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DEEP BASEMENT EXCAVATIONPREPARED BY:ASHISH SHYORAN(2013JE0876)

UNDER GUIDANCE OF:PROF. LOHIT KUMAR NAINEGALICONTENTSINTRODUCTIONANCHORS OR STRUTSDIFFICULTIES ASSOCIATED WITH DEEP EXCAVATIONREATAINING WALLS COMMONLY ADOPTEDSOIL MOVEMENT DUE TO EXCAVATIONALTERNATIVE EXCAVTION SCHEMESCASE STUDY FOR EXCAVATION IN SOFT CLAY

INTRODUCTIONNEED: Recent upsurge in commercial/residential multi-storied buildings, hence, increasing requirements of car parking and other utilitiesDIFFICULTIES:-Presence of congested roads around the site-Conventional technique of sheet pile or diaphragm wall -cant be used -Providing anchors or strut- difficult-open unsupported excavations - often not possible

ANCHORS OR STRUTSBasically a pre-stressing tendon embedded and anchored into soil or rock Provide resistance to structural movements by a tying back" principleCommon applications are :-General slope stabilization-Tying back/stabilizing a retaining structure-Tying back/stabilizing for diaphragm walls, but for a temporary nature during excavation-Tying back the entire building from up possible uplifting

DIFFICULTIES ASSOCIATED WITH DEEP EXCAVATIONMechanized excavation preferredCarried out either with mechanical excavators or dozers which operate within the excavation area

DIFFICULTIES ASSOCIATED WITH DEEP EXCAVATION(contd.)

Ground water table - often very high - requires large scale dewatering -To reduce water pressure on the retaining walls -To make the excavation stable from sand boiling/piping failureSuch large scale dewatering - subsidence in the surrounding areaIn many countries, large scale dewatering for such construction propose - not permitted, hence, the excavation scheme has to be designed considering the hydrostatic pressure on the retaining structure.SAND BOILING & PIPING FAILURE

DIFFICULTIES ASSOCIATED WITH DEEP EXCAVATION(contd.)

Natural strata below the excavation level - often comprising of loose sand or soft marine clay deposit - dont provide adequate passive resistance to the retaining structure to act as a cantilevering wall ,hence, requires either ground improvement or additional anchors/struts.The plan dimensions of some of the commercial buildings are very large exceeding 50 to 100m. Design of strut for such span with large l/r is not possible. Also presence of struts significantly affects the construction activities.

DIFFICULTIES ASSOCIATED WITH DEEP EXCAVATION(contd.)

Fairly good waterproofing of the basement walls and floor requiredRCC diaphragm wall - the joint between the panels has to be made water tight either using a PVC rubber stopper or extensive grouting along the entire depth of the jointIn several cases, the water tightness of RCC diaphragm wall is questioned and as a result permanent wall is made using in-situ concrete with formwork after excavating with temporary support In such case, appropriate waterproofing treatment can be provided on the outer side of the wall before backfilling, but this increases the cost.

RETAINING WALLS COMMONLY ADOPTEDSTEEL SHEET PILE WALL

Earth retention and excavation support technique that retains soil, using steel sheet sections with interlocking edgesInstalled in sequence to design depth along the planned excavation perimeter or seawall alignmentEasy installation & subsequent retrieval for reuseIdeally suited for temporary application where the bending moment expected is not very highBeyond certain depth (3 to 4m) this will require either anchors or strut to reduce the bending momentINSTALLATION VIBRATORY HAMMERS, IMPACT HAMMERS, HYDRAULIC PUSHING

RCC DIAPHRAGM WALLUsed either for temporary use or for permanent use as basement wallUnlike steel sheet pile cant be retrievedHowever there are cases where RCC diaphragm wall has been used as a temporary wall Due to much higher rigidity compared to steel sheet pile, this wall can cantilever for a large height

SECANT PILE WALLBored-cast-in-situ piles, almost touching each other in a row Depending on depth of excavation, the piles can be provided with intermittent support with anchors or strutsIf the soil retained is cohesionless with high water table, the zone between the piles may need cement grouting or inserting additional pileTop of all the piles is normally connected with a common copping beam which makes all the piles as an integral wallTotal waterproofing is very difficult to obtain in jointsIncreased cost compared to steel sheet pile wallsINSTALLATION OF SECANT PILES

BERLIN WALLWide flange steel sections are inserted along the excavation line with a centre to centre spacing of about 1mSections are either driven into the ground or they are lowered in a pre-bored holeGap between the bore hole wall and the section is filled with concrete from the bottom upto the excavation level. Beyond this the gap is filled with soil.Excavation is carried out in stages of 0.5 to 1m and as the excavation progresses, wooden plank or steel formwork plates are inserted between the steel sections to retain the soilThe horizontal thrust of retained earth is transferred to the steel section through the flange.

