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Four dam gate bays will be removed to make room for the lock. This will cause a more constricted water flow which may not be acceptable. During high river stages there will be increased swellhead (piling up of water) at the dam. This could have adverse effects on both upriver developments and, possibly, the dam and stilling basin design. Four more gate bays could be installed, thereby maintaining the present swellhead. However, neither the Illinois DOT nor the TAMS-Adjusted Illinois DOT plans provide for this.

The hydraulics, operating, and maintenance features of the sheet-pile cellular lock structure vs the conventional concrete lock should be studied in greater detail. Although sheet pile locks have been built at other locations, the maximum water differential of 24-ft at Locks and Dam 26 is about twice that at these other locations on the Extensive model testing will thus be necessary to

inland waterways. show that the proposed sheet pile lock filling and emptying system will work satisfactorily. Also, the average life-expectancy of a sheet pile structure is generally shorter than a concrete structure, and this should be considered in deciding on the structure type.

f. The lock and dam structure will be suspect, regardless of the rehabilitation work that is done. The extent of the uncertainty of stability (and therefore the risks) will depend upon the type and extent of rehabilitation work that is accomplished.

III COMPARISON OF ALTERNATIVES PROVIDING

THE SAME LOCKAGE CAPACITY

As noted in the Introduction, numerous alternate plans have

been developed for repairing and replacing Locks and Dam 26. This

chapter presents direct comparisons of alternatives providing the same

lockage capacity.

A. BASIS OF COMPARISONS

The following ground rules and assumptions were used in developing the plans which we consider satisfactory alternates based o sound engineering principles.

1. The first set of comparisons (Alternates A, B, and C, Table II) is based on one 1,200-ft-long and one 600-ft-long lock. This will match the lock sizes of Locks and Dam 27 downriver and permit full use of the existing upriver capacity. The second set of comparisons (Alternates A', B', and C', Table III) is based on providing only one new 1,200-ft-long lock. This corresponds with the minimum new lockage capacity deemed necessary by the Corps, and recommended by the Chief of Engineers.

2. The structures, whether new or rehabilitated, should have an approximately 50-year life expectancy without major structural maintenance or repairs. We believe this dictates using concrete locks, and this is the basis for our cost estimates. Sheet-pile cellular locks are unproven for the design and operating conditions as encountered at Looks and Dam 26, and constructing the more conventional concrete locks is especially preferable at this strategic site just below the confluence of the Illinois and Mississippi Rivers.

3. For a suitable 50-year rehabilitation project, the dam structure should be upgraded to a design level compatible with a new lock through or alongside the dam. Accordingly, a new batter pile foundation resisting 100 percent of the unbalanced hydraulic load should be provided. This, and only this, will meet present-day design standards, and the Corps' plan calling for additional horizontal restraint in the amount of 100 percent of the unbalanced hydraulic load is used for costing herein. On this basis, the WRA, Illinois DOT, TAMS-adjusted WRA, and TAMSadjusted Illinois DOT plans are deficient and were not considered further in the present study.

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These alternates are nearly the same as one of the rehabilitation alternates considered by the Corps and presented in the Board of Engineers for Rivers and Harbors report of February 1976. The lock will be located on the Missouri bank just downstream from the Clark highway bridge. The plan provides for a 250-ft-wide approach channel to the new 1200-ft concrete lock. A new lift span will be provided for the Alton railroad bridge crossing. The dam will be rehabilitated by cofferdamming, constructing a new batter-pile-supported apron, repairing gates and machinery, and filling scour holes downstream from the dam. existing 600-ft lock will either be rehabilitated (Alternate A) or

The

closed (Alternate A'), and the existing 360-ft lock will be closed in

either case.

A rehabilitated facility will still be suspect to some degree, and there will be risks and uncertainties in rehabilitating the old structure that will not be present in constructing a new facility. These are somewhat intangible points, but they should be considered in deciding whether to rehabilitate the old structure or replace it with a new one. Also, for Alternate A, there will be greater operating costs with two locks along both banks of the river because of duplicate manning requirements and having to maintain two approach-channels. Alternates B and B' New 1200-Ft Lock in the Existing

2.

Dam

This plan--constructing a new lock in the existing dam--is similar to the Corps' 1968 rehabilitation alternative, which was rejected in favor of a replacement structure. It is also similar to the Illinois DOT plan, but calls for a concrete lock and considerable rehabilitation work. The plan provides for removing part of the dam to allow for constructing the new 1200-ft concrete lock, rehabilitating the remaining dam by cofferdamming, constructing a new batter-pile-supported apron, repairing gates and machinery, and filling scour holes downstream from the dam. The existing 600-ft lock will be rehabilitated (Alternate B) or abandoned (Alternate B').

Constructing a new 1200-ft lock through the dam has even more uncertainties and greater risks than Alternate A--constructing a new lock on the Missouri bank. Deep excavations, dewatering during construction, pile driving, and removing part of the dam all add to the risk of dam failure. Swellhead at the dam will be increased because four gates

will be removed to install the new lock. Four additional tainter gates

e thus provided for, two to be located at the 360-ft lock and two at he Missouri shore end of the dam for Alternate B, or at the site of the 00-ft lock for Alternate B'. This scheme is deemed feasible with roper construction procedures but, again, these additional risks and ncertainties should be considered in evaluating rehabilitation vs eplacement.

3.

Alternates C and C' New 1200-Ft (and 600-Ft) Lock(s) and
Dam Two Miles Downstream

The Corps recommends constructing a new dam and a new 1200-ft

ong lock about two miles downriver from the present dam at an estimated cost of $391,000,000, based upon January 1976 prices. Alternate C, Table II, calls for an additional 600-ft lock with all other provisions the same as the Corp-recommended plan for a single 1,200-ft replacement structure, Alternate C', Table III.

Problems, risks, and uncertainties will be minimized with this plan. The new structure will fulfill all the requirements for presentday design standards, eliminating the continuing problems with the existing locks and dam.

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1.

Provision for a 1200 and 600-Ft Lock

A summary of estimated costs for the satisfactory alternatives, each providing one 1200-ft and one 600-ft lock, as discussed in subsections above is given in Table II. All costs are based on January 1976 price levels.

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