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Note for Ground Improvement Techniques - GIT By TechnoStuff

  • Ground Improvement Techniques - GIT
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1 GROUND IMPROVEMENT TECHNIQUES UNIT – 1 Introduction – Need for ground improvement, applications, factors affecting – different mechanical, chemical, static and dynamic techniques – mechanical stabilization – blending of aggregates – Rothfutch’s Testing INTRODUCTION Ground improvement refers to a technique that improves the engineering properties of soil mass treated. The properties which are modified are – Shear Strength, permeability and stiffness. Need For Ground Improvement 1. Scarcity of suitable construction sites:- In case of any problems occurred during construction on the site, there are only two possible options. (a) Avoiding the particular site in case of structures like buildings etc., and (b) Realignment of structures in case of roads and highways. 2. Encounter With Problematic Soils On Construction Site:-The soils which possess characteristics that make them uneconomical for the construction of structures without adopting proper reclamation measures are known as problem soils. Many soils can prove problematic in geotechnical engineering, because they expand, collapse, disperse, undergo excessive settlement, have a distinct lake of strength or are soluble. Such characteristics may be attributable to their composition, the nature of their pore fluids, their mineralogy or their fabric. Examples: collapsible soils, liquefiable soils, waste materials, expansive and shrinkage soils, marshy and soft soils, karst deposits. 3. Remove And Replace Unsuitable Soils:- Remove organic top soil, which is soft, compressible and volumetrically unstable is a standard precaution in road construction or in foundation. 4. Design The Planned Structure Accordingly:- use of raft foundation supported by pies, design a very stiff structure which is not damaged by settlement. Choose a very flexible construction which accommodates differential movement or allows for compensation. 5. Attempt To Modify The Existing Ground Huda Noorien (Asst Prof, Dcet )

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2 GROUND IMPROVEMENT TECHNIQUES UNIT – 1 Example of appilcaton of GIT Underground Construction :- Ground Anchors for New Delhi Metro Project The Delhi Metro Rail Corporation project (DMRC) connects the Indira Gandhi International Airport and New Delhi Railway Station with an exclusive Airport Metro Express Line. As a part of this project, an underground metro station and multi-level car parking facility was planned near the New Delhi Railway station. The site in general consisted of silty clay, with the Quartzite bedrock varying from 5 m to 20 m deep. Typical Ground Anchor The site has been excavated to a maximum depth of 19 m to facilitate the construction activities for the station building and underground parking facility. The excavation was supported by a Soldier pile – Anchor & Strut system. The soldier pile walls were embedded 0.5 m into the Quartzite bedrock. At most locations, two levels of anchors were designed, at 2.5 m and 8 m below ground level. To facilitate the removal of anchor strands after construction of the intended wall, U-Turn retrievable ground anchors were installed. For such anchors, the steel strands are covered with a PVC jacket, turned over a U-loop (U-turn saddle) at the bottom and connected to a reinforcement rod. A hydraulic rotary drill rig (Casagrande C6) was used for inclined drilling (30 deg to the horizontal, to a maximum length of 22 m) and simultaneous installation of the casing. The anchors consisted of 7-ply 12.7 mm diameter strands conforming to IS 14268: 2005, with an ultimate tensile strength of about 187 kN. Primary and secondary grouting were performed after washing the borehole. As a part of quality control procedures, operating parameters such as flow rate, grout pressure, Huda Noorien (Asst Prof, Dcet )

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3 GROUND IMPROVEMENT TECHNIQUES UNIT – 1 total grout volume, etc. were recorded at site for each anchor. Below figure shows a drilling in progress for the second level of anchors. The anchors were designed to withstand a working load of 60 T and were tested at 1.1 times the working load (66 T). Mechanical stabilization:The prime purpose of this technique is to increase soil density by applying mechanical force in the form of static, vibratory rollers and plate vibrators as the case may be to achieve proper compaction. Compaction of the soil can be done easily if the soil fill material is well graded. Well graded soil being characterized by high uniformity coefficient Cu >15 and coefficient of curvature Cc between 1 and 3 can be compacted to greater density by rollers , tampers and other mechanical means. The optimum moisture content (OMC) should be determined and compaction should be done at or near the optimum moisture content for cohesive soils to achieve max dry density (MDD) with sheep foot rollers. However in case of cohesionless soils the compaction can be best achieved by vibrations. The various methods are listed below1. Vibro-flotation:- This technique can be effectively used for deposits of sands. In this method compaction is achieved by vibration and flooding the soil Huda Noorien (Asst Prof, Dcet )

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4 GROUND IMPROVEMENT TECHNIQUES UNIT – 1 around it with water. This technique was invented around 1930 in Germany for treatment of sandy soils. The equipment required for vibro-flotation is shown in Fig-1. It contains a vibro-float with a water sump, a crane, a front end loader, power supply etc. The vibro-float consists of a cylindrical penetrator tube about 0.38 meters in diameter and about 2.0 meter in length with an eccentric rotating weight inside the cylinder which is responsible for developing a horizontal vibratory motion. The weight can develop a horizontal centrifugal force of magnitude of about 100 KN at a speed of 1800 rpm. Vibro-Flotation Equipments Arrangement & Vibro-Float A typical vibro-float as shown in figure consists of two parts, the lower part is horizontally vibrating unit which is connected with upper part, a follow up pipe of adjustable length to suit compaction depth. The water pump provides water to sink the vibro-float into the ground by jetting action, as the vibro-float is lowered from the crane. Vibroflotation, sometimes also mentioned as vibro-compaction. Vibro-flotation may be defined as a process of rearrangement of soil grains into a denser state by use of powerful depth vibration. It creates a stable foundation soils by densifying loose sand. The loose sand grains are rearranged into a much compact state by combined action of vibration and water saturation by jetting. Each compaction sequence involves four basic steps, as mentioned below: i. The vibro-float probe is suspended from the crane, and is positioned over the ground at the spot to be compacted. Its lower jet is then fully opened. ii. Water is pumped in faster than it can drain away into the subsoil. This creates a momentary “quick” condition beneath the jet, which permits the vibro-float to sink of its own weight and vibrations. Huda Noorien (Asst Prof, Dcet )

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