Following late summer storms at Bewdley in Worcestershire the gardens rising above a house on Cleobury Road had an unwelcome moving experience, pushing retaining walls towards the house and affecting the power supplies and sewerage  pipes which passed through the garden. The home owner instigated temporary scaffolding support and Geotechnics was called in to advise the insurers’ agent on the likely cause and solutions to the problem.

The scale of the landslide was not large but the problem of access both to investigate the site and carry out remedial works was huge. This was compounded by the house being located on an extremely busy main road. Initial works involved the temporary re-routing of the sewer and power supply to limit the consequences of the failure from a health and safety perspective. Access to the garden and on to the slope limited the Site Investigation to window sampling, laboratory testing, surveying and mapping of surface features, cracks and emerging groundwater. Analysis of the failure by Habib Ur Rehman and the development of outline remedial works involving drainage, reinforced soil and proprietary facing blocks by Len Threadgold were presented in a report to the Client’s insurers’ agent Crawford and Company Adjusters (UK) limited.

On receipt of instructions for detailed design following approval of the concepts and design strategies, Paul Thurlwell of P and S Consulting Engineers (Pascoe) was commissioned to prepare a design and drawings for construction. Following acceptance of the design, the remedial works were put out to tender with the work being awarded to WM Longreach (WML) in June 2011. They devised means of doing the work using their specialist plant and careful programming of the earthworks. The process involved the formation of a working platform and ramp up from the road, the use of a mini excavator to excavate the way in to the site and the carefully timed collection of waste and delivery of materials. The design required the excavation of a large part of the garden and the failed slope, with temporary anchors being used to hold the sides to the access route into the site and the rear scarp of the excavation to provide safety for workers and stability to the land above during construction.

Work commenced on site at the end of July 2011 with the removal of garden buildings. Careful site monitoring was undertaken during construction and excavation clearly showed the extent of the main shear surface of the slip and identified seepage locations. This allowed adjustments to give further confidence that the works would provide the necessary reinforcement and drainage in the long term whilst making savings. Adaptations to the reinforced soil wall were necessary to suit variations in ground conditions adjacent to the main slip, however, and this led to costs for the designed works being very close to the original estimate.

The success of the project on site was due to good working practices, the skill of the site staff, notably Dave Ryan of WML and Paul Thurlwell of Pascoe, and close liaison with Trevor Hardie of Geotechnics. Photographs show the situation before, during and after completion of the works. The Client, Mr and Mrs Edgerton, was delighted with the outcome, with the garden rather than themselves being left in tiers.

Geotechnics were to supply geotechnical engineers aboard the MV Poseidon to carry out logging, sampling and in situ testing during vibrocore operations.  Working as part of a multi-disciplinary international team, a total of 351 exploratory hole locations along 13 cable routes within an 25,000km² area off the west coast of Germany. 

Work commenced on the 29^th August, 2011 and continued until 1^st October. The sampling was carried out using Osiris’s newly purchased GeoCorer 6000 Vibrocorer. The vibrocore locations were drilled approximately every kilometre along the proposed routes, or at areas of interest highlighted by previous geophysical surveys.

The vibrocorer was lowered by a winch from the A frame located at the stern of the ship and winched to the seabed, where the coring commenced. Once full depth of penetration or refusal was achieved the corer was brought back to the surface.

Each sample was drilled using a 6m length barrel and sample recovery varied from 3m up to 6m. Back on deck the sample liner is extracted from the barrel. On board Geotechnics Limited geologists split each tube into 1m segments, carried out thermal resistivity testing, preliminary logging, and then sealed and labelled each tube ready for transportation to Geotechnics Limited’s office in Chester. 

At the office each 1m sub-sample was split, photographed and logged in detail.  The samples were sent to Geotechnics limited’s NAMAS/UKAS accredited laboratory for particle size distribution and other tests, all to DIN Standards to comply with TenneT’s Specification and BSH Standards.

The works scheduled were routine comprising a cable percussion borehole to be drilled to approximately 18.00m together with some window sampling and dynamic probing.  The work site, however, was located in the inter-tidal area of the Afon Dwyryd Estuary, approximately 700m from the nearest road.  Access was further restricted by environmentally sensitive grass land between the coast and the tower base preventing the use of any vehicles to cross the grass land

Lifting the equipment by air was proposed but restrictions on helicopter availability and lifting capacity coupled with a suitable tidal window reduced the options of rig type.

The helicopter had a maximum lift capacity of 1100kg ruling out completely 1.50 tonne cable percussion rigs.  A Pilcon Wayfarer 1500 rig designed to fit into a 20′ container was selected as ease of leg and mast removal and re-assembly together with a maximum single combined unit weight (engine, winch drum and axle) of 1040kg which just fell within the lifting criteria.

