Relationship between Shear-Wave Velocity and Consolidation Parameters of Peat

There is a need of developing methods for evaluating material parameters for peat. It is hard and costly to obtain undisturbed samples for laboratory testing of the peat. Therefore, in this thesis the possibility of using in situ measurements of the shear wave velocity for first order estimates of the consolidation parameters has been investigated. In earlier work, relationships have been found between the shear wave velocity and consolidation parameters for Norwegian clay. One of the biggest difficulties with peat is the large compressibility making it a difficult soil for construction work and it is avoided if possible. It is characterised by its high water content. In order for the soil to be accumulated there needs to be a high amount of rainfall and poor drainage. The high water content explains why the pressure wave velocity was not analysed in this thesis. This body wave, which controls the oedometer modulus, propagates in water making the water content the dominating factor determining the pressure wave velocity in peat. There is no shear resistance in water making the shear wave velocity more suitable to use in peat. In the study, the peat was compressed in an oedometer using a constant rate of strain. This allows for a valuable understanding of the primary compression, which is formed as the effective stress increases. The peat has been characterised according to the extended version of the von Post scale. That includes determining its level of humification, fibre content and water content. The data in this thesis is from three different sites, referred to as Ageröds mosse, Färgelanda and Mullsjö. The shear wave velocity is linked analytically to the shear modulus and the density of the soil. Furthermore, it has been shown it is a function of the void ratio and the current state of stress. It has been measured at the three sites by using Down hole method. Generally, it was seen to increase with depth, which can be expected since one of the controlling factors is the vertical effective stress. The compression parameters relevant for this study is M_0, M_L, m and σ´_c. Since the unit weight of peat is close to the one of water the preconsolidation pressure can sometimes be very low. It was the case in this study where σ´_c could not be identified as well as M_0. However, both M_L and m were evaluated from the 13 oedometer tests. There was no strong correlation with V_s identified, although a weak indication for M_L was observed.Peat is an organic soil that is highly compressible and mainly consists of water. It is therefore a difficult material for construction and is avoided if possible. One method to use if peat cannot be avoided is to preload the soil before construction, of a road for instance. It is then of big interest to know how much of the long-term settlement is developing during the time of preloading. It is critical to determine the consolidation parameters to make a useful approximation of the settlement. It is hard and costly to obtain undisturbed samples for evaluation of the consolidation parameters in peat. In order to evaluate them, time demanding laboratory work must be performed. It is much cheaper and faster to determine the shear wave velocity for a soil profile. It is possible that a correlation between the shear wave velocity and the consolidation parameters for peat can be found. This thesis investigates the possible relationship. It is done by comparing the shear wave velocity to consolidation parameters for peat from three different sites. Shear waves are elastic body waves whose velocity traveling through soil can give an indication of the soil properties. Moreover, the shear wave velocity can be measured for the full peat depth. That means, if there exists a useful relationship between the shear wave velocity and the consolidation parameters, it would be possible to estimate the parameters for the full profile. There is however no analytical relationship between the shear wave velocity and consolidation parameters. Instead, as the name suggests, the shear wave velocity is linked analytically to the parameter describing the resistance to shearing. Despite this, recent research has found an empirical relation¬ship between the shear wave velocity and consolidation parameters for Norwegian clay. In this thesis 13 un¬disturbed peat samples were tested in the laboratory and the consolidation parameters were determined. This was done by compressing the samples, the displacement was kept at a constant rate during the test. The lateral displacement was prescribed to zero, displacement was only allowed vertically and water was drained from above. The undisturbed samples were excavated from three sites in Sweden and the shear wave velocity was also measured at these sites, making a comparison possible. In addition to this, the peat has been characterised by determining to what degree it is decomposed and its water content. The water content is very high for peat which allows for a large consolidation, thus making it important to measure. The data in this work was not sufficient to make any definite conclusions about a possible relation. However, a weak indication of a relationship was seen for one of the parameters. Moreover, the shear wave velocity was generally increasing with depth, as could be expected since one of the controlling variables is the effective vertical stress. Although no definitive answer of whether a relation exists between the shear wave velocity and the compression parameters was found, the data can be complemented in future studies. Generally, the knowledge of geotechnical properties for peat is limited compared to what is known about clay and there is a further need of developing methods to evaluate material parameters