Ludeke Dam. creative dam solutions

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1 Aerial view of the Ludeke Dam site, March 2011, showing main embankment, river diversion, quarry, saddle embankment and site camp Tyler Bain Design Engineer / Project Manager MBB Consulting Engineers Mike Udal Design Engineer / Project Manager MBB Consulting Engineers Ludeke Dam creative dam solutions Alick Rennie Project Director MBB Consulting Engineers [email protected] The R200 million Ludeke Dam, one of the larger dams in the Eastern Cape, is 60% complete. However, the journey from conception to completion has presented designers and project managers, MBB Consulting Engineers, with many opportunities for creative thinking INTRODUCTION Ludeke Dam is situated 16 km northwest of Bizana in the Mbizana Local Municipality at the confluence of the Ludeke and kuntlamvukazi Rivers. Its purpose is to store raw water for the Greater Mbizana Regional Bulk Water Supply Scheme, estimated at R900 m. Water will be pumped from the dam for 14 km, with a static lift of 300 m, to the Nomlacu Water Treatment Plant for purification and bulk distribution. The project will ultimately supply about people with 20 Ml of potable water a day. For the first time this remote area, which has suffered chronic water supply problems due to poor infrastructure, will have a secure source of safe drinking water. The scheme comprises the dam, a pump station and rising main, all of which have been designed by MBB Consulting Engineers. The company is also projectmanaging the construction. Additional components include the upgrade of the existing Nomlacu Water Treatment Plant and the construction of a bulk treated water supply system that will be undertaken in a number of stages. The first phase is expected to be completed during The Greater Mbizana Regional Bulk Water Supply Scheme is being implemented by Umgeni Water as agents on behalf of the Alfred Nzo District Municipality, and is funded by the Department of Water Affairs, with co-funding from the Water Services Authority. DIMENSIONS Ludeke Dam will have a 14.5 million cubic metre capacity at Full Supply Level, a depth of 33.7 m and a surface area of 140 hectares. The 1:50 year drought yield is 5.11 million cubic metres per annum at 98% assurance. The main embankment is 240 m long and 40 m high and the saddle embankment, which closes off a neck in the basin, is 280 m long and 17 m high. The catchment area is 146 km 2, producing a Safety Evaluation Flood of m 3 /s and a Recommended Design Flood of 930 m 3 /s. 52 March 2012 Civil Engineering

2 rockfill placed clay core transition Meeressa Pillay left (assistant resident engineer), Tyler Bain (project manager) and Brighton Mbiba (resident engineer) at Ludeke s clay core downstream zones chimney filter View of left abutment from spillway pump station pipeline route toe drain inspection manhole clay finger shale and clay right abutment overburden rockfill blanket drain chimney filter clay core rockfill river diversion The foundation geology was challenging due to a mixture of soft and hard rock, variable soils and a fault line running across the site. These factors, as well as the availability of rock and clay on or near the site, indicated that a clay-cored rockfill dam was the most appropriate, safe and cost effective option. Minimising seepage could have been an expensive problem, but was turned to advantage by a design that controlled it to acceptable levels, kept costs within budget and contributes water to the required environmental releases. FOUNDATION TREATMENT MBB investigated the foundations in detail at feasibility and design phases. The dam s centre line is underlain by highly fractured post-karoo dolerite bedrock on the left flank and by highly fractured and deeply weathered sandstone and shale on the right. There was no evidence of any major faulting in the rock mass, but extensive jointing was evident at the surface and in the 19 borehole cores that were taken. These showed a complex foundation underlying the proposed centre line, with medium to hard rock dolerite foundations displaying low lugeon values on the left flank and competency increasing with depth. A variable matrix of alluvium and colluvium overlying highly fractured shale was found in the stream bed. Towards the right-hand side there is a thick band of soft sandstone and a residual dolerite matrix further up, which is between 6-18 m deep. Both are overlying highly fractured and indurated shales with high lugeon values (between 40 and 100) on the right flank. A grout curtain has been specified across the river valley to provide firm foundations and control seepage under the dam. The specification was to drill and grout in four stages to 24 m, with a primary spacing of 6 m. Secondary, tertiary and quaternary holes were placed at 3 m, 1.5 m and 750 mm spacing intervals where required. The left abutment has proved to be predominantly hard and tight dolerite. Lugeon values during water tests have been low (0-3 lugeons) and grout takes have also been low, or zero. In the central section, under the river bed where softer, fractured shale was discovered, grout caps have been cast. This was a delicate task as the grouting pressure had to be controlled to avoid lifting the shale. Lugeon values and grout takes have been high and a variety of strategies have been necessary to grout up this section. Conditions finally dictated that more than twice the expected grout holes were necessary, delaying and complicating construction. Interestingly the total grout volumes were as predicted. This has delayed the contract by about two months, with additional stoppages caused by unseasonal flooding and other events. The main contractor, Rumdel Cape RBE JV, moved onto site at the end September 2009 and the project will be completed in late 2012 / early Civil Engineering March

