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Why Do We Classify Soils? • From experience and historic data, we know the engineering behavior of most soil types: • Strength when wet • Strength when loaded • Behavior when disturbed (earthquake, vibration) • From the historic data and research of our predecessors, soil classification systems have been developed. • Therefore, if a soil can be classified accordingly, we can predict its behavior under specific conditions
Silt: What is It? • Silt is VERY fine sand • Produced by the mechanical weathering of rock • Grinding by glaciers, • sandblasting by the wind, • water erosion of rocks on the beds of rivers and streams. • Silt particles are larger than clay but smaller than sand. • Mineralogically, silt is mainly quartz and feldspar • Silt is sometimes known as 'rock flour' or 'stone dust'
Engineering Properties of Silt • Little or no dry strength • Non-plastic • Volume change (settlement) under load is rapid • Moderate to low permeability • Susceptible to frost heave • Minimal changes in volume due to wet/dry • VERY DIFFICULT TO COMPACT • VERY DIFFICULT TO EXCAVATE BELOW WATER TABLE
Clay: What is it? • Produced by the chemical weathering: • Low concentrations of naturally occurring solvents migrate through rock and take mineral particles along. • Clay deposits are formed as the result of deposition after they have been eroded and transported from their original location of formation.
Engineering Properties of Clay • Clay particles are plate-shaped & have highly charged surfaces • The electrical charge on the surface attracts and holds water. • Strongest when dry due to cohesion of particles • Plastic when wet & over a range of w% • Load carrying capacity is linked to load history • Previously compressed = higher current strength • Settlement occurs over time (under static load) • Not compressible under dynamic load • Susceptible to freeze-thaw • Volume changes due to wet/dry • Easy to compact in thin layers (lifts)
Consolidation of Clay Soils • Spring analogy : Consolidation is explained with an idealized system composed of a spring, a container with a hole in its cover, and water. In this system, the spring represents the compressibility of the soil, and the water which fills the container represents the pore water in the soil.
The container is filled with water, and the hole is closed. (Fully saturated soil) A load is applied onto the cover, while the hole is still unopened. At this stage, only the water resists the applied load. (Development of excessive internal pore pressure) When hole is opened, water starts to drain out through the hole and the spring shortens. (Loss of excessive pore water) After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excessive pore water pressure. End of consolidation)
Sheeps Foot Roller or Static Roller • Applies heavy load in a slow/rolling action. • Used to compact cohesive soils in lifts
Consolidation of Granular Materials • It is necessary to densify loose granular soils to achieve acceptable foundation performance of structures. • Compaction of granular soils is achieved by vibration: • By use if a vibrator roller • By frequent drops of a large mass from a great height (deep dynamic compaction). • By insertion of a large vibrating poker into the ground (vibro-compaction
“Vibratory Roller” • Used to compact sand & gravel • Delivers a dynamic blow as it rolls • “Shakes” particles into a more dense configuration
PenetrationThe vibroprobe penetrates to the required depth by vibration and jetting action of water and/or air CompactionThe vibroprobe is retracted in 0.5 m intervals. The in situ sand or gravel is flowing towards the vibroprobe. CompletionAfter compaction the platform needs to be leveled and eventually roller compacted at the surface. Vibro-Compaction
The principle of sand compaction (Vibroflotation): The compaction process consists of a flotation of the soil particles as a result of vibration, which then allows for a rearrangement of the particles into a denser state.
Deep Dynamic Compaction • Natural soil deposits and undocumented fills can be densified by dropping large weights from great heights repeatedly on the ground surface. • The energy imparted is considerable & compaction can be achieved at significant depths below the ground surface.
This mass of concrete, weighing about 12,000 pounds, was used for deep dynamic compaction at the site of an oil storage tank in Japan.
Here the mass is lifted to a height of 50 feet and is ready to be dropped. When it hits the surface of the ground, the blow will impart about 600,000 foot-pounds of energy.
The treatment pattern, energy level, number of passes and phasing of passes are designed based on soil conditions, required bearing capacity and settlement characteristics.