Evaluation of peat strength for stability assessments

Abstract

SynopsisIn this paper guidance is given for the assessment of peat strength for stability assessments based on laboratory undrained simple shear tests (SS). When considering the stability of peat, these tests will yield a conservative estimation of the in-situ strength of the peat mass. The study was motivated by recent interest in renewable energy developments in upland peat areas. The results of more than 111 SS tests from 16 sites in Ireland, Scotland and the Netherlands were studied. It was found that peat strength is strongly influenced by its stress history and also varies as a function of the water content and degree of decomposition (fibre content). The normally consolidated normalised strength ratio (s u /σ v ') from SS tests of peat was found to be approximately 0.4, which is towards the lower bound of previously published data for peat. Comparisons of strengths derived from SS and field vane tests showed the ratio of the strength derived from the two tests was influenced by the degree of decomposition and that previously published correction factors for field vane strengths are inappropriate. Guidance is given for engineers working on future schemes on upland peat areas.

Measurement, interpretation and recommended use of laboratory strength properties of fibrous peat

“…Based on the trend in the data plotted in Figure 18, Boylan and Long (2014) reported a lower-bound ðs u DSS =s 0 vc Þ NC value of 0·40 for s 0 vc > 30 kPa, which agrees with the lower bound 0·38-0·40 range identified from earlier published DSS data for peat materials investigated by Carlsten (2000), Farrell et al (1999) and Lechowicz (1994). In other words, ðs u DSS =s 0 vc Þ NC ≈ 0Á40 is a lower-bound value for DSS testing of NC peat material (i.e.…”

“…The strength properties of peat material are strongly influenced by its degree of humification (fibre content) (Boylan and Long, 2014). For instance, the effective friction angle, f 0 TC , correlates to some extent with bulk unit weight and organic content (Den Haan and Feddema, 2013;Edil and Wang, 2000), with f 0 TC increasing with increasing organic content and reducing bulk unit weight (see Figure 2), since more fibrous peat material has a higher natural water content and, hence, lower bulk unit weight.…”

“…corresponding to t h max ) and t h max is the peak horizontal shear stress or, alternatively, is often taken as corresponding to 15% shear strain, whichever occurs first (Boylan and Long, 2014).…”

“…Other limited published s u DSS =s 0 vc data for Polish, Irish and Swedish peats (after Carlsten (2000), Farrell andHebib (1998), Farrell et al (1999) and Lechowicz (1994)) show much scatter in the range of 0·38-0·55. From analysis of the results of >111 standard DSS tests on very different peat materials sampled from 16 sites in Ireland, Scotland and the Netherlands (Figure 18), Boylan and Long (2014) suggested that the scatter in the data may be largely due to the effect of stress history, either in the field or the laboratory.…”

“…Measurement, interpretation and recommended use of laboratory strength properties of fibrous peat O'Kelly fully drained, an undrained stability assessment is often undertaken and represents the more conservative approach (Boylan and Long, 2014), assuming that the peat material is NC. Further, the strengthtesting method adopted must induce a shearing condition that is representative of the anticipated general field shear mode.…”