Grouting: Colorado's record breaking dam rises from its foundations

Nicholson Construction is delivering foundation grouting works on a record breaking dam for a major new reservoir in Loveland, Colorado.

Near the town of Loveland on Colorado’s East Slope, a major reservoir building project is underway. The new Chimney Hollow Reservoir, just west of Carter Lake, will supply a dozen north eastern Colorado water companies with 111M.m³ of dedicated storage capacity and 37M.m³ of water each year.

When Chimney Hollow is completed in 2025, it will include the largest dam built in the United States in the last 25 years and the second asphalt core dam that has ever been built in the country.

This enormous infrastructure project will also improve the reliability of the Windy Gap project, consisting of a diversion dam on the Colorado River and a 600,000m³ reservoir. In doing so, it is expected to provide the additional storage needed to meet the demands of growing populations across Northern Colorado communities.

The US$520M (£462M) reservoir building programme is being funded by the 12 water companies. Subcontractor and Soletanche Bachy’s US subsidiary Nicholson Construction is delivering a US$22M (£19.5M) package of grouting works on behalf of Montana-based Barnard Construction, the main contractor for the reservoir scheme.

Nicholson’s remit includes rock drilling and grouting a 1km long grout curtain installed at depths of between 15m and 100m.

It is also placing a shallow blanket grout cap under the 1.16km long main dam alignment and a grout curtain along a smaller 408m saddle dam which is also part of the project. The purpose of the grout curtain is to decrease seepage in the bedrock mass beneath the dam by reducing hydraulic conductivity.

It will also help decrease uplift pore pressures that could act on the dam, greatly decreasing the risk of any potential sliding stability in the dam.

A grout cap enables pressure grouting of the grout curtain beneath the cap. Blanket grouting is when grout is injected into shallow holes drilled in a grid formation. This technique can be used to form a grout cap before curtain grouting of lower zones at higher pressures takes place.

The project kicked off in August 2021, with dam construction getting underway in December 2021.

To develop the final grouting solution the Nicholson team began grout testing on the main dam site in November last year. The testing programme split the dam alignment into two zones where the geology and the water table differ, as more siltstone and sandstone were expected in test section 1 and more granite, pegmatite and gneiss were expected in test section 2.

Engineering consultancy Stantec designed the grout curtain. The testing programme was intended to refine the baseline specification developed from a geotechnical baseline report put together for Barnard by Stantec.

“With Barnard, we chose two zones in different sections along the main dam,” explains Nicholson project manager Jose Torres.

“The intent was to confirm some of the assumptions that were made in the design of the grout curtain and to complete some additional exploratory drilling as well. From there, Stantec used the information to develop an appropriate grouting programme for the rest of the dam.”

The team picked out two 60m long zones along the main dam alignment for test zones. For each of these sections Nicholson followed a super primary, primary and secondary sequence for drilling the test holes, which Torres notes is typical of a grouting programme.

He continues: “The super primaries were cored or rotary drilled to a depth of 70m and the encountered geology helped to inform the depths of the primary and secondary holes.”

In the first test section the team drilled 68 boreholes, and in the second test section it drilled 55 shallower holes.

The geology of the first test section in particular is characterised by what is known locally as the Fountain Formation – a sandstone of Pennsylvanian age that crops out along the Front Range in Colorado. This sits on a Precambrian layer of rock consisting of gneiss. The contact between the Fountain Formation and the gneiss was encountered at approximate depths of 60m in the first test area.

As a result of the test drilling, the team found features – small cracks, fissures or voids that accept water or grout – in zones that were below the sandstone-siltstone layers in some of the gneiss areas.

“There were some areas that we encountered at deeper depths where higher water permeability or grout takes [the quantities of grout accepted by the formation] were observed,” notes Torres. “So some of the holes were extended to go to just beneath those features by the designer.”

The deepest part of the grout curtain on the main dam was initially designed to be up to 33.5m deep. But because of the conditions that the team found during the testing programme, it will now be deeper in certain areas.

The depth of the curtain also depends on the height of the dam at each point and the different geologies encountered further up along the alignment.