BERLIN WALL

NAILED WALLAs the excavation progresses, the vertical face of the excavation is supported by either steel plate or wooden plank which is nailed into the ground using long reinforcement rodAfter nailing the plate, the excavation is advanced by further 0.6 to 1m and another plate/plank is placed and nailed Planks/plates as well as the nails can rerieved for reuseHowever unlike other methods, it is not possible to have a vertical cut. The face of the retained earth is normally inclined at 70 to 80 degrees with the horizontal.INSTALLATION OF NAILED WALL

SOIL MOVEMENT DUE TO EXCAVATION

SOIL MOVEMENT DUE TO EXCAVATION

ALTERNATIVE EXCAVATION SCHEMESEXCAVATION WITH PERIPHERAL SUPPORTExcavation of the central area alone, leaving soil with slope along the perimeter to support the retaining wall. Reduces the section of retaining wall but it has following disadvantages: -Construction joint is required in the basement floors-For completion of balance excavation along the perimeter, it may not be possible to use excavators EXCAVATION WITH PERIPHERAL SUPPORT

TOP-DOWN CONSTRUCTION

After completion of perimeter retaining wall (RCC Diaphragm) and pile foundation at column locations, the ground floor slab is cast first connected to the peripheral diaphragm wall and the pilesOpenings are provided at required locations(staircase, lift well or ramp) to remove the earth subsequently.Slab can be cast on the natural ground itself, hence no formwork required. After this, the soil below the slab is excavated upto the next basement levelSlab already cast serves as strut to support the wall. Process is repeatedWhile the construction of basements progress, the work of raising the building above ground level can also been taken up simultaneously.

TOP-DOWN CONSTRUCTION

CASE STUDY FOR EXCAVATION IN SOFT CLAYI. Provide cement injection grouting for a width of 2m on either side of the diaphragm wall to improve the stability of the diaphragm trench and to reduce the active pressure and to increase the passive resistance. II. Complete RCC diaphragm wall along the perimeter of the building. This will also serve as permanent basement wall. III. Complete pile construction within the building area. The piles are constructed from the existing ground level, but the concrete is poured only upto the required level of the bottommost basement. IV. The excavation is carried out for a depth of 4m throughout the building area. This is maximum height of excavation which the RCC diaphragm wall can permit as cantilever. V. Provide peripheral dewatering outside the diaphragm wall to lower the water table and reduce bending moment on the wall. Do not pumpout water within the excavation area. VI. Leaving a berm of 4 to 5m width from the diaphragm wall, excavate the central area of the building with a convenient slope to the final founding level. At this level, the piles already constructed will project out. Chip-off the extra concrete to the required cut-off level.CASE STUDY FOR EXCAVATION IN SOFT CLAY(contd.)VII. Construct the bottommost basement floor supported on piles leaving a construction joint along the unexcavated area. VIII. Raise the columns and subsequent floor of the higher basement in the central area. IX. Use the completed basement floors in the central area to provide lateral support to the diaphragm wall with steel struts. X. Remove the unexcavated soil along the perimeter to the foundation level. XI. Extract the dowel bars from the diaphragm wall and complete the bottommost floor upto the construction join. XII. Complete balance columns and floor area of higher basement along the perimeter. XIII. Remove temporary strut between the central portion and diaphragm wall.

REFERENCESBerlie Zhu and Guobin Liu, (1994), elasto plastic analysis of deep excavation in soft clay, Proc of 13th International Conference in Soil Mechanics and Foundation Engineering, New Delhi, India.Malcolm Puller (2003), Deep excavation a practical manual 2nd Edition, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD

REFERENCES(contd.)http://www.haywardbaker.comwww.slideshare.nethttps://www.google.co.inwww.deepexcavation.comoregonstate.eduhttps://hal.archives-ouvertes.frwww.youtube.comhttp://www.ask.com/home-garden/backfill-constructionwww.perfectparkusa.comhttp://www.gemech.co.uk/contiguous_secant.htmlhttp://www.plastifab.com/solutions/geofoam/retaining-wall.htmlTHANK YOU