Geotechnical project work flow

Transcription

1 Wulf Schubert INTRODUCTION The approach to solve an engineering problem is to combine experience with analysis The more complex the conditions are, the more difficult a direct analysis becomes, as appropriate inputs and models are difficult to establish On the other hand the potential for severe consequences increases with the complexity of the problem We generally talk of risk, if we are not able to precisely predict the performance or behaviour of the engineered system 2 1

2 INTRODUCTION Risk in engineering commonly is understood as the product of probability of an event and the consequences of such an event In tunnelling unfavourable events can be Damage of the support due to excessive deformations or loads Collapses with or without damage to third parties Excessive ground deformations, leading to damage of structures and utilities Safety hazards for the crew Water inflow Lowering of ground water table Immissions by blasting vibrations, noise, dust, etc. 3 HAZARDS IN TUNNELLING (examples) Discontinuity controlled overbreak 4 2

3 HAZARDS IN TUNNELLING (examples) Destruction of support by large displacements or shearing 5 HAZARDS IN TUNNELLING (examples) Daylighting collapses without and with damage to third parties 6 3

4 HAZARDS IN TUNNELLING (examples) Inflow of water with/without material transport 7 RISK MANAGEMENT IN TUNNEL DESIGN It is understood that in case a risk has an unacceptable level, mitigation measures have to be taken to either eliminate the hazard, reduce the probability of its occurrence, or reduce the consequences Using this principle for tunnel design, first of all we would have to identify the hazards, evaluate the probability of occurrence, and the associated consequences In the next step, the mitigation measures would have to be designed for cases with unacceptable risk Design thus must focus on hazard identification and hazard mitigation Structured approach required to obtain design, which appropriately adresses project specific conditions 8 4

5 DESIGN WORK FLOW GEOLOGICAL MODEL Select geomechanically relevant parameters Assess ground properties (characterization) GROUND TYPES (GT) Ground Types are defined as ground volumes with similar properties 9 GEOLOGICAL MODEL Each tunnel design should be based on a realistic geological model, where also the uncertainties should be indicated 10 5

6 CARBONATIC ROCK blocky rock mass PHYLLITE foliated rock mass FAULT ZONE Rock Type Selection of key parameters INTACT ROCK: mineral composition solution phenomena strength properties DISCONTINUITIES: block size block shape aperture shear strength and roughness - infillings - persistence INTACT ROCK: mineral composition grain size / texture strength properties anisotropy - alteration / weathering DISCONTINUITIES: block size block shape shear strength and roughness infillings qualitative and quantitative clay mineral composition cementation ratio matrix/fragments swelling properties strength properties - alteration / weathering - mineral composition - persistence 11 CHARACTERIZATION SHEET (typical) Ground Type 4b Limestone and Marble (Permomesozoic), moderately to heavily fractured Representative core photos: Short descritption Limestone and marble,moderately to heavily fractured Detailed rock mass description Engineering geological Limestone (limestone, and marble) unit secondary: dolomite Interpretation of laboratory results, filed data and core logging Parameter representative value/range KB 17/07: limestone, bedded, with joints, fractrued, with karst Lithology Grey and sometimes dark grey limestone with transitions to marble ot to domomite (marble), limestone breccia (cemented) Intact rock Mechanical properties Isotropic to low anisotropic rock with high intact rock strength UCS [MPa] (50-120*) c [MPa] 8-18 (12-20*) 45 ± 6 mi [-] 20 ± 6 (15 ± 2*) Braz. [MPa] 1,4-2,9 (range of lab results) E [GPa] (40-80*) ν [-] 0,18 ± 0,07 (0,2*) KB 06/08: Limestone marble, tectonized, fractured Abrasivity (low) abrasive CAI [-] 0,9-1,8 General description Partially bedded with spacing in a cm range, partially karst along jointsteilweise with openings of mm to some cm (signs of weathering) Sketch: Discontinuity Foliation / bedding, spacing Foliation / bedding, conditions Joints, block size and goometry bedded and thinly laminated; bedding only partially foliation 6-20 (partially <6 bzw. >20) mechanically relevant spacing φ sf [ ] planar to wavy, rough, partially oxidations, partially minor clay fillings φ sf, rest [ ] dom.: 2-20 cm size manily narrow jointing and very narrow secondary sec.: cm jointing (incomplete healing); rarely medium jointing geometry generally blocky Remarks: Die dargestellte Gefügeneigung ist beispielhaft. Die tatsächliche Gefügeorientierung zum Bauwerk kann davon stark abweichen. Joints / slickensides, conditions dom.: stepped to planar, rough partially oxidated (karstic joints) sec..: planar /rough to smooth; rarely clayey (sl) φ K,H [ ] φ K,H, rest [ ] Rock mass Joints and bedding are dominating, blocks show GSI-classification GSI [-] size of cm to some dm, rough discontinuities UCS RM [MPa] 14,3 (6,6-25,5) the rock mass strength is generally dominated by Strength properties c RM [MPa] 3,8 (1,9-6,3) the discontinuities. Isotropic to slightly anisotropic φ RM [ ] 34,4 (31,1-37,5) ν RM [-] Elastic properties E RM [GPa] 15,8 (6,4-32,7) Additional data Remarks concerning grouting Remarks concerning swelling Gouting is possible up to a certain level (along joints), locally high grouting volumes possible due to open karst discontinuities no swelling potential 12 6

