MR02

1STRUCTURAL AND GEOTECHNICAL ENGINEERING DEPARTMENT

ROCK MECHANICS 2ROCK MECHANICS 2

Giovanni Barla

Politecnico di Torino

LECTURE 2 - OUTLINE

Observational method in Geotechnical Engineering Example: Mathematical modelling and performance monitoring for the Corso Vittorio Emanuele II Underpass in Turin- see the hand-out paper by G. Barla et al., 1995

THE OBSERVATIONAL METHOD

The term observational method appears to havebeen coined by Terzaghi and Peck in 1948. In hisRankine lecture Professor Ralph B. Peck (1969) made the following comments:

if the governing phenomena are complex, or are not yetappreciated, the engineer may measure the wrong quantitiesaltogether and my come to dangerously incorrect conclusionsThis is where the observational method is particularly

useful.

THE OBSERVATIONAL METHOD IN EUROCODE 7

2 Geotechnical Geotechnical CategoryCategory 11 should only include small and relatively simple structures

- for which it is possible to ensure that the fundamental requirements will besatisfied on the basis of experience and qualitative geotechnical investigations- with negligible risk.

Geotechnical Geotechnical CategoryCategory 11 procedures should be used onlywhere there is negligible risk in terms of overall stability or groundmovements and in ground conditions which are known from comparablelocal experience to be sufficiently straightforward. In these cases the procedures may consist of routine methods for foundation design and construction.

Geotechnical CategoryGeotechnical Category 11 procedures should be used only if there is no excavation below the water table or if comparable localexperience indicates that a proposed excavation below the water table willbe straightforward.

Geotechnical Category Geotechnical Category 22 should include conventional types of structure and foundation with no exceptional risk or difficult soil or loading conditions.

Design for structures in Geotechnical CategoryGeotechnical Category 22 shouldnormally include quantitative geotechnical data and analysis to ensure that the fundamental requirements are satisfied.

Routine procedures for field and laboratory testing and for design and execution may be used for Geotechnical CategoryGeotechnical Category 2 2 designs.

NOTE: NOTE: The following are example of conventional structures or parts of structures complying with Geotechnical CategoryGeotechnical Category 2 2 :- spread foundations, raft foundations, pile foundations, walls and otherstructures retaining or supporting soil or water, excavations, bridge piersand abutments, embankments and earthworks, ground anchors and othertie-back systems, tunnels in hard, non fractured rock and not subjected tospecial water tightness or other requirements.

Geotechnical Category Geotechnical Category 33 should include structures or parts of structures which fall outside the limits of Geotechnical CategoriesGeotechnical Categories1 1 and and 2 2 .

Geotechnical CategoryGeotechnical Category 3 3 should normally include alternative provisions and rules to those in the EC7 standard.

NOTE: NOTE: Geotechnical CategoryGeotechnical Category 3 3 includes the following examples:- very large or unusual structures, structures involving abnormal risks, or unusual or exceptionally difficult ground or loading conditions, structures in highly seismic areas, structures in areas of probable site instability or persistent ground movements that require separate investigations or special measures.

The OBSERVATIONAL METHOD isproposed for application in the draft of the new guidelines of the Ministry of Infrastructures and Public Works

3When prediction of geotechnical behaviour is difficult, it can be appropriate to apply the observationalmethod, in which the design is reviewed duringconstruction

the limits of behaviour which are acceptable shallbe established the range of possible behaviour shall be assessedand it shall be shown that there is an acceptableprobability that the actual behaviour will be within the acceptable limits

The following requirements shall be met beforeThe following requirements shall be met beforeconstruction is startedconstruction is started::

a plan of monitoring shall be devised which willreveal whether the actual behaviour lies within theacceptable limits. The monitoring shall make thisclear at a sufficiently early stage and with sufficientlyshort intervals to allow contingency actions to beundertaken successfully

the response time of the instruments and theprocedures for analysing the results shall be sufficiently rapid in relation to the possible evolutionof the system

a plan of contingency actions shall be devisedwhich may be adopted if the monitoring revealsbehaviour outside acceptable limits

EXAMPLE

Mathematical modelling and performance monitoring for the Corso Vittorio Emanuele

II Underpass in Turin

Turin Railway Link

The underpassing of the Corso Vittorio Emanuele II street is one of the most difficult work carriedout in connection with the construction of the new RailwayLink in Turin (left).The interest stems from the opportunity to discuss a successful application of the observational method in conjunction with numerical modelling

4The under-passing of Corso Vittorio Emanuele II street was carried out by pushing across two heavy concrete monolithic structures (East and West Monolith) by means of hydraulic jacks. These structures were driven ahead in the near vicinityof an under-pass, which was constructed at the beginning ofthe past century and consists of a bridge truss standing on two brick masonry walls made of buttress and arches. While the West monolith was located far away from the old structure, the East one was adjacent to it and was to be pushed ahead by excavating first down to 4m approximately with respect to the old foundation level. In the following figures the work is to described in detail

STAGE 1 - CONSTRUCTION OF THE TWOMONOLITHIC CONCRETE STRUCTURES

Removal and relocation of electric and telephone lines, water pipes, etc. on metallic structures (A-A)

