This paper reviews the percussion-cup liquid limit, thread-rolling plastic limit (PL) and various fall-cone and other approaches employed for consistency limit determinations on fine-grained soil, highlighting their use and misuse for soil classification purposes and in existing correlations. As the PL does not correspond to a unique value of remoulded undrained shear strength, there is no scientific reason why PL measurements obtained using the thread-rolling and shear-strength-based fall-cone or extrusion methods should coincide. Various correlations are established relating liquid limit values deduced using the percussion-cup and fall-cone approaches. The significance of differences in the strain-rate dependency on the mobilised fall-cone shear strength is reviewed. The paper concludes with recommendations on the standardisation of international codes and the wider use of the fall-cone approach for soft to medium-stiff clays in establishing the strength variability with changing water content and further index parameters.

This state-of-the-art paper discusses determinations of remoulded undrained shear strength (Su) for municipal sludge and residue materials by the fall-cone approach, used increasingly for this purpose in practice. The strength mobilised by a falling cone as it penetrates into a soil test-specimen is related to its 'static' strength Su (used in design for most conventional loading cases) through a rate dependence parameter. From a review of the literature and analyses of existing and new experimental data, compared with inorganic soils, these highly organic soils are found to exhibit significantly greater strain-rate dependence for strength, which reduces correspondingly the value of Hansbo's cone factor K. Hence, in the absence of reported experimental K values for different organic soils, use of inorganic soil K values in strength calculations can often result in significant overestimations of undrained strength. Recommendations are made regarding the cone characteristics more suitable for such testing and experimental approaches useful for determining related K values. Other influencing factors, including the fibrous nature of some organic soils and cone roughness (adhesion), are discussed. The paper concludes by presenting semi-logarithmic and power model formulations for Su determinations from measured water contents as interim (alternative) methods until the mentioned issues are satisfactorily resolved.

Sivakumar V, O'Kelly BC, Henderson L, Moorhead C, Chow SH, Haigh SK and Vardanega PJ (2016) Discussion: Measuring the plastic limit of fine soils: an experimental study. Proceedings of the Institution of Civil Engineers, Geotechnical Engineering, vol. 169, issue 1, pages 86–89. http://dx.doi.org/10.1680/jgeen.15.00090

Road construction in peatlands is challenging. The ability to make rapid estimates of the response of construction soils derived from natural peats to changes in water content is useful for pavement and geotechnical engineers. This paper details some laboratory test results on peat soils sourced from two sites in the Southwest of England. The samples were sieved and the roots and natural fibres removed prior to laboratory testing. Water contents on the natural specimens were determined. The percentage of roots in the samples was determined. The thread rolling test was used to estimate the plastic limit of the peat soil material. A series of fall cone tests were conducted at varying moisture contents to determine the liquid limit of the peat soil as well as study the variation of fall cone undrained shear strength with the liquidity index, logarithmic liquidity index and the water content ratio. Both the liquidity index and logarithmic liquidity index are able to predict the fall cone undrained strength to within ± 40% around 90% of the time. When using the water content ratio to predict the fall cone undrained shear strength an accuracy of ± 40% is achieved around 85% of the time. The study concludes that the liquidity index and logarithmic liquidity index are better predictors of fall cone undrained shear strength but the water content ratio approach may be preferred if the engineer is less confident in plastic limit determination for peat soils.

