SOIL MECHANICS EXAM – COMPACTION 100% accurate answers

SOIL MECHANICS EXAM – COMPACTION 100% accurate answers What happens during fill operations? When fill operations are taking place the new soils are usually compacted in place to improve the strength characteristics. What is compaction Compaction in general terms is the densification of soil by the reduction in voids (mainly air) by mechanical means. How do you define degree of compaction? the dry unit weight Describe the dry unit weight - It increases with compaction - It will also increase with an increase in moisture content. This is because water acts as a softening agent. However, after a certain water content (optimum water content), additional water will reduce the dry unit weight. How to measure compaction? - Standard Proctor Test (Lab) - Modified Proctor Test (Lab) What is the history of the Standard Proctor Test? Developed in the 1930's to simulate field compaction effort, in the laboratory What is the equipment used in the Standard Proctor Test? - 1/30 ft^3 mold - 6.5 lb hammer with a drop of 12 in. What is the simplified procedure when completing the standard proctor test? - The soil is mixed with a specific amount of water - Soil is compacted in 3 layers by delivering 25 blows per layer with the hammer. - Repeat with different amount of water. For the standard compaction test, what is the one thing that keeps changing the amount of water What is the things that you keep recording? - Wt = weight total - Vt = 1/30 ft^3 - Moisture content = w How do you calculate dry unit weight? gamma d = (Wt)/[Vt(1+ w%/100)] gamma d = dry unit weight Wt = weight total Vt = 1/30 ft^3 w% = moisture content In the standard proctor test what do you do when once you record the Wt, Vt, and the w? - calculate dry unit weight - plot the dry unit weight vs moisture content curve - Identify the maximum dry unit weight (gamma dmaximum) and the optimum water content (w opt) What are some factors that affect compaction? - Moisture content - Soil type - Compaction effort - Grain size distribution - Shape of grains - Specific gravity - Clay minerals. What are the effect of compaction on the structure of clay soils? - Compacting on the dry side of optimum will result in a flocculent structure. - Additional water increases repulsion of the clay particles and thus a greater degree of particle orientation (less dispersed structure when compacting wet of optimum) - Additionally, at the same moisture content an increase in compaction effort will result in more parallel orientation. - Variations in compaction can affect properties such as hydraulic conductivity, compressibility and strength Describe the modified proctor test - Developed in the 1940's during WWII to accommodate advances in construction machinery and need for higher compaction in airfields. - Same mold as standard test (1/30 ft^3). - However, hammer was modified to a 10 lb hammer with a drop of 18 in. - Additionally, soil is compacted in 5 layers by delivering 25 blows per layer How much Compaction energy per volume does the modified proctor test has? 12,400 ft-lb/ft^3 How much Compaction energy per volume does the standard proctor test has? 56,000 ft-lb/ft^3 Field compaction is usually done by what rollers what are the types of rollers? - Sheepsfoot rollers - vibratory rollers - Smooth-wheel rollers - Handheld vibrating plates - Pneumatic rubber-tired rollers - Rolling Dynamic Compaction Describe smooth-wheel rollers - Suitable for proof-rolling subgrades and fill soils with sandy and clayey soils - Not suited for deep compaction (thick layers) and for producing high unit weights. Whats the contact pressure for a smooth-wheel rollers 45 to 55 lb/in^2 with 100% coverage What does a smooth-wheel roller look like Describe Pneumatic rubber-tired rollers - Used for sandy and clayey soil. - Compaction is done by pressure and kneading action. Whats the contact pressure for a Pneumatic rubber-tired rollers? 85 to 100 lb/in^2 with up to 70 to 80% coverage. What does a Pneumatic rubber-tired rollers look like Describe a Sheepsfoot rollers - Very effective in compacting clayey soils. - Compacts from the bottom up, as the feet "walk out". What does a Sheepsfoot rollers look like Whats the contact pressure for the Sheepsfoot rollers 200 to 1000 lb/in^2 with small coverage. Describe a vibratory rollers - Can be attached to other types of rollers. - Very efficient in compacting granular soils. Describe a Handheld vibrating plates Used on granular soils in limited areas Describe a rolling Dynamic Compaction - Depth of influence up to 3 m. - Most effective between 0.8 to 3 m. - Can shear particles leading to higher compaction. - Can assist in crushing and screening of aggregate. How do you compact then? - Soil is usually spread in layers (loose layers) of a given thickness. Water is added to soil if needed and then compacted Specifications usually include a percentage of the proctor test, and a limit of the ranges in w What are some factors that will affect compaction? - Lift thickness - Intensity/pressure - Climate (evaporation/rain) - Number of passes How to measure compaction - Sand Cone Test - Nuclear Density Test - Drive Cylinder Test - Water Ring Test Describe the nuclear density test - Nuclear moisture density gauge. - Based on empirical correlations. - Emits gamma rays and monitors how they travel through the soil: - Gamma rays that travel back correlates to the unit weight. - Alpha particles measure moisture content. - Unusual soil chemistries can give erroneous numbers, i.e. micaceous soil.