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Owner and operator of the Arapuni Dam, Mighty River Power, came up with a plan to carry out work on the structure over several years with the technical assistance of DamWatch Services. The two decided to develop and apply unproven technology to cut-off fissures in the dam foundation rock to remove the potential for seepage.

The Arapuni Dam Alliance – a union between Mighty River Power, Italian firm Trevi and local engineering contractor Brian Perry Civil – has recently finished installing a 90m deep secant piled cut-off wall at the dam, located 16km west of Putaruru on the North Island.

Ground conditions

The dam site is in an area of multiple ignimbrite flows from volcanic eruptions over the past 2M years. Details of the ignimbrite deposits are shown on Figure 1.

Two ignimbrite units form the gorge walls. The younger Mananui is in the upper part on the right abutment, while Ahuroa is found on both abutments. Both are columnar jointed weak to moderately strong point-welded tuff.

The main dam footprint founds on a 40m to 50m thick layer of Ongatiti ignimbrite – a point-welded tuff. The upper part of this is very weak, with unconfined compressive strength of between 2MPa and 6MPa, while below the original dam cut-off wall it is up to 28MPa. This is identified as the hard zone in Figure 1.

Three major sub-vertical cracks or fractures were identified during dam construction, diagonally crossing its footprint in an east-west orientation.

A fourth set of fractures was identified during foundation investigations in 2003.

These fractures (some up to 80mm in size) extend the full depth of the Ongatiti. Clay infill (nontronite, an iron-rich smectic clay with a high moisture content and low shear strength) is typically found around the fractures.

Beneath the Ongatiti, about 40m below the base of the concrete dam, older ignimbrite deposits appear, identified as pre-Ongatiti on Figure 1.Background

Arapuni Dam is a 64m high curved concrete gravity dam, with a crest length of 94m that crosses the Waikato River bed. Commissioned in 1929, the dam now forms the reservoir for a 186MW hydro-electric power station.

The original features of the dam include concrete cut-off walls and a network of porous (no-fines) concrete under drains at the dam/foundation interface shown in Figure 2.

The under drain is the main uplift control at the dam/foundation interface. The original cut-off walls, shown in Figure 1, extend 65m below the dam crest and into the left and right abutments of the dam.

In June 1930 the reservoir was completely dewatered after a large crack opened up in the headrace near the powerhouse. Workers installed a grout curtain along the upstream heel of the dam and in front of both abutment cut-off walls (although not connected to the dam) using a single row of grout holes at 3m centres.

Workers built a bitumen plug at the steep gorge walls and grout curtain extending from the crest abutments – shown in Figure 2.

Since the reservoir was refilled in 1932 periodic seepages have occurred through fissures in the ignimbrite. Targeted grouting work has been done to manage flow rates of up to 750l/min. In 2000, solid particles of clay and bitumen, together with lake life, were observed exiting the drainage system.

Investigations by Mighty River Power and DamWatch showed the very weak nontronite clay infill had eroded within a fracture, forming a pipe connecting the reservoir and the dam drainage system – this was successfully grouted in 2002.

Methodology selection

Mighty River Power required an upgrade of the dam foundation seepage control measures. This was because of highly erodible joint infill being vulnerable to piping erosion or near-lake pressure in areas under the dam caused by fractures hydraulically connected to the reservoir.

The upgrade aimed to reduce the risk of further piping incidents to an extremely low likelihood and severity level, and to control high pressures under the dam.

Remediation had to be done without interrupting power station operations. For example, the reservoir needed to be kept at normal operating levels.

DamWatch identified four fissure sets shown in Figure 3 that required treatment to either fill open fissures or replace erodible infill with suitable materials to form a durable barrier.

Had only open fissures required filling, a grouting solution would have been appropriate. However, grouting could not be guaranteed to replace the soft fissure infill clay, making it a problem to provide a satisfactory barrier to future leaks.

The team considered other options including using high pressure water to flush fissures prior to grouting and wire-sawing to cut the rock and form a cut-off that intersects the fissures.

Instead it chose a secant pile method based upon technical objectives, constructability, cost and dam safety. By drilling overlapping secant piles through fissure zones a verifiable and durable concrete cut-off could be formed.

Data on foundation rock condition enabled DamWatch to see the four features as individual and not interconnected systems. Therefore, the design is for four discrete cut-off walls instead of a continuous one.

These walls are located as far upstream as possible to reduce uplift pressures over the largest dam foundation area to provide the best stability enhancement.
Methodology development

The adopted method comprised installing 400mm diameter secant piles at 350mm diameter centres down to a maximum of 90m from the dam crest. Workers had to maintain and verify pile overlap to obtain the necessary cut-off.

A drilling accuracy or verticality better than 1 in 3600 would ensure adjacent piles drilled independently of one another would maintain that overlap. But 1 in 200 is considered difficult to achieve using conventional piling equipment and even when using directional drilling the overlap was far from guaranteed.

The alliance came up with the idea of using a guide attached to the drill string, inserted in the previously drilled hole.

Each panel is divided into slots of up to eight overlapping piles to reduce stresses in dam concrete while the slot was open. Site workers
completed each slot one at a time, checking for continuity and backfilling with concrete before moving on to the next.

Trevi equipment manufacturer Soilmec built the guide and drilling system in Italy. It uses a reverse circulation system with a tricone bit and was selected because of its accuracy and low impact on sensitive clay in the fractures.

Anchors, straps and limiting the rate of rise of concrete helped maintain the integrity of the un-reinforced dam face during the drilling and backfilling.

Localised gallery backfilling or strutting of galleries, forming underdrain connections and temporary plating across contraction joints, allowed workers to avoid dam features, such as contraction joints, instrumentation, shafts, galleries and underdrains.

Directional drilling with real-time steering gave an accurate alignment for the 150mm diameter first holein each panel. Rig operators then reamed the hole out to a diameter of 400mm and used it to guide drilling for the next hole.

Site workers checked holes by taking inclinometer measurements and used CCTV and a steel frame template to verify continuity between each one. The team planned to use a stop-end in the penultimate hole of each slot to maintain continuity. However, it was unable to develop this method. Instead workers used a simple 150mm diameter pilot pipe cast into the last hole and re-drilled the hole using a reaming tool.

Cross-hole flow logging down each hole ensured that even with the drainage managed, high flows and potential washout would not occur.

Site workers installed piles with the help of a tremie and checked concrete quality by drilling down through the completed cut-off wall to take core samples.

Work began to prepare the site in May 2005 with drilling for the 136 holes following in September of the same year. Initially it was thought the works would take a year. However, the team had a number of challenges including guide systems jamming and drill strings shearing.

Alliance project manager Marco Lucchi says: "The project has been more challenging than expected because we are drilling to depths of 90m and the hole has to be technically precise.

"The concrete of the dam is very strong and we needed more time drilling this part of the hole than was anticipated – even with modifications to the Soilmec R312 rig hydraulic components, rotary head and drilling tools needed for the reverse circulation drilling system."

Completing the project while the reservoir behind the dam remained full meant site workers had to take great care and vigilance to avoid disturbing the foundation and generating further seepage.

An expert dam safety team from DamWatch played an important part to deal with this problem, monitoring a comprehensive real-time instrumentation system in the dam foundation around the clock to identify the start of unacceptable conditions in the foundation.Outcome

The Arapuni Dam Alliance built the four discrete concrete cut-off walls under the dam while in operation, without adversely affecting dam safety or electricity generation.

The method of installation provides a long-term solution aligned with treatment and project objectives. After two years of working on the project, early monitoring indicates that the cut-off is doing its job.

Nick Wharmby is manager of engineering at Brian Perry Civil

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