ASSOCIATION FRANÇAISE DES TUNNELS ET DE L’ESPACE SOUTERRAIN Organization member of the AFTES www.aftes.asso.fr
AFTES Recommendations Watertightness of precast concrete lining segments GT9R6A1
AFTES – WORKING GROUP 9 GUIDELINES ON
WATERTIGHTNESS OF PRECAST CONCRETE LINING SEGMENTS AFTES welcomes comments on this article before final approval
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Version 1 – 1998 - Approved by Technical Committee 7 April 1998 Drafted by
LONGCHAMP (Bouygues), MERCIER (SOFRETU), and MEYNIEL (RATP)
PREAMBLE
AFTES Working Group 9 on tunnel watertightness has previously drafted and published Guidelines on the watertightness of precast segmental concrete tunnel linings.
(excluding gaskets) - Mass porosity
- Cracking due to temporary or permanent loads
FT
They dealt with gaskets and other points directly concerned with them.
TYPES OF DISTRESS LIABLE TO AFFECT WATERTIGHTNESS
Since tunnel watertightness does not depend solely on the gaskets used in this type of construction, it was found necessary to commit to writing fur ther ideas on the other causes potentially affecting water tightness performance and propose recommended leakage preventive methods.
2. Segment casting
- Flaws or distress affecting the gasket groove or gasket contact surface. ANALYSIS OF CAUSES OF DISTRESS LIABLE TO AFFECT WATERTIGHTNESS OF SEGMENTAL LININGS
This list clearly illustrates that, at each stage of the life of the lining, events may adversely affect tunnel watertightness. For clarity, details of the analysis of the causes and possible remedies are presented in tabular form.
Excessive porosity
Insufficient concrete density Gap graded aggregate
Special laboratory tests on concrete formulation with porosity checks
Concrete deterioration
Cement and admixture proportions
Identification and analysis of natural and artificial (contamination) factors in surrounding ground liable to detract from concrete durability Second lining and watertight system Protection applied to lining extrados (coal tar)
A
1. Materials
- Concrete shrinkage cracking (insignificant in precast lining segments)
The causes addressed arise from: 1. Constituent material of segments 2. Segment casting 3.Transport and storage of segments 4. Segment installation 5. Loads due to shield thrust 6. Grouting of annular space between lining and tunnel bore 7. Llining ring and segment performance during tunnel operation.
Physical/chemical reaction with environment inside or outside tunnel
Non-homogeneous mix (honeycombs, segregation)
Hydraulic binder quality
Test cement and concrete for compatibility with environment
Compatibility between binder and aggregate
Cement selection
All causes connected with concrete casting, especially insufficient VIBRATION
Compatibility of reinforcement design with concrete composition (bar spacing ver sus aggregate size) Check vibration intensity Experiments on effects of vibration and influence zone and efficiency
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Watertightness of precast concrete lining segments For large items, resilient mould mounting that does not dampen vibration Avoid leaky moulds Rough machined mould surfaces Form release agents
Specify mould roughness
Poor segment geometry
Mould manufacturing tolerances
Specify finished mould manufacturing tolerances as segment design volume, not just as tolerances on lengths and flatness See Fascicule 56A Special attention to tolerances on angles of bearing surfaces between segments and between rings
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Bubbles (at gasket position)
Spalls, chipped arrises, cracking
Mould not rigid enough, deforms
Include effects of vibration in mould design
Inadequate kinematics of moving demoulding parts
Demoulding kinematics to be addressed in segment mould design to specify draft angles
'Thermal shock' especially when demoulding (Dt between concrete and outside temperature)
Analyse segment stresses with reference to handling equipment effectively used Consider suction stresses due to demoulding. Protect segments by inser ting compressible material under gripper jaws
Concrete sticking to mould
Specify mould roughness Choice of release agent and/or mould lining Avoid sharp edges
Inadequate concrete early strength
Test effective strength on demoulding
Flaws in reinforcement; tolerance on cage assembly
Specify tolerances on concrete cover on drawings Position reinforcement cages with accurate templates specially designed for each segment type Specific testing and inspection
