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BRACED EXCAVATION IN SOFT CLAY A CASE STUDY OF KOLKATA METRO CONSTRUCTION BY CUT AND COVER METHOD
N.N.Som Consulting Geotechnical Engineer Former Professor of Civil Engineering Jadavpur University, Kolkata
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MAP OF WEST BENGAL SHOWING LOCATION OF CALCUTTA METROPOLITAN DISTRICT
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CALCUTTA METROPOLITAN DISTRICT
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RAPID TRANSIT SYSTEM FOR KOLKATA METRO - FIRST PHASE OF CONSTRUCTION BY CUT AND COVER METHOD
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CALCUTTA METRO ALIGNMENT
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GEOLOGICAL FEATURES OF CALCUTTA SOIL
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SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT
Soil Properties (average) Description of Strata N blows/30 cm W LL PL Cu mv (%) (%) (%) (t/m2) m2/t Light brown/brownish grey silty clay/clayey silt with occasional lenses of fine sand 5 – 8 4 II. Grey/dark grey silty clay, clayey silt with semi-decomposed timber pieces 2 – 4 14 III Bluish grey silty clay with calcareous modules DEPTH (M) 10 20 IV. Brown/yellowish brown sandy silt/silty fine sand with occasional lenses of brown and grey silty clay 20 --- 25 V. Mottled brown/grey silty clay with laminations often with rusty brown spots 20 – 30 30 VI Brown /light brown silty fine to medium sand 40 / = 350 40 SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT
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3 25 30 I. Light grey/brownish grey silty clay/clayey silt
I. Light grey/brownish grey silty clay/clayey silt 3 IIA. Grey / light grey fine to medium sand DEPTH (M) N = 15 – 30 blows / 30 cm 25 Mottled brown /grey silty clay with laminations often with rusty brown silt 30 SUBSOIL OF KOLKATA : RIVER CHANNEL DEPOSIT
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LENGTH OF FIRST LINE OF KOLKATA METRO – 16. 633 Km
Elevated – Km. Surface – Km. Underground – Km Sheet Piles : Km. H-Pile with Timber Lagging : Km. Diaphragm Wall : Km. Shield Tunnel : Km.
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OPEN CUT IN MAIDAN AREA
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Dessiccated Brownish grey silty clay
0 (m) STRUT 1 Dessiccated Brownish grey silty clay - 1.5 - 2.0 Cu = 4 t/m 2 - 6.0 Grey / dark grey silty clay with decomposed wood - 6.5 3 - 9.0 - 9.5 Cu = 2.5 t/m2 - 13.2 4 - 11.0 - 11.5 - 13.5 Bluish grey silty clay with kankar Cu = 6 t/m2 - 17.2 Yellowish Brown sandy silt with clay N = TYPICAL EXCAVATION PROFILE OF CUT-AND-COVER CONSTRUCTION WITH DIAPHRAGM WALL AND STRUTS
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SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR
F I L L + 1.80 = 1.9 t/m Cu = 3 t/m2 Brownish grey clayey silt - 0.12 Grey silty clay / clayey silt = 1.9 t/m3 - 2.00 Silt with Decayed Wood LL = PL = Cu = 2 t/m2 W = 50 T u n n e l s - 8.00 = 1.9 t/m Cu = 3 t/m2 Bluish grey silty clay LL = 1.9 t/m PL = 3.6 t/m2 Brown silty fine sand SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR
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BRACED CUT WITH DIAPHRAGM WALL
Excavation & Strutting Box Construction Backfilling
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STABILITY AGAINST BOTTOM HEAVE IN STRATIFIED SOIL
C N + γ3D + γ D + å C (H + D + D ) /D F = u 4 c 2 4 f u 2 f 1 å γ (H + D + D ) 2 γ STABILITY AGAINST BOTTOM HEAVE IN STRATIFIED SOIL
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GUIDE WALL FOR DIAPHRAGM WALL TRENCH
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REINFORCEMENT CAGE OF DIAPHRAGM WALL
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DESIGN OF BRACED CUTS
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Adjacent building movement
Buckled struts Wall position Movement of wall (a) (b) Water pressure Bottom heave (c) (d) Sand and/or gravel bottom BRACED CUT : MODES OF FAILURE (WINTERKORN & FANG 1972)
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GROUND SETTLEMENT FOR EXCAVATION BELOW 1ST STRUT
