FIAT LUX UNIVERSITY OF LIVERPOOL SCHOOL OF ENGINEERING Year 1 Laboratories PMB Constant Head Permeability Test CIVE120: Geomechanics 1 Lecturer: Dr Paul Shepley Demonstrator Name: Demonstrator Email: UNIVERSITY OF LIVERPOOL Health and safety PMB Lab Script // CIVE120 Instructions by lab demonstrators should be followed exactly and information regarding the labs fire instructions made known. Lab coats can be worn during the experiment and this lab does not require steel capped boots. You are reminded that you are required by law to comply with the School's basic rules of lab safety, as outlined at the start of the first year. 1 UNIVERSITY OF LIVERPOOL 1 Aims and objectives 1.1 Aims PMB Lab Script // CIVE120 This laboratory experiment aims to help students understand how to measure soil permeability. 1.2 Objectives Describe the soil type and measure the permeability of two sands, previously prepared into relatively dense packings. At the end of this lab experiment, students should be able to: Describe and classify soil • Conduct a constant head permeability test to measure soil permeability, and Determine the coefficient of permeability from lab results 2 Theory 2.1 Darcy's law Darcy's law governs fluid flow in porous media and can be written as follows: where: q=k.i where Δη i = AL - q is the average flow velocity (m/s) over the cross-sectional area of the sample, A (m²) - k is the coefficient of permeability of the soil (m/s) i is the hydraulic gradient, measured as water head loss, h (m), over a given length, L (m) 2.2 Permeability The coefficient of permeability, k, is used to characterise the permeability of granular materials. It depends on a range of factors, including: - Soil type and particle size Void ratio / pore size / density of the soil deposit Pore fluid properties (e.g. viscosity) Saturation ratio of the soil Homogeneity, soil layers and fissures An approximation for the coefficient of permeability can be obtained from the grain size of the soil, particularly the d10 particle size (i.e. the size at which ten percent of soil by mass passes through a sieve). This is given by Hazen's law (1930): k≈ 0.01 × d10 where d10 is given in mm 2 UNIVERSITY OF LIVERPOOL PMB Lab Script // CIVE120 Both Darcy's law and Hazen's law require laminar flow conditions and reasonable homogenous soil conditions to be valid. 2.3 Measuring permeability The permeability of a soil can be measured in numerous ways, either in the field using in-situ permeability tests or in the laboratory, either using a falling head permeameter (for fine-grained, low permeability soils) or a constant head permeameter (for coarse-grained, high permeability soils). In this laboratory class, you will be using a constant head permeameter. This has been set up in advance of the class, with the following schematic in Figure 1. From constant head device h₁ h₂ h3 1 AL12 Soil sample Area A 2 AL23 To drain / measuring cylinder 3 Figure 1 - Simplified apparatus set up Ah12 Ah23 3 UNIVERSITY OF LIVERPOOL 3 Experimental procedure PMB Lab Script // CIVE120 You will determine the coefficient of permeability for two materials during this laboratory. - these are the same two materials you will/have used during the GSD Grain Size Distribution Note laboratory. 1) Take a few minutes to handle the dry soil provided and write a soil description for the sand. A BS5930:1999 chart has been provided to help you describe the material. Note the material type, colour and another other points of interest. 2) Measure the height of sand in the cell to enable the dry unit weight of the specimen to be determined. The dry weight of the specimen is marked on the side of the cell. Note that the internal diameter of the cell is 76 mm. 3) Adjust the flow to the constant head device to keep it fully charged throughout the tests. 4) Pass water vigorously through the specimen from the constant head device for 5 minutes to remove as much air as possible from the sand. 5) Control the flow using the drain tap, allow 2-3 minutes for the flow to reach steady state and measure the head drops in the standpipes. 6) Determine the rate of flow using a stop watch and a measuring cylinder. 7) Increase the flow using the drain tap to increase the pressure head differences and again measure the flow rate. 8) Swap the apparatus and repeat the whole procedure - including the soil description - for the other sand sample. 4 Result recording Use the tables overleaf to record your results during the laboratory class. Note that the units need to be changed during calculations. Units in the tables are given in the same format as they are measured. 5 Interpretation and presentation of results You are required to: 1) Plot the values of flow rate q against the associated hydraulic gradient and obtain the coefficients of permeability for the two materials. (Hint - you may choose to plot the hydraulic gradient between points 1-2, 2-3 and 1-3 separately and discuss their differences.) 2) Describe why the coefficients of permeability are different for the two materials.