7 ASSIGNMENT OF GROUND TYPES TO PROJECT Tunnel alignment is segmented into sections with similar ground- and boundary conditions 9, , ,00 RMT 6,00 5,00 4,00 3, overburden 2, ,00 0, station 0 13 GEOLOGICAL MODEL geomechanically relevant properties (key parameters) GROUND TYPE (GT) ground water stresses orientation size & shape of opening GROUND BEHAVIOUR Ground behaviour is defined as the reaction of the ground to the excavation of the tunnel in its full profile without consideration of support or other construction measures 14 7

8 DESIGN PROCESS Assessment of boundary conditions Definition of requirements (RQ) Selection of construction concept Evaluation of system behaviour in excavation area Detailed determination of construction measures and evaluation of SYSTEM BEHAVIOUR (SB) Geotechnical design no yes SB complies w RQ BASELINE CONSTRUCTION PLAN Determination of tunnelling classes Distribution of tunnelling classes TENDER DOCUMENTS Secifications payment clauses 15 IMPORTANCE OF GROUND BEHAVIOUR ASSESSMENT Potential failure modes are identified Combinations of different failure modes can occur Examples: stress induced failure + discontinuity controlled overbreak, ravelling and large volume overbreaks, etc. Information is used for a preselection of feasible construction and support methods 16 8

9 Rock Mass Type Parameters of RMT - B Parameters of RMT - D Influencing factors Geometry of excavation Relative orientation of discontinuities to excavation Primary stress condition Water condition Modeling of ground behavior 17 BEHAVIOUR TYPE CATEGORIES Basic Category Rock burst Buckling failure Characteristic behaviour Stable with the potential of minor overbreak Potential for systematic discontinuity controlled overbreak Stress induced failure of rock mass with limited depth Stress induced failure of rock mass involving large volume Large volume overbreaks in crown with potential to daylighting failure Ravelling dry flow of poorly interlocked ground Flowing flow poorly interlocked ground with water Swelling physical/chemical process leading to large volume increase in the presence of water Strong variations in stresses and displacements due to heterogeneous ground 18 9

10 GEOLOGICAL MODEL geomechanically relevant properties (key parameters) ROCK MASS TYPE (RMT) ground water stresses orientation size & shape of opening GROUND BEHAVIOUR boundary conditions (BC) & requirements (RQ) design of excavation & support SYSTEM BEHAVIOUR (SB) SB = RQ? excavation & support classes TENDER DOCUMENTS 19 DISTRIBUTION OF GROUND BEHAVIOUR TYPES Behaviour Type overburden, watertable (m) station [m] 20 10

11 CHECK SB ON COMPLIANCE WITH REQUIREMENTS / OPTIMIZATION Check System Behaviour with respect to stress & strain compatibility and compliance to requirements Optimization of excavation and support methods Note: usually more than one solution exists for each problem. The selection criteria are a function of constructability, costs, and local regulations and experience 21 CONSTRUCTION As real ground and system behaviour cannot be predicted with sufficient accuracy during the design stage, design has to continue during construction Important for safety, updating ground model and optimizing of excavation and support methods is an appropriate observation and monitoring program Contractual and organizational setup must allow observational approach 22 11