Development and advancement of CPT
As part of Lankelma’s commitment to the development and advancement of CPT technologies, it is assisting Dr Mike Long, of the University College, Dublin with the objective of assessing the suitability of using novel flow penetrometers, to more fully characterise soft Irish soils and
peat in-situ.
Undrained shear strength challenge
The measurement of undrained shear strength in very soft soil poses a challenge due to the low tip resisitances being measured. Also, the deformation mechanism around the cone is asymmetric in the vertical plane, hence a correction for overburden pressure and pore water pressure is required.
The empirical and theoretical solutions relating undrained shear strength to cone tip resistance are difficult to apply and resulting estimates can therefore be erroneous.
Many of the above limitations can be overcome by varying the shape of the penetrometer, so that it causes symmetrical flow during penetration. Two such devices are the T-bar and spherical ball cone penetrometers.
Encouraging results
The results presented are from a site at Nenagh, Co. Tipperary, with additional work now being performed at Lochrea, Co.Galway.
The initial results are extremely encouraging, with the flow penetrometers displaying a higher degree of definition within the peat deposits as well as providing a more accurate assessment of the undrained shear strength. The work is now being included as part of a Phd study at UCD.
Fig 1: (above) Undrained shear strength from ball cone
Fig 2: (below) PWP from flow penetrometers and 600
Fig 3: (bottom) Tip resistances from flow penetrometers
