C150331 Feasibility Study - Laserfiche WebLink

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Ken Nelson December 20, 2011 Page 9 rehabilitation alternatives included slip-lined pipe, precast concrete box culvert and structural tunnel liner plate, each with grout backpacking. The preliminary analysis did not include engineering assistance, but did represent a comprehensive range of construction methods. Based on input from local Contractors related to construction Costs, and input froju affected property owners above the tunnel alignment, horizontal tunneling along a new alignment was determined unfeasible not only because of significantly higher construction and administrative costs, but also due to major complications with obtaining new easement(s) for the tunnel realignment. Alternatives for removing the spalled debris and for tunnel rehabilitation need to consider methods of construction that protect the health and safety of personnel. Based on the apparent instability, physical entry into the tunnel must either be avoided, or conducted with structural tunnel reinforcement. A viable preliminary alternative method to reach the debris included installation of fabricated Corrugated metal liner plate beginning at the west portal and proceeding toward the debris in approximate 18" segments. The void between tunnel and Structural plate is backpacked with grout as the plate is installed and allowed to cure. A bulkhead at the open end of pipe would be needed to prevent grout from spilling into the working at-ca. This liner system is considered safe ,and stable to work within, so long as grout installation proceeds immediately behind steel plate installation, for tunnel depths Up to 80-feet. Installatiou at additional depth requires geotechnical engineering with respect to mechanics of the overlying soils, discussed below. Based on inspection of the debris and roof span faikire by NDIC maintenance personnel, it may be possible to remove fallen material with a dragline attached to a plow/bucket mechanism without personnel access to the debris site. Structural strength requirements of tunnel liner, and/or slip-line pipe, relate to the soil mechanics of the tunnel overburden and probable soil pressure/pipe loading. The critical issues are whether the tunnel is excavated through solid, formational material or less stable geomorphology. Since a spall did occur, it is understood that at least a portion of the tunnel is not in solid rock and is less stable. It is this iloti-foriliatiolial.niateri,,iI that is the basis for liner design criteria, Overburden is then either categorized as "passive", "at rest", or ",,active". Passive earth pressure generally describes a worst. case scenario of greatest loading on a tunnel liner/pipe, ",here the overlying soil has no lung-term structural integrity, and the full weight of overburden is the basis for the conduit strength requirements. In this event, the soil column above the tunnel would impose thernaximum vertical pfisni load on the tunnel insert, Active earth pressure generally describes a best case scenario of the lowest loading, where the overlying soil has bridging or arching characteristics that minimize earth forces on the tunnel conduit. Depending on soil reactions, active earth pressure may be about 50% of passive earth pressure, At rest earth pressure generally describes conditions between the two extremes. Project geotechnical engineers visually inspected the ground surface around the tunnel portals, and the exposed hillsides, to ascertain whether overlying soils maybe categorized as passive, at rest, or active. According to the geotechnical engineers' assessment of the visual and anecdotal evidence, portions of the tunnel may be stable, but the history of rockfall strongly indicates that the tunnel structure is weathered, and less stable material. Consequently, the theoretical maximum vertical load of passive soil mechanics is the recommended design criteria for the entire length of tunnel.