How old dams are reborn10 June 1999
Performance data supports RCC as an economical alternative to other methods of rehabilitation. With many of the more than 75,000 dams in the US nearing the end of their design life, RCC is expected to continue to play a major role in ensuring they stay in operation, as Kenneth Hansen* and Randall Bass** explain
The first major project to use large volumes of roller-compacted concrete (RCC) worldwide was Tarbela dam in Pakistan. Between 1974 and 1986, John Lowe III calculated that 2.7M m3 (3.5M yd3) of what he termed ‘rollcrete’ was used for eight separate rehabilitation applications at the world’s largest volume engineered embankment dam (see ‘Roller compacted concrete dams - an overview’, published in Roller Compacted Concrete III, ASCE February 1992).
Rehabilitation began with emergency repairs to restore abutments surrounding collapsed outlet tunnel No 2. RCC was then used for massive placements to modify the plunge pools for both the service and auxiliary spillways; a cofferdam for the auxiliary spillway; plunge pool repairs; backfill and lining for an outlet tunnel plunge pool; and a gravity wall constructed in two stages in the powerhouse area.
Rather than signalling the start of the use of RCC for the rehabilitation of existing dams, the repairs at Tarbela had a major impact on the development of RCC gravity structures for new dams. The high production rates desired for RCC dam construction and the positive erosion resistance of RCC were proven at Tarbela. During the repair of the collapsed concrete-lined 13.7m (45ft) diameter outlet tunnel an average rate of RCC placement of nearly 8,400m3 (11,000yd3) per day was achieved. This high production rate remains a record for an average daily RCC placement rate today. However, this record is expected to be exceeded when the Stage III cofferdam is built for the Three Gorges dam in China.
Even though most of the RCC in the rehabilitated service spillway area was faced with higher strength conventional concrete, an RCC buttress or strut in the plunge pool area was left exposed. The entire repaired area was subjected to a test flow down the spillway of 11,300m>=/s (400,000cfs) for six hours. Visual inspections and soundings following the test flow indicated basically no erosion of the protective works, including the exposed RCC strut.
Now, 25 years after the start of the RCC rehab experience at Tarbela, RCC for new dams has made great strides worldwide. By the end of 1998, 184 RCC dams with a height greater than 15m (50ft) had been completed and 25 more were under construction. However, further use of RCC for upgrading existing dams has not progressed to any extent throughout the world, except in the US. This, then, may be a good time to learn from the US experience on the use of RCC for rehabilitating existing dams of all types.
While the US remains one of the leaders in the construction of new dams, with 31 greater than 15m (50ft) in height now completed, the number of existing dams rehabilitated using RCC is more than double the number of new RCC dams. By the end of 1998, the number of dams rehabilitated using RCC totalled 67, located in 24 states. Of this number, 58 are existing embankment dams while nine are concrete or masonry dams.
Of the 31 new higher RCC dams and 17 smaller ones completed in the US, 13 replace or partially replace failed or deteriorated existing dams. Therefore, the number of dams in which RCC was a part of the rehabilitation process is even greater (see right).
All but four of the upgraded embankment dams have used RCC to increase spillway capacity by taking floodwaters safely over an RCC armoured downstream embankment slope. This type of application is termed RCC overtopping protection.
For the existing concrete dams, the main use of RCC has been to improve the structural stability of the dam by adding an RCC buttress section, invariably downstream of the existing structure. The buttress provides additional safety under all loading conditions, including seismic, and at times also provides a durable covering for a freeze-thaw damaged downstream face.
Most of the 67 rehabilitation projects in the US use small volumes of RCC. The total volume of RCC used in the 67 projects is 778,000m3 (1,018,000yd3) or an average of about 11,600m>= (15,200yd3) per dam. The total RCC volume used for the 67 US dam rehabs is still less than 30% of RCC volume needed for the repairs at Tarbela alone.
Many embankment dams in the US need to be upgraded to meet current hydraulic safety standards. They are unable to safely store or pass design floods that have increased considerably since the dams were originally designed. Current criteria indicate that the spillway capacity should be at least one-half the theoretical Probable Maximum Flood (PMF) and in many cases, the full PMF.
Solutions proposed for accommodating such high volume floods include:
•Raising the dam and spillway crest to increase storage.
