The University of Toledo Soil Mechanics Laboratory

Transcription

1 The University of Toledo Soil Mechanics Laboratory 1 Soil Moisture-Density Relationship Standard and Modified Proctor Tests Introduction For earthork construction it is important to compact soils to a dense state so that the soils ill attain satisfactory engineering properties. It is also desirable to kno the optimum soil conditions for compacting a given soil. According to compaction theory, hen samples of a soil are compacted at different ater contents using the same compactive energy, there is optimum ater content at hich the soil ill reach a maximum dry density. The optimum ater content and dry density depend on the soil composition and the amount of compactive energy used. The moisture-density relationship of a soil is a graph of dry density versus ater content, for a given compactive effort. The data points obtained from compacting several samples at different ater contents form a smooth curve, called the compaction curve, hich is used to obtain the optimum ater content and maximum dry density. The to standardized tests in use today, the standard and modified Proctor tests, differ only by the amount of compactive energy. In practice, the standard or modified Proctor test is conducted on a soil and specifications are ritten stating: 1) that the soil moisture content during compaction should be ithin a specified percent of the optimum ater content; and 2) that the dry density of the compacted soil should be 90 to 100 percent of the maximum dry density. Apparatus 1. No. 4 sieve for Procedure A or /8-in sieve for Procedure B 2. Mixing tools including bol, spoon and spray bottle. Mold assembly including a base, a mold ith a 4.0-inch diameter and volume of 1/0 cubic foot (944 cm ) and a collar 4. Manual rammer as follos: a) Standard Proctor (ASTM D 698) free fall distance of 12.0 inches and mass of 5.5 lbm (2.5 kg) b) Modified Proctor (ASTM D 1557) free fall distance of 18.0 inches and mass of 10.0 lbm (5.54 kg. 5. Rubber mallet 6. Straightedge tool 7. Sample extruder including a jack and frame 8. Platform balance 9. Water content containers and drying oven 1 ASTM D (Reapproved 1998); ASTM D (Reapproved 1998) Proctor Tests - 1

2 Procedure A. Preparation (one eek before test) 1) Pass approximately 8 lbs. of air-dried soil through a #4 (Procedure A). It ill be necessary to pulverize the soil. This can be done using a Proctor mold and rammer. 2) Using the spray bottle, add approximately 250 ml of ater to the soil and mix thoroughly in a pan until the soil is uniform in color. This ill increase the ater content of the soil by approximately 7%. Transfer the soil to a plastic bag and close the bag. B. Laboratory 1) Determine and record the eight of the mold. 2) Assemble and secure the mold to the base and the collar to the top of the mold. ) Compact the soil in the mold according to the standard or modified Proctor procedures as follos as directed by the instuctor: a) Standard Proctor (ASTM D698) Standard rammer using layers and 25 blos per layer; b) Modified Proctor (ASTM D1557) Modified rammer using 5 layers and 25 blos per layer. Soil should be mixed thoroughly in the mixing pan and placed loosely in the mold and lightly tamped before compaction. After compacting each of the layers belo the top layer and before placing soil into the mold, use a sharp object to loosen the soil on the surface of the soil and around the edge of the compaction mold. Observe the compaction behavior of the soil in order to estimate the amount of loose soil that should be placed in the mold so that the top layer ill extend at least ¼-inch but not more than ¾-inch into the collar after compaction. The compacted layers should be approximately equal in thickness. 4) When compacting the soil, place the mold on the concrete floor of the laboratory and stand adjacent to the mold. Hold the sleeve of the mold in one hand slightly above the surface of the soil. Use the other hand to apply the blos hile holding the sleeve of the rammer in a nearly vertical position. Move the sleeve of the rammer around the surface of the soil after each blo. With a little practice the blos can be applied at a rapid rate of approximately 25 blos/min. 5) After compaction, place the mold on the counter and carefully remove the collar from the mold. It is important to prevent the soil from breaking off belo the surface of the mold. This can be accomplished by tapping on the collar ith a rubber mallet and by pushing don on the surface of the soil ith the thumbs hile pulling up on outside of the collar. Proctor Tests - 2

