Serious Headworks Design Flaw - Inducing Shear Cracking in Gate Piers - Risks Stresses in Trunnion column
An overlooked design detail in footing loading upon the substructure noted "competent rock" at the FCO Headworks and Spillway Bridge "Lift" column reveals a "Diagonal Cracking" or "Shear Cracking" Flaw mechanism. This "Flaw" risks a "Failing" of Gates 1 and 8 Pier supports via a potential shifting of the Trunnion pin axis alignment to the Radial Gate Skin Plate radius. A "shift" in the axis alignment is amplified by the radial gate radius length. Thus a small "shift" could "bind" the gate. Anchor tendons, below within the pier columns, traverse across this shear cracking "plane" centered at the corner of the spillway bridge support notch. Thus, the anchor tendons will absorb "Shear Cracking" forces transferred from crack expansion. This would create an additional load to the Anchor Tendons beyond the original design loads of Radial Trunnion Gate Hydrostatic pressures in flood release operation.
The overlooked design detail was the "footing" settling forces upon the "competent rock". The FCO Gate Structures (Piers, Headworks, Footing) are all secured with reinforcement (rebar). Two FCO Gate Structures were built with a contraction seam where these meet. Gates 1-4 form one FCO reinforced Monolith structure, and Gates 5-8 form the second reinforced Monolith structure. Both have their own individual large reinforced footings constructed of thick concrete as further anchorage. However, the spillway bridge "lift" columns (5ft+ thick) have their own footings separate from the FCO reinforced Monolith structures. Blueprints do not show any design of footings wider than the original width of the Bridge Abutment (5ft thick, thus 5ft wide in drawings). Differences in settlement of the Bridge Abutment "lift" column vs the FCO Gate structure from - Phreatic pressure, "rock competency", and heavy bridge crossing loadings, all could influence the forces concentrated into the corner of the notch in the Pier. The evidence of the large diagonal cracks originating at this corner in the notch confirms the presence of a "shear force". This is the exact location that this design flaw would manifest if the Bridge Abutment were to "settle" deeper than the FCO structure.
DSOD Inspection reports confirm that both end piers have the same "Shear Cracking" problem at the same corner location(s). DSOD 1998 Inspection report also notes that this dual "shear cracking" condition has been noted in prior inspections. Thus, this "flaw" cracking result has been developing for many years (& prior to 1998). This means that for 19 years this issue either was not considered a "structural risk" to the Headworks by DWR, or that the issue may not have been understood, or ?.
The immediate "arresting" of this Shear Cracking would require the shutting down of the spillway bridge and removing the corner load forces on the end pier corner notches. With all of the extreme tonnage of emergency repair work loads across the spillway bridge, this likely has risked further aggravation of the shear cracking flexing on these existing cracks. With nothing but red paint as a mechanism in addressing these "cracks", and all of this heavy stress loading placed upon this "flaw", is there proper engineering judgement being applied? DWR has placed metal plates across these end junctions, but should they even be operating this bridge knowing that they are risking further structural damage potential? (including stress loading the end pier Anchor tendons).
"Flaw" in headworks design. Different settlements of the Bridge Abutment concrete support column between the FCO Structure will create a focus force in the corner notch in the 5ft thick Gate end Piers 1 & 10. This is where DSOD inspectors are watching "Shear Cracks" grow."
DSOD Inspectors documenting the "Shear Cracking" at the corner notch of both end piers. Cracks have been present and growing for 19+ years. Why hasn't this "flaw" been figured out and addressed? (instead of using red paint).
After an information "tip".. I've been moderately puzzled over the large block of concrete at the end of the spillway chute blocks. In going back to re-analyze the "large concrete" block -that was pushed down to the end of the spillway- there looks to be evidence that this large block may be a 6ft deep "void" fill.
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.
