Relationship between Shear-Wave Velocity and Consolidation Parameters of Peat

Abstract: 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.