3 Various aspects of the right abutment rockfill transition material blanket drain chimney filter and core cut-off drain drain inspection manhole drain pipe blocky shale foundation with clay interstices placed clay core Shotcrete and soil nailing in the cut face above the spillway trough at the Ludeke Dam site The grouting, soil nails and shotcrete are 90% complete and the geotechnical contractor, WEPEX, is due to dis-establish site in February The next big challenge will be closing off the Ludeke River. On the right flank the soft sandstone and dolerite layer, from 0-18 m, and overlying highly fractured shale at depth, have required special treatment as it was not practical or economical to cut the usual deep core through this layer. MBB commissioned ARQ Consulting Engineers to carry out various studies to determine the quantum of seepage under different scenarios, within a confidence envelope. As a result a shallow clay core cut-off trench is specified, leaving the blocky weathered material, with clay in the interstices, in situ. Disturbing these blocks to cut a deep core would have potentially created more problems than it would have solved. This type of design was pioneered in the USA on permeable foundations and was selected by Heinrich Elges, leader of the MBB design team. Elges has used this concept successfully elsewhere in South Africa, for example on the Tzaneen Dam. Professor A Monte van Schalkwyk was commissioned to review this design. The anticipated seepage is less than 7 l/s, at a high level of confidence. The required minimum environmental release, in the driest month of a 1:100 dry year, is 21 l/s and hence this flow can easily be sustained. This design element precluded excessive expenditure to minimise seepage, only to release the same water through the outlet works. A variety of measures have been incorporated into the right flank to minimise and control seepage while ensuring that the embankment is stable, safe from piping failure and economically viable. These features include: A core cutoff drain, downstream of the core, extending mm below the base of the core, including a drainage pipe, and tied into the blanket drain. A 1 m deep blanket drain over the full area of the solum downstream of the core, with clay cut-off fingers and drain pipes. A 3 m deep toe drain along the downstream toe, tied into the blanket drain, with inspection chambers to allow measurement of seepage. Pressure relief wells at the downstream toe, to relieve uplift pressures and alleviate any hydraulic exit gradient that may exist. 54 March 2012 Civil Engineering

4 Grouting of the underlying rock structures, by installing sleeves through the soft overlying layer, thus leaving this layer largely intact and undisturbed. EMBANKMENT DESIGN The difference in material types between the left and right flanks, and divergence in lugeon values, indicated the variable nature of the foundations, and that long-term differential settlement could be anticipated across the embankment. Rigid structures such as a concrete wall or faced dam would not have been suitable due to the uneven strengths of the foundations. These factors together with the discovery of good quality, unweathered dolerite in the basin, influenced the selection of a clay-cored rockfill embankment, with the ability to absorb minor uneven settlement over time. The missing ingredients were good quality clay for a core and suitably graded sand for drains and filters. A source of clay was established just outside the basin in an area used for agriculture adjacent to the saddle embankment. Permission to mine the clay was obtained provided the site was rehabilitated thereafter. A depth of 0.5 m of topsoil is being stripped and stockpiled, and the land will be graded to drain freely once mining is complete. Suitably graded sand is not available on site and is being imported from commercial sources 130 km away, at considerable expense, for use in filters, drains and concrete. Based on the foundations and availability of materials, both embankments were initially designed as clay-cored rockfill dams, with upstream slopes of 1H:1.8V, and downstream slopes of 1H:1.5V. However, when the quarry was opened the layer of weathered material overlying the hard dolerite was found to be greater than investigations had shown. This is intermediate material and can be stripped using excavators, whereas the hard rock dolerite requires blasting. This overburden material has different physical characteristics from the hard dolerite, notably cohesion c and friction angle φ. These different parameters required MBB to recalculate the stability analyses of the embankments, and it was found that the slopes did not meet the factor of safety requirements. The apparent quantity of each material was determined and a cut/fill balancing exercise was carried out with the quantities required in each embankment zone in an attempt to use the poorer quality overburden material where possible. A solution lay in using this weathered material in the saddle embankment, flattening the upstream slope from 1V:1.8H to 1V:2H, and the downstream slope from 1V:1.5H to 1V:1.8 A portion of overburden has also been used as a wedge in the downstream face of the main embankment; however, ultimately a fair amount will have to be spoiled. This good quality G5 material is suitable for road works and is in short supply locally. Hence the spoil dump is situated outside of the basin, and the South African National Roads Agency Ltd (SANRAL) will be using it to upgrade sections of roads in the region. Civil Engineering March