 Pre weighed stillages of drilling and window sampling equipment were made up to speed up the lifting and recovery process, with all equipment moved in eight lifts each way.  The works were completed over a period of 3 days.

Geotechnics Limited are currently undertaking a ground investigation at Wimbleball Dam in North Somerset. The dam, which is some 50m in height, was constructed between 1974 and 1978 using mass concrete buttress foundations benched into the underlying bedrock, and has a storage capacity of around 21,500M litres.

The geology around the dam comprises the Upper Devonian Pickwell Down Beds which are formed of sandstones, siltstones, mudstones and slates. In places these beds are faulted and fractured, which necessitated the formation of a grout curtain at the time of the dam construction, to form a seal around the dam shoulders.

A new grout curtain is now proposed for the southern end of the dam to address suspected leakage through the fractured rocks. In order to optimise the possible design solutions, Halcrow Group approached Geotechnics Limited to devise a digitally controlled hydrostatic test to determine the permeability and pressure head data at a range of test depths up to 50m bgl. Given the rock variability, it was important for equipment to be able to assess a wide range of potential rock mass permeability, with flows in the range 1 litre/min up to 20 litres/second, thereby requiring flow control and pressure measurement systems to suit. Halcrow also had a requirement to improve data quality and digital data storage to a level greater than that currently expected by British Standards, to ensure accurate determination of the permeability values.

Geotechnics have taken the required Specification for the tests and designed and constructed a digitally controlled Borehole Packer Equipment with data storage capacity.

At the surface a digital flow meter is clamped to the valve control assembly which allows the flow rate to be monitored and adjusted by either a larger diameter valve control assembly, or at very low rates of flow, the use of a smaller diameter bypass valve arrangement.

Pressure transducers/thermistors, placed above and below the Packer, are connected to a laptop computer at the surface, where real time pressure and flow rates are presented on screen continually throughout the test. The ability to also measure the water temperature above and below the packer provides additional data on the time taken for water to pass through the fracture zones, and around the packer.   

On this particular contract, the packer tests are being carried out in a rotary cored borehole, with test sections of up to 5m. Injection pressures are set at hydrostatic pressure plus 1 bar for high flow situations, and up to a maximum of 10 bar for low flow situations.  The tests are commenced at the set pressure and flow rate until a constant pressure of 5 mins is achieved, or for a period of up to 30 minutes.

Calculation of permeability results are made using Jacob and Loham (1952), the ’straight line method’, or by using BS5930. 

Geotechnics Limited were appointed to carry out investigation works comprising of four over water boreholes to measure the depth of silt and sediment which would need to be removed from the bed and to prove the depth and condition of the underlying sandstone strata. Cable percussion boring and rotary drilling work was carried out from a spudded pontoon barge using a standard Pilcon Wayfarer boring rig and a Mobile Drill B24 rotary rig.

The condition of the dock walls, their depth and foundations were critical to the scheme designer R G Parkins and Partners Limited. Quayside level is +12.2m Chart Datum, the base of the wall was expected to be at about -3m Chart Datum, and the proposed dredged level is -2.9m Chart Datum,  so there were serious concerns that the walls would lose frontal lateral support.

The investigation called for rotary core holes to be drilled from coping stone level, through the Victorian mass concrete wall and into the foundation materials. Geotechnics Limited used a Comacchio 205 track mounted rig, working as close as one metre from the waters edge and chose a T6/116 water flush, coreline set, drill barrel, and Series NF drill bits supplied by JKS Boyles UK Ltd. The wall structure proved to be variable but intact and the drilling equipment successfully cored granite coping stones, mass concrete and sandstone blocks.

Laboratory testing is underway and design options and costings are being finalised.

 Geotechnics Ltd were commissioned to carry out the works as ground investigation marine specialists under the AX5 KitFramework Agreement Term Contract.

Construction woks will comprise:

• Dredging out of Wellington Dock,
• Removal of dock bed silts/clays to either the River Mersey (Dependent upon a Marine Management Organisation Licence Application) or to a suitably licensed landfill, Backfilling of Wellington Dock with engineered fill.
• Construction of a new process treatment works to reduce Chemical Oxygen Demand (COD) of waste waters prior to being discharged into the River Mersey via the new outfall structure.

United Utilities commissioned a bathymetrical and magnetometer survey to determine river bed levels and any anomalies such as UXO and obstructions that could be a risk to both the investigation and construction schemes.

