Ground improvement technique
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1) The document discusses ground improvement techniques of preloading and vertical drainage. Preloading involves applying a surcharge load to improve soil strength and reduce settlements before construction. 2) Vertical drains are often used with preloading to accelerate consolidation by shortening the drainage path. Common types are sand drains and prefabricated vertical drains. 3) Vacuum preloading is described as an alternative to conventional preloading using surcharge loads, applying atmospheric pressure via a membrane system instead. This requires an effective drainage and vacuum maintenance system.
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Ground improvement technique
- 1. GROUND IMPROVEMENT BY PRELOADING AND DRAINAGE Prepared By:- Sagar Halwawala (11BCL014) Himanshu Manolkar (11BCL015)Himanshu Manolkar (11BCL015) Satish Kambaliya (11BCL016) Abhinav Karmilla (11BCL017) Civil Engineering Department Institute Of Technology Nirma University, Ahmedabad
- 2. Introduction • In times of urbanization, growth of population and associated developments, construction activities are more and more focused on soils which were considered unsuitable in the past decades. These soft soil deposits have a lowdecades. These soft soil deposits have a low bearing capacity and exhibit large settlements when subjected to loading. It is therefore inevitable to treat soft soil deposits prior to construction activities in order to prevent differential settlements and subsequently potential damages to structures.
- 3. Introduction cont. • According to Bergado et al. (1996) ground improvement techniques can be divided broadly into two categories. 1. Technique which require foreign materials and utilisation of reinforcements. They are based on stiffening columns either by the use of astiffening columns either by the use of a granular fill (stone columns), by piling elements which are not reaching a still soil stratum (creep piles) or by in situ mixing of the soil with chemical agents (deep stabilisation). 2. Technique which includes strengthening of the soil by dewatering, i.e. preloading techniques often combined with vertical drains.
- 4. Preloading OR Precompression • Preloading is surcharging the ground with a uniformly distributed surface load prior to the construction of the structure such as Buildings, Embankments, Motorways, and Runways Tanks etc.)etc.) • The purpose is to take up the settlements under the Civil Engineering structures before they are built. • Soils suitable for preloading: Compressible soft to medium soft saturated clays and silts, organic clays, peats.
- 5. Preloading cont. Primary consolidation using Surcharge loading Primary consolidation by using surcharge loading
- 6. • Types of preloads: Earth fills (Most common), Water in tanks or ponds, vaccum application under a membrane, special anchor and jack systems, groundwater lowering, electro osmosis. • The surcharge results in; Preloading cont. • The surcharge results in; 1. Primary Consolidation Settlement 2. Secondary Consolidation Settlement 3. Increase in the undrained shear strength of the soil.
- 7. Preloading Techniques A. Conventional Preloading • The simplest solution of preloading is a preload, e. g. by means of an embankment. • When the load is placed on the soft soil, it is initially carried by the pore water. When the soil is not very permeable, which is normally theis not very permeable, which is normally the case; the water pressure will decrease gradually because the pore water is only able to flow away very slowly in vertical direction. • In order not to create any stability problems, the load must mostly be placed in two or more stages.
- 8. Conventional Preloading cont. The principle of Conventional Preloading If the temporary load exceeds theload exceeds the final construction load, the excess refers to as surcharge load.
- 9. Conventional Preloading cont. • The temporary surcharge can be removed when the settlements exceeds the predicted final settlement. • This should preferably not happen before the remaining excess pore pressure is below the stress increase caused by the temporary surcharge. • By increasing the time of temporary overloading, or the size of the overload, secondary settlement can be reduced or even eliminated.be reduced or even eliminated. • This is because by using a surcharge higher than the work load, the soil will always be in an over consolidated state and the secondary compression for over consolidated soil are much smaller than that of normally consolidated soil. This will benefit greatly the subsequent geotechnical design.
- 10. B. Vacuum Preloading • Sometimes soft soil may be so weak that even a common 1.5 m embankment might cause stability problems. Then it can be suitable to use the method of vacuum preloading. • In 1952 Kjellman was the first who introduced vacuum preloading to accelerate consolidation. Invacuum preloading to accelerate consolidation. In vacuum consolidation the surcharge load is replace by atmospheric pressure. • In its simplest form the method of vacuum consolidation consists of a system of vertical drains and a drainage layer (sand) on top.
- 11. Vacuum Preloading Conti…. • The common advantages of vacuum preloading are that there is no extra fill material needed, the construction times are generally shorter and it requires no heavy machinery. Moreover, norequires no heavy machinery. Moreover, no chemical admixtures will penetrate into the ground and thus it is an environmental friendly ground improvement method.
- 13. Vacuum Preloading cont. Possible problems associated with vacuum preloading are: • To maintain an effective drainage system under the membrane that expels water and air throughout the whole pumping duration. • Keeping non-water saturated medium below the• Keeping non-water saturated medium below the membrane. • To maintain an effective level of vacuum. • To maintain a leak proof system in particular at the pumps / membrane connections and over the entire membrane area.
- 14. Vacuum Preloading cont. • Anchoring and sealing of the system at the periphery. • Reducing lateral seepage towards the vacuum area.area.
