With live webcasts of a selection of papers at Dam Safety 2011 in September, the ASDSO expanded its distance education offering, reports Patrick Reynolds
Sitting at a computer screen anywhere in the US at the end of September, or anywhere in the world for that matter, there were chances to tune-in to the first live webcasts from an annual conference of the Association of State Dam Safety Officials (ASDSO). Virtual attendees could listen to the likes of a briefing on alliance procurement on the Hinze dam in Australia, design tool development for labyrinth weir spillways and also a primer on computational fluid dynamics (CFD), amongst other key topics like levees and use of geomembranes.
The move by ASDSO at its Dam Safety 2011 conference, in Washington DC over 26-28 September, was to expand the offering of its online, e- and distance education programmes. It already has live, online training seminars by expert engineers that are archived for later access, usually a year, then would be made available on disc.
For the new webcast initiative at the Dam Safety 2011 conference, each online purchase of the flat-rate daily fee bought an email link, username and password to access coverage of an afternoon session of six presentations. ASDSO selected afternoons only to time the webcast offering to the West coast business day, making the coverage available to the widest potential audience in its core, US, territory.
The live webcast was presented through a split-screen window dominated by two visuals – live video of the presenter and the slides, respectively – and audio. Proving effective, the webcasts, provided by Mediasite, also covered the valuable Q&A sessions, and there was particular merit in the system with speakers’ presentations complementing and not duplicating either their slides or papers, the bane of many a good conference, but not here.
Dam Safety 2011 – webcasts
At Dam Safety 2011, the first afternoon’s two sessions that were webcast live focused on the US levee safety programme, at the state and local levels and also the national and international perspectives, respectively.
For the former, examples and experience were drawn from Louisiana, Pennsylvania and California. From an international perspective, spotlights were thrown on levee performance in Japan following the offshore earthquake early this year, safety lessons from The Netherlands and also development of the international levee handbook.
ASDSO’s second of three afternoon webcasts focused on hydraulic modelling and spillway design.
In the first of the two sessions the focus was on hydraulic modelling with presentations giving a primer on CFD that complemented a more detailed paper, spillway analysis using 3D CFD, and composite modelling for hydraulic structures. In the spillways session, the presentations looked at labyrinth weirs, stepped spillways and vegetation in auxiliary capacity.
The final day’s webcast offered presentation on geomembrances in dam and rehabilitation design, and some project cases studies in dam improvements in US and internationally.
Geomembrane topics covered in the first session looked at the Occoquan dam in Virginia, general experience in embankment dams, and a presentation on the 2010 icold bulletin on their use for dams. Case studies were examined in the final session of the conference – seismic and seepage rehabilitation at Abbott Brook dyke in Maine, levee improvement in the Natomas Basin in California, and the Hinze dam Stage 3 works in Australia.
Hinze dam raising and rehab
A key message from the presentation by Richard Davisdon of URS on the latest heightening works on Hinze dam, near the Gold Coast in Queensland, Australia, was the benefits of alliance procurement, not least for avoiding expensive contractual claims. He added that there was also potential to explore the use of the approach elsewhere, such as the dams sector in the US.
Kicking off the second session of the afternoon on the last day, Davidson gave an excellent presentation that complemented his paper through an engaging broad discussion on the key aspects of the project, and honing in on particular points and the prime take-away possibilities for the audience. The slides and paper both offer a balanced mix of text and images, including photos and diagrams.
Hinze dam is an 80m high earth core and rockfill embankment structures built on the Nerang River in southeast Queensland in the 1970s. Raised in the following decade to 65m, the height of the dam is almost finished being increased further by the owner, Seqwater. The project is the latest – Stage 3 – improvements to the water supply storage volumes in the region, and will also help flood mitigation and meet dam safety guidelines including the revised estimate of the probable maximum flood (PMF), which was up 75% to approximately 4200m3/sec.
The alliance approach was important to keeping costs down on the Stage 3 works at Hinze dam, which included a number of challenging aspects, such as dealing with the right abutment using a cut-off wall to manage piping risks, increasing the spillway capacity for the increased PMF, and raising the main embankment by almost 15m and increasing its length by 1100m – which involved partial, stepped excavation into the structure while meeting safety and fisheries needs as full reservoir storage had to be maintained due to water scarcity in the region.
Then, to complicate the construction further, after the region had suffered about 13 years of drought there were 13 flood events during the project – and also during that time concrete placement works had to continue at the spillway, the level of which was being raised as well as the flood capacity increased for the PMF though outflow for more frequent events, such as 1 in 100 return period was reduce by half. The solution to pass the flows involved creation of a low-level gate.
As Asia-Pacific has seen a number of problem with design-build procurement for dams, a different approach for bringing people together was tried for Hinze – an alliance model, explained Davidson. He added that something can really only be tried when money is put behind it on a project, and then it is tested when problems arise. It is a recognition that money is at risk, in three aspects – direct cost, overheads & profits, and performance grading pain/gain against targets.
Key members of the alliance for Hinze are the owner (initially Gold Coast City Council [GCCC], then in 2009 responsibility was assumed by the Southeast Queensland authority, Seqwater), consultants URS and Sinclair Knight Mertz (SKM), and contractor Thiess. The alliance team, which came together in late 2006, was responsible for all aspects of developing and delivering the project.
