Organic soils are susceptible to creep. When predicting settlement of organic deposits due to small load increments, the initial conditions, particularly the initial strain rate should be assessed carefully. Recent developments in predicting creep behaviour of clays are tested in an ongoing study on organic soil behaviour. A series of incremental loading tests on peat and organic clay shows that a framework of undisturbed isotachs cannot be used to predict strain rates in overconsolidated conditions. This complicates the applicability of conventional incremental loading test results for assessing field conditions. Additionally, the tests show the relevance of a large strain approach when dealing with soft organic soils.@en

In practical engineering the direct simple shear, DSS, tests are a valuable tool for parameter assessment, despite its known shortcomings. The shear stress mobilised at the end of the test is often used as a proxy for critical state strength. Several aspects complicate conducting reliable tests at large strain, in specific for clay samples. One of these effects is slippage between the sample and the platens. When slippage occurs, the measured resistance follows from the interface properties rather than soil behaviour. To better understand the critical state behaviour of clays during a DSS tests a test series is conducted on an anthropogenic stiff silty clay. The test series not only demonstrates that critical state conditions are reached before the end of the tests, but also that slippage tends to occur after reaching critical state conditions. Complimentary series of direct shear, DS, tests were conducted to further confirm critical state conditions are obtained. This confirms the applicability of DSS tests to obtain a soil specific proxy for critical state conditions without the influence of slippage. Hence, the presented method to derive critical state strengths from DSS tests, for anthropogenic clay, and thereby discarding the influence of slippage provides useful and reliable strength parameters for design.@en

As the population in cities all over the world is increasing, the effects of climatic action on the resilience of communities is becoming more and more important. Cities situated in deltas are also under strong urbanization demands and these demands have to be met with due consideration of the challenges presented by climate change, land subsidence and sea-level rise. Within this context, the local geological conditions in the Netherlands present a particularly pressing challenge where water pressure under a clay cover may increase. The static equilibrium of the cover layer might be adversely affected, and uplift failure can be imminent. A recent research program into this failure path has been initiated. Besides the field tests and advanced numerical modelling approaches, the research program also made use of centrifuge tests to quantify the extent of uplift, cracking, and deformation phenomena. This contribution intends to exhibit the aspects of the centrifuge tests conducted as part of this study. The experimental setup design and design considerations will be explained as well as the instrumentation methodology and the reasoning behind the instrumentation choices. This contribution aims to improve the stability assessment of dikes under uplift conditions.@en

A series of plate loading tests on clay has been conducted in the centrifuge. The aim of the tests is to create a data set, which is freely downloadable, to validate numerical tools that account for geometrical nonlinearities. The tests include two sources of geometrical non-linearities. The first source is the reducing clay layer thickness below the plate, which causes an increase in resistance. The second source is the backflow of the clay around the tip of the plate. The backflow has a reducing effect on the plate resistance. This paper outlines four tests: two involving a wide plate and two with a small plate. Each plate geometry is investigated under both smooth and rough side model boundaries. An material point method (MPM) schematization is used for numerical analysis. The schematization and parameter selection are initially validated by comparing the MPM results against CPTu data in each test. The numerical analysis examines the impact of a finite layer thickness by analyzing various layer thicknesses. Furthermore, the analysis shows the influence of the backflow on the plate resistance by analyzing different ratios of shaft to plate width. In this study, the pore pressures below the plate and vertical and horizontal displacement fields are considered in addition to the load displacement curves. The MPM simulations are in good agreement with the centrifuge data.

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Sub soil stratum in deltaic regions typically include a succession of permeable and non-permeable layers. Consequently, dikes built to protect delta areas, should be able to resist not only extreme surface water levels, but also an increase in hydraulic head in permeable layers. The rise in hydraulic head might result in uplift of the cover layer, inducing either backwards erosion piping or slope stability. A large research programme, including the combination of numerical analysis, centrifuge testing and field trials, is started to study the uplift phenomenon. Preliminary results show that uplift has a clear influence on dike stability. Horizontal compression of the cover, during uplift, might trigger slope failure. Large cracks in uplifted cover layer were not found in the field trials, although some local failure, resulting in sand boils, was observed. The test results provide valuable experimental data to further validate numerical tools and stability assessment procedures.@en

This study aimed to assess the suitability of lime treatment for use in dikes in the Netherlands. The effect of this technique on the behavior of a Dutch clay was addressed by comparing the detailed response of lime-treated and natural samples at different lime contents (1.25% and 2.25%) and curing periods. A series of laboratory tests consisting of index classification, constant rate of strain, and triaxial and hole erosion tests were performed. The results demonstrated that lime treatment altered the soil response. Differences were observed in the physical, compressibility, strength, and erodibility properties. It was found that lime improved considerably the resistance to compression and erosion, but the effect on hydraulic conductivity was limited. The triaxial test results showed that lime treatment was particularly effective at low stress (<25 kPa) and low strain levels (<10%). During shearing, lime-treated samples exhibited dilative tendencies and enhanced effective strength properties until a stress-strain state was reached that was believed to be related to the breakage of the bonding structure of the sample. The findings of this study demonstrate that the merits of lime treatment can be of particular benefit in dike applications, particularly when the focus is on improving soil erosion resistance.