That survey information was provided by Geotechnics Ltd to aid the selection of a suitably large jack up platform to work in up to 20m of water tidal range and to withstand currents of up to 5m/s in the river. Specialist marine contractors, Red7 Marine were asked to mobilise Haven Seajack 1 – an 18 x 18m modular platform with 27m long jack up legs to Wellington Dock in late January 2011.

Eight HGVs duly arrived and work began on erecting the giant meccano set in time to ensure the lock gates from the dock system in to the river were open at or around ’slack water’ ! Carmet Tug Company supplied two tugs, ‘Audrey’ and ‘Vigour’, to tow the Seajack through the docks system and out into the River Mersey. A safety vessel, ‘Vigour’ was also supplied for both staff crew changes at near by Pier Head and to act as a support vessel in the case of an emergency.

Potentially the most difficult aspect of the investigation was the outfall works in the river. Careful planning and timing was required to ensure that the boreholes in the deepest water were completed at neap tides. Over a period of 5 days four 10m boreholes, positioned along the route of the proposed 300m long outfall were successfully drilled through the river bed sediments and into the underlying Sherwood Sandstone.

Haven Seajack 1 came back into the dock system on 31st January – mission accomplished and bang on programme !

Meanwhile, within Wellington Dock Geotechnics Ltd were busy carrying out over water works on a smaller scale using a spudded pontoon barge. Water depth here was generally less than 9m deep, and a total of eleven boreholes were carried out, again using a combination of cable percussion and rotary techniques to depths varying between approximately 13.40m and 24.90m below dock base level.Samples were taken at pre-determined depths within the dock silts/clays for chemical analysis to determine whether materials could be removed from site to the River Mersey Estuary (subject to a Marine Management Organisation Licence Application) or whether they would have to be removed to a suitably licensed landfill.

Landside boreholes around Wellington Dock were also required – a total of seventeen were needed to be drilled to investigate the backfilled nature of the dock walls and to determine contamination levels of both the soils and groundwater. Each borehole was drilled with a combination of cable percussion and rotary techniques to depths of between 20.40m and 29.70m below ground level. Dual monitoring installations, one deep and one shallow, were installed to monitor ground water and gas regimes over a period of three months. Eight automatic water level loggers were installed to monitor tidal variations over a period of one month.

The wealth of experience exhibited from Geotechnics Limited’s own staff teamed with approved specialist contractors resulted in a high quality and professional investigation in a demanding and potentially risky environment.

All site works were supervised by Senior Engineer, Lawrence Page and Geo- Environmental Engineer, Jon Hutchinson with project management by Paul Hayes.

The impact of European legislation and directives are being felt in the site investigation industry, and the recent introduction of Eurocode 7 has had significant implications for  technical reports, in-situ testing, and sampling techniques.

There has been considerable debate in the geotechnical press about the benefits and drawbacks of Eurocode compliance, but one thing is clear- the Eurocodes are here to stay. Consequently the major players in the geotechnical industry are learning to live with the requirements, and developing working methods to suit.

Geotechnics Ltd has been at the forefront of the drive toward quality and consistency in the site investigation industry since being founded in 1983. A recent project undertaken from its Coventry head office illustrates the nature of the Eurocode compliant working methods now being adopted in the engineering reporting carried out by the company.

Highway upgrade project

In July 2010, Geotechnics was awarded a site investigation contract in Stourport on Severn, UK for a highway upgrade project related to a proposed development of an approximate 8ha site by Tesco. It was important from both the client and the designer’s perspectives that the investigation and the subsequent design work were seen to be Eurocode compliant.

In accordance with the requirements of the Highways Agency, a Preliminary Sources Study Report (PSSR) was undertaken prior to the main ground investigation works. This is the equivalent of the desk study of existing information and site reconnaissance which has been the corner stone of high quality site investigation practice in the UK for a number of years. It forms part of most projects and is considered to be critical for a successful and efficient investigation design.

The proposed highway works are to include the upgrade and resurfacing of an existing road, and the construction of a new link road running on embankments up to 2m in height in accordance with the highway design scheme devised by the client’s design engineer (Arup). The proposed link road will cross a former factory site and a flood plain, and will involve the construction of a new road bridge and a footbridge over the River Stour.

The PSSR showed that the site is underlain by soil comprising Recent alluvial deposits and Pleistocene River Terrace sediments. These superficial strata are underlain by rocks of the Wildmoor Sandstone Formation of Triassic age.

