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Ground Improvement Tehnique: Issues, Methods and their Selection

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  1. Ground Improvement Tehnique: Issues, Methods and their Selection Dr. J.N.Jha, Professor and Head (Civil Engineering), Guru Nanak Dev Engineering College, Ludhiana, Punjab-141006

  2. Present Day Scenario • Best buildable lands not available for construction • Available sites are having low strength because : • Filled up sites, • Low lying water logged, • Waste lands, • Creek lands with deep deposits of soft saturated marine clays • Another problem: Design loads are high and the site is situated in seismic zones

  3. What are the options? • Traditional foundation techniques sometimes costlier than the super structure and in many situations can’t be built • when a poor ground existsat the project site, designer faces following questions: • Should the poor ground be removed and replaced with a • more suitable material? • Should the weak ground be bypassed laterally by • changing the project’s location or vertically by the use of • deep foundations? or • Should the design of the facility (height, configuration, • etc) be changed to reflect the ground’s limitations?

  4. Development of ground improvement, gives the designer/bulder has a fourth option • To “fix” the poor ground and make it suitable for the project’s needs • Now the designer/builder faces new questions: • Should the problematic ground at the project site be • fixedinstead of bypassed? • What are the critical issues that influence the successful • application of a specific fixing tool? And • Which fixing tool to be used from comprehensive • and diversified set currently available in the tool box?

  5. What are the major functionsof Ground improvement in soil ? • To increase the bearing capacity • To control deformations and accelerate consolidation • To provide lateral stability • To form seepage cut-off and environmental control • To increase resistance to liquefaction • Above functions can be accomplished : • by modifying the ground’s character - with or without the addition of foreign material

  6. The current state of the practice: • Densification • Consolidation • Weight reduction • Reinforcement • Chemical treatment • Thermal stabilization • Electrotreatment • Biotechnical stabilization

  7. Ground Improvement by Densification • Methods of Application : • Vibrocompaction • Dynamic Compaction • Blasting • Compaction Grouting • Key Issues affecting densification: • Percent of fines in the soil, • Ability of the soil to dissipate excess pore water pressure, • Energy felt by the soil, • Presence of boulders, utilities and adjacent structures, and • Mysterious phenomenon of ageing.

  8. Vibrocompaction • Loose granular soils are densified at depth by • insertion of vibrating probes into the ground • Compaction is achieved by impact and vibration, • Compaction is achieved by with or without the • use of a water jet or compressed air, • Compaction is achieved by with or without the • addition of granular material. • Densification can be achieved to up to 30 m in • depth

  9. Vibrofloatation Schematic View and Equipment Used 1.Vibrofloat 2. Crane for Suspending Vibrofloat 3. Power Supplying Unit (75-150 KW)

  10. Real Time Photos

  11. Vibrofloat Nose Cone Lower Most Part helps in Penetration in to the soil Eccentric Weight Provides Weight, helps in lowering Vibrofloat Water Jet Creats a Quick Condition facilitating the vibrrofloat unit to sink Electric Motor Develops a huge Centrifugal Force by its Rotational Motion Vibration Insulator Prevents the vibration s to reach Hollow Tube Follow Tubes Facilitates the Vibrating Unit to Reach Desired Depth Diameter and Length Diameter- 300-400mm Length- 2-3m.

  12. Procedure-Step Step-1-Jet at the bottom of Vibrofloat is turned on and is gradually lowered into the ground Step-2- Water Jet creats a quick condition in the soil thus facilitating the vibrating unit to sink under its own weight

  13. Step-3-Granular material is poured from the top into the hole through the annular space between the hole and vertical pipe. Water from the lower jet is transferred to the top of Vibrating unit which carries the granular material down to the hole. Step-4- Vibrating unit is raised gradually in lifts (1 feet) and held for vibration for 30 seconds. This process is continued which compacts the soil to the desired density

  14. Grain Size Distribution Zone-B Most suitable for Vibroflotation since Quick condition can easily be created Zone – C and Zone –D Contains excessive amount of Fines, Difficult to compact and requires considerable effort Zone- A Appreciable amount of Gravel, Compaction by Vibroflotation is uneconomical

  15. Suitability of the Backfill Suitability NumberSN= 1.7 {3/(D50)2 + 1/(D20)2 + 1/(D10)2}1/2 Where D50 ,D20 ,D10 are particle sizes corresponding to 10, 20 and 50 % finer of the backfill material

  16. Comparison of CPT Test CPT Test Performed Before Densification of Sand Fills CPT Test Performed After Densification of Sand Fills

  17. Dynamic Compaction • Repeated lifting and Dropping of Weight at a location • Tamping Weight (Concrete/Cast iron/Steel)- 80 to 120 kN • Ht. of Drop- 10 to 15m • No. of drops (same location)- 8 to 12 times • Formation of Crater like Depression-Filled with Extra Soil • Process Repeated- Grid Pattern at a spacing of 2-4 m • Densification of Soil- 4-8m below GL

