Cone Penetration Testing & Seismic Testing

In August 2023, we were commissioned to undertake a comprehensive Geotechnical and Geoenvironmental Ground Investigation to provide data at trenchless crossing locations as part of National Grid’s proposal to replace the existing overhead lines at Glaslyn with underground buried power lines. The purpose of the project was to remove the negative visual impact of the existing overhead power cable infrastructure over a 6km linear route and enhance the landscape at Snowdonia National Park.

In order to carry out the assessments, we undertook an investigation comprising In-Situ CPT testing, boreholes, trial pitting, monitoring, laboratory testing and reporting. The site works were completed between August and October 2023.

We successfully undertook a multi-faceted investigation that utilised the different areas of expertise within the company. The collaborative approach within our team allowed the In-Situ Testing, Ground Investigation & Laboratory Testing departments to all contribute to the investigation in a meaningful way to promptly deliver high-quality data that the client can rely upon. We were able to provide a single point of contact for these services, improving the efficiency of our service delivery and ultimately reducing the clients programme and costs.

Technical Details of the work

The fieldwork was carried out between August and October 2023, in and around the area of Glaslyn, near Porthmadog. The 6km long site was linear, generally at a low level and in areas that were liable to flooding. The exploratory hole test locations were spread across highly sensitive areas that included Agricultural Land, Ancient Woodlands, Conservation Areas, Priority Habitats, Sites of Special Scientific Interest, Nature Reserves, and Special Areas of Conservation.

Our team worked collaboratively with the client, land agents, conservation groups and ecologists to ensure works were completed with the minimum disruption possible to both landowners and the environment.

The intrusive investigation works included:

•   A series of CPT & Seismic Tests to depths of up to 20m
•  25 no. cable percussion boreholes to depths of up to 20.45m below ground level (bgl) › Nine machine excavated trial pits
• Extensive field testing – including thermal resistivity, falling, and rising head permeability and dynamic cone penetration
• Monitoring installations – including groundwater, gas and magnetic extensometer.

CPT test control and data acquisition was carried out using CPTask software. Cone end resistance, sleeve friction, porewater pressure and inclination were recorded at one centimetre intervals with the groundwater levels estimated from examination of the porewater pressure measurements and the general groundwater monitoring data from borehole installations. Datgel software was then used to derive values of shear strength, relative density, friction angle, SPT N60 value, soil behaviour type and initial stratum descriptions which were further refined by comparison with other site data collected as part of the investigation. The results were presented as plots relative to both the depth below ground level and the Ordnance Datum.

Seismic Cone Penetration Tests (SCPT) were completed using a pair of inline triple axis geophones. Testing was carried out in accordance with ASTM 7400 Standard Test Methods for Downhole Seismic Testing. Seismic velocity profiles were derived using the surface to downhole technique in which the source was fixed to the ground surface and the detector system was advanced into the ground in a cone penetration assembly.

Shear (S) waves and Compressional (P) waves were generated in the ground surface working downwards in 1.00 meter increments, during the test data quality was monitored in real time on screen to ensure that the records being obtained were of suitable quality.

Values of small strain shear modulus (Gmax) based on an assumed bulk density were included on the figures for each test location together with seismic velocity details.

Collaborating with you as a Trusted Partner

From the outset, our team and the client’s project team were in constant liaison, keeping the client fully up to date with progress and enabling feedback from the client team in real time. Whilst carrying out the assessments, the project staff came across a number of technical challenges and went above and beyond the normal call of duty to deliver for the client