Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat

Abstract

Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat C. ZWANENBURG Ã a n d R . J. JA RD IN E †The low submerged unit weights of peats usually lead to low effective self-weight stresses, stiffnesses and undrained shear strengths. These features, in combination with high compressibility, a propensity to creep and the uncertain effects of fibrous inclusions, make foundation stability hard to assess reliably. It is usual to apply high safety, or strong material reduction, factors in foundation design. However, over-conservatism can lead to undesirable environmental and financial costs. This paper describes full-scale field tests conducted on peat, with and without pre-loading, at Uitdam on the borders of Lake Markermeer, north of Amsterdam. The experiments investigated the peat layers' consolidation behaviour and their response under loading, including full shear failure. Noting the complex final test geometries and the large displacements developed, simple numerical analyses were undertaken to help interpret the failures within a Tresca and 'consolidated undrained shear strength' framework. The trials that included modest pre-loading developed large vertical consolidation strains (up to 35%) and significant bearing capacity improvements. The field experiments provide a rich resource for testing advanced numerical techniques. They also allowed a range of practical characterisation techniques to be assessed and calibrated for flood dyke applications.

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

“…(Ajlouni, 2000), with s u TC defined at the phase transformation point on the ESP trace described earlier, whereas ðs u fTC =s 0 vc Þ NC values of 0·62 for CK 0 UC (see Figure 11), 0·61 for CIUC and 0·59 (±22%) for CIUC (Zwanenburg and Jardine, 2015) were deduced based on estimated s u fTC values for Marken peat, Booneschans fibrous peat and Uitdam sedge-reed peat materials, respectively, where s 0 vc is the vertical consolidation stress at the start of the compression stage in the CU TC tests.…”

“…According to Helenelund (1967), the type of the surface moss has a considerable influence on the tensile strength of the peat mat; for instance, Polytrichum commune produces greater tensile strength than Polytrichum strictum does, with both of these moss types apparently having greater tensile strength than Sphagnum moss. O'Kelly (2015b) reported nominal undrained shear strength profiles determined from field vane measurements (s u FV ) for Ballydermot Raised Bog, with this peat deposit comprising three distinct sublayers: a desiccated zone within a Sphagnum peat layer that was underlain by fen/reed peat ( Figure 1), with the reported strength values calculated as m FV × s u FV , where µ FV is a correction factor with a value of 0·5 (Mesri and Ajlouni, 2007;Zwanenburg and Jardine, 2015). The significant difference in the shear strength profiles for the waterlogged Sphagnum and fen/reed peat layers is related to their different botanical compositions and is also partly due to the hydrological conditions at the test site, with the fen/reed peat layer underlain by higher-permeability subartesian inorganic layers that tended to act as a bottom drainage boundary, inducing downward flow in the lowermost portion of the peat deposit.…”

“…Further, the principal stress ratio ðs (Den Haan and Kruse, 2007;O'Kelly, 2015a) (true also for the other approaches to shear strength measurement described earlier), with low c 0 TC values ranging 0-6 kPa typically reported for fibrous peats (Edil and Zhang, 2000). In addition, according to Zwanenburg and Jardine (2015), K 0 is difficult to estimate reliably for peat materials, and anisotropic reconsolidation of the peat test specimen is difficult to apply for the very low effective stresses often encountered in situ.…”

“…s 0 r ¼ 0 condition), the ESP traces for CU TC of fibrous peat specimens will then follow up along the TCO line with an inclination M of 3:1 (e.g. see Den Haan and Kruse (2007), Yamaguchi et al (1985a), Zwanenburg and Jardine (2015) and Zwanenburg et al (2012)), making it difficult to define when specimen failure occurs. Note that M = 3 gives f 0 TC ¼ 90°, indicating an axial tensile failure mode, which has been observed experimentally for CU TC testing of large fibrous peat specimens (400 mm dia.…”

“…Although not the subject of this paper, it is also useful to determine the soil parameter values for geotechnical designs by using in situ tests and geophysical techniques that can measure pertinent soil parameters continuously to the required depth (e.g. see Long and Boylan (2012) and Zwanenburg and Jardine (2015)). This is important since a slip surface can go from weak spot to weak spot within the peat deposit, resulting in an overall mobilised strength value lower than the mean of the measured strength values (progressive failure).…”