Deep Excavation Support Systems Speed Plant Construction

As part of constructing the recently commissioned We Energies’ Oak Creek Power Plant Elm Road units, four remarkable below-ground structures were built. Each unique structure required creative designs and meticulous construction techniques to meet the project’s distinctive requirements.

Bechtel Power Corp. (Bechtel) is currently completing a contract to engineer, procure material for, and construct (EPC) the We Energies’ 1,135-MW Oak Creek Power Plant Expansion Project (OCPP, also known as the Elm Road Generating Station). Construction started in 2005 on the two-unit plant located on Wisconsin’s western shore of Lake Michigan. OCPP is an expansion of the existing Oak Creek coal-fueled power plant. For this achievement, POWER recognizes OCPP as a 2010 coal-fired Top Plant. A companion article (p. 44) provides more project-specific details.

One of the unique challenges encountered by Bechtel on this project was the construction of several complex below-ground structures. In this article, we describe four of these structures and discuss why a particular earth retention system was selected for each. We believe that there were lessons we learned during the design and construction of this project that will be useful to others involved in similar projects.

Four Structures, Four Solutions

OCPP required construction of a series of three deep excavations related to the water supply system and one excavation related to the delivery of coal to the plant. Each project required unique engineering and construction approaches, especially where the selection of the earth retention support systems was concerned. As always, economic and schedule considerations played a significant role in the final selection. The four structures discussed are:

  • A series of three vertical shafts was constructed for a rock tunnel that extended out approximately 2.7 kilometers (km) under Lake Michigan. Each shaft was constructed by building and sinking a precast concrete section to the top of rock at a depth of about 30 m. Two of the three shafts were constructed on land, while the third was constructed over water.
  • An intake water pump house was constructed using slurry wall techniques to a depth of approximately 20 m with multiple levels of tie-back anchors.
  • A multiple-layered braced sheet pile system was used to construct a gate structure in Lake Michigan as part of the intake water scheme to shut off water flow to the existing plant intake structure.
  • A rotary railroad car unloader was installed that incorporated the use of two soldier pile and lagging walls, the upper wall having a single level of tiebacks while the lower wall had multiple tieback levels. The total depth of this structure was also about 20 m. The upper wall was added during construction to protect an operating rail line that was jeopardized by the originally planned open excavation.

All the structures were successfully completed and are now in operation.

Tunnel Shaft Construction

POWER reviewed the design aspects of OCCP’s upgraded cooling water system in “Oak Creek Power Plant Upgrades Cooling Water System,” March 2009 (available from POWER’ s online archives at https://www.powermag.com). In summary, We Energies elected to upgrade the circulating water system for the entire plant to comply with Section 316(b) of the Clean Water Act by constructing a new common system to serve the four existing units plus the two new 615-MW OCCP units.

However, not discussed in that article were the engineering and construction challenges of drilling a new 8.25-m-diameter intake tunnel system that projects approximately 2.4 km offshore to collect cold water from near the bottom of Lake Michigan. This new structure replaced both the existing shoreline intake and supply water for the new plant.

The first of the three shafts constructed as part of the tunnel water supply system was the construction shaft. This shaft, which was only used during construction, was built to allow the tunneling equipment to be lowered down to the tunnel level for drilling the tunnel and for removal of the spoil material.

Construction of the tunnel system—which included the shafts, tunnel, dike wall, and intake pump house—was part of Bechtel’s EPC contract for constructing OCCP. The method chosen for building the initial construction shaft was to pour a 10-m-diameter by 6-m-high reinforced concrete caisson above ground at the location of the shaft. The caisson was designed to withstand the design lateral earth pressures and hydrostatic pressures present at the project site. The upper soils at the site consisted mainly of very stiff clays with some sand lenses.

Once the initial caisson was poured and cured, excavation was started inside the caisson shaft that allowed the caisson to sink progressively under its own weight as the excavation advanced. Upon reaching a depth of about 6 m, excavation work was halted and another 6-m-high section of caisson was added on top of the first section. This process continued until the concrete caisson was sitting on top of the underlying rock at a depth of approximately 22 m to 25 m below grade (Figure 1). Once the caisson was positioned on the rock, the shaft was extended down to the tunnel elevation using drill and blast techniques. Rock anchors and wire mesh were used to support the shaft construction through the rock portion of the shaft.