  18. In-Situ Dynamic Compaction

  19. Blasting • Weight of Charge(Rough Guideline)- W= 164CR3 • W = Weight of Explosive (N) • C = Coefficient (0.0025 for 60% detonator) • R = Radius of influence (m) • Arrangement of Explosive- Grid Pattern • Firing Pattern – From outside to inside • First Blast- At the corner of Periphery Line of First Grid from outside • Second Blast – At the Centre of Periphery Line of First Grid from outside • Third Blast - At the corner of Periphery Line of Second Grid from outside • Spacing – 3 to 8 m (Less than 3 generally avoided) • Depth of Stratum to be densified – 10m or less • Depth of Explosive- 2/3 of depth • Compaction - In one tier only • Depth of Stratum to be densified – More than 10m • Depth of Charge – Greater than Radius of Sphere of Influence (R) • Compaction- More than one Tier

  20. Materials & Equipments • Dynamite sticks. • Electric detonator. • Drilling equipment. • Backfill material (Sand). • Waterproof packets.

  21. Method • Series of boreholes are drilled and Pipe of 7.5 to 10 cm is driven to the required depth • Dynamite sticks and detonator are wrapped in a water proof bundle and is lowered through casings • Casing is withdrawn and a wad of paper or wood is placed against the charge of Explosive (To protect it from misfire) • Boreholes are backfilled with sand to obtain full force of blast • Electric circuit is closed to fire the charge • The charge is fired in definite pattern • For deeper deposits blast is done in stages • Repeated shots are more affective than single larger one • Each successive blast in a given area will cause less densification than the one preceding • Top 1m surface get disturbed and needs surface compaction

  22. Compaction Grouting • Step -1 Predrilled Compaction Grouting hole to • desired depth • Step-2 Insert Compaction Grouting Casing in • Predrilled hole • Step-3 Begin Pumping Low Slump Compaction • Grout Mix in Stages and withdraw at • Controlled rate • Step-4 Withdraw casing as stages are complete • until the hole is complete

  23. Key Issues Affecting Densification • Key Issues : • Percent of fines in the soil, • Ability of the soil to dissipate excess pore water pressure, • Energy felt by the soil, • Presence of boulders, utilities and adjacent structures, and • Mysterious phenomenon of ageing.

  24. Presence of Fines • Fines act as lubricantsreducing the frictional • resistance between the rearranged soil particles • of the densified mass. • In vibrocompaction, a fines content of 20 • percent renders the process ineffective. • The amount of fines in the soil also affects • drainage properties • (A key factor when the densified soil is in a • saturated state)

  25. Pore Pressure Dissipation • In a saturated cohesionless material, during • densification a micro- liquefaction process takes place • allowing the soil particles to rearrange themselves • If excessive fines or cohesive soils are present, • dissipation of the excess pore water pressure • generated by the densification process is slowed down • (or maybe prevented) thus affecting the feasibility of • the applied method

  26. Level of Energy • Factors affecting Level of Energy: • Characteristics of the equipment (Vibrating frequency, Tamper • weight, amount of explosives, etc) • Configurations of application (probe spacing, drop height, depth of • charge, etc) • Ground characteristics, effectiveness of the applied procedures and • the experience of the equipment operator • Degree of saturation and the way weight is dropped (a crane drop • is less efficient than a free drop) • Presence of soft cohesive layers or peat has a damping effect on • the dynamic forces penetrating the soil, and thus the depth of • influence is reduced

  27. Proximity to Structures • The impact of the process 9 vibrocompaction or • dynamic compaction) on adjacent structures - A major • concern with no set criteria in present practice as to • how close can it be implemented next to an existing • structure • A recent project involving vibrocompaction of • hydraulic fill adjacent to a bulkhead structure: • Horizontal deflection less than 10 mm when the • vibroprobe was 3 m away from the bulkhead • Horizontal deflection increases to more than 50 mm as • the probe got within 2 m of the bulkhead

  28. Ageing • Mechanism • Increase in strength and deformation modulus with time to the possible • action of silica bonding between grains • Rearrangement of the sand particles during secondary compression, • resulting in gradual increase in particles interlocking • Example Ageing: • Strength increase of 35% from the second to the sixth week after a • major vibrocompaction application in Hong Kong • On a recent project in Norfolk, Virginia, USA, a 15-30 percent increase • in the relative density of a hydraulic fill, approximately 20 days after • vibrocompaction was measured due to ageing

  29. Ground Improvement by Consolidation • Methods of application: • Preloading with or without vertical drains • Electro-osmosis • Vacuum consolidation

  30. Preloading with or without vertical drains • Preloading is usually accomplished by placing surcharge fills • To accelerate consolidation, vertical (sand or prefabricated • wick) drains are often used with preloading

  31. Principle and Mechanism Coefficient of Surcharge: Ratio of weight used in preloading and wt. of the final structure to be constructed on the improved soil Using a surcharge higher than work load, soil always remains in an overconsolidated state secondary compression for overconsolidated soil is much smaller than that of normally consolidated soil Increasing the time of temporary overloading or size of the overload, secondary settlement can be reduced /eliminated.

