If true, this provides an answer as to how a couple of sidewall drain outlets could have been completely plugged. The blowout failure area is in a location that has been revealed to have highly weathered rock. Upthread post (copied here below for a "combined post") identified a "diagonal" percolation seam of water near the blowout failure region of the spillway. This water presence infers a history of a spring like water presence. Thus, this would note that there could be a highly erodible area of "weathered rock".
Upthread posts have noted "fixing" of the spillway in the areas where 2 sidewall drains were not functioning. The BOC has also verified that "washable" material was underneath the concrete slabs where piping water flow could wash and erode. If "escape" channels of this "pressurized" washable erosion effect developed, to where material was being eroded and carried away underneath the sidewall berms, then large voids could develop that would not be detectable from a surface observation. The most vulnerable locations to this "effect" would be the highly erodible weathered rock near the seam of water was in the early construction postcard photo.
The information "tip" noted that the larger voiding was near 6 feet deep. If this size of void or even a shallower size void occurred underneath a drain, there is the possibility that the "coupled" drains could fall into the void. A drain coupling would then be fully "open" at a disconnection point. Subsequent "void repair" in pumping in grout/concrete mix could likely flow into the "open" drain pipe and including flowing into the longitudinal collector drain. This certainly could plug a drain if the internal pipe voiding fill reached the last longitudinal drain coupling junction. (note: the BOC had noted of "holes" cut into the slabs for repairs - perhaps these "holes" were to provide a volume of concrete emplacement such as to fill a large void).
Another possibility is if tree roots assisted in this "voiding" disruption of the drain pipe disconnect. Grout "void fill" repairs could enter into the drain pipe and end up in the longitudinal drain & seal it off.
The key tip to this "large block" of concrete is the clean seam of the slab line and the bonding concrete below. A somewhat good bond would be necessary for the slab layer & under material formed block to survive a push near 1000+ feet down to the end chute blocks. BOC reports and early design photographs (below) reveal that a "grade layer" of material was placed before the drains were laid and then the concrete slabs were poured over. Only by a pressurized "cleaning" or washing of the underside of this material below the slab could a sufficient bond of a "void fill" repair attachment form. The irregularity of the slab underside surface would also affect the resulting bond.
Original design specifications noted that concrete fill to "grade level" was to be done in areas of less than competent rock. However, the photographs do not reveal any surface areas that are "concrete" in sections - only a uniform "grade fill" of a material that has percolation water filtering within. The photographic evidence of a number of view angles of this large block also notes the absence of any anchor rebar that the original design specs noted - IF this was a "fill to grade" concrete layer. Further examination of this large block reveals that the fractured face is slightly curved. Thus this likely isn't a section that was a junction directly abutting a sidewall seam. DSOD Inspection report did note a "drum" sounding in the spillway during inspection. Apparently, a method of dragging chains were used to "sound" the surface of the slab. I'll have to dig back to find which year and date that DSOD "drum" sounding was reported. But, this reveals that there has been an awareness of voiding to the degree that should have triggered further investigations - i.e. rather than "pumping" until the slab "gas tank" is "full".
Possible 6ft "void" repair fill from "blowout" area
prior upthread post:
Subsurface Seam at Blowout Failure location gives Strong Clue
A 1967 postcard image reveals a natural "Water Percolation Seam" at/near the Blowout Failure location. (note: the postcard was digitally enhanced to filter the "texture dots" to provide this clearer forensic image).
As this photograph was taken prior to the pour of the spillway concrete slabs, the image reveals the "grade" construction of which the slabs were to be emplaced upon. This forensic "seam" presence provides geologic insight, especially regarding the question of "Where's the Missing Water?" from the spillway drains in this pre-blowout failure location. The photograph also provides insight into how "un-captured" waterflow in the sub-par drain design could "wash" a void layer between the bedrock and the concrete slab pour. The spillway "chute" design has embankment fill. There were no signs of embankment erosion near this failure area. The only sign was a preference of large tree growth (roots + growth = subsurface water). The "Seam" answers the question of a "deeper" subsurface flow capability. Over time, the erosion of this flow could increase the underslab voiding.
Given these conditions, the deep waterflow + erosion could have created substantial voids (note: persistent spillway "missing water" at equal levels of the "working drains" observed flow rates = a high capacity deep flow "seam" likely was created in time; exploiting this location's natural formation). Each time the spillway was operated, it eroded and enlarged a void area. Until a higher structural stress condition was applied - such as a higher flow rate - did the right conditions of the voiding stress the failed slab to its limits. Part of this equation is the other dynamic stress condition of hydraulic jacking (one is up the other is down).
Subsurface Seam at Blowout Failure location. Photograph provides insight into how "un-captured" waterflow in the sub-par drain design could "wash" a void layer between the bedrock and the concrete slab pour.