- 15. Principles of Preloading Figure illustrates schematically a vertical stress profile when a vacuum load (assuming 100 % efficiency) is applied to the ground surface in comparison with initial conditions and conventional surcharge. a) initial in situ conditions
- 16. Principles of Preloading cont. b) conventional surcharge c) vacuum induced surcharge
- 17. Vertical Drains • Preloading technique may not work sometimes alone due to a thick uniform soft clay layer or permeability of the clay is very low. • so time for precompression is very long and not practical or surcharge will be very high for reasonable waiting periods.reasonable waiting periods. • Because of its low permeability, the consolidation settlement of soft clays takes a long time to complete. To shorten the consolidation time, vertical drains are installed together with preloading either by an embankment or by means of vacuum pressure.
- 18. Vertical Drains cont. • Therefore, the vertical drain installation reduces the length of the drainage path and, consequently, accelerates the consolidation process and allows the clay to gain rapid strengthprocess and allows the clay to gain rapid strength increase to carry the new load by its own.
- 19. Vertical Drains cont. Preloading with vertical drains
- 21. Types of Vertical Drains • Sand drains are basically boreholes filled with sand. As for the displacement type of sand drains, a closed mandrel is driven or pushed into the ground with resulting displacement in both vertical and horizontal directions. • The installation causes therefore disturbances, especially in soft and sensitive clays, whichespecially in soft and sensitive clays, which reduces the shear strength and horizontal permeability. • The low- or non-displacement installations are considered to have less disturbing effects on the soil. Drilling of the hole is done by means of an auger or water jets. In terms of jetting, however, installation is very complex.
- 22. Types of Vertical Drains conti. • The installation of prefabricated vertical drains is also done by a mandrel and it is a displacement installation. The dimensions of the prefabricated drains are much smaller compared to sand drains and subsequently are the dimensions of the mandrel. Thus, the degree of soil disturbance caused by the size of the mandrel during installations is lower. by the size of the mandrel during installations is lower. • At the tip of the mandrel is detachable shoe or anchor made of a small piece of metal (given in following figure). Sometimes it might also be a piece of drain itself. The purpose of the anchor is to prevent soil from entering the mandrel and plugging it during penetration. It also keeps the drain at the desired depth as the mandrel is withdrawn.
- 23. Some disadvantages of sand drains • To receive adequate drainage properties, sand has to be carefully chosen which might seldom be found close to the construction site. • Drains might become discontinuous because of careless installation or horizontal soil displacement during the consolidation process. • During filling bulking of the sand might appear which could lead to cavities and subsequently to collapse due tocould lead to cavities and subsequently to collapse due to flooding. • Construction problems and/or budgetary burdens might arise due to the large diameter of sand drains. • The disturbance of the soil surrounding each drain caused by installation may reduce the permeability, the flow of water of water to the drain and thus the efficiency of the system. • The reinforcing effect of sand drains may reduce the effectiveness of preloading the subsoil.
- 24. Drain Properties • Equivalent diameter for band-shaped drains The theory of consolidation with vertical drains assumes that the vertical drains are circular in their cross-section. Since most of the prefabricated drains are rectangular in cross-section, the rectangular drain has to be converted into an equivalent cylindrical shape. That implies that the equivalent diameter has the same theoretical radial drainage capacity as the band-shaped drain. The equivalent diameter (dw) of a band-shaped drain with width (a) and thickness (b) as given below.
- 25. Discharge Capacity The discharge capacity depends on the following factors: • Lateral earth pressure: By increasing lateral pressure, the filter passes into the core and subsequently decreases the discharge capacity due to a reduction of the cross-sectional area available for flow. • Large settlements: During consolidation, the ground will be subjected to large settlements. Thus, the drains tend to settle together with the ground which will result in bending of folding of the drain. • Clogging of drain: In the initial filtering process of flow from the soil• Clogging of drain: In the initial filtering process of flow from the soil through the drain filter, the displaced water will contain a small portion of fine particles. These may be deposited with the core channels and may lead to clogging of the drain. • Time: The discharge capacity may be reduced due to aging in the soil after installation, possibly due to biological and chemical activities. • Hydraulic gradient: The measured discharge capacity varies with different hydraulic gradients and is smaller when a higher hydraulic gradient is used. This might be due to the loss of flow energy as a result of turbulent flow at a high hydraulic gradient.
- 26. Properties of the filter The filter has to meet the following requirements: • The permeability of the filter should be high enough not to influence the discharge capacity of the drain system. • On the contrary, the permeability of the filter should be low enough to retain fine soil particles. The soil particles might penetrate through the filter into the core, which eventually might be filled with soil and getcore, which eventually might be filled with soil and get clogged. • The filter needs to be strong enough to withstand high lateral pressure in order not to be squeezed into channel system of the core. • The filter should be strong enough not to break during installation, and • The filter should not deteriorate with time because this would reduce the discharge capacity of the drain.
- 27. CONCLUSION • A system of prefabricated vertical drains(PVDs) combined with vacuum preloading is an effective method for accelerating soil consolidation. • It is clear that the application of PVDs combined with vacuum and surcharge preloading has become common practice, and is now considered to be one of the most effective ground improvement techniques. • Analytical and numerical modelling of vacuum preloading is still a• Analytical and numerical modelling of vacuum preloading is still a developing research area. There has always been a discrepancy between the predictions and observed performance of embankments stabilised with vertical drains and vacuum pressure. • Vacuum assisted consolidation is an innovative method which has recently, and successfully, been used for large scale projects on very soft soils in reclamation areas. The extent of surcharge fill can be decreased to achieve the same amount of settlement and the lateral yield of the soft soil can be controlled by PVDs used in conjunction with vacuum pressure.