GROUND IMPROVEMENT TECHNIQUES UNIT-1, Exams of Design of Wood Structures

Download GROUND IMPROVEMENT TECHNIQUES UNIT-1 and more Exams Design of Wood Structures in PDF only on Docsity! GROUND IMPROVEMENT TECHNIQUES UNIT-1 Introduction Need for Engineering ground Improvement When a project encounters difficult foundation conditions, possible alternate solutions are: Avoid the particular site Design the planned structure accordingly. Use a soft foundation supported by piles, design a very stiff structure which is not damaged by settlements Remove and replace unsuitable soils. Attempt to modify the existing ground Classification of Ground modification Techniques: 4 Groups of Ground Improvement techniques Mechanical Modification: Soil density is increased by the application of mechanical force, including compaction of surface layers by static vibratory such as compact roller and plate vibrators. Hydraulic Modification: Free pore water is forced out of soil via drains or wells. Course grained soils; it is achieved by lowering the ground water level through pumping from boreholes, or trenches. In fine grained soils the long term application of external loads (preloading) or electrical forces (electrometic stabilization) Physical and chemical modification: Stabilization by physical mixing adhesives with surface layers or columns of soil .Adhesive includes natural soils industrial by-products or waste. Materials or cementations or other chemicals which react with each other and/or the ground. When adhesives are injected via boreholes under pressure into voids within the ground or between it and a structure the process is called grouting. Soil stabilization by heating and by freezing the ground is considered thermal methods of modifications Modification by inclusions and confinement: Reinforcement by fibers, strips bars meshes and fabrics imparts tensile strength to a constructed soil mass. In-situ reinforcement is achieved by nails and anchors. Stable earth retaining structure can also be formed by confining soil with concrete, Steel, or fabric elements. Suitability, Feasibility and Desirability The choice of a method of ground improvement for a particular object will depend on the following factors. Type and degree of improvement required Type of soil , geological structure, seepage conditions cost Availability of equipment and materials and the quality of work required Construction time available Possible damage to adjacent structures or pollution of ground water resources Durability of material involved ( as related to the expected life of structure for a given environmental and stress conditions) Toxicity or corrosivity of any chemical additives. Reliability of method of analysis and design. Feasibility of construction control and performance measurements If soil is moist, freezing is applicable to all type of soil. Dewatering Dewatering involves controlling groundwater by pumping, to locally lower groundwater levels in the vicinity of the excavation. The application of sumps and ditches within an excavation is one of the elementary method of dewatering employed in construction. The water entering these installed units can be pumped out. The general procedure of dewatering with sumps and ditches is depicted in the figure-1. Fig.1. Dewatering Method by the Installation of Sumps and Ditches The sump is located below the ground level of the excavation as shown in figure-1, at one or more corners or the sides. The procedure involves the cutting of a small ditch around the bottom of the excavation, that is falling towards the sump. The sumps is the name given for the shallow pits that are dug along the periphery of the excavation or the drainage area, which is named as ditches. Under the action of gravity, the water from the slopes will flow to the sumps. The sumps collect the water and are later pumped out. Significant amount of seepage can result in raveling or sloughing or softening of the slope in the lower part. The slump bottom may also be subjected to piping. A deep well system consists of an array of bored wells pumped by submersible pumps. Pumping from each well lowers the groundwater level and creates a cone of depression or drawdown around itself. Several wells acting in combination can lower groundwater level over a wide area beneath an excavation. Because the technique does not operate on a suction principle, large drawdowns can be achieved, limited only by the depth of the wells, and the hydrogeological conditions. The wells are generally sited just outside the area of proposed excavation, and are pumped by electric submersible pumps near the base of each well. Water collection pipes, power supply generators, electrical controls and monitoring systems are located at the surface. This is shown in figure-1. Here the excavation can be pre*drained for the complete depth. Fig.1: Deep Well Dewatering System Working and Arrangement of Deep Well System The deep wells arrangement for the purpose