•Increasing spillway capacity.
•Providing for safe overtopping.
•Breaching the dam.
•Some combination of the first three.
Site restrictions or economics frequently make the first two options unfeasible or unattractive to the owner. The functional use for the reservoir usually makes breaching unacceptable as a permanent solution.
Thus, providing embankment overtopping protection may be the only reasonable solution available to the design engineer. In many instances it is the most cost-effective solution, even when other alternatives are available. This is especially true for small embankment dams required to pass low-to-moderate depths of overtopping flows.
The most accepted method for providing overtopping protection to older embankment dams in the US is the use of RCC, placed in a stair-stepped manner on the downstream slope. In addition to being erosion resistant, the RCC can be placed quickly at low cost. This increase in spillway capacity to accommodate infrequent flood events can be accomplished without lowering the reservoir water surface.
The height of the dams that have increased their hydraulic capacity, and thereby safety, using RCC overtopping protection has ranged from 4.6m (15ft) to 33.5m (110ft). The volume of RCC used for this purpose has ranged from less than 750m3 (1,000 yd3) to 132,000m3 (160,000 yd3) - used to modify the Alvin J Wirtz dam in Texas (see p41).
The basic design concept for RCC overtopping protection is to provide an overlay of the downstream embankment slope with sufficient weight and durability to resist displacement and erosion due to overtopping flows. The exposed RCC surface also must have sufficient strength to withstand climatic changes such as freeze-thaw cycles.
For ‘over the embankment’ spillways that will operate infrequently at low depth of overflow, the design of the RCC section is simple and conservative. The construction method determines the thickness of the overlay. A minimum lane width of 2.5m (8.2ft) is required for proper hauling, spreading, and compacting 300mm (1ft) thick horizontal lifts of RCC. This, then, produces a minimum thickness of concrete, measured perpendicular to the slope, of about 0.6m (2ft) for a 3H:1V embankment slope.
The simple section used to provide overtopping protection for Brownwood County Club dam in Texas is shown above. This was the first earth embankment dam in the US rehabilitated using RCC. In this design, the RCC is cut into the shale foundation rock to prevent undercutting of the RCC due to head cutting during overtopping.
On other projects, the RCC terminates in an RCC apron that continues downstream a sufficient distance to control the hydraulic jump. At the end of the apron a cutoff wall is usually installed, or an RCC sill section constructed. If the RCC spillway is designed to pass flows greater than 2m (6.6ft) in height or will operate frequently for a sustained time, a structural analysis similar to that required for a reinforced concrete spillway may be warranted. Jointing and crack control play a more important role when the spillway’s planned operation approaches that of a primary spillway.
Downstream slope treatment
The selection of the type of downstream slope geometry and finish is based primarily on hydraulic design and the owner’s expectations. A number of scaled hydraulic models of stepped spillways have confirmed that steps can reduce exit velocities by up to 70%, compared with a smooth concrete surface. Reducing velocities translates directly into smaller stilling basins.
The construction of the steps has been accomplished by several different methods. For those projects where hydraulic design and aesthetics are important the steps are generally formed with a vertical face. Forming will typically add several dollars to the unit price of the RCC and may affect the productivity of RCC placement operations. In lieu of forming, steps can be created with sloping faces at a 45-60° slope by compacting the face with hand compactors or using a vibratory plate attached to a backhoe which can create a sloping face with very little handwork. Performance data has shown that the greater the compactive effort at the step face, the more durable the RCC.
The second selection criterion is the owner’s expectations. Some voids at the RCC surface can be expected when compacted against forms. Owners sometimes have the misconception that, because RCC is concrete, the finished product will resemble that of conventional concrete. Owner conditioning during the planning and design phase is important so everyone’s expectations are met.
Recent trends have the RCC overlays covered with soil and grassed so that the modifications to the dam are not visible (see left). This allows greater leniency in surface tolerances and surface appearance during construction, because the RCC will only be exposed if the dam were to overtop and erode away the veneer of topsoil
A wide range of RCC mixtures have been used in the 53 RCC overtopping protection projects. This is not unusual, considering the varying site conditions, size, and design criteria for each project, as well as the fact that 34 different private consulting firms or government agencies were involved with the design of the rehabilitations.