The investigation was designed to comply with Eurocode 7 and the new European and International testing standards, and employed a wide range of techniques. These included both cable-tool and rotary drilled boreholes, window sample boreholes, trial pits, static cone penetration testing (CPT), pressure, meter testing, dynamic cone penetration (DCP) testing and CBRs. Selection of the most appropriate techniques to suit the expected ground conditions was critical to optimising sample quality.

Inspection pits were used to investigate the foundations and sub-grade of an existing retaining wall, and bridge foundations. Cored samples were taken of the existing pavement structure, and the presence of a number of exposures of the sandstone allowed geological mapping. The ground investigation work was carried out under the supervision of Geotechnics’s senior engineers, Habib Rehman and Leigh James.

In accordance with BS EN 1997-2: 2007 and BS EN ISO 22745-1: 2006, Class 1 undisturbed samples were required for compressibility and shear strength testing. These were made possible by utilising a piston sampler in the soft alluvium and thin-walled, 100mm diameter open-tube samplers in firmer clays. Laboratory testing was undertaken at Geotechnics’ NAMAS accredited testing laboratory in Coventry.

The full ground investigation report was drawn together by principal engineer Trevor Hardie, using the protocols embedded in the Eurocodes.

The field work data and laboratory results were presented in a substantial two-volume report, together with a summary of the ground and groundwater conditions encountered. Engineering and geotechnical Parameters for each material type were also presented for use in detailed design.

Equipment

On a wider front Geotechnics Limited has also recognised the need for the development and availability of Eurocode-compliant sampling equipment and has liaised with a number of industry-leading manufacturers to ensure that high standards are maintained.

Eurocode 7 has particular implications relating to Class 1 sampling and the calibration of Standard Penetration Tests (SPTs). Together with industry suppliers, Geotechnics has helped to make UT100 thin-wall steel sampling tubes, catcher boxes and cutting shoes widely available. Recent experiences with the new UT100 tubes have been mixed- they sample soft to firm clays reliably, but the cutting shoes and tubes can buckle when driven into stiff gravelly clays such as glacial till.

Geotechnics has also recognised the need to comply with SPT hammer calibration requirements under the Eurocode. All hammers used by the company are fully calibrated and certificates are included in reports where necessary. The company also has mandatory arrangements in place to make sure that approved subcontractors in its supply chain use calibrated hammers. Other factors which affect the potential variability inherent within the test, such as operator skill and equipment condition are also monitored by site audits. Interpretation of the results, not in isolation, but as one part of the assessment of the ground model is crucial.

The AGS Format allows direct transfer and receipt of data without the need for a printed interface, provided that the producing and receiving software can export/import data using the protocols embodied in the format. This concept avoids wasteful re-entry of data and allows efficient use of them all and a means to create an electronic archive which can be updated and used in the future. The AGS Format not only embraces geotechnical and geological data but also instrumentation and geoenvironmental data.

The latest manifestation is AGS4 which recognises the need to comply with Eurocodes and QA processes and to accommodate concepts such as bar coding for example. This was launched at a conference held in Birmingham where over 100 enthusiastic practitioners from all over the world presented and discussed the use and development of the Format. It has provided designers, specialist contractors and clients with the potential to make significant savings in costs and time and to make much better, more comprehensive and timely use of the data obtained.

However, whilst it is widely available, research by the AGS and FPS has shown that AGS Format data is not being passed down the supply chain in many cases. Reports in .pdf form may well be useful but this is not the same as data in AGS Format. Various educational initiatives by members of the AGS working group and in particular Chris Power are promoting its use to Universities and other bodies.

Jackie Bland, Geotechnics Limited’s IT manager, has been involved with the Format since its inception and now chairs the Data Management Group which has responsibility for maintaining and developing the Format. She follows on from the previous chairmanship of Steve Walthall who led and inspired the continually enthusiastic team up to the production of AGS4.

Jackie wrote Geotechnics’ project management system “GeoCentric” which deals with the whole Site investigation management process from first quotation or proposal, through ordering, site work and laboratory test scheduling to report production and accessible referenced archiving. Her founding principles are “enter it once, use it many times”; data entry is critical and must be a simple process to follow; outputs must be appropriate to the question being asked and easy to use otherwise their value is diminished. She regularly extends the system to accommodate the latest “good ideas”. As would be expected, she has a particular focus on ensuring that reporting outputs of the data are compatible with the AGS Format.

Whilst the Digital Age is unlikely to feature in geological chronology it is one which is very significant to both the understanding and application of the geosciences.

Close to Tedburn St Mary near Exeter, measurement and assessment of embankment instability was required as there are signs of landslipping. Inclinometers were important to allow lateral movements within the slope to be monitored. Instruments previously installed by Geotechnics Limited over two earlier phases included Vibrating Wire Piezometers and standpipes and these were indicating very high groundwater pressures which were concerning the Client, Enterprise Mouchel. Movements were also being identified by reference to marker pegs which the client had placed on the embankment.