of dewatering is similar to that for commercial water wells. These systems will make use of a screen that have a diameter of 6 to 4 inches with lengths ranging up to 300 feet. When such a system is installed, a filter is placed around the screen. This arrangement helps to prevent the infiltration of the foundation materials into the well. The installation of filter also helps to improve the yield. In order to dewater small deep excavations, the deep well systems can be used in conjunction with the deep wells. This is applied for related works of tunnels, caissons sunk, shafts and the areas with fine grained sand or stratified soils that are pervious. In areas, there are rock layer below the ground table this method work best. An increase in hydraulic gradient to the well because of the use of vacuum creates a vacuum within the surrounding. This phenomenon avoids seepage from the perched water into the excavation. The installation of deep well system incorporating vacuum is shown in figure-2. This type requires adequate vacuum capacity to undergo the dewatering operation efficiently. Fig.2. The use of deep well with vacuum systems to dewater a shaft over a stratified ground material. To have sufficient wetted area of intake in the aquifer, adequate well depth have to be provided. This helps to produce yield and interactive drawdown. In most of the civil engineering applications, a depth of 60m with a typical depth value of 20m is used. For a limited distance say 1 to 2m, the well might penetrate an impermeable layer lying below the pumped aquifer. This is to behave as sump for the fines. The pump must be placed such a level in the well so that the water circulation helps it to remain cool. The site layout decides the spacing of the wells. But most commonly, the spacing used is 10 to 30m. The deepening of the well creates drawdown in areas. Sometimes these might be the areas where the wells cannot be sited. Special care and precaution must be taken so that with increase in drawdown no kind of settlement is happening to the adjacent buildings. Groundwater Engineering provides complete deep well dewatering solutions: Design of dewatering systems Well drilling and installation Pumping tests Equipment sales and rental Monitoring systems On-site operation and maintenance WELLPOINTS Wellpoint dewatering is widely used for excavations of shallow depths, especially for pipeline trench excavations. In appropriate ground conditions a wellpoint system can be installed speedily and made operational rapidly. A typical wellpoint system consists of a series of small diameters wells (known as wellpoints) connected via a header pipe, to the suction side of a suitable wellpoint pump. The pump creates a vacuum in the header pipe, drawing water up out of the ground. For long pipeline trenches, horizontal wellpoints may be installed by special trenching machines. Wellpoints are typically installed in lines or rings around the excavation, and are pumped by diesel or electrically powered pumps, with associated header mains, water discharge pipes, power supply generators, electrical controls and monitoring systems. There are two types of well point system, namely single stage well point system and multi- stage well point system. These systems are briefly described in the followings. 1. Single Stage Well Point system 2. Multistage Well Point system Single Stage Well Point system – A well point consists of a pipe about 1 m long and 50 mm in diameter. It has perforations, which are covered with a screen to prevent clogging in. At Vacuum wells are an adaptation of deep well systems. Each well in the system is pumped by a submersible pumps, but a vacuum is also applied to each well via a vacuum pump located at the surface. The application of a vacuum allows the wells to be more effective in reducing pore water pressures in poorly draining fine grained soils. In appropriate ground conditions vacuum wells can be a viable alternative to eductors. Groundwater Engineering provides complete vacuum well dewatering solutions: Design of dewatering systems Well drilling and installation Pumping tests Equipment sales and rental Monitoring systems On-site operation and maintenance HORIZONTAL WELLS Horizontal wells for dewatering are of two principal types: Horizontal drains installed by specialist trenching machines Horizontally directionally drilled (HDD) wells. Horizontal drains installed by specialist trenching machines This technique uses a horizontal flexible perforated pipe, pumped by a wellpoint pump, to lower groundwater levels. The perforated pipe is installed by a special trenching machine. One end of the pipe is unperforated and is brought to the surface and connected to a wellpoint suction pump. The method can be very effective for dewatering long pipeline excavations. Horizontally directionally drilled (HDD) wells HDD wells are used where groundwater must be abstracted from beneath inaccessible areas or from areas where