Tackling conduit replacement15 June 2006
The removal and replacement of existing outlet works at Pablo dam in the US demonstrates a good example of the complexities of conduit replacement at an embankment dam
Generally, removal and replacement of an existing conduit through an embankment dam consists of excavating the dam down to the existing conduit, stockpiling the material, removing the existing conduit, constructing a new conduit and possibly new entrance and terminal structures, installing a filter around the downstream portion of the conduit, and replacing the embankment material. A cofferdam may also be required if the reservoir cannot be drained during construction.
Removal and replacement of a deteriorating conduit can be time consuming and expensive compared to other renovation methods. Typically, construction costs for removal and replacement may be five to 10 times higher than for other renovation methods, such as sliplining of the conduit. Construction costs rapidly rise as the height of the embankment dam increases. However, if the embankment dam is small and the downstream impacts to users are acceptable, this method may be more advantageous. Often, removal and replacement is the alternative of choice for low hazard embankment dams, since it is generally less expensive. This is especially true on older low hazard embankment dams, where they may have been built without the benefit of modern design and construction techniques and often lacked proper quality control. The safer and more efficient solution may be to remove and replace the conduit and possibly the entire embankment dam.
Modern principles should be employed for design and construction of the new conduit. A new publication is available through the Federal Emergency Management Agency (FEMA) titled ‘Technical Manual: Conduits through Embankment Dams’ which provides extensive information on the best practices for design, construction, problem identification and evaluation, inspection, maintenance, renovation, and repair of conduits.
Removal of an existing conduit
The advantages of removal and replacement of an existing conduit through an embankment dam include:
• Evaluation – the exposed foundation of the conduit can be fully examined and evaluated.
• Repairs – areas along the existing conduit that may have been damaged by internal erosion or backward erosion piping can be repaired.
• Seepage – extensive seepage control measures along the conduit can be installed.
• Design modifications – the new conduit can be designed to provide increased discharge capacity to meet current or future operational and emergency release requirements.
The disadvantages of removal and replacement of an existing conduit through a high embankment dam include:
• Cofferdam – unless the reservoir can be drained, the construction of a cofferdam is generally required. Inflows into the reservoir will need to be diverted. In some special cases a downstream cofferdam may also be required.
• Costs – construction costs for removal and replacement are generally higher than for other renovation methods.
• Reservoir operations – construction may impact reservoir operations and add risk to the downstream community.
• Seepage paths – if proper compaction of the embankment closure section is not obtained, potential seepage paths may exist along the junction of the closure section and existing embankment.
The first step in removal and replacement of the existing conduit is usually to excavate the embankment dam to the invert of the conduit and remove it. An excavation transverse to an existing embankment dam centreline increases the potential for hydraulic fracture of the replacement embankment material from arching. Because hydraulic fracture poses special hazards when the reservoir is subsequently refilled, special care is required for designs that involve excavation transverse to the existing embankment dam. Excavations should be wide enough at the bottom to ensure adequate working room for removal of the existing conduit and replacement with the new conduit, and compaction of earthfill materials.
A qualified professional engineer or engineering geologist should carefully observe and document the excavation required for the removal of the existing conduit to verify that any damaged embankment or foundation materials have been fully removed and/or treated prior to construction of the new conduit and replacement of embankment materials. Often, removal of the entrance structure, terminal structure, or other structures may be required due to age or deterioration, or to ease construction of the replacement structures. Occasionally, where removal of the existing conduit is difficult and expensive, the existing conduit may not be removed, but will be abandoned by backfilling the conduit with grout and installing a new conduit at a separate location.
A new filter should be designed to extend upstream into the embankment dam. Frequently, the filter installed in this situation is larger than that used for first time construction of an embankment dam. The filter should extend to both sides of the new conduit and key into the existing embankment dam. If the existing dam has a chimney filter, the filter should be designed to be a part of that system where feasible.
If the conduit is being replaced in a zoned earthfill embankment dam where a central core is substantially different in properties than the outside embankment shells, backfill for the conduit should coincide with the zoning for the dam. Core zone backfill should only be used around the conduit through the core section, with shell backfill soils used through those sections of the conduit. An exception to this recommendation is where rock shell zones include large angular rocks that could impose point loads on the conduit that exceed its strength. For that condition, cushioning soil with small sand and gravel should encircle the conduit to prevent the problem.
