CONSTANT HEAD AND FALLING HEAD PERMEABILITY TEST

Transcription

1 CONSTANT HEAD AND FALLING HEAD PERMEABILITY TEST 1 Permeability is a measure of the ease in which water can flow through a soil volume. It is one of the most important geotechnical parameters. However, it is probably the most difficult parameter to determine. In large part, it controls the strength and deformation behavior of soils. It directly affects the following: quantity of water that will flow toward an excavation design of cutoffs beneath dams on permeable foundations design of the clay layer for a landfill liner. For fine grained soil Falling head permeability test is done, whereas constant head permeability test is done for the coarse grained soil. Application Estimation of quantity of underground seepage water under various hydraulic conditions Quantification of water during pumping for underground construction Stability analysis of slopes, earth dams, and earth retaining structures Design of landfill liner Equipment Combination Permeameter assembly Stop watch Graduated cylinder (250 or 500 ml) Balance sensitive to 0.01 lb Moisture cans Drying oven Thermometer Figure 1 Sketch of the combination permeameter

2 2 Figure 2 Sketch of the combination permeameter assembly Figure 3 Sketch of the combination permeameter test: Falling head (left), constant head (right)

3 Constant Head Permeability test 3 Procedure The following steps are already done: Mix sufficient water into the sample to prevent segregation of particle sizes during placement into the Permeameter. Enough water should be added to allow the mixture to flow freely, forming layers. Remove both the chamber cap and upper chamber from the unit by unscrewing the three knurled cap nuts and lifting them off the tie rods. Position one porous stone on the inner support ring in the base of the chamber. Using a scoop or funnel, pour the prepared specimen into the lower chamber, using a circular motion to fill the lower chamber to a depth of 1.5 cm. A uniform layer should be formed. Use an appropriate tamping device to compact the layer of soil to the desired density. Repeat the compacting procedure until the sample is within 2 cm of the top of the lower chamber section. Replace the upper chamber section, placing the rubber gasket between the chamber sections. Be careful not to disturb the test specimen. Continue the sample placement operation until the level of compacted material is about 2 cm below the rim of the upper chamber. Carefully level the surface of the specimen and place the upper porous stone on it. Place the compression spring on the porous stone. Replace the chamber cap and sealing gasket, securing it firmly with the cap nuts. The spring will restrict upward sample movement. Measure and record the sample length. Assemble the constant head funnel, rod and meter stick. Use the rod clamp the funnel's lower portion. Adjust the level of the funnel to allow the constant water level in it to remain a few inches above the top of the specimen. Connect the flexible tube from the tail of the funnel to the bottom outlet of the Permeameter. Keep the valves on top of the Permeameter open. Place a receiver at the top outlet to collect any water that may come out. If preferred, a piece of tubing may be connected to the outlet, leading the water to a sink. Open the bottom outlet valve and allow water to flow into the permeameter. As soon as water begins to flow out of the top control (deairing) valve, close the control valve, letting the water flow out the outlet for a time. Close the bottom outlet valve and disconnect the flexible tubing at the bottom. Connect the constant head funnel to the top side port. Open the bottom outlet valve and raise the constant level head (funnel) to a convenient height to get a reasonable steady flow of water. Accurately measure the vertical distance between the funnel overflow level and the chamber outflow level. Measure and record the length of the specimen, L.

4 You need to perform the following steps Allow adequate time for the flow pattern and/or specimen to stabilize. After equilibrium flow has been established, measure the time taken to have specified volume of water flowing out. Use a measuring cylinder and a stop watch. Repeat three or more times, calculating the average time. Calculations 4 The lab reports shall include the following: Sample calculations. Table showing the calculations pertinent to the permeability of the soil. Average value of permeability Calculate the void ratio by oven drying the specimen and taking the dry mass. EQUATION TO BE USED k = VL Aht (1) Where, K = Coefficient of permeability V = Collected volume of water L = Length of soil column (13.25 cm) A = Area of the soil column (31.65 cm 2 ) h = Head difference (34.3 cm) t = Time required to get V volume