the disruption associated with surface drilling is undesirable. Applications for HDD wells include: Installation of permanent dewatering systems beneath existing built up (urban) environments. Pumping for remediation of contaminated groundwater without the risks of cross- contamination associated with vertical drilling Dewatering for tunnel construction Recharge wells to re-inject water as part of artificial recharge schemes. Groundwater Engineering provides complete horizontal well dewatering solutions: Design of dewatering systems Well drilling and installation Pumping tests Equipment sales and rental Monitoring systems On-site operation and maintenance Drains: A Drain consists of filter, conduit and disposal system. A filter is necessary for continued efficiency of the drain and to prevent seepage erosion. The collection of water is done in the drain conduits. Normally the size of the conduit is 5 to 10 times larger than its hydraulic dictate. The commercial pipes have perforations of diameter 8 to 9mm and are need of 12 to 15mm gravel filter. Classification: a. Open drains b. Closed drains c. Horizontal drains d. Foundation drains e. Blanket drains f. Interceptor drains Open drains Drainage systems may be open or closed (subsurface), depending on the drainage method. In the open system, open channels are used as the regulating network, whereas in the closed system, the regulating network is made up of closed collectors and has underground drains and small channels. In both systems, the main conducting and protective channels are open. Open drainage systems are used for the initial drainage of marshes and forests and sometimes also of hayfields and pastures. Their drawbacks include a reduction in land available for cultivation, interference with mechanized farming operations, and overgrowth with weeds and other types of interference in the channels. Closed drains Closed drainage systems are technically more advanced and long lasting. They do not have the shortcomings of open systems, and they offer great potential for irrigation of drained lands during dry periods of the growing season. For every 30m to 50m there must be openings to flush out the pipe. At 100m to 150m intervals, the manholes must be provided at changes in direction along straight sections. Such systems are built for intensive use of drained lands. The Criterion For Selecting Of Fill Material Around Drains The criteria for selecting material as suitable for use as fill shall be based on achieving adequate strength, stiffness and permeability after compaction. These criteria shall take account of the purpose of the fill and the requirements of any structure to be placed on it. Suitable fill materials include most graded natural granular materials and certain waste products such as selected colliery waste and pulverized fuel ash. Some manufactured materials, such as light aggregate, can also be used in some circumstances. Some cohesive materials may be suitable but require particular care. The following aspects shall be considered when selecting a fill material: - grading; - resistance to crushing; - compactibility; - plasticity; - organic content; - chemical aggressivity; - pollution effects; - solubility; - susceptibility to volume changes (swelling clays and collapsible materials); - the effect of frost; - resistance to weathering; - the effect of excavation, transportation and placement; - the possibility of cementation occurring after placement (e.g. blast furnace slags). If local materials are not suitable for use as fill in their natural state it may be necessary to adopt one of the following procedures: - adjust the water content; - mix with cement, lime or other materials; - crush, sieve or wash; - Protect with appropriate material; - use drainage layers. When the selected material contains potentially aggressive or polluting chemicals, adequate provisions shall be adopted to prevent these attacking structures or services or polluting the groundwater. Such materials shall only be used in large amounts in permanently monitored locations. Blanket Drains A drainage blanket is a very permeable layer of material. It can be used to remove water from beneath pavement structures when applied as a permeable base or can be used effectively to control groundwater from cut slopes and beneath fills. In slope stability applications drainage blankets improve slope stability by preventing a seepage surface from developing on the slope and by providing a buttressing effect. Drainage blankets are also used as an interface between embankment and soft foundations to provide drainage during foundation consolidation. GROUTING Grout is a fluid form of concrete used to fill gaps. Grout is generally a mixture of water, cement, and sand, and is employed in pressure grouting, embedding rebar in masonry walls, connecting sections of pre-cast concrete, filling voids, and sealing joints such as those between tiles. Grouting is a high-cost