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The dynamic behavior of a peat deposit in the north of the Netherlands is described. The organic content ranges from 70% to 95%, which is high compared to the organic content generally presented in publications on the dynamic behavior of peats. Shear wave velocities vs and correspondingly small-strain shear moduli G0 closely match values stated in the literature. Correlations stated in the literature for predicting G0 proved to be applicable. Resonant column and cyclic direct simple shear tests were performed to establish the shear modulus reduction curves and damping curves. Excess pore pressure development during testing indicates dilatant behavior. The general trend shows nearly flat shear modulus reduction and damping curves at small strains regardless of organic content. Cyclic direct simple shear tests on humified material showed a larger pore pressure buildup than found in tests on non-to-moderately humified material. Differences in degree of humification did not result in significant differences in the shear modulus reduction curve, including G0 values. Large scatter was found in the damping curves. For the humified material, tested at low stress level, a discontinuity in the damping curve is found at shear strain of 3%, which corresponds to a rapid pore pressure buildup in the tests.

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A series of undrained cyclic direct simple shear (CDSS) tests on dense Toyoura sand has been performed with the aim to investigate the influence of the stiffness of the DSS device on test results. To this end, springs were installed to reduce deliberately the stiffness of the apparatus. It is shown that the cyclic resistance of the sand depends strongly on the rigidity of the apparatus frame. In particular, as the stiffness of the DSS device increases, the number of loading cycles required to reach liquefaction decreases. This pronounced apparatus-stiffness dependence is of great practical concern in geotechnical engineering because it directly implies that the CDSS response of a soil sample can be predominantly controlled by the stiffness of the apparatus and not by the soil behavior alone. In addition, the test results indicate that the effect of equipment compliance in cyclic undrained DSS testing can be minimized when the ratio of the stiffness of the tested sand sample to the stiffness of the apparatus has a significantly low value.

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Direct observation of gas in peat layers, generated by slow degradation in anoxic conditions, raised concern in the Netherlands about its potential impact on the geotechnical response of dykes founded on peat. To address this issue, an experimental investigation was initiated, aimed at quantifying the main consequences of the presence of gas on the mechanical response of peats. The results of a series of triaxial tests on natural peat samples flushed with carbonated water are presented and discussed. Controlled amounts of gas were exsolved by undrained isotropic unloading, and the samples were sheared under undrained conditions. During gas exsolution, the samples suffered volumetric expansion, at a rate which is ruled by the relative compressibility of the fluid and the soil skeleton. The gas in the pore fluid dominates the stress-strain response upon undrained shearing, causing lower excess pore pressure compared to fully saturated samples. The experimental results suggest that local fabric changes occur during gas exsolution. However, for the amounts of gas investigated, these fabric changes seem to be almost reversible upon compression. Although the ultimate shear strength is hardly affected by gas, the reduction in the mobilised shear strength at given axial strain thresholds is dramatic, compared to fully saturated samples. The study suggests that the presence of gas must be cautiously accounted for at low stresses, when a reference stiffness is chosen for serviceability limit states, and when operative shear strength definitions, based on mobilised strength for given strain thresholds, are chosen in the assessment of geotechnical structures on peats.

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Peats are soils containing a significant component of organic matter. Biochemical degradation of this fraction generates gases such as CO₂, H₂S and CH₄, which tend to saturate the pore water eventually resulting in exsolution and expansion. The effects of these gases on the hydro-mechanical behaviour of peats are under investigation at Delft University of Technology. The results of a series of triaxial tests are discussed, in which gas was exsolved under controlled conditions by flushing natural samples with carbonated water, and undrained isotropic unloading and shear were performed. A significant reduction in the effective stress acting on the soil skeleton was observed during undrained unloading due to gas exsolution. However, different stages were observed in time, which appear to be ruled by the very high compressibility of peat. The mechanical response upon shearing is dominated as well by the ratio between the compressibility of the fluid and the soil skeleton. Although the ultimate strength does not differ much between the samples tested, the mobilised shear strength for a given axial strain does, which has to be accounted for cautiously in the choice for an operative shear strength.

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