Analysis of Excavation-induced Deformation with Different Soil Models
Abstract: The monitored case of deep excavation works located in the centre of Warsaw is described and back analysed as a boundary value problem with the finite element method. The excavation was carried out in over-consolidated clayey layers under the support of braced diaphragm walls. Accurate simulation of such soil-structure interaction problems requires advanced soil constitutive models – especially for the pre-failure range of small strains. On the other hand, such models should not be very complex and their material parameters should be relatively simple to obtain from laboratory and in situ surveys. By means of a case study, this paper examines several simple elasto-plastic constitutive models for the simulation of the behaviour of soil layers. The influence of such characteristics of the soil behaviour, as anisotropy and non-linearity of stiffness, is studied in the paper. The discussion concerns the problems related to the application of the models for a practical example. The comparison is focused on the obtained and measured displacements of the neighbouring building and diaphragm walls.
Pure elastic stiffness of sand represented by response envelopes derived from cyclic triaxial tests with local strain measurements
The elastic stiffness of a fine sand at small to moderate strains ( $$\varepsilon \le 2 \times 10^{-4}$$ ε ≤ 2 × 10 - 4 ) has been studied based on cyclic triaxial tests on cube-shaped samples with local strain measurements. Three samples with different densities (between very loose and very dense) were tested at up to 21 average stresses in succession (variation of mean pressure and stress ratio). At each average stress cycles were applied in six different directions, as a result of a simultaneous sinusoidal oscillation of the axial and lateral effective stress. Ten preconditioning cycles with larger strain amplitudes $$\varepsilon ^{\mathrm{ampl}}= 3 \times 10^{-4}$$ ε ampl = 3 × 10 - 4 per polarization were intended to induce a shakedown. An almost purely elastic response was then observed during the 5 subsequent cycles per polarization with lower strain amplitudes $$\varepsilon ^{\mathrm{ampl}}= 2 \times 10^{-4}$$ ε ampl = 2 × 10 - 4 . The stress and strain paths measured during the latter ones were used to derive the components of the incremental elastic stiffness tensor $${\mathsf{E}}= \{E_{PP}, E_{PQ}, E_{QP}, E_{QQ}\}$$ E = { E PP , E PQ , E QP , E QQ } at a given average stress. The analysis of $${\mathsf{E}}$$ E is based on all 60 reversal points and the corresponding stress and strain points lying in a certain distance $$\Delta \varepsilon$$ Δ ε (e.g. $$10^{-4}$$ 10 - 4 ) from the reversals. A graphical presentation of $${\mathsf{E}}$$ E by means of response envelopes is shown. They are used to calibrate the hyperelastic stiffness incorporated into a new hypoplastic constitutive model.
Non-uniformity of triaxial samples due to consolidation with radial drainage
This note describes a limited series of tests in which 38 mm diameter triaxial samples of kaolin clay were consolidated isotropically with radial drainage and the variation in water content across the diameter of the sample was measured. The results showed that significant non-uniformities can occur during consolidation of soil samples in routine triaxial tests in which side drains are used. A consolidation increment of 200 kpa gave rise to non-uniform water contents which imply substantial variations in effective stress. The problem can be avoided by removing radial drainage or it can be minimized by slow drained loading if radial drainage is necessary. (TRRL)