8. OTHER USES FOR RCC IN DAM CONSTRUCTION
8.5. FOUNDATION REPLACEMENT
It is especially important that the foundations of hydraulic structures, including dams, are shaped in a proper manner to avoid sharp offsets that could result in stress concentrations. Foundations can be notoriously variable, often with large areas of weaker material that needs to be removed in order to expose a competent material.
8.5.2. Design considerations
To meet design requirements or to address changing field conditions, engineers will often specify the use of RCC, which lends itself well to of mass foundation replacement and to massive footings, etc, for hydraulic structures. For the following reasons, RCC is particular suitable for this purpose.
a. Strength - RCC can be designed to have ultimate strength characteristics of a “slightly-to- moderately weathered” rock foundation, with a low degree of permeability and high erosion resistance, or it can be designed to simulate much more competent rock.
b. RCC as a foundation replacement can typically be constructed without contraction joints, allowing the material to crack in an unplanned, random pattern that is not expected to introduce any inherent weaknesses.
c. Relaxed foundation treatment – RCC foundation replacements can generally be approached in a manner that is less rigorous than required for a dam foundation, with typical receiving surface treatments ranging from nothing to general preparation using light air and water blasting cleaning.
d. Lift Joints – treatment of lift joints is important in any application of RCC that is going to depend on the mass characteristic properties of the completed structure and accordingly, horizontal lift treatments should generally be similar to those applied for dams.
e. Quality Testing – due to the less-critical nature of RCC used for foundation replacement, testing of materials and placed concrete can be reduced to approximately half that expected for a new dam construction.
8.5.3. Representative projects
Olivenhain Dam – Shaping Blocks
The foundation criteria for Olivenhain Dam was “slightly weathered” granodiorite rock. An extensive geotechnical and geological field investigation programme was conducted during the planning and design phases to establish this horizon.
As shown in Figure 8.6, the foundation profile included three primary geological lineaments crossing the dam axis, with low points in the left minor valley, the central main valley, and the right minor valley. The three valleys were formed due to the local presence of weaker, weathered rock. The design required excavation of a portion of the weathered areas and replacement with large, concrete
“foundation shaping blocks”. The design optimized the dam’s lateral performance during an earthquake by minimizing differential movement between monolithic sections.The foundation shaping blocks augment and restore the structural and topographic integrity of the “saddles” in the foundation at the
“tonalite” body (in the vicinity of Sta. 23+50) on the left abutment and in the topographic low (in the vicinity of Sta. 5+00) near the right abutment.
A structural analysis established the dimensional outlines of the shaping blocks. The shaping blocks simulate the properties of a semi-infinite plane of foundation relative to the dam with the properties of a spread footing relative to the foundation. A series of parametric studies were conducted to achieve optimum stresses that minimize the volume of the blocks. The dam and optimized shaping blocks were analysed as an integrated structure using the selected MCE loads. The complete Left Abutment Shaping Block is shown in Figure 8.7.
Fig. 8.6
Plan and profile of Olivenhain Dam
Fig. 8.7
Foundation replacement "Shaping Block" at Olivenhain Dam (Escondido, California, USA) (Photo: San Diego County Water Authority, USA)
Portugues Dam Valve House Foundation (Puerto Rico)
Another application of RCC is a straight-forward foundation replacement for missing or eroded foundations. At the construction of the Portugues RCC Dam, engineers utilized the RCC operations of the dam to construct a foundation pad for the outlet valve structure. The RCC foundation of the structure provides both structural support and erosion resistance against flows from the valves and in the adjacent
Fig. 8.8
RCC foundation for Portugues Dam valve house (Puerto Rico, USA) (Photo: Ibañez-de-Aldecoa, 2013) Oroville Dam Foundation (United States)
The extensive repairs following the Oroville Dam spillway incident in 2017 (ICOLD, 2018) provide another example of the use of RCC for replacement of missing or eroded foundations. At a height of 234.7 meters, the Oroville Dam is the highest dam in the United States. On February 7, 2017 during spillway operations following a very large precipitation event, the project’s 54.5 m (179- foot) wide, 1006 m (3300- foot) long high- velocity Flood Control Outlet spillway suffered a catastrophic failure of the lower chute area. Finally, approximately 427 m of the lower spillway chute was lost and 1.2 million m3 of rock and soil materials was eroded. During the same event, the Emergency Spillway was used for the first time since the project was completed in 1967. This overflow discharge caused significant erosion and scour, which led authorities to fear for the safety of the emergency spillway structure, resulting in the activation of the Emergency Action Plan and the evacuation of approximately 188,000 persons from downstream communities.
As part of the recovery design for the project, RCC was used as replacement of the spillway channel foundation and to form an interim chute in a portion of the reconstructed main spillway, as shown in Figure 8.9. RCC was also used as a buttress and apron for the rehabilitated Emergency Spillway
Fig. 8.9
RCC used in the spillway reconstruction at Oroville Dam (Photo: California Department of Water Resources, USA, 2018)
8.6. COFFERDAMS