3 6) Carefully trim the soil level ith the top and the bottom of the mold using the straightedge. Care must be taken so that the soil does not break off belo the top of the mold. Best results are achieved by initially trimming the soil from around the edges of the mold and gradually orking toard the center. Fill any holes on the surface of the mold using the trimmed soil by pressing don ith the ide edge of the straightedge and then scrapping the surface of the soil again. 7) Determine the eight of the soil and mold as accurately as possible. 8) Use the extruder to remove the soil sample from the mold. Slice or break the compacted soil in half by slicing axially through the sample from the outside through the center. Obtain a ater content sample from the cut face of each half of the specimen. Obtain the ater content of the samples as per previous labs. 9) After compacting the soil, increase the ater content of the soil by adding approximately 75 ml. of ater and mixing thoroughly. 10) Repeat steps 2 through 9 so as to obtain five data points ith five ater contents using either the standard and modified Proctor procedures. Note: It is not permissible reuse soil after it is compacted in a Proctor mold. This procedure is folloed in a teaching laboratory in order to reduce the amount of soil required for the testing. Calculations The density of the soil can be computed using either the English or the Standard International (SI) metric system. For the English system, the density or unit eight is given in units of lbf/ft. For the SI metric system, the density is given in kn/m. If the soil mass is determined, then the unit eight,, (kn/m ) is computed using Neton s second la of motion (F M a) and the folloing conversions and calculations. 2 N W ( N) M ( kg) g kg 9.81m / sec 2 kg m / sec 6 kn W N kn 10 cm ( ) m V 944cm 1000 N m lbf kn ( ) m ft 6.66 (1) (2) () Proctor Tests -

4 The dry density or dry unit eight is then obtained by d d ( 1+ ) ( 1+ ) (4) (5) here is the ater content expressed as a fraction. For any given ater content, the associated dry density for zero air voids (degree of saturation equal to 100%) is computed. This is a useful verification of the compaction curve since, in theory, it is not possible to have a higher dry density. Dry densities greater than the dry densities on the 100% saturation curve may be an indication of error or an incorrect value of specific gravity. The equations for the dry density and unit eight at 100% saturation are d + s (6) d Gs 1+ G s (7) Supplementary Calculations The folloing computations can be completed using phase relationships that are derived from the defined relationships. Results e G s d e n 1+ e G S e s -1 Complete the folloing tables and figure using either the English or SI system of units. Compute the dry unit eight and ater content for each point and sho the results in Table 1. Compute the dry density at 100% saturation (zero air voids curve) for five assumed ater contents using Equation 6 or 7, respectively and sho the results in Table 1. Sho the curves for density (dry unit eight) vs. ater content and dry unit eight at 100% saturation vs. ater content from Table 1 on Figure 1. Determine the optimum moisture content and maximum dry unit eight. Compute the void ratio, porosity and degree of saturation for each sample and sho the results in Table 2. (8) (9) (10) Proctor Tests - 4

5 Conclusions Estimate the optimum ater contents and maximum dry unit eight that ould be obtained if the other procedure, standard or modified, had been used. Did the results come out as expected? Explain. What is the range of ater contents for compacting the soil in the field if the specified dry unit eight is 99% of the modified Proctor maximum dry unit eight? Ho do void ratio, porosity and degree of saturation vary ith dry density? Table 1 Proctor Test Calculations Proctor Compaction Test Soil Description Specific Gravity (Assumed) Volume of Mold Group Date Data Point No. Point - 1 Point - 2 Point - Point - 4 Point - 5 Wt. Soil + Mold Wt. Mold Wt. Soil Wet Density Dry Density Water Content Sample Top Bot. Top Bot. Top Bot. Top Bot. Top Bot. Tare No. Tare + Wet Soil Tare + Dry Soil Mass of Tare Mass of Dry Soil Mass of Water Water Content % Average Water Content Dry Density at S100% Void Ratio Porosity (%) Degree of Saturation (%) Proctor Tests - 5

6 Sample Calculations: Dry Unit Weight (lb/ft ) Water Content (%) Dry Unit Weight (kn/m ) Figure 1 Proctor Compaction Curve ith Zero Air Voids Curve (G s 2.7) Proctor Tests - 6

7 Picture 1, 2 and Proct Picture 1, 2 and - Proctor Compaction Apparatus Proctor Tests - 7