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

MAP OF WEST BENGAL SHOWING LOCATION OF CALCUTTA METROPOLITAN DISTRICT

CALCUTTA METROPOLITAN DISTRICT

RAPID TRANSIT SYSTEM FOR KOLKATA METRO - FIRST PHASE OF CONSTRUCTION BY CUT AND COVER METHOD

CALCUTTA METRO ALIGNMENT

GEOLOGICAL FEATURES OF CALCUTTA SOIL

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 30 54 24 4.0 0.002 4 II. Grey/dark grey silty clay, clayey silt with semi-decomposed timber pieces 2 – 4 50 55 28 2.5 0.007 14 III Bluish grey silty clay with calcareous modules DEPTH (M) 10 30 60 25 6.0 0.003 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 65 25 10.0 0.002 30 VI Brown /light brown silty fine to medium sand 40 / = 350 40 SUBSOIL OF KOLKATA : NORMAL CALCUTTA DEPOSIT

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

LENGTH OF FIRST LINE OF KOLKATA METRO – 16. 633 Km Elevated – 1. 228 Km. Surface – 0. 270 Km. Underground – 15. 135 Km Sheet Piles : 0.356 Km. H-Pile with Timber Lagging : 0.684 Km. Diaphragm Wall : 12.687 Km. Shield Tunnel : 1.090 Km.

OPEN CUT IN MAIDAN AREA

Dessiccated Brownish grey silty clay 0 (m) STRUT 1 Dessiccated Brownish grey silty clay - 1.5 - 2.0 Cu = 4 t/m2 - 3.8 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 = 21 - 37 TYPICAL EXCAVATION PROFILE OF CUT-AND-COVER CONSTRUCTION WITH DIAPHRAGM WALL AND STRUTS

SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR F I L L + 1.80 = 1.9 t/m3 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 = 58 PL = 32 Cu = 2 t/m2 W = 50 T u n n e l s - 8.00 = 1.9 t/m3 Cu = 3 t/m2 Bluish grey silty clay LL = 1.9 t/m3 PL = 3.6 t/m2 - 13.00 Brown silty fine sand - 19.50 SHIELD TUNNELLING BETWEEN BELGACHIA AND SHYAMBAZAR

BRACED CUT WITH DIAPHRAGM WALL Excavation & Strutting Box Construction Backfilling

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

GUIDE WALL FOR DIAPHRAGM WALL TRENCH

REINFORCEMENT CAGE OF DIAPHRAGM WALL

DESIGN OF BRACED CUTS

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)

GROUND SETTLEMENT FOR EXCAVATION BELOW 1ST STRUT

WALL DEFLECTION IN SEQUENTIAL EXCAVATION AND STRUTTING (CONCEPTUAL)

APPARENT EARTH PRESSURE DIAGRAM IN BRACED CUT (PECK 1969)

GROUND SETTLEMENT IN BRACED CUT WITH SHEET PILE SUPPORT (PECK 1969)

INSTRUMENTATION

OBJECTIVE To Study Earth Pressure on Diaphragm Wall Diaphragm Wall Movement Strut Load Ground Settlement

CALCUTTA METRO CUT : LOCATION OF INSTRUMENTED TEST SECTIONS BETWEEN GIRISH PARK AND M. G. ROAD STATION

LOCATION OF EARTH PRESSURE CELLS IN DIAPHRAGM WALL

FLUID PRESSURE MEASURED BY EARTH PRESSURE CELLS IN DIAPHRAGM WALL TRENCH BEFORE CONCRETING

GROUND SETTLEMENT MEASURING POINTS Concrete Cover G.L Pit 600x600 600 50 mm Lean Concrete Settlement Stud GROUND SETTLEMENT MEASURING POINTS

PORE PRESSURE DISTRIBUTION AS MEASURED BY VIBRATING WIRE PIEZOMETERS

EARTH PRESSURE ON DIAPHRAGM WALL BEFORE COMMENCEMENT OF EXCAVATION

VALUES OF IN – SITU KO Depth Location Location Location Average (m) 1 2 3 2 1.65 0.77* 0.55* 1.0 4 1.70 0.85 0.80 6 0.95 0.85 0.73 8 0.83 1.16 0.70 10 0.70 0.84 0.65 0.70 12 0.67 0.90 0.60 13.5 0.50 0.80 0.70 16 0.37 0.48 0.30 0.50 17.5 0.44 0.65 0.45 * Low values due to inadequate seating

DIAPHRAGM WALL DEFLECTION AT DIRRERENT STAGES OF EXCAVATION

MEASURED STRESSES ON DIAPHRAGM WALL AT DIFFERENT STAGES OF EXCAVATION

SUMMARY OF MEASURED EARTH PRESSURE AT DIFFERENT STAGES OF EXCAVATION

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) 10.0 Peck 10.8 0.80 2.5 Observation 12.5 0.55 6.0 12.0 Peck 12.0 1.00 3.0 Observation 17.0 0.50 7.2 13.6 Peck 15.0 1.00 4.4 Observation 18.0 0.65 8.2

FORCES ON DIAPHRAGM WALL

FORCES ON DIAPHRAGM WALL

DISTANCE FROM EXCAVATION MAX. DEPTH OF EXCAVATION VARIATION OF GROUND SETTLEMENT AT DIFFERENCE TEST SECTIONS

CONSTRUCTION CONTROL

CONTROL PARAMETERS PARAMETERS Effect on Adjoining Buildings Settlement at Different Stages of Excavation Angular Distortion of Buildings Placement of Strut Time Effect

FIELD MEASUREMENTS Observations Settlement of Plinth / floor Tilt Cracks in Walls / Floors

EFFECT ON ADJOINING BUILDINGS Observations Settlement of Plinth / floor Tilt Cracks in Walls / Floors

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

GROUP B BUILDINGS 28 Buildings Required Detailed Study Moderate to severe damages Temporary Protective Measures Final Rectification after Construction

MAXIMUM SETTLEMENT OF BUILDINGS

Time of Construction

Building Settlement at Different Stages of Excavation

TIME EFFECT

SETTLEMENT OF ADJACENT BUILDINGS : EFFECT OF TIME OF CONSTRUCTION

SETTLEMENT OF PREMISES NO. 161, C. R. AVENUE : EFFECT OF TIME