C. There is a risk of losing part of the dam structure, or of causing severe cracking and displacements in the dam structural elements by driving cofferdam piles and by dewatering within the cofferdam, both of which will be required for the new lock construction. Raymond Technical Facilities, Inc. (RayTech) studied this problem and concludes the cofferdamming could possibly be accomplished without seriously damaging the existing structure if the foundation below the dam were chemically grouted to solidify the soil, and any voids below the foundation slab were filled with cement grout" RayTech states: "There has been a history of structural distress, movement, piping, and scouring and/or settlement of underlying soil strata. We believe there is a possibility that the cofferdamming can be accomplished by this method (noted above) without seriously damaging the existing structure..." In summary, we believe there is some risk of losing part of the dam by building a new lock through it. The dam will undergo greater loadings than ever before because of the procedures required in cofferdamming, dewatering, and removing part of the dam itself to make room for the new lock. d. Constructing a lock through the middle of the dam will introduce the following problems, which are understood to be of particular concern to the Corps and should be addressed in greater detail: 2. The dam continuity will be broken. Deep excavations and dewatering, necessary for new lock construction, will add to the risk of dam failure. See Item c. above. Locating the lock at or near mid-stream where the river currents are greatest is contrary to usual Corps of Engineers practice and may increase navigation risks for tows approaching the lock. Feasibility Study, Cofferdamming Existing Locks and Dam No. 26, e. 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 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 on 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 Locks 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. 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 closed (Alternate A'), and the existing 360-ft lock will be closed in either case. The 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 ever 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 |