and the installation of the 54 inch diameter force main river crossing were completed in March of 2008. The final phase of the project is in progress including the non-tidal wetland restoration and tidal wetland mitigation.

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