Properties of Danish Clay Tills determined from laboratory testing

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1 Properties of Danish Clay Tills determined from laboratory testing S. B. Jessen 1 GEO, Lyngby, Denmark ABSTRACT As a part of the soil investigations for the Fehmarnbelt Fixed Link project (tunnel or bridge), a large series of more than 100 advanced laboratory tests has been executed on clay till by GEO (former Danish Geotechnical Institute) and partners. Likewise a large number of tests has been executed on Danish clay till for other projects, for example wind farms and the Great Belt Fixed link. The results from many of these tests have been collected in a database. Clay Till, which is one of the most common soil types in Denmark, is by nature a very mixed soil, due to its origin as a glacier deposit. At the Femern site, the underlying deposits consists of highly plastic Palaeogene Clay and Cretaceous chalk, thus the Clay Till consists of a mixture of these two, as it usually is in Denmark. Consequently, a large variability in the properties of Clay Till must be expected. In the laboratory, strength and deformation parameters have been determined as well as classification characteristics for each of the tested specimens. Due to the mixed geology of the Clay Till, a spread in the plasticity from non-plastic to very highly plastic and in the strength from weak to very competent has been measured. In this paper, the classification properties of the various Clay Till formations are presented and the strength and deformation properties are related to the classification characteristics for the Danish clay till in general, based on the database. Keywords: tunnel, Danish clay till, geology, laboratory test, classification, undrained shear strength, preconsolidation stress 1 INTRODUCTION Glacial materials constitute more than 95 % of the quaternary deposits in Denmark and approximately one third of the topsoil in general, mostly consisting of clay till. The properties of clay till are therefore of great interest to Danish geotechnical engineers. Despite seemingly identical soil descriptions, the clay till deposits found in Denmark emerge with highly variable strength properties due to the geological formation pattern during glaciations. Clay till often appears heavily preconsolidated due to the weight of the ice cover, but due to a number of both mechanical and chemical processes e.g. high pore pressure beneath the glacier sole, weathering and melting of interbedded ice, normally consolidated (disturbed) clay till can be experienced, in the worst case. Local variations must be foreseen within a few meters. The aim of this paper is to provide an overall understanding of the classification properties of Danish clay till and to describe a possible correlation between the classification properties and the undrained shear strength determined from triaxial testing. A large number of advanced laboratory tests on Danish clay till have been executed by GEO (former Danish Geotechnical Institute). During 1 GEO, Maglebjergvej 1, 2800 Lyngby, Denmark. sbj@geo.dk

the past approximately 50 years, GEO has established a large database, containing correlated measurements of classification parameters, deformation parameters and strength parameters, respectively,

2 the past approximately 50 years, GEO has established a large database, containing correlated measurements of classification parameters, deformation parameters and strength parameters, respectively, from more than 500 advanced tests on Danish clay till samples, among others from the Fehmarnbelt Fixed Link project in the planning phase, wind farms and the Great Belt Fixed link. The analysis conducted in this paper will be based on the test results from this database. 2 CHARACTERISTICS OF DANISH GLACIAL DEPOSITS 2.1 General Tills are directly deposited by the inland ice moving from the Scandinavian mountains and by meltwater streams from the same ice. By friction, particles frozen to the sole of the glacier rip of material from the subsoil, whereby these are transported along with the glacier. Glaciers are able to transport all sizes off particles, even boulders with a diameter of more than 10 m. A more detailed description of the depositional history of the soil genesis in the glacial environment is described in ref. [1] and ref. [2]. Opposite wind and water, glaciers are nonsorting. Consequently, tills are very mixed soil types, consisting of a mixture of the soiltypes passed by the glacier. Clay till deposits usually appear in Denmark as unsorted silty, sandy, calcareous clay of low plasticity with a variable amount of gravels, cobbles and boulders. A till is described as clay till when the soil exhibits a plastic (cohesive) behavior, often with clay content of %. A typical geological description of clay till could read: CLAY TILL, sandy, silty, gravelly, calcareous, grey. This description covers clay tills with a great variety in the composition. In Figure 1, two different clay tills with the same geological discription are shown. As can be seen from the figure, the difference in clay content of the two clay tills is significant and they vary from sorted (a) to unsorted (b). Plasticity index (%) 5.7 CaCO 3 (%) 67.1 Plasticity index (%) 48.7 CaCO 3 (%) 19.6 b) a) Figure 1. Different grain size distributions for clay tills with seemingly identical geological descriptions. A different approach than the geological description is therefore crucial in order to classify the clay till. 2.2 Classification properties In Figure 2 the distributions in the measured classification properties of the Danish clay till are shown. Due to the large variation in clay content, Danish clay till is often classified by the plasticity index and the moisture content. The moisture content of clay till typically approaches the interval of 8-14 %, but as can be seen from Figure 3, where the moisture content is plotted versus the vertical effective in situ stress, there is no correlation between the moisture content and the deposition depth.