treatment method and should be used where there is adequate confinement to handle the injection pressures. The typical applications include control of groundwater during construction, filling voids to prevent larger amounts of settlement, soil strengthening, stabilization of loose sands, foundation underpinning, filling voids in calcareous formations and strengthening soils for protection during excavation. Selection of the most suitable method for stabilization will depend on the type of soil, degree of improvement and depth and extent of treatment required. Another factor to consider is whether the treatment is required for a new or existing structure. Grouting especially with some chemical grouts may present risks to the public health and environment that must be considered. Considerations for utilizing a treatment method include energy use, maintenance costs, requirements for excavation and adequate treatment performance. Environmental risks include mismanagement of surface and groundwater drainage and incomplete treatment. Leachates and migration of contaminants can contaminate subsoil, groundwater, water wells and nearby surface water unless properly managed. There are several ground barrier methods used to control seepage, which include slurry-trench cutoff walls and grout curtains. The advantages of grouting include: a. Can be performed on almost any ground condition b. It doesn't induce vibration and can be controlled to avoid structural damages c. Improvements to ground formations can be measured d. Very useful for confined spaces and low headroom applications e. Used for slab jacking to lift or level distorted foundations f. Can be installed adjacent to existing walls. g. Can be used to control seepage, groundwater flows and hazardous waste plumes The primary Objectives of grouting ground formations are to: a) Increase the strength and bearing capacity or the soil stability, b) Reduce seepage and control groundwater during construction, c) Form groundwater barriers and d) rehabilitate or reinforce structures. The Different Investigation Methods Carried Out Before Grouting: Drilling and direct inspection to accurately locate and determine local conditions. Taking coring samples for laboratory tests. Drilling with drilling data recording to locate fissured zones, voids and the interface between structure and surrounding ground Borehole logging with BHTV Scanner examination (optical/seismic) Non-destructive geophysical investigations (seismic resistivity) Water testing (constant head or falling head tests conducted in borehole) Underground flow & temperature measurements Pumping test to assessment of initial hydraulic conditions. The Applications Of Grouting Grouting May Be Used In The Following Applications: Filling Voids To Prevent Excessive Settlement To Increase Allowable Pressure Of The Soil Both For New Structures And / Or Additions To Existing Structures. Control Of Groundwater Flow Prevention of Loose - Loose to Medium Sand Densification under Adjacent Structures (i.e. both for Vertical and Lateral Movements) Due To Adjacent Excavations, Pile Driving Etc. Ground Movement Control during Tunnelling Operations. Soil Strengthening To Reduce Lateral Support Requirement. Soil Strengthening To Increase Lateral and Vertical Resistance of Piles. Stabilization of Loose Sands against Liquefaction. Foundation Underpinning. Slope Stabilization. Volume Change Control Of Expansive Soils Through Pressure Injection Of Lime Slurry (Only For Some Expansive Soils Not All) The Systematic Representation Of Different Methods Of Grouting: Different Grouting Materials Grout Materials: 1. Suspensions: Small particles of solids are distributed in a liquid dispersion medium. Example: cement and clay in water 2. Emulsions: A two phase system containing minute (colloidal) droplets of liquid in a disperse phase. Example: bitumen and water. Foams created by emulsifying a gas into the grout material, which could be cement or an organic chemical. Foaming agents increase surface tension; assist in forming bubbles by agitation. 3. Solutions: Liquid homogeneous molecular mixtures of two or more substances. Example: sodium silicate, organic resins, and a wide variety of other so called chemical grouts. Classification Of Grouting other finely ground solids that undergo a hardening process with time. These materials may be used to fill pores and joints in soil and rock, provided the grout particles are small enough to be carried through the pore or joint openings. A good rule of thumb is that the effective particle diameter in the grout suspension should be less than the dimension of the pore or joint aperture divided by 5. Slurry grout mixes used for permeation grouting are designed primarily to promote passage of the grout particles into the porous medium. The grain size of the slurry is matched to the pore aperture and steps are taken to assure the grout particles are properly dispersed in the grout. To eliminate