  32. Electro-osmosis • The process of dewatering assisted by the application • of a direct electric current is known as electro- • osmosis thus resulting in consolidation • soft clays whose moisture content cannot be reduced by • conventional dewatering methods. • In Electro-osmosis method, Electrodes are installed in • the soil and a DC current supplied which results water • movement from anode to the cathode • A wellpoint system or ejector well system used as cathode which collects and removes the water from the ground.

  33. Mechanism Electro-osmosis transports water of the clay pore space to the cathodically charged electrode When these cations move toward the cathode, they also bring water molecules along with them These water molecules clump around the cations as a consequence of their dipolar nature Macroscopic effect of this process is reduction of water content at anode and an increase in water content at the cathode Free water appears at the interface between the clay and the cathode surface

  34. Molybdenum Electrodes Graphite Electrode Metallic Electrodes Shapes of Electrodes

  35. Connecting Wires Electrodes Before Installation DC Current Source

  36. Flow of Water under Electro-osmosis

  37. Advantages and Limitations Advantages Limitations Practical application limited since very costly. Before actual application on site Laboratory tests and site tests are imperative. Huge amount of electricity. needed Highly skilled labour needed Electrodes replacement needed from time to time. Method becomes ineffective If the moisture content of the soil is extremely low • Can be used for dewatering of silty and clayey soils which are difficult to drain by gravity. • Method is fast and instantaneous. • Environment-friendly method • Equipments required are few in number and easy to carry to the site. • Method useful for all types of soils. • Efficiency of this method is very high. • Less man-power required to implement this method.

  38. Vacuum consolidation • Vacuum consolidation, • Both liquid and gas (water and air) are extracted from the • ground by suction • This Suction is induced by the creation of vacuum on the • ground surface and assisted by a system of vertical and • horizontal drains • Vacuum is applied to the pore phase in a sealed membrane • system • The vacuum causes water to drain out from the soil and • creates negative pore water pressure in the soil • This leads to an increase in effective stress to the magnitude • of the induced negative pore water pressure, without the • increase of total stress

  39. For rapid pre-consolidation, vertical drains (Prefabricated Vertical Drains) along with the vacuum preloading are used Vertical drains helps to distribute the vacuum pressures to the deeper layers and drain out water from the sub soil Vacuum preloading with PVD substantially reduces the lateral displacement and potential shear failure Maximum achievable vacuum pressure in the field is only about 80kPa Schematic view vacuum consolidation technique

  40. Advantages of Vacuum preloading technique over the Surcharge preloading technique • Ground improvement with vacuum preloading does not require any fill material and there is no need of heavy machinery • Construction period is generally shorter • The increase in effective stress under vacuum preloading is isotropic. Therefore, the corresponding lateral displacement is in the inward direction and there is no risk of shear failure • Application of Vacuum Preloading improves Bearing capacity of soil by 100% in the case of soft clays and eliminates 70% of the total estimated settlement of design load • The overall cost of vacuum preloading is only about 2/3rd of that with surcharge preloading

  41. Ground Improvement by Consolidation • Key Issues associated with consolidation: • Stability during surcharge placement, • Clogging of vertical drains, and • Maintenance of the vacuum.

  42. System Stability • To safeguard against stability problems, the surcharge loads • are often placed in stages • Each stage of loading is added only after the soil has acquired • sufficient strength under the influence of the previous stage to • support the new load • Build up and dissipation of the excess pore water pressure and • the accompanying soil deformations are monitored to pinpoint • the time for stage placement • In case of electro-osmosis or vacuum consolidation no stability • problem is anticipated

  43. Clogging of Drains • Clogging of the vertical drain is a key issue affecting the • feasibility of the system of ground improvement • Major advantage of the plastic wick drains over sand drains is • their flexibility and ability to sustain large deformations of the • consolidating cohesive soil, which may otherwise shear and clog • the sand drains, rendering them ineffective • The hydraulic conductivity of the wick drains is influenced by • the potential crimping of the material when large deformations • take place or clogging of the drainage channels due to an • ineffective filter jacket • A non-wooven geotextile fabric is usually used to provide • filtering and ensure the hydraulic conductivity of the • prefabricated drains

  44. Maintenance of the Vacuum • Maintaining the vacuum by providing an all-around seal is • critical for the successful application of vacuum consolidation • Resistance of the membrane to tear during and after placement • is an important factor • Membrane is covered by a layer of soil or by water ponding to • prevent its tear by vehicles, animals or birds attacks, or • vandalism • Vacuum is generated by circulation of air through a series of • specially designed drains, installed to the depth of the layer to • be consolidated (new system developed recently in France • eliminated the need for the membrane)

  45. Ground Improvement by Weight Reduction • Methods of Application: • Placing lightweight materials over the native • soil in one of three ways: • Spread in a loose form, then compacted • Cut in block forms, then stacked according to a • certain arrangement, or • Pumped in a flowable liquid form

  46. Lightweight material used for ground improvement