5 Tower crane working on the outlet tower with the rock quarry and Ludeke River in the background COST The dam was designed to be the most economical structure for the site, with the least cost per unit of water yield. Various economic models were developed which provided embankment fill quantities and spillway cut quantities for different volumes of water stored and hence related to safe yield. MBB s goal was to optimise the dam s yield against the embankment costs. A larger dam could have provided a higher yield, but the cost per unit of water would Preparing shuttering for the first concrete pours of the ogee weir have been significantly higher. Although the dam was designed to a 30-year planning horizon, it may potentially fall short of projected demand by 2031, and an augmentation scheme from the Umtamvuna River may be necessary. INLET/OUTLET TOWER To keep water treatment costs to a minimum a 40 m tall, multi-level wet well inlet tower has been designed. This has four sluice gates, installed at 6 m vertical intervals, to allow water to be drawn from a range of depths. This is drawn off via a 762 mm diameter steel pipe, which sits vertically above a mm diameter steel scour pipe that connects directly to the bottom of the dam through the tower base. As the site is isolated, an appropriate approach was used in the design of the system which is sturdy, simple and easy to operate and maintain. A pedestrian bridge, supported by four columns, services the tower, allowing access to the platform to operate and maintain the sluice gates, which are controlled by handwheels and spindles. Manual hydraulic actuators, on top of the anchor block that houses the outlet works, are used to open and close the sleeve valves. On the downstream end of the service outlet the pipe branches into three. One branch conveys water across the toe of the dam to the raw water pump station; the other two branches reduce in diameter and terminate in a 400 mm diameter sleeve valve, and a 100 mm diameter gate valve. These are sized to allow for a range of environmental flow releases from 21 l/s, up to 630 l/s. The mm diameter scour pipe releases water back into the river via a 750 mm diameter sleeve valve. The installation of sleeve valves helps to oxygenate, warm, and aerate the water, while reducing the erosive energy, and benefiting the health of the river downstream. The outlet works have been designed to withstand seismic events applicable to the 56 March 2012 Civil Engineering

6 Eastern Cape; while not a major seismic zone, this has still resulted in considerable reinforcing being used in the tower bases. SPILLWAY DESIGN AND SCALE MODELLING OF THE SPILLWAY CAPACITY One of the most important safety aspects of any dam design is the spillway, which must pass a flood of such magnitude that it seldom operates to capacity. Physical scale modelling provides an effective method of testing a design to ultimate capacity prior to construction. The topography of the Ludeke Dam site did not provide much space for a spillway as the valley is narrow and the valley sides rise steeply from the river. The variable geology of the foundations precluded a rigid concrete overspill section on the main embankment, and the depth to competent rock ruled out the right flank. The saddle embankment was considered for a wide bywash spillway, but erodible soils, high flood volumes, the social importance of the area and a wetland further down the valley eliminated this option. MBB opted for a side channel spillway with a 60 m long uncontrolled ogee weir entrance, discharging into an 11 m deep concrete lined trough. This returns to stream via a 17 m wide, 200 m long, concrete-lined chute, with a skijump type energy dissipater discharging into a stilling basin. The spillway system is anchored into rock on the left bank and has extensive sub-surface drainage. The design of the spillway required some fairly complex hydraulic analysis, and to test the design the Hydraulics Laboratory of the University of Stellenbosch was commissioned to build a 1:25 scale model of the spillway and chute. The model confirmed MBB s theoretical design to its fullest and verified the performance of the spillway under extreme conditions. The MBB team has appreciated the importance of a pro-active, creative response to real world challenges exposed during construction, and the need for design review as the unexpected is brought to light. PROFESSIONAL TEAM Implementing Agent Umgeni Water Designers and Project Managers MBB Consulting Engineers Pietermaritzburg (Assisted by H Elges as APP) Geotechnical Consultants Drennan Maud and Partners Main Contractor Rumdel Construction (Cape) & Rob Business Enterprises Joint Venture Geotechnical Subcontractor WEPEX Blasting Subcontractor Brauteseth Blasting Aggregate Crushing Subcontractor BLH & C Dam Foundation Specialist Consultant Prof A Monte van Schalkwyk AVS Consultants Specialist Seepage Studies ARQ Consulting Engineers Civil Engineering March