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Peat Dikes and Strength Parameters – Unexpected Results By A. Tomczak 1 About 26% of the Netherlands lies below sea level while 55% is susceptible to flooding, therefore dikes are an essential part of the Dutch infrastructure. Of the approximately 16000 km of dikes, roughly 3500 km contain sig-nificant amounts of peat. In order to guarantee the safety of the inhabitants living behind these dikes these are checked regularly according to national standards. Strength parameters of the peat dikes’ soil form an important input for geotechnical stability calculations. Collecting data for these calculations is costly and labour intensive. Therefore the idea that a useful relationship might exist between the botanical composition and other properties of the peat and their mechanical strength was worthwhile to investigate. To this aim geotechnical descriptions of a large number of peat soil samples have been compared with results from laboratory tests to investigate if there is indeed a useful relation between the botanical composition, de-gree of weathering, amount of fibres and strength parameters. These characteristics have been determined by two traditional tests, according to Dutch standard geotechnical methods.

Background Approximately 2000 years ago a large part of the Netherlands was covered with peat, see figure 1. From the 10th century onward people started to drain peatlands to make them accessible and suitable for agriculture. At the end of the 15th century a large part of these areas was transformed into agricultural land. A dense complex network of canals drained surplus water to the sea.

Figure 1. Surface cover around 2000 BP, the purple areas are covered with peat.

1 Alicja_Tomczak(at)gazeta.pl

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As living peatlands contain large amounts of water the draining was not without consequences. Over time the used land surface subsided significantly, also due to the oxidation of the top peat layer. Eventually the system of canals that transported the drainage water to the sea by gravitation became less adequate as the subsiding agricultural lands became closer to sea level. This rendered part of agricultural land unsuitable for wheat pro-duction; these lands were increasingly used as grasslands for dairy. Due to further subsidence additional measures became necessary to be able to use the peatlands. Polders were created where windmills on the dikes that surrounded them were used to raise the water from the ditches in the agricultural plots to the canals that transported the water to the sea, see fig. 2.

Figure 2. Development of polders in the Netherlands In the 17th and 18th century large amounts of peat were used for fuel. During this period Holland prospered due to trade. As the cities grew larger the demand for fuel increased. The availability of wood from the higher sandy areas diminished, further increasing the pressure on the peat production. At the end of the 16th century the method of wet mining was introduced. A landscape with wide ditches and long narrow strips of land developed in the low laying peat lands, see fig. 3. The narrow strips of land were used to dry the peat excavated from be-low the water level. These strips were kept as narrow as possible. Wind and waves often demolished these un-stable strips of peat land over time, resulting in vast inland lakes.

Figure 3. Peat mining below water level and resulting landscape

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Due to the risks that these large inland lakes posed and the demand for agricultural land, many lakes and other low laying areas were also drained by windmills and later pumping stations, resulting in the characteristic Dutch polder landscape. A substantial part of the densely populated western part of the Netherlands is situated below sea level as is shown by the purple-coloured areas in figure 4.

Figure 4. Surface elevation of the Netherlands Of the 3500 km of dikes that consist largely of peat most are situated in the western part of the Netherlands. It was not well known that these dikes were not designed and built as such, but were strips of peatland left over after the removal of peat on both sides, often centuries ago when geotechnical knowledge was not yet well de-veloped. This is clearly not an ideal situation as history shows. To reduce the risk of dike failure to acceptable levels it is essential that the dikes are stable. Strength parame-ters of the dikes’ soil are important properties in this respect. Correlations between various parameters could probably reduce the costs of investigations to obtain reliable data for safety management.

Geotechnical description of peat dikes After a dike failure near Amsterdam in 1960 a technical commission for flood defence was established in 1965. Its task was to check the conditions of the Dutch dikes and provide recommendations as to increasing safety against flooding. Over the years many reports were produced. One of the reports produced by the commission was a technical report regarding the geotechnical classification of peat (1996). This classification was based on the geotechnical insights at the time and based upon Canadian and Finnish classification methods that were adapted for the Dutch conditions as to allow a first indication of the strength and compressibility of the peat. This geotechnical description of peat comprised the following aspects:

- botanical composition (sphagnum species, reed, sedge, wood, cotton grass peat) - degree of weathering according to the Von Post2 and PVI indices3; - water and ash content, specific mass; fibre content; - anisotropy of shrinkage: vertically more than horizontally.

2 Field method of determining the weathering degree, depending on a visual assessment of squeezed samples of peat. The humification degree is a scale from H1- H10 where H1 stands for young peat and H10 is completely decomposed peat. 3 PVI index – laboratory method for determination the weathering degree, based on the absorbance of a solution of organic matter extracted by solution of sodium pyrophosphate, measured by photo spectrometer

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Figure 5 Zones along dike failure slope

Figure 5 shows a typical surface failure for a dike. Along a potential failure slope three zones can be distin-guished. In the active zone the horizontally oriented peat fibres act as reinforcement. The resulting vertical

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strength is usually tested by so-called Triaxial Compression tests (TC-test). In the neutral zone the fill material between horizontal fibres plays an important role. The horizontal strength is usually tested by Direct Simple Shear tests (DSS-test). In the passive zone the fill material also plays an important role, because stretching of the fibres may occur. Here the vertical strength is usually tested by Triaxial Extension tests (TE-test). The TC-test and DSS-test will be explained below. For the current investigation Triaxial Compression tests and Direct Simple Shear tests were conducted. Both give shear strength of soil (the τ-axis, measured in kPa, see graph 1) as a function of confining or consolidation stress. The σ-axis represents the surrounding pressure (kPa) the sample would experience within the soil or body of the dike. Shear stress can be regarded as the resistance to shearing or failure. Failure occurs when shear stress surpasses the maximum shear stress the soil can withstand.