the effect of bleed on Portland cement grout, additives are used to hold the cement grains in suspension at water to cement ratios that would otherwise be quite unstable. The most common additive is a water suspension of bentonite. Even small amounts of bentonite increase the inter particle forces dramatically and hold the cement particles in suspension. Permeation grouting is also known as Penetration grouting. Displacement Grouting: Displacement grouting is the injection of grout into a formation in such a manner as to move the formation, it may be controlled, as in compaction grouting or uncontrolled, as in high-pressure soil or rock grouting which leads to splitting of the ground, also called hydro fracture. Displacement grouting involves the use of grout to displace soil Such displacement can fill voids, cap sinkholes, deal with poor soils and leave grout mass in place. Displacement grouting constitutes a method of introducing support elements into a soil which cannot otherwise be modified readily. Such grouting takes on a number of names such as pressure grouting, cement grouting, slurry grouting, all of which are designations of both grouting and grout. The grouts involved in Displacement grouting range from compaction grout, through low mobility non-cohesive grout, to thinner and less viscous materials. Many are cement based because of low cost although ground rubber, walnut shells, oyster shells, and many other available things have been used in grout, depending on the problem. Jet Grouting Jet grouting is a technique where high-speed water jets emanating from a drill bit cut into alluvial soils; as the drill bit is withdrawn, grout is pumped through horizontal nozzles and mixes with or displaces the soil. The original foundation material is thus replaced with a stronger and/or more impermeable grout-soil mixture. Jet grouting may be used to form cut-off walls, do underpinning, or form deep foundations similar to grouted auger piles. The high-pressure water or grout is used to physically disrupt the ground, in the process modifying it and there by improving it. Jet grouting blasts extremely high pressure fluids into the ground at ultra-high velocities. The soil is broken up and mixed with the fluids to become one mass which then hardens. Depending on the application and soils to be treated, one of three variations is used: the single fluid system (slurry grout jet), the double fluid system (slurry grout jet surrounded by an air jet) and the triple fluid system (water jet surrounded by an air jet, with a lower grout jet). The jet grouting process constructs soil create panels, full columns or anything in between (partial columns) with designed strength and permeability. Jet grouting has been used to underpin existing foundations, construct excavation support walls, and construct slabs to seal the bottom of planned excavations. Jet grouting is effective across the widest range of soil types of any grouting system, including silts and most clays. Because it is an erosion-based system, soil erodibility plays a major role in predicting geometry, quality and production. Cohesion less soils are typically more erodible by jet grouting than cohesive soils. Since the geometry and physical properties of the soil create are engineered, the properties of the soil create are readily and accurately predictable. Jet grouting’s ability to construct soil create in confined spaces and around subsurface obstructions such as utilities, provides a unique degree of design Flexibility. Indeed, in any situation requiring control of groundwater or excavation of unstable soil (water-bearing or otherwise) jet grouting should be considered. Stages in grouting flow Grouting in stage may be in a descending or ascending direction. In descending method, impregnation of the ground occurs in advance of the borehole, which is advantageous in loose soil or rock. In ascending technique, grouting follows drilling as a separate phase Water pressure testing is possible immediately prior to grouting, allowing for a choice of the most suitable grout type, pressure and quantity of grout for that particular stratum. Following points to be taken care of: Minimum wastage of grout. Least damage to the ground. Maximum gain in strength or reduction in seepage. Grouting Techniques and controls The hole is drilled and cased A steel or plastic tube, slotted at regular intervals is inserted. The vertical slots are covered with a rubber sleeve. As the casing is withdrawn, the space between the sleeve tube and the borehole wall is sealed with a cement-bentonite grout. After the seal has set, the grouting tube is inserted grout exist between two packers allowing injection through selected slots with increasing pressure, the rubber sleeve bursts and grout flows into the soil. With the sleeve tube technique, grouting can be repeated in the same hole using different viscosity grout or different chemicals in a planned sequence. Post Grout Test