Positioning of cages in mould
Support reinforcement cages
FT
Spalls, chipped arrises, cracking
Effects of segment handling systems (gripping and turning)
A
Poor gasket bonding
3. Segment transport and storage
Inadequate concrete/adhesive/gasket complex Poor workmanship
Write detailed procedure for adhesive preparation and application and gasket fitting
Adhesive out of date
Use-by date to be shown on packaging Use within stated time Storage conditions to be stated and met
Inadequate protection to hydrophilic gaskets
Gaskets expanded before fitting
Proper protection from the weather
Spalls, chipped arrises, cracking
Impact damage (accidental or when positioning)
Write detailed procedure for each handling and storage operation Use compressible supports on storage, transport and handling equipment
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Watertightness of precast concrete lining segments
Spalls, chipped arrises, cracking
Provide suitable supports during storage Handling machines to have inching mechanism or proportional speed control Attention to storage support material, positioning and levelling, and base Stock management to avoid excessive handling
Stacks too high
Provide enough storage yard area
See sections 2 & 3 Segment installation kinematics
Plan movements of segment in its immediate environment when homing into place Detailed study of key, counterkey and machine interfaces when designing lining ring
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4. Segment installation
Supports poorly positioned or adjusted (especially with newly-cast segments)
Insufficient reinforcement
Allow for stray stresses often associated with introduction of key between counterkeys
Excessively tight ring
Check free space between counterkeys before inserting key Accurate control of segment installation from first to last
Gasket damaged by rubbing or snagging
Excessive friction Poor bond
Grease longitudinal gaskets before installing segment Check gasket bond
Key dislodges when nearly home
Key self-guiding system not working
Provide for guidance between key and counterkeys in segment design
FT
Broken arrises on key and counterkey
5. Loads applied by shield
Chipped arrises, cracking, broken edges, misalignment
A
5.1.Thrust
5.2. Shield rubbing against lining rings
Chipped arrises, cracking
Thrust cylinder pads poorly seated
Use resilient thrust pads Check kinematics of cylinder movement in tightest curves
Poor transmission of thrust loads between lining segments
Analyse stress paths in successive rings Keep segment casting tolerances as tight as possible Arrange for loads to be 'filtered' between rings
Rear effects of thrust cylinders in tight curves
Mount cylinders on resilient ring or similar device to allow enough angular movement between cylinders and shield
Excessive thrust load from cylinders
Control pressure to groups of cylinders rather than on/off control
Excessive pressure from thrust cylinder pads
Check bearing area Check pad stiffness Check segment tightness near pad positions and contacts between rings
Clearance between lining and tailskin too tight
Specify clearance with sketch Use tailskin with free articulation
Lining ring off-centre
Procedure for setting out rotational position of ring High performance guidance/navigation system
Shield guidance
In the event of errors, specify correction curve compatible with design,TBM and pinching of ring
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Watertightness of precast concrete lining segments
6. Grouting stresses
Excessive grouting pressure
Provide active and passive overpressure devices on grout pumps
Out-of-round Dislodged key segment
Imbalance between grouting holes
Provide balanced grouting device Grout mix suitability study
Upward movement of lining due to buoyancy
Incomplete grouting Grout formulation unsuitable
Control grouting pressure Malfunctioning grouting plant and equipment
Other forms of distress (unusual)
Effects of combined stress due to groundwater, soil, and internal and external loads
Review design assumptions
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7. Segmental lining during operation of tunnel
Distress at contact surfaces between segments
Electrical continuity between segments (bonding) Cathodic protection Materials quality Reinforcement completely surrounded by concrete
Various
Dowels between rings
A
FT
Displacements between segments or between rings
Ageing
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