Graph 1. The estimated relation between the maximum shear stress of soil samples as a function of the surrounding pressure.

Triaxial compression tests can be carried out under various conditions depending of the required soil parame-ters. For the so called Consolidated Undrained TC-test (a type of TC-test) the sample is first consolidated under the desired stresses by applying the chamber pressure (confining pressure) which can be regarded as the con-fining stress at a certain depth of the soil. All tests are executed as single or multi-stage, isotropic consolidated, undrained TC-tests (different types of TC-tests) on a cylindrical soil sample with a diameter of 38 mm. These types of TC-tests can be regarded as stan-dard type of tests for cohesive soils in geotechnical labs in the Netherlands. The specimen is vertically enclosed by a thin rubber membrane and placed between two pressure heads at the top and bottom of the specimen inside a pressure chamber. After finalizing the consolidation phase, the shear phase is started by deforming the sample vertically with a constant speed. During this phase horizontal stress is kept constant, while the vertical stress gradually increases until a maximum is reached. The sample then fails or collapses, called “failure of the sample”, represented by the point (σ’,τf). It has reached its maximum shear strength. Two types of tests were performed on the samples: multi-stage and single-stage TC-tests. For a multi-stage TC-test, a single specimen is tested under three different confining pressures. The test consists of multiple load-ing stages where each stage is terminated before failure, when deformation exceeds e.g. 2 %. In single-stage test three specimens from the same sample are tested under different confining pressures until they do fail. In each case three tests are used to estimate the strength parameters of the soil. The three test results on speci-men from the same sample of a dike give three failure points in the graph. The red line, the best fitting straight line through these points, estimates the soil behaviour under a range of loads.

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The shear strength depends on two components: cohesion (c) between the particles and frictional resistance between particles. Shear strength can be expressed with or without taking pore pressures into account. When pore pressures are taken into account, drained or effective parameters are calculated. The “internal friction angle” φ’ is the slope of the straight line; this parameter represents the increased shear strength of the soil when the confining pressure of the sample is increased. In other words, the strength of the sample increases with increasing confining stresses. Multi stage TC-tests have often been used in the last decades as they are less expensive than single stage TC-tests. An important disadvantage however is that the strength of the samples of at the two higher confining stresses is influenced by the previous stages. Comparison of the results of both methods has shown that single-stage TC-tests produce higher values of internal friction angle at low cohesion. This corresponds with the strength decreasing effect of the multi-stage tests: at higher confining stresses the maximum shear strength is lower, resulting in an increasing cohesion and decreasing angle of internal friction. Nowadays multi-stage TC-tests are considered less reliable than single stage TC-tests.

Graph 2. Results from multi-stage and single-stage TC-tests

For comparison of TC-test results with the results of DSS-tests, only single-stage TC-tests has been taken into account, with the assumption that peat is a non-cohesive soil. In the DSS-test the resistance to horizontal deformation of the test sample is controlled by a flexible wire-reinforced rubber membrane inside metal rings and it is placed between the base pedestal and the top cap. Vertical stress is applied to the sample from the top cap and sheared by horizontal movement of the top cap with a constant speed. In this test failure is forced to be more or less horizontal. Drainage can be controlled to a certain level. Undrained tests are simulated by preventing vertical deformation (the volume of the sample is kept constant). Pore pressures are deduced from the changes in vertical stress. The ASTM- and NEN-standards used for all tests are provided in the references at the end of this article.

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Looking at the results For this article the test results of 252 samples from undisturbed peat from the body of peat dikes were used. The samples were taken at different locations in the Netherlands using a standard Ackerman sampler (shelby tube Ø 70 mm, length 400 mm) in combination with cable tool percussion drilling. The laboratory tests were car-ried out at the geotechnical laboratory of Wiertsema & Partners in Tolbert, the Netherlands. The conviction that the degree of humification and the botanical composition influences the strength of peat soil is very strong. Considering the structure of peat soil, its heterogeneity, the variation in the occurrence of fibres and the different age of the peat samples, experts were almost sure that there has to be some interconnections between them. The graphs below are quite surprising as well as disappointing. First, the degree of weathering of soil samples is considered. The Von Post degree is the most popular method to estimate the weathering of peat. To do it in a proper way requires a lot of practical experience and instruc-tions from experienced people in order to produce reproducible results. Using the method that determines the weathering degree which is less affected by human error, the PVI index was also tested to determine whether it has a useful relation to strength parameters. For this experiment both parameters from 30 samples were deter-mined, see graph 3. Unfortunately no clear relation between the two techniques, which aim to estimate the same property, can be found.