The project was run through two leadership teams – the senior, which was almost like a board of directors, and the project management, which had operational control. There were no lawyers, emphasised Davidson – ‘partners must work things out’.
Target out-turn cost
Broadly, procurement at Hinze Stage 3 sought partners based on qualifications, experience and questions in interviews, and not bid prices; the difference was a team was to be formed, and melded. Then, with partners aboard and signed up to the alliance approach, with its implications and opportunities, work began to develop the first part of the two-stage fulfilment of the project – establishing the Target Out-turn Cost (TOC), founded upon open-book accounting and also independent audits.
The TOC was reached at the 30% point in the design process, said Davidson. That is a ‘hold point’, he adds, because depending on what figure has been developed ‘the owner can stop everything’ or will be comfortable with the upper limit. In addition to cost, greater certainty is also provided in terms of schedule. At that early stage, the client was GCCC.
To help firm up the TOC, a specialised groundworks contractor – bauer Foundations Australia – was brought in early as a sub-alliance partner to give more certainty of methods and costs on works, such as the cut-off-wall at the right abutment. The move is similar to Early Contractor Involvement (ECI), observed Davidson.
After formal approval of the TOC, backed by supporting documents, in mid-2007, the alliance then had to carry out all investigations required, the detailed design, pursue regulatory approvals, and perform construction the commission the facility.
Cut-off wall
A cut-off-wall solution was chosen to counter the piping risks at the geologically complex right abutment.
The wall is 200m long, up to 55m deep and 830mm thick, and was constructed as an alternating sequence of primary and secondary panels. Bentonite slurry was used for excavation support until it was displaced by tremie-delivered concrete. Construction of the wall included excavating through the dam core with the reservoir at full capacity.
Constructed mostly over the second half of 2008, the wall was completed ahead of schedule, in January 2009, and also under budget, the savings being split between the owner and alliance partners.
Embankment construction
Tasks undertaken during the Stage 3 works also included numerical modelling and dam safety plans to manage deformation and maintain stability. Careful examination was required as the works involved removal of the existing rockfill stability berm to expose the foundation and Stage 2 filters – the key interface with Stage 3. A dam safety management plan (DSMP) was developed to control the construction risks.
Comparative case studies were examined as well as computer simulations were needed as significant deformations were recorded on the main embankment while the rockfill berm was under construction from late 2008 to late 2009 – but not stability related as the slopes had good quality rock and the foundation had sufficient factor of safety of above 1.3.
Monitoring revealed that deformation was higher was the berm build increased in height, adding to the load on the existing dam and foundations, but the rate of both it and settlement reduced as construction activity lessened, and eventually ceased. Limited rigger levels were incorporated into the DSMP, helping hazard observation and evaluation by site personnel.
Lessons learned, noted Davidson, included the value of numerical modelling (2-D) both for prediction and helping to confirm mechanisms causing deformation. Coupled with empirical predictive methods, the numerical modelling boosted the overall assessment and prediction of deformation – and, overall, the model has compared reasonably well with observation.
In exposing the dam core, a clearly defined construction staging sequence was vital. Only the downstream section of the core was exposed at the outset, and the excavation limited to allow rapid reinstatement within eight hours if merited under the particular features of the DSMP for this phase of the works. The DSMP drew data from weather and reservoir monitoring and also trigger levels on monitored parts of the structure.
In building up the Stage 3 core from the exposed Stage 2 level, processing and compaction of the clay was vital, not least as the latest works were taking material from another location, with naturally different characteristics. Mixing of the materials was the way forward and so monitoring moisture content and density of the reworked and combined was important to compaction control. A key lesson, noted Davidson, is to have design representatives on site full-time for quality control testing. With the alliance model in force, this led avoided disputes over test results and contractual claims, as can happen under traditional procurement methods, and had the team focused instead on solutions.
Spillway
Modelling studies were important to the upgrade of the spillway, the geometry of which was ‘complicated’ and the hydraulics ‘tricky’, Davidson said. Both numerical and physical models were used.
Three spillway alternatives were considered for Stage 3 and CFD analyses were the only way in the rapid early project schedule to choose the option for budget purposes, but physical models for each would have been too expensive anyway. But a physical model was also used for detailed design of the chosen option – a two-level ogree crest with converging chute, steps and baffles.
Building up from and continuing the seven concrete monoliths of the Stage 2 spillway, the new works established both low and high level overflows, and has a stepped downstream face. Designed as single composite structure, the construction challenge was temperature control of the mass concrete to minimise cracking in itself and next to the existing concrete.
Used hydro-demolition to prepare the existing concrete surface for bonding with the fresh pours, which had been cooled with varying combination of cooled aggregates, chilled water and also, in Summer, injections of liquid nitrogen at the batching plant to obtain a mix temperature at placement of 18 degrees Centrigrade.
Next webcasts
Following the success of the pilot webcast, the system will be given more lead-time to build awareness when used for Dam Safety 2012, which will be attended over 16-20 September, in Denver – except for virtual delegates.
In the meantime, apart from accessing the archived sessions from this year, ASDSO will be broadcasting live (then archived) two-hour webinars focusing on risk, in October and December, to add to further add to its distance learning portfolio.