Abstract

Most published research on CPT based SBT classification is on mineral soils. Consequently, these classifications do not accurately capture the classification of soft organic clays and peats. Organic soft soils are frequently present within the Holocene deposits in the Netherlands and in other deltaic areas worldwide. Organic soils can be identified by a specific combination of CPT parameters such as a high friction ratio, low cone resistance and low pore pressure response. In contrast to other soft soils, the strength is not necessarily low. This paper presents an updated CPT based classification with focus on organic soils, for the non-normalized SBT chart (Robertson, 2010, Lengkeek et al, 2022) as well as a new classification based on the stress normalized SBT chart (Robertson, 2016). In the new proposed classifications, additional boundaries are set based on the CPT pore pressure measurements, as this appears to be successful to separate organic soils from mineral soils. The performance of the classifications can be quantified by metrics such as the F1 score. The F1 score of the new proposed classifications all show significant improvement.

Abstract

This paper reviews the use of piezocone (CPTU) testing to characterise and identify peats and organic soils. Examples of data for peat sites from several countries are given, including some experience of the use of T-bar and piezoball penetrometers in peat. These “full flow” devices show smoother resistance profiles than the equivalent from the CPTU and are perhaps representative of the peat mass with a reduced fibre effect. In CPTU tests, organic soils and peat are often characterised by low corrected cone resistance (qt) and high friction ratio (Rf). CPTU penetration in peat is often drained with data showing low pore water pressure coefficient (Bq) values. Rf broadly decreases with increasing degree of decomposition of the peat. However care needs to be taken in using Rf values in peat given the characteristically very low sleeve friction values (fs) encountered. It would seem that it is not always easy to distinguish between peat and underlying soft soils using CPTU alone. There may be some promise in inclusion of CPTU “add on” sensors particularly a seismic element to yield shear wave velocity (Vs). However there is some uncertainty in measuring Vs in peat both offshore and onshore so care is needed in this regard. Recently developed Soil Behaviour Type (SBT) charts from the Netherlands have also been trialled on several sites. This new formulation shows promise and warrants further study

Abstract

Accurate evaluation of undrained shear strength of soils is crucial in geotechnical design and assessment. In the practice, undrained shear strength is obtained most frequently from CPT data, dividing the net cone tip resistance by a cone factor, 𝑁 . For organic soils, values between 8.6 and 15.3 are reported, depending on the stress history. The cone factor can be conditioned to the results of laboratory tests, although uncertainties remain on the variety of stress paths followed by the soil elements around the tip of the cone, compared to the ones tested in the laboratory. Non-uniqueness in the definition of the cone factor may lead to either unsafe or over-conservative choices, partly undermining both the reliability and the sustainability of the design. This contribution analyses numerically the inversion technique used to determine the undrained shear strength of organic clays, exploiting data from an extensive in situ and laboratory investigation. The adopted constitutive model was calibrated on the results of laboratory tests. Cone penetration tests were simulated performing coupled hydro-mechanical numerical analyses via G-PFEM, developed in the last decade at CIMNE-UPC. The role played by initial stress state and previous stress history upon stress distribution at failure, cone factor and sleeve friction is discussed. The numerical results suggest how the sleeve friction could be used to condition the cone factor depending on the over-consolidation ratio and demonstrate how combining the different available CPT readings with the aid of numerical results may reduce the uncertainty in the estimation of undrained shear strength.