A H M 531 The Civil Engineering Center

Transcription

1 Compaction test By: Ahmed Essam Mansour Objective: To measure the optimum water content at which maximum unit weight of a soil occurs at standard compaction energy. General discussion: General approach for compaction process: Compaction of a soil can be defined as the process of reduction the volume of soil by expelling the air in the voids and so reducing the voids ratio. Soil compaction in engineering projects is desirable for three main reasons: To prevent future settlements. To increase shear strength. To decrease permeability. Compaction is considered as the cheapest method for soil stabilization which can be defined as the process of improving the physical properties of a soil to achieve the desired shear strength, structure and voids ratio compared to other methods of using chemicals such as lime, lime fly ash admixtures, cement and phosphoric acid compounds. In general soil stabilization is achieved by the input of energy through compaction, in the laboratory this energy is provided by the impact of a rammer, but in the field there are several methods of compaction: Static. Vibrating; preferred for sands and gravels. Impact; the one used in laboratories. Kneading; preferred for cohesive soils. Water content effect on the compaction process: Water content in soil plays a great role in the compaction process of fine grained soils. When adding water into a dry soil, soil particles adsorb water and form films of water around each particle, these water films gets thicker by the addition of more water and provides lubrication to the soil which aids the process of compaction. The water films effect on coarse grained soils is negligible as thickness of water films are small compared to the particles diameter. The addition of water above a certain limit Called optimum water content will increase the voids ratio in the soil and cause less compaction, that because excess water will take the place of air voids and occupies spaces that could be filled with soil. Compaction methods used in laboratories: In the laboratory test, the used compaction method shall give a degree of compaction similar to that of available field compaction. As construction equipments was small and gave relatively low densities in the past, small compaction energy was needed in the laboratory. But as the construction equipments developed and provided high densities, it became necessary to increase the amount of compaction energy. For the purpose, the main available compaction standard Standard Proctor was developed to Modified Proctor which provides a compaction energy 4.5 times of the standard proctor. The compaction energy is a function of: Weight of soil, number of blows, weight of the rammer, height of fall and the volume of the mold. 1Visit for more lab reports and lecture notes!

2 Compaction test By: Ahmed Essam Mansour Table 1 shows a comparison between the 2 standards. Table 1; Comparison of standard proctor and modified proctor. Standard Wt of hammer (kg) # of layers Height of hammer drop (cm) # of blows per layer Compaction energy (kj/m) Standard Proctor Modified Proctor Main purpose of this experiment: The main purpose of this experiment is to find out the optimum water content of a soil sample that provides the highest unit weight, which provides more control for the field work of compaction of a soil fill. This optimum water content is found by drawing a semi log graph, relating the water content (the ordinate) and the unit weight (the abscissa), this graph should have a zero slop point which represent the optimum water content and the maximum density. For the graph to be drawn three points at least should be obtained, one of them representing the zero slope point. For more accurate graph more points should be obtained. An inspection of the optimum water content shall be done before the starting the experiment, this inspection reduces the number of trials the have to be done. The inspection of this point is based on past experiments or on the Liquid limit and the plastic limit of the soil. General approach for the procedure: The method applied in the lab was standard proctor. The optimum water content for the soil sample was approximated as 21% from past experience, 5 point on the graph was planned to be obtained, so considering a water increment 3% at each trial we started from 16% water content keeping in mind an approximation of 3% initial water content. The water is added to soil and mixed thoroughly, the a sample is filled in the mold of known volume 944 cm 3 at three layer and rammed 25 times for each layer. The sample is weighed (W ms ), this weight represents the weight of mold + wet soil, small sample is taken to find the water content at each trial by oven drying it and calculating the difference between the wet weight and the dry weight. The dry unit weight is calculated using Formula 1; * Where; γ d : Dry unit weight Wt dry : Dry weight of soil filling the 944 cm 3 mold, and is calculated using Formula 2; * Where; Wt wet : Weight of wet soil filling the mold. W%: Water content 2Visit for more lab reports and lecture notes!

3 Compaction test By: Ahmed Essam Mansour Sample identification: Light brown clay sample passing sieve #4. Equipments: Compaction mold of 944 cm 3 volume with base plate and collar. Compaction rammer of 2.5 kg weight and 30.5 cm fall. Empty cans. Sample extruder. Steel straight edge. Large mixing pan and large spoon for dispensing soil. Weighing device. Procedures: 1. A 3 kg of air dried soil was used for the experiment, after the optimum water content was estimated as 21% and the initial water content of the soil as 3%, 13% (390 ml) of water was added to the soil and mixed thoroughly. 2. The inside layer of the mold was brushed with oil. 3. The soil was placed in the mold at 3 layers and rammed 25 times for each layer. The last layer is dealt with differently, where it is rammed first 10 times and then the layer was checked that is covers a distance a little bit above the collar, if lower more soil was added and the left 15 rams are continued. After finishing the ramming process of the three layers, the collar is removed and the surface of the mold was leveled by using the steel straight edge. The weight of mold + wet sample (W mw ) was obtained. 4. The sample was removed out of the mold by the sample extruder, and small sample was taken in a can, and the weight of the Can + Sample was obtained (W 1 ). And the cylindrical shaped soil was dispersed into soil particles passing sieve #4. 5. The previous steps were repeated till a noticed drop in the weight of the weight of mold + wet sample (W mw ) decreased. And at each trial an increment of water of 3% was added. 6. The cans containing wet samples were placed then in the oven for 24 hours. The weight of can + dry sample (W 2 ) was obtained. 3Visit for more lab reports and lecture notes!

4 Compaction test By: Ahmed Essam Mansour Calculations: Calculation of water added to the sample: Calculation of water content in soil sample: Calculation of unit weight: 4Visit for more lab reports and lecture notes!

5 Compaction test By: Ahmed Essam Mansour Discussion and results: As can be seen from the drawn curve between the dry unit weight and the water content, it increases at the beginning as was expected until it approached a point of zero slope which represents optimum water content and max dry density, after that point the curve decreases downward as was expected too. From the curve the optimum water content was specified as 27.9% which is higher that the expected value of 21%, this considerable increment may be a result of none curing the air dry soil used in our test. Sources of errors: 1. Ramming the soil on the mold over a non rigid pan, this caused dissipation of compaction energy. 2. The scale of the graph makes it hard to define the Optimum water content accurately. 3. The used air dried sample wasn t cured before the test. Improvements: More accurate result could be obtained if the manually compacting hammer was replace with a mechanical one, besides the development of a rigid, well-fixed bench to ram the sample over, in order to avoid loss of compaction energy. 5Visit for more lab reports and lecture notes!