08-02-2010, 08:11 AM
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SOIL NAILING
INTRODUCTION
Soil Nailing is a procedure to reinforce and strengthen ground adjacent to an excavation by installing closely spaced steel bars called nails ,as construction proceeds from top down.
It is an effective and economical method of constructing retaining wall for excavation support, support of hill cuts, bridge abutments and high ways.
This process is effectively in cohesive soil, brocken rock, shale or fixed face conditions.
HISTORY OF SOIL NAILING
Soil nail technology was first used in France to build a permanent retaining wall cut in soft rock. The project, undertaken in 1961, was the first where steel nails were used to reinforce a retaining wall.
The first soil nail wall to use modern soil nailing techniques was built near Versailles in 1972. The technique included installing high-density, grouted soil nails into a 60-ft.-high wall and facing it with reinforced concrete.
APPLICATIONS
Stabilization of railroad and highway cut slopes
Excavation retaining structures in urban areas for high-rise building and underground facilities
Tunnel portals in steep and unstable stratified slopes
construction and retrofitting of bridge abutments with complex boundaries involving wall support under piled foundations
Stabilizing steep cuttings to maximize development space.
The stabilizing of existing over-steep embankments.
Soil Nailing through existing concrete or masonry structures such as failing retaining walls and bridge abutments to provide long term stability without demolition and rebuild costs.
Temporary support can be provided to excavations without the need for bulky and intrusive scaffold type temporary works solutions
NAILS
Driven Nails
Grouted Nails
Corrosion Protected Nails
Jet grouted Nails
Launched Nails
Driven nails
Generally small-diameter nails (15-46 mm) with a relatively limited length (to about 20 m) made of mild steel (about 50 ksi) that are closely spaced in the wall (two to four nails per square meter). Nails with an axial channel can be used to permit the addition of grout sealing. Driven nails are the quickest (four to six per hour) and most economical to install (with a pneumatic or hydraulic hammer).
Grouted Nails
Steel bars, with diameters ranging from 15 to 46 mm, stronger than driven nails (about 60 ksi). Grouted nails are inserted into boreholes of 10-15 cm and then cement-grouted. Ribbed bars are also used to increase soil adhesion.
Corrosion-protected nails
For aggressive soils as well as for permanent structures.
Jet-grouted nails
A composite of grouted soil and a central steel rod, up to 40 cm thick. Nails are installed using a high-frequency vibropercussion hammer, and cement grouting is injected during installation. This method has been shown to increase the pullout resistance of the composite, and the nails are corrosion-resistant.
Launched nails
Nails between 25 and 38 mm in diameter and up to 6 m or longer are fired directly into the soil with a compressed-air launcher. Used primarily for slope stabilization, this technique involves the least site disturbance
MACHINERIES USED FOR SOIL NAILING
Drilling Equipments
Grout Mixing Equipments
Shotcreting / Guniting Equipments
Compressor
Drilling Equipment
There are few common types of drilling equipment, namely rotary air-flushed and water-flushed, down-the-hole hammer, tri-cone bit. It is important to procure drilling equipment with sufficient power and rigid drill rods.
Grout Mixing Equipment
In order to produce uniform grout mix, high speed shear colloidal mixer should be considered. Powerful grout pump is essential for uninterrupted delivery of grout mix. If fine aggregate is used as filler for economy, special grout pump shall be used.
Shotcreting/Guniting Equipment
Dry mix method will require a valve at the nozzle outlet to control the amount of water injecting into the high pressurized flow of sand/cement mix. For controlling the thickness of the shotcrete, measuring pin shall be installed at fixed vertical and horizontal intervals to guide the nozzle man.
Compressor
The compressor shall have minimum capacity to delivered shotcrete at the minimum rate of 9m3/min. Sometimes, the noise of compressor can be an issue if the work is at close proximity to residential area, hospital and school.
MATERIALS USED FOR SOIL NAILING
Steel Reinforcements
Grout Mix
Shotcrete/Gunite
Steel Reinforcements
For corrosion protection, all steel component shall be galvanized. If machine threading after galvanization is unavoidable, then proper zinc based coating shall be applied onto the thread. For double corrosion protection, the PVC corrugated pipe used shall be of good quality and adequate thickness. Preferably galvanized corrugated steel pipe shall be used.
Grout Mix
For conventional soil nail, the water cement ratio of the grout mix ranges from 0.4 to 0.5. As most cementitious grout will experience some grout shrinkage, non-shrink additive can be used to reduce breeding and grout shrinkage. The resistance at grout-soil interface of nail will significantly reduced when the grout shrink.
Shotcrete/Gunite
Shotcrete or gunite can be continuous flow of mortal or concrete mixes projected at high speed perpendicularly onto the exposed ground surface by means of pneumatic air blowing for dry mix or spraying for wet mix.
Initial excavation
This initial excavation will be carried out by trimming the original ground profile to the working platform level where the first row of soil nails can be practically installed. The pre-requisite of this temporary excavation shall be in such a way that the trimmed surface must be able to self support till completion of nail installation. Sometimes, sectional excavation can be carried out for soil with short self support time. If shotcrete/gunite is designed as facing element, the condition of the trimmed surface shall be of the satisfactory quality to receive the shotcrete
Drilling of holes
Drilling can be done by either air- flushed percussion drilling, auguring or rotary wash boring drilling depending on ground condition. The size of drilled hole shall be as per the designed dimension. Typically, the hole size can range from 100mm to 150mm. In order to contain the grout, the typical inclination of the drill hole is normally tilted at 15º downward from horizontal. Flushing with air or water before nail insertion is necessary in order to remove any possible collapsed materials, which can potentially reduce the grout- ground interface resistance.
Insertion of nail reinforcement and grouting
The nail shall be prepared with adequate centralizers at appropriate spacing and for proper grout cover for first defense of corrosion protection. In additional to this, galvanization and pre- grouted nail encapsulated with corrugated pipe can be considered for durability. A grouting pipe is normally attached with the nail reinforcement during inserting the nail into the drilled hole. The grouting is from bottom up until fresh grout return is observed from the hole. The normal range of water/cement ratio of the typical grout mix is from 0.45 to 0.5
ADVANTAGES
With the right soil and site conditions, a rapid and economical means of constructing earth retention support systems and retaining walls.
Shorter drill holes.
Smaller diameter bars at shorter lengths.
Retaining walls are secured laterally into the soil, eliminating piles and foundation footers.
Grouting only once is required, saving time and labor.
The technique is flexible, easily modified.
Creates less noise and traffic obstructions.
Less impact on nearby properties
Allow in-situ strengthening on existing slope surface with minimum excavation and backfilling, particularly very suitable for uphill widening, thus environmental friendly,
Allow excellent working space in front of the excavation face,
Can be used for strengthening of either natural slope, natural or man- made cut slopes,
DISADVANTAGES
Nail encroachment to retained ground rendering unusable underground space,
Generally larger lateral soil strain during removal of lateral support and ground surface cracking may appear,
Tendency of high ground loss due to drilling technique, particularly at course grained soil,
Less suitable for course grained soil and soft clayey soil, which have short self support time, and soils prone to creeping,
Suitable only for excavation above groundwater
CONCLUSION
Since this process is effective in cohesive soil, broken rock, shale, or mixed face conditions it permits flexibility to conform to a variety of geometric shapes to meet specific site needs.
Due to its rather straightforward construction method and is relatively maintenance free, the method has gained popularity in India for highway and also hillside development projects. Soil nailing is an economical means of creating shoring systems and retaining walls.