3 Figure 3. Natural moisture content versus vertical effective in- situ stress. Figure 2. Percentage distribution of the measured values of moisture content, plasticity index and calcium carbonate content, respectively. A similar lack of correlation can be demonstrated for the void ratio. A correlation between the preconsolidation stress and the depth is therefore not realistic. The rather large scatter in void ratio and moisture content is most likely dictated by the grain size distribution, as unsorted clay till, like the one on Figure 1.b, usually reveals lower moisture content than clay till like the one in Figure 1.a. The plasticity index is influenced by the clay content, the mineralogy (type of clay minerals) and the calcium carbonate content. In figure 4 the measured plasticity for each sample has been plotted in a traditional Casagrande plot, showing the plasticity index, I p, versus the liquid limit, w L. The Casagrande plot is described in ref. [3]. CL: Low plasticity clay CM: Medium plasticity clay CH: High plasticity clay CV: Very high plasticity clay Figure 4. Plasticity index versus liquid limit. Both Figure 1 and Figure 4 show that the plasticity index is typically lower than 20 %, and as can be seen from Figure 4, Danish clay till mostly occurs as low plasticity clay but even high plasticity clay till can occur. 3 DISCUSSION OF STRENGTH PROPERTIES IN RELATION TO CLASSIFICATION PROPERTIES 3.1 Estimation of the preconsolidation stress According to Danish laboratory practice for triaxial testing, the clay till is preconsolidated to the estimated preconsolidation stress before shearing. Thus, a reliable estimate of the preconsolidation stress is therefore of great importance.

4 Besides the different geological formation patterns during glaciation, two mechnisms tend to hamper a reliable estimate of the preconsolidation stress. Sample disturbance can occur due to the lack of capillary suction needed to maintain in situ stress conditions in so called intact samples. The preconsolidation stress can therefore be quite difficult to verify in the laboratory by standard oedometer tests, which very often underestimate the stiffness severely. Sample disturbance can be eliminated by retracing the stress history of the specimen before shearing, which models the in situ conditions and allows evaluation of the actual deformation and strength properties of the soil. Estimating the preconsolidation stress is also difficult for chalk cemented soil (e.g chalk till), as this tends to behave as a soft rock. Figure 5. Undrained shear strength, c u, versus vertical effective in-situ stress. As indicated earlier, there is no clear correlation between undrained shear strength and the vertical effective in situ stress (depth), as clearly demonstrated in Figure 5. An attempt to classify the different types of clay till is instead based on evaluations on the undrained shear strenth determined from triaxial testing, in relation to the classification properties of the clay till. No test results from UU tests or UCS tests are included in this analysis, as these test types are considered to be inadequate for Danish Clay till according to ref. [4]. 3.2 Undrained shear strength determined from triaxial testing The distribution of the measured values of undrained shear strength determined from triaxial testing is shown in Figure 6. Figure 6. Distribution of measured values of c u determined from triaxial testing. It is seen that the typical value interval of c u is between 100 kpa and 400 kpa, but it can easily reach values higher than 500 kpa. In Figure 7 c u is plottet versus the in-situ moisture content, w. For a moisture content less than approximately 13 %, i.e. most Danish clay tills cf. Figure 2, a decrease in c u is seen for increasing moisture content. The correlation is though not very pronounced and a different approach is therefore desirable. Above 13 % moisture content, no clear correlation is seen. A similar plot for c u versus the void ratio can be obtained. Figure 7. Undrained shear strength, c u, versus moisture content, w.