5 Falling Head Permeability test 5 Procedure The following steps are already done: Compact the sample in the lower chamber section of the Permeameter, in layers approximately 1.5 cm deep, to within about 2 cm of the lower chamber rim. Use an appropriate tamping device to compact the sample to the desired density. Remove the upper section of the chamber tie rods and place the upper porous stone on the specimen, securing the upper section of the chamber with spring to the unit. Measure and record the length of the specimen. Use the clamp to attach the falling head burette to the support rod. Position the burette as high as is possible for practicality. Place the meter stick directly behind the burette, so the height of water in the burette above the chamber outflow port may be read. Saturate the specimen, following the steps outlined above. Measure the heights of the two levels from the outflow level. You need to perform the following steps After a stable flow has been established, note the drop in head (Δh) in 2 hours. (use a stop watch). Calculations The lab reports shall include the following: Sample calculations. Table showing the calculations pertinent to the permeability of the soil. Average value of permeability Calculate the void ratio by oven drying the specimen and taking the dry mass. EQUATION TO BE USED al k = ln At h h 0 1 (2) Where, K = Coefficient of permeability a = Area of the burette (1.695 cm 2 ) L = Length of soil column (13.25 cm) A = Area of the soil column (31.65 cm 2 ) h 0 = Initial height of water (56 cm) h 1 = Final height of water = h 0 - Δh t = Time required to get head drop of Δh

6 6 Temperature Correction k 20 C = k T C η η T C 20 C (14) Where, k ToC = measured permeability at the actual water temperature in the lab = permeability at the standard temperature of 20 O C k 20oC Table 1 of Correction Factors for Water Temperature Test Water Temperature, T ( C) η T C /η 20 C Test Water Temperature, T ( C) η T C /η 20 C Table 2 Typical permeability coefficients for different soils Typical Permeability, k Soil Type (cm/sec) Gravels and Coarse Sands > 10-1 Fine Sands 10-1 to 10-3 Silty Sands 10-3 to 10-5 Silts 10-5 to 10-7 Clays < 10-7

7 7 HYDRAULIC CONDUCTIVITY OF GRANULAR SOIL UNDER CONSTANT HEAD (ASTM D2434) LABORATORY DATA SHEET I. GENERAL INFORMATION Tested by: Date tested: Lab partners/organization: Client: Project: Boring no.: Recovery depth: Recovery date: Recovery method: Soil description: Sand II. TEST DETAILS Specimen diameter, D: 6.35 cm 2 Specimen area, A: cm 2 Specimen Length, L: Volume of soil, V: Dry mass of soil, M s : Specific gravity of soil solids, G s : 2.65 Dry unit weight, γ d : Void ratio, e: Scale type/serial no./precision: Saturation method: Constant head Saturation duration: 48 hours Specimen preparation method: Dry packing Notes, observations, and deviations from ASTM D2434 test standard: III. MEASUREMENTS AND CALCULATIONS Test Head Hydraulic Flow Time Flow Rate Hydraulic No. Difference Gradient Volume (q) Conductivity (h) (i) (Q) (t) (k)

8 HYDRAULIC CONDUCTIVITY OF GRANULAR SOIL UNDER FALLING HEAD LABORATORY DATA SHEET 8 I. GENERAL INFORMATION Tested by: Lab partners/organization: Client: Boring no.: Recovery date: Soil description: SM Date tested: Project: Recovery depth: Recovery method: II. TEST DETAILS Specimen diameter, D: 6.35 cm 2 Specimen area, A: cm 2 Burette area, a: cm 2 Specimen length, L: Dry mass of soil, M s : Volume of soil, V: Specific gravity of soil solids, G s : 2.68 Dry unit weight, γ d : Void ratio, e: Scale type/serial no./precision: Saturation method: Constant head Saturation duration: 48 hours Specimen preparation method: Dry packing Notes and observations: III. MEASUREMENTS AND CALCULATIONS Test No. Initial Initial Hydraulic Final Time Hydraulic Head Gradient Head Conductivity (H 0 ) (i i ) (H 1 ) (t) (k)