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Slopes: Drain to gain

Innovative geogrids are not only helping earthworks using marginal soils to be delivered over the winter but are also aiding construction on an historic landslide complex.

Constructing an earth embankment to support a new 230m section of road, which will improve traffic flows and open up new areas fo rbusiness development to the north of Hastings, may not sound too challenging. However, add in a high pressure water main (see box), a periglacial landslide and clay fill material and the complexity of the work increases, particularly with a wet summer and earthworks extending into winter.

These are the issues that contractor Breheny and consultant Campbell Reith have been dealing with on the Queensway Gateway in Hastings for Sea Change Sussex. Careful planning and use of an innovative geogrid product means that the potential problems on the scheme have not caused the headaches that might have been expected.

“We carried out a walkover survey of the site in the early stages of the project, and the geomorphology suggested that there had been periglacial landslide activity affecting the site, which had left the Ashdown Beds in a residual condition,” says Campbell Reith partner Adam Fisher.

Queensway Gateway Hastings

Queensway Gateway Hastings

The gegrid promotes drainage

“However, in order to get the road gradients to adoptable standards, a 9.5m high embankment was needed which had the potential to reactivate the ground movement.”

Fisher consulted with slope specialist Eddie Bromhead and carried out specific ground investigation with Geotechnical Engineering to confirm the problem and clearly identify the shear surfaces.

“We wanted to know where the shear surfaces were and how many they were,” says Fisher. “The investigation found them and that has governed the design.

“During the ground investigation we took samples of the shear surface and carried out Bromhead Ring Shear tests to compare with existing data on the Ashdown Beds and the classic index testing results.”

Embankment solution

Options considered for the new road included piled platforms, lime stabilised ground improvement – with and without cement, and reinforced earth wall structures. The final solution uses a mix of all three.

“Drainage improvement was a major element of the design,” says Fisher.

CA Blackwell started installing a network of drains last summer.Fisher says these are not to lower the groundwater but to manage it if it rises. Information gained during the drainage work was used to further develop the ground model and modify the ground improvement design, which resulted in treatment depths being reduced in some areas.The ground improvement used 2% lime and 2% cement to create a stronger embankment foundation to benefit the subsequent work on site.

Materials for the embankment were being brought over from another Sea Change Sussex project – the North Bexhill Access Road – as part of a balanced cut and fill design. “The clay-based material was not great for this work, but it is what was available and we were able to build the parameters into the stability analysis,” says Fisher.

Queensway Gateway Hastings

Queensway Gateway Hastings

The new road will ease traffic flow

“The embankment slope was set at 1:2 due to land take issues but limit state analysis showed that this wouldn’t work. ”The solution was to use Maccaferri’s Paradrain geogrid that was first developed over 20 years ago for the Asian market, but which has only recently been deployed on UK construction schemes.

The geogrid looks conventional from above but underneath each band is a special drainage layer.

“Low-permeability marginal fills can be used to construct reinforced slopes and walls, provided adequate reinforced drainage is provided within the reinforced fill,” says Maccaferri technical manager Nico Brusa.

“Without drainage, excess porewater pressures would typically build up in the fill during construction.“

Pore water pressure reduces the internal shearing resistance of the fill, and reduces the interface shear strength between the reinforcementand the fill.

”Use of Paradrain not only allowed for winter working – the embankmentis scheduled to be finished just before Christmas – but also allowed the height to be raised in 500mm sections,which is larger than normal for a soil reinforced structure using marginal materials.

“Using Paradrain resulted insignificant programme benefits,” saysFisher. “It dealt with the pore pressure build up during the construction and the life of the structure.”

According to Fisher, the results of using the geogrid are clear to see.

“You can actually see moisture coming out of the embankment along the geogrid and there is a line of vegetation growing along the lines where the geogrid outcrops on the flank of the embankment.”

Fisher’s team carried out detailed analysis of the design before work moved onto site, to ensure the innovative solution would work. “We used Slope W and Maccaferri’s Macstars software to check for rotational, three part wedge and relatively shallow slide failures,” says Fisher.

“We carried out a lot of back analysis to check the parameter we’d used, particularly for φR. 

Queensway Gateway Hastings

Queensway Gateway Hastings

Marginal fills were used to build the embankment

“We carried out the analysis underpermissible stress first and then checked the design under Eurocode 7.

“In my opinion, it is easier to understand the behaviour of a slope this way than through factoring as you can easily see the main factors affecting failure.

”While Fisher and Brusa both believe that rigorous design was a major part of the success of the work so far, they also credit contractor Breheny’s careful attention to detail in controlling the compaction and site management .

“Breheny has achieved dry densities of at least 95% and in some cases they have reached 100%,” says Brusa. “It is very hard to achieve 95% in clay materials without drainage.”

According to Fisher, the benefits of the work at Hastings are wider reaching than the scheme itself, and the Paradrain design could be an advantage for other earthwork projects.

“We are already proposing to use the solution on another scheme as it allows you to work with marginal soils and have confidence in the programme,” he says.

Under pressure

Queensway Gateway Hastings

Queensway Gateway Hastings

Ground stability issues were not the only challenge – the site was already crossed by a high pressure water main that could not be moved or buried in case future access was needed. The water pipe has remained in place – and undisturbed –throughout the work as a result of a 3m exclusion zone, which was imposed during the ground treatment and a major piling scheme to support a steel arch to enable the embankment to span the utility.

“We used temporary works to protect the pipe during the ground improvement phase and carried out monitoring throughout the work to ensure it wasn’t compromised,”says Fisher.

“The steel arch, which is built off of a piled ground beam, relies on hoop stresses so construction of the embankment around the arch is being carefully planned. The 70m long arch is supported by 18m deep continuous flight auger piles installed by Simplex Westpile.

“The pile lengths may appear excessive but there is up to 12m of embankment above the arch and the piles are not designed to transfer load in the top 4m to 5m, due to the residual nature ofthe Ashdown Beds,” says Fisher

 

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