Structural design of tunnel lining

STRUCTURAL DESIGN OF

TUNNEL LINING

PRESENTED BY:

Mahesh Raj Bhatt

M.E. In Structural Engineering

Kathmandu University31th march-2017

Content outline:

Team work in tunnel design

Fundamental approach of design

Lining in tunnel

Mode of failure in lining

Design of lining

Initial design

Final design

Conclusion

Team work in tunnel design:

i. Engineering Geologist: geo-exploration

ii. Hydraulics engineer: tunnel pressure, profile

iii. Structural engineer: lining type and material selection

iv. Geotechnical engineer: ground support requirement

v. Civil engineer: construction, location, layout

vi. Surveyor: maps, geometric frameworks

vii. Environmentalist: Environmental issues

viii.Construction engineer: construction issue

ix. Other experts: estimation, costing, quality expert etc.

Fundamental approach of design:

• Engineers promise that the structure will neither suffer structurally nor collapse during its projected lifetime.

• Design must be

• Safe

• Stable

• functional

• Possible failure modes should be analyzed.

Lining in tunneling:

Steps in structural design construction

• Excavation: Blasting / mechanical tools

• Initial ground support: safety during work

• Final ground support: structural safety

what is tunnel lining?

• timber, brick, concrete, or steel supports erected in a tunnel to maintain dimensions and safe working conditions.

• And finally for permanent safety of the system.

Mode of failure in tunnel

Can be categorized

i. Failure due to discontinuity: fault, joint, fold

ii. due to stratified rocks

iii. Rock fail by stress: squeezing, in fault zone, weak and expansive soil in deeper zone.

iv. Fail by mineral control

v. Water pressure control: swalling

vi. Seismic effects

vii. Other permanent soil land displacement

viii. Failure in shortcrete

ix. Failure in rock bolt and dowel system etc.

Fig: tunneling in Rock stratification:

Design of lining

Types:

a) Design of initial support

installed after excavation to support the opening safely.

b) Design of final lining:

to make the structural safety and stability

a) Design of initial support:Based upon: how to select material for support

i. empirical method: Terzaghi (1946)

ii.Theoritical method/ analysis method

iii.Fundamental approach: based on potential failure mode

i) Imperical method: (design of steel ribs)

-based on the experience

-terzagi (1946) gave written rule for selection based on Rock loads and RQD-index of the rock.

(RQD=rock quality designation)=degree of joint in rock mass

•Skinner , Wickman, Tiedeman (1972)

• Gave Rock structure rating (RSR)

Rock type

Joint

Discontinuity

Fault

Fold

Orientation based

ii) Theoretical method:

It deals with design of

• rock bolts

•Dowels

•Shortcrete

•Steel ribs design

•Rock bolts: based on 3-D wedge analysis

•Shortcrete: to make immediate stiffness to excavated rocks

•Dowel: to support mass / and reduce deflection

Design of steel ribs:

• Bent I-section or H-section made circular (steel)

Ribs are oriented to act as arch action which reduces moment in the section.

b) Design of final lining:

• Common options are:

•Un reinforced concrete

•Reinforced concrete

•Segmental concrete

•Steel backfill with concrete

•Concrete pipe with backfill

•Steel backfill with grout

Note: interaction between rock and lining material is important to design the lining in tunneling.

• Unlined tunnel: water excess to rock

• Shot-crete: to provide ground support, hydraulics

• Un reinforced line : to protect rock exposure, smoothen

• Reinforced line:

1-layer:temperature stress and shrinkage

Multi layer: swelling, squeezing, high stress

• Steel lining: when internal pressure > external pressure/

ground or ground water

Load cases for design:

Design requirements:

• Lining thickness: tmin>300 mm

Min cover>100mm

Guideline EM-1110-2-2104 for concrete cover

• concrete mix design:

•Most cases=M21 (21 mpa)

•Thin line =M35

•Segmental line=M42

•Reinforcement: 0.28% of X-section of line

0.4% for blast type tunneling

note: reinforce for crack control due to temperature/pressure etc.

Lining for external water pressure:

• If P external water< P internal water

Than take

P ex=Pin ( because leakage increases

external pressure)

•if Tlining< R/10 (R-radius of tunnel)

•If T lining>R/10

Circular tunnel with internal pressure:

Concerns are mainly on

1)Rock lining interaction

2) estimation of lining leakage

1) Rock lining interaction

-thin shell equation ( radial displacement of lining)

-cylinder theory for thick lining

To find radial displacement of lining:

2) Lining leakage estimation:

Lining for bending and distortion:

Note: moment and forces analysis must be done using Finite element analysis tools

Design of steel liners:

• Amstutz (1970)

• Jacobson(1974)

• Vaughan (1956)

• Moore (1960) computer solution:

•To find the critical buckling pressure .

Linear ring lining design: basic approach

• A liner ring is statically indeterminate.

• A one pass lining is designed for transport and erectionloads, loads during grouting, and ground loads

• For a circular ring of constant cross section

symmetrically loaded

the thrust at the crown C is

• Note: in ring segmental design all the analysis procedure is same as like as parabolic/circular arch.

• Based upon the fined analysis result rebar must be designed .

(horizontal thrust/ shear / and moment are governing parameters for the lining rebar design)

CONCLUSIONS:

• Lining design should be safe , stable andfunctional

• Lining type depend upon the property of theinterior rock or soil.

• To save the rock portion to be exposure, lining isdone.

• To give additional support to rock mass lining isneeded.

• There are various methods for lining design thesemay be not structural sometime these may beonly based upon the constructional approach.

Thank you!!

THANK YOU!!

Tunnel spillway in Kanogawa dam Japan

References:

• Engineering and design tunnel and shaft in rock, CECW-EG

Engineer Manual1110-2-2901 Department of the Army U.S. Army

Corps of Engineers Washington, DC 20314-1000

• TUNNEL ENGINEERING, 20 Lars Christian F. Ingerslev , Arthur

G. Bendelius Parsons Brinckerhoff New York, New York

• PRACTICAL TUNNEL, CONSTRUCTION Gary B. Hemphill PhD,

PE, JOHN WILEY & SONS, INC.