The soil removed from the embankment dam as the existing conduit is excavated is frequently reused to backfill the notch in the dam. Designers should carefully evaluate the water content of these soils and determine if drying or wetting is required for satisfactory reuse. The excavated slopes in the existing embankment dam may remain exposed for a period of time before they are backfilled. The time over which the excavation made to replace the conduit is left exposed may be hot, dry weather. In this case, the exposed soils on the face of the excavation may desiccate to considerable depths. Before commencing backfilling of the excavation in the dam, any desiccation cracks in the existing dam must be removed, and the earthfill surface disked and moistened. This process will probably have to be delayed until immediately before backfill of an interval of the dam is ready to commence. If backfilling of the excavation is interrupted during hot weather, the surface of the reconstruction backfill also should be closely inspected for desiccation features before placing new fill. Poorly bonded lifts can occur during interruptions of fill placement. They provide an avenue for possible internal erosion.
Designers should consider these important points:
• Testing – soils used to rebuild the embankment dam should be evaluated by the same tests that would be used to evaluate soils for a new embankment dam. The water content, plasticity, gradation, compaction properties, and dispersivity of clay fines are important evaluations. If the replacement fill is in a zoned embankment dam, similar zoning should be used.
• Water content – soils used to rebuild the dam should usually be placed wet of Standard Proctor optimum water content to improve their flexibility and resistance to cracking and arching. Compacting soils at water contents that are 1 to 3% wet of optimum significantly improves their flexibility. At the same time, the likelihood that pore pressures could be generated in medium to high plasticity clays in fills of significant height should also be evaluated. Designers must weigh the advantages of compacting soils wet of optimum against the disadvantages of this wetter compaction water content. The lower shear strength and potential pore pressures generated by wetter compaction water contents must be considered in the design stability evaluations. Many designers consider excessive pore pressures to be a lesser long term danger to the successful performance of an embankment dam than the danger of arching and hydraulic fracture if the soils are placed dry.
• Exposed filler – special care to remove desiccation cracks in exposed fill surfaces is important. This applies to the exposed excavation slopes and to layers of fill used in reconstructing the embankment dam.
Generally, the construction period for a complete removal and replacement of a conduit will require more time than other renovation methods. Mitigating the impacts of a longer construction period may require consideration of: (1) diversion and downstream water requirements (i.e. irrigation); (2) traffic control measures (lighting, signs, etc.), road closures, construction of detours (such as detouring dam crest traffic); (3) larger disturbance areas and potential environmental issues; and (4) draining or drawing down of the reservoir.
An example of the replacement of a conduit is the Pablo dam modification.
Pablo dam is located on the Flathead Indian Reservation near Polson in Montana, US. The dam is operated and maintained by the US Bureau of Indian Affairs. The embankment dam is an earthfill structure consisting of a main dam and dikes, which flank both sides of the dam, south and north. The crest elevation of the main dam is at 981.5m, and the dikes are at 980.5m. The main dam has a structural height of 13.1m, a crest length of 3215.6m, a crest width of 6.1m, a 3:1 upstream slope and a 2:1 downstream slope. The north dike has a crest length of 1783.1m, and the south dike has a crest length of 3124.2m. The crest width of both dikes is 3.7m.
Pablo dam was constructed in three phases over 24 years. In 1911, the embankment was constructed to elevation 976m. The second construction in 1918 raised the embankment dam to elevation 978.1m, and the final construction in 1934 raised the dam to the present elevation of 981.5m. Pablo dam is an offstream structure that is fed by the Pablo Feeder canal, and its purpose is to impound water for irrigation. The reservoir has a capacity of 35Mm3 at elevation of 978.7m.
The original outlet works were situated at the maximum section of the dam and consisted of a 12.8m high concrete intake structure with two 0.9m by 1.52m slide gates. The original outlet works consisted of three box shaped conduits; the middle and south conduits were 52m long and 1.37m wide by 1.52m high. The north conduit was about 41.4m long and 31.37m wide by 1.52m high, but was abandoned prior to the third phase of original construction.