Haigh, S.K. & Vardanega, P.J. (2012) Discussion of “Re-examination of undrained strength at Atterberg Limits Water Contents” by H. B. Nagaraj, A. Sridharan and H. M. Mallikarjuna. Geotechnical and Geological Engineering, 30(6): 1389-1391 https://doi.org/10.1007/s10706-012-9543-0

CITATION: O'Kelly, B.C. (2016) “Briefing: Atterberg limits and peat”. Environmental Geotechnics, Volume 3, Issue Number 6, pages 359–363. http://dx.doi.org/10.1680/envgeo.15.00003 The Atterberg limits are the most common tests specified by practising geotechnical engineers worldwide. These tests, which have physical meaning for fine-grained mineral soils, are regularly specified for peat and other highly organic soils despite fundamental issues regarding their appropriateness for such materials. These issues are explored in this briefing from which it is concluded that Atterberg limit concepts are inappropriate for peat and are unlikely to meaningfully correlate with mechanical (strength) behaviour. In assessing likely engineering behaviour of peat, a more useful suite of index tests is natural water content, organic content, fibre content and degree of humification. For correlations with strength and compressibility parameters, Dutch organic soil practice has found that water content and bulk density are usually sufficient.

This paper reports on the challenges associated with the determination of the Atterberg limits for peat, fundamental issues regarding the appropriateness of Atterberg limit concepts applied to peat and peaty soils and their use in characterising the engineering behaviour of these materials. As demonstrated in the present study, different sample preparation methods and preloading of the peat material (which gives the organic solids some stress history because of their compressible nature) can result in significantly different Atterberg limit values being measured. The significance of reinforcement and scale effects related to the peat fibres for the thread-rolling method is investigated. It is concluded that the Atterberg limit tests are not appropriate for peat in that the deduced plastic range for the peat test material is notional and the calculated liquidity index values are not reliable indicators of its consistency. In assessing the likely engineering behaviour of peat material, a more useful suite of index tests is its natural water content, organic content, fibre content and degree of humification.

Citation: O’Kelly BC (2016) Assessing the shear strength of municipal sludges and residues for landfill disposal. Proceedings of the Third Symposium on Urban Mining and Circular Economy (SUM2016), Old Monastery of St. Augustine, Bergamo, Italy, 23–25th May 2016. CISA Publisher: Italy. 11 pages. http://urbanmining.it/ This paper describes various issues from a geotechnical standpoint concerning the strength of biosolids (sewage sludge) and water-treatment residue (WTR) materials for landfill (monofill) disposal. These materials must be sufficiently dewatered at the municipal works to reduce transportation and landfill-disposal costs, to provide adequate shear strength for efficient handling, placement and trafficability equirements at the landfill site, and for geotechnical stability of the landfill slopes. Topics covered in this paper include: (i) the characteristic behavior and properties of these challenging geomaterials; (ii) undrained strength requirements for landfilling; (iii) in-situ and laboratory strength measurement techniques and interpolation of the strength data; (iv) strength predictions using existing undrained strength–water content correlations. Since these correlations are material specific, with the geotechnical properties of biosolids and WTR materials varying between treatment plants, they generally cannot be applied more widely with confidence. A new (different) approach, which uses a power-law relationship to predict values of remolded undrained strength mobilized for different water contents, is presented.

Vardanega, P.J. & Bolton, M.D. (2016). Discussion of “Characterization of Model Uncertainty for Cantilever Deflections in Undrained Clay” by D.M. Zhang, K.K. Phoon, H.W. Huang and Q.F. Hu. Journal of Geotechnical and Geoenvironmental Engineering (ASCE), 142(1): [07015036] https://doi.org/10.1061/(ASCE)GT.1943-5606.0001395

A database of 641 fall cone tests on 101 soil samples from twelve countries has been analysed to determine the best mathematical relationship linking undrained shear strength with liquidity index. From the database, it is shown that the use of a linear relationship linking liquidity index and the logarithm of undrained shear strength that uses the commonly assumed 100-fold factor increase in strength from liquid to plastic limit over-predicts the measured data of soil strength. The use of a factor of about 35 for the ratio between the strength at liquid limit and that extrapolated to plastic limit is shown to be more realistic. Logarithmic liquidity index is examined and found to also correlate strongly with the logarithm of undrained shear strength, however it is shown that no great statistical improvement is present compared with the semilogarithmic formulation. When considering data of individual soils a power law fitting is shown statistically to be the preferred mathematical function.