5 In Figure 8, c u is plotted versus the calcium carbonate content, CaCO 3. A weak tendency of increasing c u versus increasing calcium carbonate content is indicated, especially for a content of CaCO 3 greater than approximately 25 %. I p against the calcium carbonate content results in a plot similar to Figure 9 with the limit of I p /CaCO 3 = 0.5 instead of I p = 14 %, see figure 10. Figure 8. Undrained shear strength, c u, versus calcium carbonate content, CaCO 3 Regarding the plasticity index, no clear correlation between c u and I p can be demonstrated in general, but as seen in Figure 9, a correlation between c u and the combined effect of I p and the CaCO 3 content is indicated. Figure 9. Undrained shear strength, c u, versus plasticity index, I p For I p < 14 %, the clay till samples can be divided into two groups based on their calcium carbonate content, where the specimens containing more than 25 % of CaCO 3 seem to obtain a higher undrained shear strength, probably due to chalk cementation. For I p > 14 %, c u is approximately constant even though specimens with a content of CaCO 3 greater than 25 % are experienced is this group. Normalizing Figure 10. Undrained shear strength, c u, versus plasticity index normalized against the calcium carbonate content, I p /CaCO 3 Based on Figure 9 the correlation between c u and the combination of I p and CaCO 3 given in Table 1 is suggested by the author to be used as a first estimate when designing triaxial tests on Danish clay till. Table 1. Author s suggestions of c u values based on classification properties Classification properties I p < 14 % and CaCO 3 < 25 % I p < 14 % and CaCO 3 > 25 % I p > 14 % c u [kpa] 100 kpa 400 kpa > 300 kpa 100 kpa 400 kpa Some deviations from these suggestions are of cause experienced, which can probably be related to the preconsolidation stress of e.g. Ablation till, which has only been subject to a very limited preconsolidation by the glaciers, or sample disturbance if no retracing of the stress history has been performed in the laboratory. 4 CONCLUSION Clay till is a very mixed soil type with a very complex stress history. The strength properties of clay till are directly related to the stress history, which is reflected in the preconsolidation of the soil. For Danish clay tills there is no correlation between strength properties and the vertical

6 effective in situ stress. As a result, clay tills with seemingly identical soil descriptions emerge with highly variable strength properties. Instead it is suggested to relate the undrained shear strength to the plasticity index and the calcium carbonate content of the soil. Additionally, high moisture content will also give an indication of the expected undrained shear strength. ACKNOWLEDGEMENT The author would like to direct special thanks to Helle Foged Christensen from GEO for her guidance, comments and suggestions during the research and for reviewing this paper. REFERENCES [1] B. Dinesen and B. Larsen, Sedimentgeologi, Institut for geologi og geoteknik, (In Danish) [2] N. Foged and J.S. Steenfeld, An Engineering Approach to Preloaded Clay Till Properties, dgf-bulletin 9 (1992), [3] N.K. Ovesen, L.D. Fuglsang and G. Bagge, Lærebog I Geoteknik, Polyteknisk Forlag, Lyngby, 2007 (In Danish) [4] J.L. Christensen, E. Schjønning and N. Foged, Comparison of Clay Till Strength Parameters using BS and Danish Test Procedures, dgf-bulletin 9 (1992),

Table of Contents Chapter 1 Introduction to Geotechnical Engineering 1.1 Geotechnical Engineering 1.2 The Unique Nature of Soil and Rock Materials

Table of Contents Chapter 1 Introduction to Geotechnical Engineering 1.1 Geotechnical Engineering 1.2 The Unique Nature of Soil and Rock Materials 1.3 Scope of This Book 1.4 Historical Development of Geotechnical