Federal spread for RCC3 June 2014
A range of RCC works are planned, underway or recently completed by two major US federal agencies. Report by Patrick Reynolds
In the US, the number of roller compacted concrete (RCC) works on dams and hydraulic structures has been growing in range as well as size for its federal agencies, notably the US Army Corps of Engineers (USACE) and the US Bureau of Reclamation (USBR).
The biggest project is the Portugues dam in Puerto Rico, undertaken by USACE and completed recently. Coming in at smaller scale are a number of other RCC projects, including, for USACE, works to build a berm downstream of a saddle dam at Center Hill reservoir, in Tennessee; and repairing the plunge pool at Fort Peck dam, in Montana.
For the USBR, a key project is the extensive modification works at the overflow spillway and dykes required at Minidoka dam, in Idaho.
RCC dam construction gained a foothold in the US in the early 1980s when the US Army Corps of Engineers (USACE) built the Willow Creek dam in the Pacific Northwest. Since that beginning, its work with RCC hydraulic structures has varied in scale and type.
Portugues dam: a changed design
Undertaken by the USACE Jacksonville District, completion of the 67.1m (220ft) high Portugues dam was officially celebrated at a dedication ceremony in early February. Construction work was undertaken from early 2008 to the end of 2013, and the US$386 million project is a key component in the Portugues and Bucana flood defence scheme for the city of Ponce.
The dam has a total volume of 267,584m3 (350,000cy), and construction was undertaken by Dragados to a design developed by USACE. Portugues dam is a single-centred thick arch dam - the first RCC structure of its type in the US, notes USACE. However, it was not always planned to be built to that design.
The Ponce area has suffered from seasonal flooding due to hurricanes and rainfall. Initial assessment for a major flood control scheme began in the 1940s, and by the mid-1960s there was a plan for two multi-purpose reservoirs and storm channels. The US Congress approved pursuit of the concept in 1970.
By the 1980s, the chosen design concept for the Portugues dam element of the overall scheme envisaged a three-centred, double curvature thin arch concrete structure - which would have been a first for USACE. With the design approved, work began on site at the abutments with major test grouting and curtain grouting - including the new "duration grouting" procedure.
Meanwhile, by the late 1980s into the early 1990s, the other major part of the flood management scheme was built - the Cerillos embankment dam. Stormwater channels were also constructed in the city.
With respect to Portugues dam, however, development of the project came to a halt in late 2000 after bids were called for the construction contract: the procurement efforts attracted only one offer, and that tender was 'significantly above' the cost estimate, says USACE.
Following the setback, a five-year programme of field studies and redesigns was launched. The programme was supported by USACE's Computer Aided Structural Engineering Task Group, which also included engineers from USBR. The group developed arch dam software and design criteria.
The extra studies led to a switch in design to the single-centred thick arch dam, which USACE says was a lower cost option. Unlike the 1970s vision though, the project would be for flood control only.
Portugues dam is the first time the construction method had been used by USACE in Puerto Rico. It says the project served to help educate many engineers and students on the benefits of the technology.
USACE adds that it has used the project as part of its "Dam Safety University" programme - an initiative to transfer knowledge to the next generation of engineers from those nearing retirement. It says this is important as fewer dams are constructed across the US but more repair and remediation work will be needed.
Puerto Rico Department of Natural and Environment Resources is the local project sponsor. It expects to take operational control of the dam and reservoir in 2015. The optimum pool level for the reservoir is El.439.8ft with allowable drawdown to approximately El.410ft and the maximum to El.372ft to attenuate flood flows. The spillway crest level is El.512.75ft. USACE will act as the regulating agency for the scheme.
In the Cumberland basin in Tennessee, USACE's Nashville District is preparing plans to construct a RCC berm at the toe of a saddle dam and fuse plug at Center Hill reservoir. The project concept, recognised as needed back in 2006, is to have a backup structure to retain the reservoir pool should the saddle embankment dam fail - through seepage or piping, or if used as an emergency spillway during a Probable Maximum Flood (PMF) event.
Options considered to address the problem include: do nothing; grout the saddle dam; construct a cut-off wall within the structure; or place a backup berm immediately downstream of the dam.
The selection of RCC berm option was finalised last year following design development investigations. Key benefits of RCC for the berm structure are described as its resistance to overtopping, and also internal erosion due to seepage or piping - unlike an embankment structure. The RCC mass would be able, therefore, to span possible sinkholes in weakened foundations without catastrophic failure, says USACE.
Typically for concrete generally - and RCC, although a mature technology - a key challenge in planning for the project has been the mix design. In this instance, the particular challenge comes from the local sources of aggregate - limestone rock, which has the potential to chemically react with cement over the long-term. Therefore, USACE is undertaking a major test programme to assess the risk for the berm project, and so help finalise the design criteria.
Design development is almost complete. The design arrangement is for the reinforcing berm to be placed at the toe of the saddle dam's downstream slope, and the space behind is to be backfilled to create level ground from the top of the RCC structure back to embankment crest, on top of which sits the fuse plug.
Presently, the berm design anticipates the structure will be up to 29.9m (98ft) high with a base width of up to 36.6m (120ft). At the top of the dam, the crest is 7.9m (26ft) wide, and stretches 228.6m (750ft) - or 81% - of the 282m (925ft) total length of the structure. The downstream face of the RCC berm is to be lined with conventional concrete-faced steps, typically 1.5m (5ft) wide by 1.8m (6ft) high. The vertical upstream face is to have minimum 300mm (1ft) thickness of grout-enriched RCC for most of the height. An inspection gallery is to be built within the berm.