Differential settlement between the intake tower and the outlet works conduits caused some offset in ‘sliding joints’. This settlement was expected, as sliding joints (no reinforcement crossing the joint) were included in the original design. However, continued settlement of the intake structure and the first 4m of the conduits required grouting of the foundation shortly after construction. No further settlement had been detected in the last 50 plus years. The first sliding joint is displaced vertically about 5cm and sprays water at high reservoir head. Mortar filling in all sliding joints was disbonded, cracked, and deteriorating; tensile cracks were also discovered along the length of the conduit. Water was commonly leaking from both the cracks and the sliding joints, and there are signs of possible internal erosion of embankment material occurring in a few areas. Furthermore, spalling concrete had been discovered in the walls of the conduits. The concrete in the centre wall at the downstream end of the conduits was deteriorated, resulting in exposed aggregate and rebar.
Dam safety modifications began in 1993, consisting of the injection of polyurethane grout into cracks and conduit joints. A two-man crew from McCabe Brothers Drilling of Idaho Falls, Idaho, mobilised to the job site. They installed ventilation ductwork into the two outlet works conduits and began drilling injection holes in the south conduit. Existing cracks (mostly at construction joints) upstream of station 0+38.71 were injected with polyurethane resin grout to stop leakage through the cracks. This was done prior to repairing spalled concrete in the conduits. The subcontractor used a ratio of polyurethane to water of 1.3:1, which effectively stopped 90% of the seepage. However, after completing injection of cracks in the south conduit, seepage began to migrate downstream and appear in cracks that were previously dry.
During drilling of the injection holes, two voids were discovered, one in the crown of each conduit at station 0+3.96. The voids were approximately 30.5cm deep and 61cm wide and seemed to be connected to each other. Old construction drawings showed this as the location where concrete counterfort walls, which support the intake tower, meet the conduits. No voids were found behind any of the other cracks. The voids at station 0+3.96 were injected with polyurethane. As injection of the south conduit was completed, some migration of polyurethane was noted through the crown and divider wall of the middle conduit.
In mid-November, McCabe Brothers Drilling finished injecting polyurethane resin into cracks in the outlet works conduits. They injected a total of 1154.5 litres into the two conduits (the specified quantity was 189.3 litres). As the injection operation progressed from upstream to downstream, cracks that had been previously dry near the canal outlet began to seep water. Therefore, these cracks were injected also. Because the seepage appeared to be following the exterior of the conduits and exiting farther downstream, the seepage continued to be unfiltered and may increase the internal pressures in the embankment. A decision was made to install weep drains in the conduit and to construct a filter collar about the exterior of the walls. A modification to the contract was issued to provide for this additional work. After the polyurethane injection was completed, the conduits were unwatered and inspected. Repair areas were marked, and the contractor began chipping out and preparing the surfaces of the repair areas for epoxy-bonded concrete. Approximately 30 small repairs and one large repair at the conduit outlet (splitter wall) were done to complete the conduit repairs option of the work. Smaller and shallow areas were repaired using an approved two-part epoxy material. Larger areas were repaired with epoxy-bonded concrete.
Areas of concern
During an inspection of the interior of the conduits in April 2001, it was discovered that material had been deposited inside the middle conduit near an opening in a construction joint. This was occurring through a hole in the floor of the middle conduit at a construction joint near station 0+39.62. Approximately 2.83cm3 of silt and fine sand were deposited on the floor. However, this deposit was observed during the winter when no irrigation releases are made. More deposition may have occurred during irrigation season that was washed downstream and not observed. Consequently, the total volume of material could have been much greater than the 2.83cm3 observed in 2001. The US Bureau of Reclamation (USBR) theorised that plugging this opening could result in redirecting the erosion through a different hole or crack in the conduit. Also, redirecting the erosion might cause a more dangerous path to develop along the foundation contact of the conduits, and an erosion exit might develop downstream of the embankment dam. If the exit point was located within the outlet channel, early detection would be very difficult.