Graph 3. The PVI index and the Von Post degree of humification unexpectedly do not show a clear relationship. Other possibilities of potentially useful relations were graphically explored:

- Density – Von Post degree of humification - Water content – ash content - Water content - density - Ash content – density - Ash content – Von Post degree of humification - Water content – Von Post degree of humification - Water content – effective internal friction angle - Ash content - effective internal friction angle

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- Density - effective internal friction angle - Van Post degree of humification , content of wood, fibres with effective internal friction angle - botanical composition – water content - botanical composition – ash content - botanical composition – effective internal friction angle - Absorption (PVI-index) with Von Post degree of humification

However, these parameters also showed no clear useful relationship, as the graphs 4 and 5 exemplify.

Graph 4 The relation between the PVI index and the Von Post degree of humification classes of weathering also depends on the ash content (the ratio in % of the weight of material that remains after

calcination at 500˚C and that of the dry material before calcination).

Graph 5. The relation between the maximum shear stress of soil samples as a function of the surrounding pressure φ’ and the Von Post degree of humification class is not obvious.

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Except for a relation between ash content and wet density (see graph 6) no generic relations could be estab-lished.

Graph 6. The ash content of samples as a function of their density shows a clear rising trend. Factors that have to be taken into account to explain the relation between ash content and density are:

- compression and reduction of watercontent; - difference in composition (% mineral parts); - oxidation and escaping of methane (CH4).

Conclusions and discussion Contrary to the expectations, none of the graphs depicting combinations of the various measured parameters (not shown here) suggests a useful relation between the botanical description and internal friction angle. It is possible that only peat samples from the same locations and for the same type of peat the results of determin-ing the strength parameters and botanical description have a useful relationship. Also the influence of the size and diameter of the samples may have influence on usefulness of the tests. For each test an undisturbed sam-ple is required, but fibrous peat the fibres are cut anyway so they can behave differently than in situ. Another issue may be the way the samples are stored, it may influence the properties of peat that are relevant to the current investigation Due to the heterogeneity of the material closer examination and sufficient test results are required to deduct a realistic set of strength parameters. Oversimplification may lead to an underestimation of strength and higher costs to guarantee the same level of assumed dike safety. For technical questions related to the laboratory tests carried out both the author and head of the geotechnical laboratory of Wiertsema and Partners (j.vdkaap(at)wiertsema.nl) can be contacted.

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Recent Dutch investigations about peat dike strength Due to the heterogeneity of peat, safety factors and statistics play an important role in evaluating the strength of the Dutch dikes. Safety calculations according to the present standards show that dikes that have been here for centuries nevertheless do not always meet the present safety standards. In order to find more realistic methods to determine safe strength parameters based on laboratory tests full scale dike failure tests are carried out at several locations. The results of these tests are expected to be published in international literature over the coming years. Apart from this, general guidelines are formulated concerning the carrying out of laboratory test in the case of reviewing stability of dikes in the Dutch professional magazine Geotechniek (Greeuw et al.). This protocol gives guidelines concerning problems often experienced when geotechnical lab tests are executed. Some of them may be the related to the results presented in this paper.

Acknowledgements I would like to thank H. van Essen and C. Laban from Deltares, P.J. van Boven, C.J. van der Made and J. van der Kaap of Wiertsema & Partners (www.wiertsema.nl) and A.J. Schilstra of the University of Groningen for their valuable contributions during the investigation and reviewing of this article.

References B.H. Sheldric Analytical Methods Manual, 1984, Land Resource Research Institute, Canada M. Klavins, J. Shire, O. Purmalis and V. Melecis, Approaches to estimating humification indicators of peat. Faculty of Geographical and Earth Sciences, University of Latvia. G. Greeuw, T.A. van Duinen, H.M. van Essen, Deltares; Geotechniek: Protocol Laboratorium-onderzoek voor Toetsing Macrostabiliteit van Dijken (in press). Technische Adviescommissie voor de Waterkeringen, 1996, Technisch Rapport Geotechnische Classificatie van Veen. T.J. Malterer, E.S. Verry, J. Erjavec, 1992, Fibre Content and Degree of Decomposition in Peats: Review of National Methods. Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Cohesive soils. ASTM D 6528-07. American Society for the Testing of Materials. Geotechnics. Determination of Shear Resistance and Determination Parameters of Soil. Triaxial Test. Neder-landse Norm NEN 5117, 1e druk, December 1991. Gloeiverlies, Organisch Stofgehalte en CaCO3-gehalte. Proef 28. RAW 2010 Standard Conditions of Contract for Works of Civil Engineering Construction 2010. CROW Ede. Standard Test Method for Specific Gravity of Soil Solids by Gas Pycnometer ASTM D5550 – 06 American Soci-ety for the Testing of Materials. Geotechnics – Laboratory Determination of Moisture Content. Nederlandse Norm NEN 5112, 1e druk, August 1995.