LANKELMA RESEARCH

T-Bar Cone for Soft Soils: Lankelma research with University College Dublin and Norwegian Geotechnical Institute

Lankelma CPT Limited has recently completed a programme of field works in Ireland with Dr. Mike Long at the Department of Civil Engineering, University College Dublin (UCD), to assess the performance of a novel cone penetrometer to measure the undrained shear strength of soft soils. Three ‘high’ plasticity soft clay sites were selected for investigation. This data will augment the data of the NGI sites and has predominantly been obtained from sites comprising soils of ‘low’ plasticity.

The T- Bar consists of a short cylindrical bar measuring 250mm in length and 40mm in diameter that replaces the conventional 60 degree cone and is attached at right angles to the penetrometer rods just below the calibrated load cell. Also included in the shaft is an inclinometer to indicate any deviation from the vertical during penetration. The device can also have incorporated pore water pressure transducers. The T- Bar cone has two major advantages over the conventional electric cone. Firstly, the load cell measures what is essentially a differential force (or net pressure)on the bar, so that no corrections need be made for the overburden stress and ambient pressure.

Secondly, the correlation between net pressure on the bar and the shear strength of the soil is by an exact plasticity solution, with a potential range of bar factor of less than 10% (due to different roughness of the bar surface), compared with cone factors that range from as low as 7 for sensitive soils to over 15, that is a range of 35%.

An additional use of the T-Bar is to assess the remoulded strength of the soil, by monitoring the bearing resistance during extraction along the same path as insertion.

Hopefully, the combined UCD and NGI data will provide a reliable correlation between the undrained shear strength and T-Bar penetration resistance for soft soils that range from low to high plasticity. The results of this work are to be published in the near future. For further information contact Brian Georgious at our Regional Office 01244 289 356.

Soil Moisture Probe (SMP)

The soil Moisture Probe (SMP) is installed directly behind the electric cone and permits the simultaneous acquisition of resistivity, soil moisture and geotechnical data. The SMP comprises two ring shaped and carefully isolated electrodes and an Application-Specific Integrated Circuit (ASIC). The ASIC uses synchronous detection for accurate measurements, free from electric interference Dielectric soil properties, capacitance and electrical conductivity, are governed mainly by the water content of the soil mass. By measuring these parameters between two electrodes, with the soil dielectric in between, the soil moisture can thus be determined.

The capacitance of dry soil is determined by its dielectric constant. For dry soil this constant is of the order of 3 to 6 and for water the constant is 80, that is, the capacitance between the two electrodes is dominated by the volume fraction of water.

In polluted soils non-aqueous phase liquids (NAPL’s), such as oil or chlorinated solvents, will replace part of the water in the soil matrix. As they do not dissolve easily in groundwater, they will occur as a film of pure product, often just above the groundwater table. Dense NAPL’s such as chlorinated hydrocarbons, which are denser than water, will penetrate the groundwater table and are typically found on top of impermeable layers. These pollutants affect the dielectric properties of the soil significantly and are detectable using the Soil Moisture Probe. Graph above of water content v. dielectric constant. By using this cone technique, time and cost can be saved.

Special Guests

Lankelma CPT Ltd was very honoured to have two well respected guests visiting their Iden Offices. Two of the authors of the book Cone Penetration Testing in Geotechnical Practice, Tom Lunne form NGI in Norway and John Powell of BRE in the UK came over to discuss developments in cone testing.

On the photo you can see Lunne and Powell with technical director Andy Barwise and geotechnical engineer Darren Ward in Lankelma's cone workshop.