A rig capable of drilling through the embankment into natural, stable Carboniferous mudstones into which the inclinometer could be socketted was needed. Accessibility to the desired locations was the main obstacle as the embankment itself is fairly heavily vegetated and slopes down at a gradient of around 1:2. It is locally steeper at the investigation area due to the features of the slip. Further constraints due to land ownership at the base of the embankment meant that access was only available from the top of the embankment, directly off the A30.

Two options were looked at by Geotechnics’ Matt Yates. One involved a slope climbing rig which would travel down the slope to the top of the slip to allow drilling to proceed. However, to ensure that the rig and crew wouldn’t slide down the embankment, the rig would need to be tethered to anchors at the top of the embankment. This risked anchoring into the failing material itself and potentially helping to pull the embankment down!

The option adopted was to mobilise a tracked excavator with a modified boom and a rotary drill attachment. Under a Lane 1 closure the rig, together with compressor, water bowser and support vehicles, was mobilised to site and the rig tracked along Lane 1 into position. The boom reached over the crash barrier down to the proposed borehole position and, using rotary solid-stem augering techniques, the drilling commenced. It was expected that, upon encountering natural bedrock, the drilling technique would need to switch to down-the-hole-hammering. However the auger managed to penetrate to the required depths successfully at the two borehole locations, reaching between 13.5m and 15.5m below surface level.

Once the augers had been removed from the ground, 58mm outside diameter click-lock inclinometer access pipe was placed into the open borehole and filled with water to counteract buoyancy caused by subsequent grouting. This was achieved using a mixture of bentonite powder and cement, mixed on site and pumped into the borehole. The installations were finished with raised lockable barrel covers cemented at the surface. This work was completed in two working days – slightly ahead of schedule – and all plant was removed from site and the Traffic Management removed the following morning, as planned.

Around one week later, three sets of base readings were taken at each borehole using a biaxial inclinometer probe equipped with Bluetooth which transmits data directly to a weatherproof PDA. Data was then taken back to the office and downloaded into In-Site inclinometer software and results shown as tabulated readings and graphs. On-going monitoring of the instruments is being done by the Client.

Geotechnics sees instrumentation as a key means of delivering significant geotechnical benefits and the innovative installation techniques and monitoring used at this site shows its commitment to providing solutions to the access problems which slopes so often pose.

The anticipated geology comprised Terrace    Gravels, London Clay, Lambeth Group and Thanet Sands with Chalk at depth. The tender for the work required rotary coring of most boreholes for much of their depth. However, Geotechnics’ experience of rotary drilling in the Lambeth Group and Thanet Sands was such as to lead them to submit an alternative tender with significant cost savings whilst maintaining or even enhancing the quality of the technical output. The Thanet Sands are infamous for the problems they cause during drilling but at this site it was considered practical to use predominantly cable percussive techniques with only two boreholes required to penetrate   to significant depths into this stratum.

The Client and the Engineer, Buro Happold, were interested in the alternative and the contract was awarded to Geotechnics Limited. The work also involved dynamic sampling, pavement coring, trial pitting and in situ CBR testing. Instrumentation included shallow standpipes, deep Vibrating Wire Piezometers and water level monitoring transducers for continuous monitoring of shallow ground water levels. Geotechnics Engineers for the project were Rob Webster and Dan Fry and the management of the site work was by Ian Boyle.

Prior to and during site work Geotechnics Limited had to maintain close liaison with both the Highway Authority and the occupants of business units on the site to ensure the minimum disruption. Laboratory testing was undertaken at Geotechnics’ UKAS accredited laboratory in Coventry and a factual report was provided in November 2009. Monthly site monitoring has continued over the twelve month period following the site work.

In February 2010 Geotechnics Ltd was invited to prepare a quotation for the Phase 2 works which built on the knowledge gained from the first phase and addressed developing features of the design. Particular attention was given to dynamic sampling as standard equipment had limited success in the dense Terrace Gravels due to limited rates of penetration and instability at depth. In order to overcome these problems and to permit access to the concourse areas, a mini rotary rig with hollow stem auger was mobilised and sampling carried out through this. Site works were successfully completed May 2010.

This project extends the Company’s track record in the investigation of major stadia in the north London area having undertaken the complex investigations for the new Arsenal Emirates stadium some years ago. We are proud to have contributed to this prestigious scheme and hope that it Spurs the club to greater success in the future!