Bubbling under

An industrial site in Nelson, Lancashire, required stabilisation after suffering significant ground movement. Nia Kajastie reports.

The Lancashire factory of an international beverage producer is set on a site that had historically suffered from instability prior to the company taking ownership of it.

This was extremely evident in the staff car park, located on an embankment next to the river Pendle Water, where significant ground movement had occurred over a length of approximately 200m. This had rendered the use of the car park unsafe and raised questions about the long term stability of the site as a whole. An added complication was that a United Utilities 900mm diameter foul sewer crossed beneath the site at a depth of 3-4m, which required careful consideration and consultation.

The design brief from the client was to stabilise the site boundary enabling reconstruction of the staff car park and to safeguard the factory before any additional investment was made on it.

The client approached Byland Engineering in 2018 to provide specialist geotechnical advice and design services for the remediation works. The principal contractor chosen for the project was Neil Taylor Contractors.

Ground conditions

The first stage, as with any project, was a thorough ground investigation to identify the ground and groundwater conditions, and specifically to establish the depths at which ground movement was occurring. Byland designed and specified the site investigation works and guided the client through the tender process, with the works awarded to Dunelm Geotechnical in the winter of 2018.

The works comprised 10 cable percussion boreholes with rotary coring follow on along the full length of the slope. In order to identify the shear zones continuous U100 samples were taken over the top 10m, which were later extracted and examined in the laboratory. Field instrumentation comprised biaxial inclinometers and vibrating wire piezometers installed in all holes where conditions allowed. A detailed series of laboratory testing was undertaken to identify the geotechnical properties of the strata.

The ground conditions on site comprise granular made ground, used to form the level car park area, underlain by 15m of notoriously weak glacial laminated clays, underlain by stiffer cohesive glacial till and lower coal measures bedrock at around 30m depth.

“Of particular interest were the groundwater conditions,” says Byland Engineering director James Binns, “which were artesian, above ground level by some 5-6m, when the bedrock was penetrated. This proved for very tricky conditions for the ground investigation works and was a particular constraint on the design and final construction methods proposed.”

The combination of relic shear surfaces, low strength laminated clays and artesian groundwater conditions presented the designer with a unique combination of challenging ground conditions. This required a combined team approach, Binns notes. Early in the design process when artesian conditions were encountered Byland advised the client to seek specialist advice from Preene Groundwater Consulting. Additionally, early specialist geotechnical contractor involvement was key to identify which techniques would be most suited to the constraints of the site.

Design

Following assessment of the ground investigation data, back analysis of the existing condition was undertaken to verify the proposed ground and groundwater parameters on the basis that the site was marginally stable, meaning a factor of safety of approximately 1.0. This enabled a series of slope stability models to be analysed using Oasys Slope in order to assess the geometry of the critical failure surfaces, linked to both the observed features at surface and the data being received from the inclinometers. Additionally, a Plaxis 2D finite element analysis was undertaken using a phi-c reduction to provide further verification. From these analyses it was evident that the failure surfaces principally occurred within the laminated clay layers.

“Since the land had suffered an existing slip, the remedial works required must penetrate beyond this surface to prevent reoccurrence. Mitigation of ongoing ground movements was also key in respect of the United Utilities foul sewer. Thus, from the outset an embedded wall forming a robust engineering structure at the site boundary to enable remedial works to the sewer and reconstruction of the car park,” says Binns. “It was also identified that a permanent dewatering system would provide significant benefit to overall stability.”

Detailed design was undertaken by Byland using a number of different methods:

  1. initially the additional required “restoring forces” to achieve a satisfactory factor of safety were extracted from the slope stability analysis and applied to the embedded wall
  2. detailed analysis of the retaining wall was undertaken using the Geosolve computer program Wallap
  3. and final validation was undertaken using Plaxis 2D finite element analysis.

Construction

The construction for the embedded pile wall was originally conceived as a reinforced concrete contiguous bored pile wall. However, Binns explains: “Further into the design process this was discounted due to the risks of siting plant at the crest of a failed slope, the cost of forming a safe working platform and the programme speed.

“Instead, the sheet pile wall enabled the plant to be sited away from the crest of the slope and the sheets were able to project above ground level, to then be backfilled to enable the construction of passive tension micropiles and formation levels for car park reconstruction.”

Working on land that has failed, required detailed consideration of the existing stability and how that is affected by the construction sequence.

He adds: “The works had to be undertaken in a particular sequence which resulted in an extended construction sequence. For example, the car park could not be finished until the foul sewer was re-laid, the United Utilities foul sewer could not be relaid until the ground was stabilised and then the stabilisation works could not start until the de-watering was in place and effective. Added to that environmental permitting was necessary requiring groundwater pumping tests and further monitoring in advance of dewatering.”

Sheet Piling UK served as the sheet piling contractor on the works, while Bachy Soletanche was the self-drill tie contractor. The sheet piles were 16m long Emirates Steel ESZ26-700 z-profile ones that were installed by vibratory and percussive hammer techniques to meet the required toe level some 3-4m into the stiff glacial till layer. In order to mitigate forward deflection of the wall, especially given the proximity of the United Utilities foul sewer, the sheet piles were tied back using Ischebeck Titan 52/29 Titan self-drill plain, ungalvanised hollow threaded bars in a tension micropile application.

“Ischebeck Titan hollow bars were selected on the basis of the conditions present, bar capacity, size of drill head and the unique thread orientation enabling the grout to be used as an effective corrosion barrier, meaning, the bars did not require to be galvanised, or any calculation for an assumed sacrificial corrosion allowance,” notes Binns.

Ischebeck Titan UK and Ireland manager for ground engineering Colin Thompson adds: “Ischebeck Titans’ unique hollow bars, with a unique thread type that controls crack widths under load comes in varying capacities. The thread form, with the groove taken out of the crown at manufacturing, gives the bar the tried and tested properties that ensures crack widths of 0.1mm are achieved when the bar takes up load. Having this thread form, controls corrosion of the bar, hence the reason galvanising is not required. This also means that the designer can still follow BS and EN guidelines”

Various piling techniques were considered for the stabilisation works; however, due to the restrictive working room, difficult ground conditions and the high loading required for each bar, the self-drilling process was chosen as the most suitable. The stabilisation works were completed in late 2020, followed by reconstruction of the United Utilities foul sewer in 2021 and finally reconstruction of the car park in December 2021.

*Information taken from GE Magazine