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The Cutting crew

Investigation of a remediated cut slope failure presented a challenge to the team working on the M1 widening scheme, says Gifford technical director Andy Rose.

Since it began in October 2007, widening of the M1 between junctions 25 and 28 has presented many challenges to the project team of joint venture contractor MVM (Morgan Est, Vinci and Sir Robert McAlpine) and designer Gifford.

This is not surprising considering the project involves the engineering of 48km of embankment and cutting slopes to accommodate additional traffic lanes on both the north and southbound carriageways.

The main design challenges on the Highways Agency’s £335M ECI project were developing buildable solutions within the working space, which was constrained by the existing motorway, and site clearance limitations set down in the environmental statement.

Ground challenges for the scheme included widening through and over backfilled opencast coal sites, highly fissured limestone, and areas of historic mining.

Particularly interesting was the investigation and subsequent excavation of a remediated cut slope failure in what at first sight appeared to be a benign cut slope (Figure 1).

The cutting was approximately 10m deep with a side slope of 1V to 3.5H.

Ground conditions did not appear to vary throughout the cutting and as a result we were concerned that similar features might develop elsewhere.

The ground profile within the cutting comprised firm to stiff gravelly sandy clay (glacial deposits) overlying middle coal measures.

The middle coal measures within the cut slope comprised mudstone and silt stones with subsidiary clay layers, coal bands and associated seat earths dipping gently into the cut slope.

Fig_1

Piezometers were installed in the cutting slope which suggested two groundwater regimes: one associated with the superficial deposits, the other below the base of the cutting.

Historical maps showed a series of springs that form a linear feature on an adjacent back slope behind the location of the slope failure.

Both the geology and topography suggested groundwater flow would be favourable for stability once the cut had been formed.

The maps also showed the location of several abandoned collieries and an opencast coal site next to the cutting.

The former highwall slope lies immediately adjacent to the site boundary and opencast operations ceased in 1986.

When Gifford developed the widening design, slope stability assessments were carried out to determine possible causes of the first slope failure.

This analysis, with design parameters determined for the site in combination with groundwater information from preconstruction boreholes, found that when reasonable parameters were considered failure would not be predicted.

Bands of residual strength within coal measures or seat earths due to inter bedding translocation, possibly as a result of subsidence from deep mining or perhaps due to the proximity of the opencast highwall, were considered but the inclination of the coal measures suggested this was unlikely to influence stability.

The combination of weathered and/or reworked higher plasticity soil and the geometry of the fluvio-glacial channel was the most likely explanation for the initial failure.

The cut slope was inspected in detail and a series of trial excavations were carried out.

This revealed that the remedial works comprised a geogrid reinforced slope repair (see Figure 2).

A layer of fine to coarse sand at the base of the remedial works was encountered and it is likely that this was intended as a drainage layer.

Surprisingly, a tension crack on the edge of the cut slope crest was also discovered and a possible shear surface at the toe of the cutting below the basal sand layer.

These suggested the slope was showing signs of further distress.

As part of the widening works the slope failure was excavated, which revealed a fluvio-glacial channel within the coal measures coincident with the failure.

Higher plasticity clay, possibly reworked or highly weathered coal measures material, was also found at the channel-coal measures interface.

Remnants of this material were also found below the sand layer.

Fig_2

As the slope was excavated the channel deepened towards the cutting with groundwater seepages emanating from granular lenses within the channel infill.

This led Gifford to believe the combination of weathered and/or reworked higher plasticity soil and the geometry of the fluvio-glacial channel was the most likely explanation for the initial failure.

The subsurface geometry of the channel is likely to have led to continued recharge of groundwater towards the cutting slope.

The subsequent slope movement is more difficult to explain, but may have been a result of remnants of the highly weathered/reworked material below slope repair and a deteriorating drainage layer.

As a result the final design reflected the need to ensure adequate groundwater drainage leading to specification of a free draining material in the cutting along with removal of the weaker lining to the buried channel.

Thanks to the Highways Agency and MVM for allowing publication of this article

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