1. Creative caisson installation. This caisson was designed to extend compressed air, water, and power lines down to the bottom for construction workers. The caisson was built at ground level and then workers dug out underneath and let gravity sink it into the ground. This process was repeated two more times, until the bottom caisson literally hit rock bottom. Courtesy: Bechtel Power Corp.

The second shaft constructed into the tunnel was for water supply to the existing power plant. Construction of this shaft was performed within the existing intake channel in open water. As a result, it was first necessary to construct a sheet pile cofferdam by driving steel sheet piles into the underlying channel bottom (Figure 2).

2. Construction of the sheet pile cofferdam for shaft two. Courtesy: Bechtel Power Corp.

Once the cofferdam built with steel sheet pilings was in place, the area outside of the sheet piling was backfilled within the outer sheet piling structure to allow for ease of construction (Figure 3). This fill would later be removed as part of the final design of the existing plant’s modified intake scheme. The cofferdam sheet pile structure adjacent to the shaft location (above the shaft in Figure 3) was for construction of a new lift station for the existing plant, and the cofferdam below the shaft location was for construction of the dike wall, which is discussed later.

3. Backfilling around the cofferdam of shaft two. Courtesy: Bechtel Power Corp.

After the area was backfilled, the cofferdam was dewatered down to the bottom of the channel to allow for construction of the first section of the caisson. After this section was constructed in place, the shaft was constructed in a similar manner to the original construction shaft by excavating inside the shaft and allowing the caisson to sink until solid rock was reached. Unlike the initial shaft, uneven rock made sinking the shaft more difficult than planned. Some additional excavation work at the bottom of the shaft and support of the excavation using ring beams and vertical wood lagging was also required.

As this shaft was part of permanent construction on the site, the entire shaft from the interface with the tunnel to the top was lined with cast-in-place concrete (Figure 4). The original caisson was used as the outside formwork for the permanent liner in the shaft to allow for a uniform inside shaft diameter. After the shaft was completed, sheet piling for the cofferdam was removed.

4. A concrete lining is placed inside the second concrete shaft. Courtesy: Bechtel Power Corp.

The last of the three shafts was constructed at the base on the intake pump house for the new plant. Construction of the pump house is discussed later. At this location, because the shaft only needed to extend approximately 4 m to 5 m from the bottom of the structure to the top of the rock, the shaft was constructed using a series of ring beams and vertical wood lagging, similar to that used at the base of the second shaft. Figure 5 shows the base of the pump house and the system used to support the shaft as the concrete formwork is being placed for the permanent shaft. Figure 6 shows the final
completed shaft.

5. The ring beam and lagging are added to the third shaft. Courtesy: Bechtel Power Corp.

6. A new pump house will be constructed later on top of the finished third shaft. Courtesy: Bechtel Power Corp.

Dike Wall Construction

A dike wall was constructed across the width of the existing intake channel to shut off the flow of water into the plant along the shoreline and enable cooling water to come through the newly constructed tunnel and connecting shafts. The dike wall incorporated gate structures to allow for water flow in the event that the tunnel supply was temporarily lost due to frazzle ice forming around the offshore intake shafts at the bottom of Lake Michigan at the far end of the tunnel (a very remote possibility).

The dike wall was constructed as a rectangular sheet pile cofferdam (shown partly constructed at the bottom of Figure 3). Support for the dike walls consisted of four levels of removable bracing that were placed at strategic elevations to allow for safe construction. The bottom level of bracing was removed after a thickened mud mat was poured to allow for construction of the structural mat and the wall up to the level of the first construction joint.

Once the bottom section of each wall was in place, the second level of bracing was removed to allow construction to continue. This process continued until construction of the dike wall was completed. Figure 7 shows the completed cofferdam when rebar was placed for the base mat and lower sections of the wall. At that time the bottom level (fourth level) of internal bracing had already been removed after the thickened mud mat had been allowed to cure.