Nicholson vice president of operations Ron Hall explains: “As you get higher in elevation along the dam, we start to see that contact between the siltstone to sandstone and gneiss is much shallower. So the whole of the second test section, which is almost at the furthest extent to the dam, goes up to 50ft [15m] and 80ft [24m], because we’re not treating as much of that siltstone-sandstone layer. There the geology kind of changes; you’re in a bigger bed of that Precambrian gneiss.”

The test programmes ran until early May. Then the team started grouting the blanket cap on the main dam foundation later that month.

The Nicholson team carrying out grouting work along the concrete foundation of the main dam

As the dam is in a valley, its elevation changes dramatically and rises to a height of 128m. The team has started installing the grout curtain from the lowest elevation of the dam at ground level because this was where the main contractor started work on the dam’s concrete foundations.

It has been working out from the first test section towards the abutments of the bottom of the dam.

The spacing between each hole is 3m from centre and like this along the whole length of the dam. There is an upstream and downstream line for the grout curtain. On the main dam there are three rows of blanket grout lines 1.5m apart which are just for shallow treatment and are between 9m and 11.8m deep. There is another row on the upstream side.

To bore the super primary holes for the grout curtain the team used the rotary drilling method to drill 70m deep into the bedrock. These holes were flushed out using water to remove cuttings from the bore.

Stantec’s design for the grout curtain dictates the location of the holes and that each hole has an angle and an inclination that it must be within two degrees of.

Once the team had cored the grout holes, it ran a deviation survey, which measured the boreholes’ departure from the vertical to ensure that the team had hit its target on each of the holes during the grouting.

After this the team carried out water testing on the bored holes to assess the permeability of the rock that it will be treating with grout.

A grout injection tool known as a single packer was hooked up to a rubber hose reel, which enables the team to get water into the hole. The packer was lowered into the hole and inflated to effectively isolate the bottom 6m of the hole for water testing.

The team then removed everything from the hole and installed a double packer assembly, which was again lowered into the hole and used to isolate it in 6m stages up to the top. The double packers are used to conduct permeability testing during core drilling.

Once the team had all the information from the water testing, it went back to the hole and lowered a single packer assembly into it to isolate it, again in 6m stages, for the grout injection. This grout treated any fractures or features in the bored holes.

After each hole was water tested and grouted, Nicholson moved onto the next hole in the sequence. It did all the super primaries – the deepest holes – first, then the primary holes, and the secondary holes last.

Nicholson engineers monitored in real time the pressure and flow in the bored holes and their permeability as well.

Torres adds: “When each section is completed and we feel that the grout curtain is done, we’ll do a verification borehole. We’ll then run some water tests through the hole to get information about how effective the grout curtain is. For every 12m section that we finish, we’ll run one of the verification cores. That will give us confidence in the performance of the ground during any monitoring.”

Once it has finished grouting for the main dam, Nicholson will begin curtain grouting on the smaller saddle dam. This grout curtain will be shallower because it is at the tail end of the reservoir and this dam is not as tall.

The Nicholson team next to a drilling rig used to bore the grout curtain holes

Nicholson was under pressure to finish grouting the bottom zone of the dam by the end of September so Barnard and the subcontractor could come in and start placing asphalt for the asphalt core of the dam.

“It’s a fine balance,” notes Torres. “Working in sequence and keeping up with Barnard, or having them stay ahead of us is a challenge.”

Barnard is building the base section of the dam and the concrete plinth – or the working platform – as Nicholson is grouting. Given that Nicholson is sandwiched between several different construction activities, it has had to collaborate closely with Barnard and the other construction partners to coordinate the sequencing and scheduling.

Although Nicholson’s contract is not a huge portion of the project, Torres says that it “sits very close to the critical path because everything has to follow what we are doing”.

Later this year, the team will begin installing the grout curtain for the tallest sections of the dam which is up to 128m tall in some places.

When it does so, it will face a new challenge of working on a much steeper slope in sub-zero Colorado winter conditions.

The slope is at a 43° angle, on which the team will have to construct a working platform that can be tied down to anchors and can hold equipment weighing up to 4.5t.

“We imagine the grouting work will go much slower just from virtue of being on a steep slope,” notes Torres, “and that presents a challenge in terms of keeping to the schedule but also keeping everyone safe.”

When constructed, the main dam will be the tallest dam constructed in Colorado in 50 years. Nicholson’s work on the project is expected to finish by the end of summer of 2023.

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