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WALL DEFLECTION IN SEQUENTIAL EXCAVATION AND STRUTTING (CONCEPTUAL)
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APPARENT EARTH PRESSURE DIAGRAM IN BRACED CUT (PECK 1969)
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GROUND SETTLEMENT IN BRACED CUT WITH SHEET PILE SUPPORT (PECK 1969)
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INSTRUMENTATION
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OBJECTIVE To Study Earth Pressure on Diaphragm Wall
Diaphragm Wall Movement Strut Load Ground Settlement
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CALCUTTA METRO CUT : LOCATION OF INSTRUMENTED TEST SECTIONS BETWEEN GIRISH PARK AND M. G. ROAD STATION
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LOCATION OF EARTH PRESSURE CELLS IN DIAPHRAGM WALL
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FLUID PRESSURE MEASURED BY EARTH PRESSURE CELLS IN DIAPHRAGM WALL TRENCH BEFORE CONCRETING
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GROUND SETTLEMENT MEASURING POINTS
Concrete Cover G.L Pit x600 600 50 mm Lean Concrete Settlement Stud GROUND SETTLEMENT MEASURING POINTS
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PORE PRESSURE DISTRIBUTION AS MEASURED BY VIBRATING WIRE PIEZOMETERS
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EARTH PRESSURE ON DIAPHRAGM WALL BEFORE COMMENCEMENT OF EXCAVATION
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VALUES OF IN – SITU KO Depth Location Location Location Average (m) * * 1.0 0.70 0.50 * Low values due to inadequate seating
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DIAPHRAGM WALL DEFLECTION AT DIRRERENT STAGES OF EXCAVATION
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MEASURED STRESSES ON DIAPHRAGM WALL AT DIFFERENT STAGES OF EXCAVATION
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SUMMARY OF MEASURED EARTH PRESSURE AT DIFFERENT STAGES OF EXCAVATION
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COMPARISTON OF EARTH PRESSURE BY PECK’S METHOD AND OBSERVED DATA
Depth Method Maximum earth m Depth at which pressure linear variation changes to constant pressure m (t/m2) (m) Peck Observation Peck Observation Peck Observation
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FORCES ON DIAPHRAGM WALL
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FORCES ON DIAPHRAGM WALL
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DISTANCE FROM EXCAVATION
MAX. DEPTH OF EXCAVATION VARIATION OF GROUND SETTLEMENT AT DIFFERENCE TEST SECTIONS
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CONSTRUCTION CONTROL
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CONTROL PARAMETERS PARAMETERS
Effect on Adjoining Buildings Settlement at Different Stages of Excavation Angular Distortion of Buildings Placement of Strut Time Effect
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FIELD MEASUREMENTS Observations Settlement of Plinth / floor Tilt
Cracks in Walls / Floors
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EFFECT ON ADJOINING BUILDINGS
Observations Settlement of Plinth / floor Tilt Cracks in Walls / Floors
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GROUP A BUILDINGS : 60 Buildings Monitored
Small and Minor Damages in the form of * Cracks & Minor Tilts * No Structural Distress * No Detailed Observation done except monitoring cracks & tilts
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GROUP B BUILDINGS 28 Buildings Required Detailed Study
Moderate to severe damages Temporary Protective Measures Final Rectification after Construction
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MAXIMUM SETTLEMENT OF BUILDINGS
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Time of Construction
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Building Settlement at Different Stages of Excavation
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TIME EFFECT
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SETTLEMENT OF ADJACENT BUILDINGS : EFFECT OF TIME OF CONSTRUCTION
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SETTLEMENT OF PREMISES NO. 161, C. R. AVENUE : EFFECT OF TIME
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