Based on observed field performance, it appears adequate durability and erosion resistance of the RCC material is obtained by providing a mixture that reaches a minimum compressive strength of 21MPa (3,000psi) in 28 days in areas subjected to many freeze-thaw cycles. For less critical environments, a lower strength may be specified. With well-graded sound aggregate, 21MPa (3,000psi) can usually be achieved with nearly 200kg of cement per m>= (about 330lb/yd3).
Because many of the projects require relatively small volumes of RCC, a commercially available highway base course has been used as aggregate for many projects. Maximum size aggregate is usually limited to 38mm (1.5in) to minimise segregation.
Performance of overlays
Because RCC overlays have generally been selected for use as protection from failure due to infrequent flood events, very few projects have been called upon to perform under design conditions. Most were designed to experience overtopping only when the 1 in 100 year event is exceeded. Still, some have been designed to accommodate more frequent flows. Thus, the performance of Ocoee 2 in Tennessee, and six other dams which have experienced flow over the RCC surface, provide a degree of confidence that RCC overlays will perform well when overtopped.
Ocoee 2, a 9.1m (30ft) high rock-filled timber crib dam constructed in 1913, was the first dam to use RCC to provide better safety during overtopping. The initial plan by the Tennessee Valley Authority (TVA) to improve the structural stability of the dam while overtopped was to place a rock riprap berm at the structure’s vertical downstream face to buttress the structure. After the rock washed out four times when flood waters overtopped the dam, TVA decided it was time to adopt another method of rehabilitation.
The new solution in 1980 was to place RCC lifts in a stair-stepped configuration over the riprap on a 2H:1V slope. In order to accommodate white water rafters, the dam is now intentionally overtopped at least 82 times per year. The rafting operators pay TVA for lost power revenues as the water that overtops the dam is diverted from a square wooden flume that feeds a hydro powerhouse downstream. In 1996, the number of planned overtoppings doubled, as the stretch of the Ocoee river upstream of the dam was the site for the canoeing and kayaking events for the 1996 Atlanta Olympic Games.
After more than 1500 overtoppings, the well-compacted RCC remains basically unaffected by the water flows and exposure to the weather. Poorly compacted RCC placed at the downstream toe has eroded and the compacted layer above the eroded area has been undercut by about 300mm (1ft) in places. While the depth of flow over the dam has generally been 300mm (1ft) or less, the RCC rehabilitated structure was overtopped by more than 3.8m (12ft) during a flood in 1990 that in effect inundated the structure.
On other projects where the RCC is exposed, some minor weathering of step faces has been noticed. This weathering was found to be generally confined to the outer 50mm (2in) of the RCC surface, particularly where coarse aggregate segregation and/or low density had occurred. Uncontrolled transverse cracking is typical of RCC overlays, however, the rather narrow cracks have not adversely affected either the structural integrity or hydraulic performance of these spillway upgrades.
Most of the US dam rehabilitation projects using RCC have increased the hydraulic capacity of embankment dams, but RCC has been used to upgrade nine concrete or masonry dams. The RCC has been used to provide increased lateral support and thereby greater safety from normal, overtopping and seismic loads, as well as sliding at the foundation contact.
The two most significant RCC buttressed dams were upgraded to improve their seismic resistance. Both Gibraltar and Littlerock dams are located in earthquake-prone California. Gibraltar is a 59m (194ft) high single curvature thick arch dam originally completed in 1920, while Littlerock, an Ambursen type multiple arch completed in 1924, reached a maximum height of 53m (174ft). Neither dam met current seismic design criteria prior to strengthening.
Several alternatives were investigated for rehabilitating the dams: an RCC buttress downstream of the existing structure was the most cost effective solution. The buttress in effect changed the structural action of the arch or multiple arch to that of a concrete gravity dam. Each buttress required about 70,500m3 (92,500yd3) of RCC.
The downstream face of Gibraltar dam is exposed RCC on a 0.8H:1.0V slope, while Littlerock has steps of conventional concrete on a 0.65H:1.0V slope. The latter steeper slope required forming.
Originally, the steps for Littlerock were 0.6m (2ft) high. However, the owner wanted and received 1.8m (6ft) high steps in order to discourage climbing on the dam and thereby possibly incurring liability should someone injure themselves. The completed Littlerock rehabilitation is shown left.