7. Holding back the lake. A dike wall was constructed across the width of the existing intake structure to shut off the flow of water into the plant through the original intake channel. The dike wall was constructed with four levels of bracing, starting from the highest level. Shown is the rebar base mat and final level of shoring in the cofferdam. Courtesy: Bechtel Power Corp. 

Intake Pump House

The intake pump house for the new plant was set at a depth of about 20 m below finished plant grade. The pump house is a six-sided structure that is approximately 45 m long and 39 m wide. Bechtel elected to use a concrete diaphragm slurry wall with multiple layers of tieback earth anchors to support the walls. The slurry wall construction with tiebacks allowed for an entirely open excavation. This made for easier construction of the complicated internal structure of the pump house (Figure 8).

8. Slurry walls. The slurry wall construction with tiebacks allowed for an entirely open excavation that reduced the difficulty of constructing the complicated internal structure of the pump house. Courtesy: Bechtel Power Corp.

Construction of the slurry wall consisted of first building a concrete guide wall around the entire exterior of the foundation. The wall panels were then excavated using a grab bucket and slurry, reinforcement in the wall was placed, and then the concrete was placed by “tremie method” to displace the slurry. In the tremie method, concrete is poured by inserting the tremie pipe to the bottom of the slurry. The lower end of the tremie pipe is kept immersed in the fresh concrete so that the rising concrete from the bottom displaces the slurry but without the cement content being washed out. The panels were constructed in a sequence where one panel was placed and the next panel was skipped.

The intermediate panel was poured after allowing sufficient time for the two outside panels to have initial set but while they were still green enough to enable the grab bucket to scarify the surface of the completed panels to allow for some locking of adjacent panels. Once the wall was completed, excavation inside the walls was started until the depth of the first level of anchors was reached and the 64 multi-strand anchors were installed.

Excavation then continued, with each level of anchors being grouted in place, post-tensioned, and locked off at the face of the wall. Installation of the anchors included load testing and proof testing. This sequence continued until the bottom of the excavation was reached.

Figure 8 shows four levels of tiebacks installed. The original design of the slurry wall system called for placement of five levels of anchors; however, a reevaluation made during excavation indicated that the bottom level of anchors could be eliminated by allowing the bottom concrete mat to pick up the loading that would have been transferred to the fifth level of anchors.

Rotary Car Dumper

The existing plant was originally constructed to accept coal deliveries by ship. Facilities for rail delivery, added as part of the OCCP addition, included replacement of the existing railcar unloader with a new rotary car dumper (RCD) constructed adjacent to the old unloader.

Slurry wall–type construction was originally planned for the 20-m-deep excavation for the RCD. Later, the excavation plans were modified for an open excavation of the upper, larger portion of the RCD and for use of a soldier pile and lagging wall for the lower, smaller portion of the building. The plan also assumed that groundwater could be handled using a series of sumps and pumps, due to the clayey nature of the soils.

After the initial portion of the excavation work was started, groundwater flow was found to be a bigger problem than anticipated. This was considered highly significant, because the flow could make the upper slopes unstable, and such instability could endanger the rail line that provided the sole source of coal to the existing plant. As a result, work was halted and plans were made to install both an upper soldier pile and lagging wall and a dewatering system. The soldier pile system incorporated a single level of earth anchors.

Once the upper portion of the excavation was completed and stabilized, soldier piles for the lower excavation were installed along with a second level of dewatering wells. After these installations, excavation was completed down to its base, as can be seen in Figure 9. The completed RCD is shown in Figure 10.

9. Lining up. A soldier pile wall was used for the lower excavation for the new rotary car dumper. Total excavation was about 20 m deep. Courtesy: Bechtel Power Corp.

10. Running on time. The completed rotary car dumper is ready to unload unit train coal deliveries. Courtesy: Bechtel Power Corp.


The authors appreciate the support from Bechtel project management and We Energies in the preparation of this article.

K.R. Bell, PhD, PE (krbell@bechtel.com) is chief engineer, Geotechnical & Hydraulic Engineering Services, and J.R. Davie, PhD, PE (jdavie@bechtel.com) is geotechnical engineering supervisor for Bechtel Power Corp.