Another area of concern was the condition of the north conduit that was reportedly plugged at each end prior to the final raise of Pablo dam in 1932, but was never confirmed. Therefore, it could be possible that a nearly full reservoir head could exist at the end of the north conduit, which was less than 31m from the downstream toe of the dam. After much discussion between all involved parties, it was decided to completely remove and replace the original outlet works.
As an interim measure, a temporary patch was installed over the opening to prevent additional material from being eroded into the conduit while allowing for relief of water pressures. The patch consisted of filter fabric under a metal screen. During March 2002, the geotextile portion of the patch ruptured and approximately 1.42cm3 of silt and fine sand were deposited into the conduit. The patch was repaired soon after the rupture was discovered. Reservoir level restrictions were implemented in April 2003 and were to be kept in place until the removal and replacement modifications could be completed.
New outlet works
The USBR prepared designs, drawings, and specifications for the conduit replacement. The construction of a new outlet works began in November 2004 and was completed by the spring of 2005. The major aspects of the work included:
• Construction of a cofferdam to maintain an area free of water during construction.
• Clearing, grubbing, and stripping prior to excavation.
• Removing existing embankment dam slope protection.
• Excavating embankment materials to accommodate construction of the new outlet works (Slopes transverse to the dam centreline were excavated at 4H:1V).
• Removal of the existing reinforced concrete intake structure, conduits, retaining walls, and apron. A forensics investigation was conducted during the excavation to better understand the causes and mechanisms on how embankment materials were transported into the conduits. Provisions were included within the specification paragraphs requiring the Contractor to facilitate such efforts.
• Constructing a lean concrete mudslab, on which to found the new outlet works.
• Constructing reinforced cast-in-place intake structure, conduit, retaining walls, and apron. The new conduit was double barrelled with each barrel having a 1.83m, 7.5cm inside diameter. The exterior surface of the conduit was sloped at 1H:10V below springline and was curved above springline to provide a good surface to compact earthfill against. Each conduit joint was a treated as a control joint with longitudinal reinforcement extending across the joint and 15cm polyvinyl chloride (PVC) waterstop.
• Installing two emergency guard gates and two regulating gates within the upstream intake structure.
• Constructing a chimney filter and drain system. The filter extended downstream and encased the outlet works conduit. Filter materials encasing the conduit consisted of sand processed to a specified gradation from an approved offsite source.
• Placing and compacting zoned earthfill in the embankment dam closure section.
• Replacing the embankment dam slope protection.
Sometimes repairs alone are not fully robust enough to address all the unknown erosional mechanisms existing within an embankment dam. Due to continued dam safety concerns, more extensive measures may be warranted.
The absence of fines within the embankment facilitated seepage and internal erosion of organic materials along the conduits.
Dissimilar foundation materials under the conduits at about Station 0+39.62 probably allowed for differential settlement. This settlement could have caused the conduit to crack. At this same location, a hole between the floor and the south wall allowed the internal erosion of embankment materials into the conduit.
Antiseep collars did not stop seepage or internal erosion of fines along the conduit. There was really no benefit from the collars because the embankment materials adjacent to the conduit were so pervious.
The polyurethane resin was able to travel along the sides and around the collars of the conduits. However, the polyurethane did not travel more than about 5 feet (1.52m) perpendicular to the conduits. Although there was limited opportunity to observe the foundation material under the conduits, what was seen
contained no polyurethane.
Chuck Cooper, P.E., is a civil engineer for the US Department of the Interior's Bureau of Reclamation’s Technical Service Center in Denver, Colorado, US. He served as chairman of the multi-agency national committee responsible for developing the technical manual, Conduits through Embankment Dams – Best Practices for Design, Construction, Problem Identification and Evaluation, Inspection, Maintenance, Renovation, and Repair. E-mail: [email protected]
The free manual – available in print copy (FEMA 484), CD-Rom (FEMA 484CD), and DVD (FEMA 484DVD) – contains more than 280 illustrative figures, 34 cases histories, and an in-depth glossary. The CD-Rom and DVD also contain PDF copies of all references cited within the manual that were available in the public domain or where reprint permission was obtained. In addition, the DVD has a collection of more than 150 ‘additional reading’ references in PDF format.
Copies of the manual may be obtained by calling FEMA’s Publication Distribution Center on tel: (+1) 800 480 2520.