Pre fabricated vertical drain and stone columns
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This document discusses pre-fabricated vertical drains (PVDs) and stone columns for ground improvement. PVDs are plastic strips wrapped in geotextile that shorten drainage paths and reduce consolidation time from over 15 years to 1 year. Stone columns consist of crushed stone that improve load bearing capacity, reduce settlement, and act as vertical drains. Both techniques alter soil properties, with PVDs best for clays and stone columns suitable for weak soils like clays and organics.
Pre fabricated vertical drain and stone columns
- 1. PRE-FABRICATED VERTICAL DRAIN AND STONE COLUMNS Prepared by- Bhadani Ravi (14CL004)
- 2. WHAT IS THE REQUIREMENT OF GROUND IMPROVEMENT? To alter the natural properties of rock or soil Reducing compressibility and permeability Increasing strength ,bearing capacity
- 3. What is PVD ?
- 4. What is PVD ? Prefabricated Vertical Drain - PVD Typically 95 -100 mm wide by 3 - 5 mm thick Synthetic core wrapped with geotextiles. Many types of core.
- 5. PVDs shorten drainage path 90% Consolidation time reduced from >15 years to 1 year
- 6. Introduction to PVD Prefabricated vertical drain or “Wick drain” are composed of plastic encased by geotextiles for the purpose of expediting consolidation of slow draining soils. Two main components of PVD serve the following functions; 1. Core serves as a longitudinal flow path along the drain . 2. Filter jacket allows water to pass into the core while restricting intrusion of soil particles. Prefabricated wick drains have several other purposes. They may be applied to reduce potential down drag on piles, or increase storage capacity for future landfills and waste containment sites. A developing application for PVD are the collection and extraction of contaminated groundwater, which may be coupled with cutoff walls to ensure full withdrawal.
- 7. Suitable soils. PVD are not versatile for all soil types and geological conditions. The drains can be implemented in soils that are moderately to highly compressible under static loading. Applicable soils include: 1. Inorganic silts and clays of low to moderate sensitivity 2. Organic layers 3. Decomposed peat 4. Clayey and silty sands 5. Dredge spoils 6. Varved cohesive deposits
- 9. Drain Influence Zone The drain influence zone (D) is a function of drain spacing (S) only. Vertical drains are commonly installed in square or triangular patterns. Square pattern layouts have greater ease and control in the field. However the triangular patterns are preferred to provide more uniform consolidation between drains.
- 10. Settlement Vs Time Curve
- 11. Benefits of PVD Decrease overall time required for completion of primary consolidation due to preloading Decrease the amount of surcharge required to achieve the desired amount of pre-compression in the given time Increase the rate of strength gain due to consolidation of soft soils when stability is of concern Comparison to sand drains: Economic competitiveness, fewer disturbances to the soil mass compared to displacement sand drains, and the speed and simplicity of installation. Also feasible to be installed in a non vertical orientation. Installation of PVD typically 6000 linear meter per day and result in a lower project cost. No risk of PVDs breaking during installation.- sand drain have discontinuous if mandrill is too fast. No shear failure of PVDs during settlement.
- 12. Stone Columns
- 13. What is Stone Columns ? A ground improvement technique to improve the load bearing capacity and reduce the settlement of the soil Stone column consists of crushed coarse aggregates of various sizes. The stone columns serve two basic functions, namely 1. Providing strength reinforcement to the soil and 2. Acting as vertical drains to allow subsoil consolidation to occur quickly under any given loading. Stone columns are constructed by experienced contractors using specialist equipment. The construction uses an excavator with a vibrating probe to feed stone into the ground, forming a vertical column of stone
- 14. Functions of Stone Columns Improve the bearing capacity of weak soil. Carry high shear stress by acting as stiff elements and hence increase the stability of embankment founded on soft ground. Facilitate radial drainage (by acting as vertical drains) and dissipate rapidly the excess pore water pressure leading to acceleration of consolidation process and reduced post-construction settlements.
- 15. Suitable soils. Soft, Non-compactable, Weak soil Granular with high fines content Organic soils Marine/ Alluvial clays Liquefiable soils Cohesive soils
- 18. Basic Design Parameters Stone column diameter, D Depth of stone column. Pattern. Spacing. Equivalent diameter. Replacement ratio.
- 19. Advantages of Stone Columns. Increase the bearing capacity of in-situ soils To reduced total and differential settlements Expedites consolidation settlement. Mitigates the risk of liquefaction. More economical than piling. No waiting period after installation. Homogenizes variable soil properties