The RCC berm project is scheduled to be constructed in 2015-17. No particular construction challenges are envisaged. In the meantime, the reservoir level has been lowered anyway to reduce risk as part of a larger construction project to halt seepage at the main Center Hill dam.
The planned RCC works for the side structure come, therefore, as an additional feature of the programme of engineering improvements being undertaken at the reservoir. USACE is constructing a cut-off wall in the main embankment dam.
For that large project, Nashville District is adapting lessons from a recently completed similar project on the Wolf Creek dam, in the same catchment. A key aspect of USACE's work is using its award-winning information management system to control, and ensure, the verticality and effective overlap of the piles and sections of the cut-off wall. It is expected that the barrier wall will be completed around mid-2015.
Severe flooding on the Missouri River in 2011 caused damage at the plunge pool of Fort Peck dam in Montana. The flooding had led to prolonged, excessive flows over the spillway - more than double the previous maximum discharge.
The high flows scoured the downstream end of the dam, exposing much of the cut-off structure supporting the spillway chute, says the Omaha District of USACE. Less than half of the 21.3m (70ft) embedment remained after the flood. Other repairs that were needed included addressing damage to the gates, spillway slab, emergency gate controls, relief wells and horizontal outfall pipes.
The dam was constructed in the 1930s, and is a hydraulic and rolled earth fill structure - the largest of its type in the US. It is up to 76.4m (250.5ft) high and 6410m (21,026ft) long, excluding the spillway. The reservoir was impounded from 1937, and reached minimum pool level in 1942. Five generating units were added over 1943-61.
The dam has a remote, gated spillway on the right bank. It is 250m (820ft) wide with 16 vertical lift gates, each 12.2m (40ft) by 7.6m (25ft) - the top of the closed gates being at El2,250ft asl. The design discharge capacity is 6509m3/sec (230,000cfs) at the maximum operating pool level of El.2,250ft asl, and 7783m3/sec (275,000cfs) at El.2253.3ft asl.
Prior to the 2011 floods, the record flow for outlet release at Fort Peck was 990m3/sec (35,000cfs), in 1975. By mid-June 2011, however, unprecedented runoff from record rainfall in parts of the upper Missouri basin, along with plains and mountain snowmelt, led to peak outlet releases at many dams, quite apart from the flows over the spillways. At Fort Peck, the outlet releases were 1865m3/sec (65,900cfs) - the maximum - through the bypass tunnels and powerhouse.
USACE choose RCC construction to repair and extend the plunge pool supporting structures. These works are to be completed by the end of 2015, and the other repairs should be finished beforehand.
Minidoka modification project
USBR's modification project at the Minidoka complex in Idaho includes two new RCC overflow spillways and two new RCC dykes, in addition to a new gated spillway and changes to the other existing gated spillway. The main contractor is local firm RSCI Group, construction started in late 2011 and is scheduled for completion by early 2015.
The Minidoka complex is a combined diversion, storage and hydropower scheme on the Snake River, and was constructed just over 100 years ago. During the construction phase, the original ogee-shaped overflow spillway - completed in 1906 - was effectively heightened almost immediately; piers and stoplogs were added, enabling the reservoir to be raised by 1.5m (5ft).
The overflow spillway is not a continuous structure - it was built as two sections separated by a radial gated spillway. The irregular layout came from the designers trying to fit the structure to the topography while minimising the cross-sectional areas, and therefore costs. The piers are spaced at approximately 2.1m (7ft) centres, and consequently the overflow spillway has 292 bays. Flood management calls for the stoplogs to be removed manually.
Challenges for the structure have included ice loading, which damaged the concrete and has restricted the reservoir level back to the original crest level during Winter to minimise risk. In addition to the loss of some storage volume - the extra originally intended - the seasonal restriction also demands regular removal and then replacement of the stoplogs.
However, the spillway has more concrete problems, and these are attributed to freeze-thaw cycles and alkali silica reaction (ASR) as well as old age and design. The problems include deterioration where the piers were added to the original spillway.
USBR is addressing the problems by undertaking a range of works, including replacement of the overflow spillway and piers with a new RCC overflow spillway, the crest of which is the same level as the top of the stoplogs - therefore permanently obtaining the extra 1.5m (5ft) that was wanted from the outset. With the new spillway at a higher level, though, the depth of flood flow over the crest will be less than before, and therefore the discharge capacity of the local site is reduced.
USBR says therefore that a new 12-bay radial gated spillway structure will be constructed as part of the modification, and will serve as the main flood control feature when completed.
The new overflow spillway will have a vertical upstream face, a 3.4m (11ft) wide crest, a downstream slop of 0.72H:1V, and be clad with 300mm (1ft) thick structural concrete. The reinforced concrete is to be placed with each lift of RCC, and on the upstream and top facing there will also be embedded, continuous waterstops at each contraction joint - spaced at approximately 9.1m (30ft) along the length of the spillway.
Following completion of the new spillway structures, the reservoir will no longer be lowered by 1.5m (5ft) during winter, USBR adds.
The works also include demolishing the canal headworks. Construction calls for foundation excavation, new spillway approach and outlet channels, and installation of radial gates, gate hoists and associated equipment procured by USBR.