Trong các chương trước các nguyên tắc chính của thử nghiệm ba trục, và tế bào thử nghiệm rất tương tự, đã được trình bày. Để đơn giản, nó đã được giả định rằng các tài liệu là đất khô, do đó không có áp lực lỗ chân lông, và những căng thẳng hiệu quả bằng những căng thẳng được áp dụng. Trong thực tế, đặc biệt là đối với đất sét, mẫu thường chứa nước trong lỗ chân lông của nó, và tải đất có thể làm tăng sự phát triển của áp lực lỗ thêm. Sự ảnh. | Chapter 24 PORE PRESSURES In the previous chapters the main principles of triaxial tests and the very similar cell test have been presented. For simplicity it was assumed that the material was dry soil so that there were no pore pressures and the effective stresses were equal to the applied stresses. In reality especially for clay soils the sample usually contains water in its pores and loading the soil may give rise to the development of additional pore pressures. The influence of these pore pressures will be described in this chapter. Measuring the pore pressure There are two possibilities to controll the pore pressures in a triaxial test Figure Triaxial test with measurement of pore pressures. I 1 3 either execute the test on a drained sample at a very low deformation rate so that no pore pressures are developed at all or measure the pore pressures during the test. In the first case the drainage of the sample can be ensured by filter paper applied at the top and or bottom ends of the sample together with a drainage connection to a water reservoir and taking care that the duration of the test is so long that consolidation has been completed during the test. The consolidation time should be estimated using estimated values of the permeability and the compressibility of the sample and the duration of the test should be large compared to that consolidation time. This may mean that the test will take very long and usually it is impractical. A better option is to measure the pore pressures in the sample for instance by means of an electric pore pressure meter. This is a pressure deducer in which the pressure is measured on the basis of the deflection of a thin steel membrane using a strain gauge on the membrane. The pore pressure meter is connected to the top or the bottom of the sample see Figure . An alternative is to measure the pore pressure in the interior of the sample using a thin needle. Whatever the precise system is care should be taken .
