Lock Access Road Bridge

The last picture is an old Wilson Lock Dam bridge a double leaf bascule bridge before the single leaf bridge replaced it.

TVA: "The Proposed Decision and Need

Tennessee Valley Authority (TVA) and the U.S. Army Corps of Engineers (USACE) jointly

propose to construct a fixed bridge over the lower end of the auxiliary lock at Wilson Dam

(Figure 1) to provide safe, reliable access to the main lock. The proposed fixed bridge

would replace the existing bascule bridge to access the main lock chamber and lock

operations building (Figure 2). Due to the age and condition of the bascule bridge, TVA is

concerned that the continuing operation and use of the bascule bridge would be unsafe and

finds the bridge to be unreliable. The bascule bridge currently continues to operate, but its

long-term reliability and safety are in question.

Critical components such as the trunnion bolts, counterweight attachment bolts, and gear

track anchor bolts have active corrosion and section loss (gear wear). When the bridge is

operated, binding occurs in the gear mechanism, which is designed to slide freely up and

down. It is suspected that the binding is a result of deterioration of the gear assembly.

In addition, the opening of the bascule bridge span is not functioning properly. The bridge

is designed to open to 76 degrees and 15 minutes, but due to its condition, it currently

opens only to approximately 58 degrees. The opened bridge span at either angle does not

allow adequate clearance for some oversized cargo or towboats.

Furthermore, the bascule bridge is considered unsafe for heavy loads because it is not

designed to carry the heavy equipment that must pass across it to support construction

activity at the main lock. For example, because the bridge only has toe locks and lacks

heel locks to secure both ends, when the bridge is in a lowered position, heavy equipment

has to be moved onto the heel of the bridge to prevent toe uplift as heavy equipment

passes across the bridge (Figure 3).

The existing bascule bridge would remain in place and continue to be available for main

lock area access until the new bridge is completed. The new bridge would remove traffic

from the bascule bridge, which currently provides the only access to the main lock area.

Background

The Tennessee River system is managed through a series of dams and navigation locks

owned by the U.S. government and operated by TVA and the USACE. In accordance with

the TVA Act, TVA is entrusted with the possession, operation, and control of the dams and

all associated buildings, machinery, and lands, with the exception of the navigation locks,

which are entrusted to the USACE.

2

Wilson Dam, completed in 1925, is over 100 feet tall and nearly 0.75 mile in length (Figure

4). Wilson Dam is on the National Register of Historic Places (NRHP), and it has been

officially recognized by the U.S. government for its historical significance. The dam was

designated a National Historic Landmark (NHL) on November 13, 1966. The largest mass

concrete lock and dam of its day in the U.S., it was the first federal hydroelectric project as

well as the first USACE multipurpose effort. TVA acquired possession of the dam when the

agency was created in 1933. As part of Wilson Dam, a single-leaf bascule bridge was

constructed that spanned the original lock chamber and permitted barge traffic to pass

through the chamber when in the raised position (Figure 5). The original lock chamber,

which now serves as the auxiliary lock, was the only lock until the main lock was completed

in 1959. The bascule bridge also serves as the only access to the main lock chamber and

lock operations building.

The bascule bridge is used daily for access to the lock area. In addition, during an outage

of the main chamber, it is used by vehicles carrying heavy equipment to access the main

lock for maintenance activities. Typically, the main lock undergoes routine maintenance

every three to five years, which requires the shutdown of the main lock and the use of the

auxiliary lock. When the auxiliary lock is used, the bascule bridge must be raised to permit

barges through its upper lock (Figure 6).

TVA has concerns that the bridge is unsafe and unreliable. Critical components such as

the trunnion bolts, counterweight attachment bolts, and gear track anchor bolts have active

corrosion and gear wear (Figure 7). In addition, a lack of heel locks in the original design

causes the machinery to resist live load (i.e., moving objects such as vehicles or

equipment) and induces significant stresses in them, which is an undesirable condition.

This lack of heel locks also causes uplift at the toe under a heavy load at the heel. If by

human or mechanical error the lock pins were not to engage when traffic passed over the

bridge, a catastrophic failure could occur. If a bridge failure were to occur early into a main

chamber outage, it has been estimated that the auxiliary and main locks could both likely

be closed for two months. Because heavy load vehicles using the bascule bridge could

contribute to a potential failure, discontinuing use of the bridge for access to the main lock

(once the new bridge is in place) would reduce the likelihood of a failure."