Soil treatment by cement injections underthe foundation of the old underpass

Construction by casting in place of the twomonolithic structures B and C, one to the South and the other one to the North of Corso Vittorio Emanuele II street

Corso Vittorio Emanuele II street closed down with traffic on the central way only(29 June 1994)

Excavation of trench for pushing themonolithic structure ahead

STAGE 2 - THE TWO MONOLITHIC STRUCTURES (B and C) WERE PUSHED THROUGH AS SHOWN

The traffic along Corso Vittorio Emanuele IIstreet was fully interdicted (21 July 1994) The East and West trenches were excavatedin the upper section only The East (B) and West (C) monolithic structureswere pushed alternately with excavationtaking place 1 m advance each cycle

For pushing each monolith structure ahead a totalof 32 jacks were used for a total horizontal forceapplied up to 6500 t

STAGE 3 - CONSTRUCTION OF THE TWOMONOLITHIC CONCRETE STRUCTURES

Filling up with soil between the monolith structures and the excavation walls Corso Vittorio Emanuele street is rendered totrafic in the central way on 12 September 1994 The heading of the structures is completed and the street is opened to regular traffic on 31 October 1994

5A general view of the area during construction as the two concrete monolithic structures (East and West)are driven in a direction orthogonal to Corso Vittorio Emanuele II street. The exixting monitored railway underpass is indicated by the arrow. Also indicated is the excavation zone

Existingunderpass

Excavatedzone

West Mon

olithEast Monolith

A general view of the area during construction as the two concrete monolithic structures (East and West)are driven in a direction orthogonal to Corso Vittorio Emanuele II street. The existing railway underpass is shown with a train moving through as work is under way

A view of the area during construction as the Wast monolithic structure has reache its final position.Atrain moving from Porta Susa Station toward Porta Nuova Station is shown on the left. The future railway line on the right is 4m below the level of the old railway line

The ground conditions in the work site were well documentedconsisting of gravelly-sandy soil (51 per cent gravel, 31 per cent sand, and 18 per cent silt and clay), which wascharacterised with a friction angle of 35, a short-term cohesionof 20 kPa (the long-term cohesive strength is generally takento be 0 kPa), and a deformation modulus of 100 MPa, at 10 mdepth approximately.With due consideration given to the static conditions of theexisting unerpass structure, it was decided to improve themechanical properties below the East wall by injecting the soilwith cement grout and chemical mixtures - Silacsol)

6Cross section showing the existing railway underpass and the excavation zone where the East monolithic structure has been driven in the direction from Porta susa to Porta Nuova Station

Excavation Zone

The East monolith during excavation

The East wall of the exixsting underpass

A SERIES OF PICTURES TAKEN DURING WORK IS GIVEN IN THE FOLLOWING

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8

9

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MATHEMATICAL MODELLINGMATHEMATICAL MODELLING

The deformations and stresses induced in the existing underpass structure and in particular in the brick masonrywall located in the near proximity of the East monolith werepredicted by finite element calculations well in advance ofexcavation and construction. In line with the observational observational methodmethod the purpose was to evaluate the behaviour of thestructure later to be monitored.Two Two numerical modelsnumerical models were created: a longitudinal modellongitudinal model (plane stress) a transversaltransversal modelmodel (plane strain)always in two dimensional conditions.

MATHEMATICAL MODELLINGMATHEMATICAL MODELLING

Stress distribution in the brick masonry wall with a vertical displacement induced at the bottomof buttresses A and B

Longitudinal SectionLongitudinal Section

Stress distribution in the brick masonry wall for the same displacement conditions introducedin the previous figure, however with prior filling of the arches by light concretebuttresses A and B

MATHEMATICAL MODELLINGMATHEMATICAL MODELLING

Longitudinal SectionLongitudinal Section

MATHEMATICAL MODELLINGMATHEMATICAL MODELLING

Also computed with the transversal model were displacements of the wall as due to excavtion. Tipically these resulted to be as follows: horizontal displacement at the top of the wall: 5 mm toward the excavation change in vertical displacement at the top of the wall: - 5 mm opening of the key of the arch: negligible change in tilt angle: 72 (mean value along the wall height)

Predicted mobilized strengths near the underpass at the end of excavation (the soil is given a cohesive strength of 20 kPa)

Transversal SectionTransversal Section

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PERFORMANCE MONITORINGPERFORMANCE MONITORING

In order to analyse the deformational behaviour of the brickmasonry wall durin excavation, a monitoring programmehas been established comprising the following instrumentations: nr. 3 inclinometers located along bottresses 1,3 and 9 nr. 3 double point borehole extensometers nr. 7 VW crackmeters installed at the key of the arch nr. 2 VW tiltmeters a number of topographic targets placed along the brick masonry wall and at the surface

see the following figure showing the monitoring systemLongitudinal section of the brick masonry wall located toward the excavated zone where the Eastmonolith is driven in the Porta Susa to Porta Nuova Station direction. Also shown is the instrumentationinstalled for monitoring purposes

PERFORMANCE MONITORINGPERFORMANCE MONITORING

tiltmeter

crackmeter

tiltmeter

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crackmeter

ACCEPTABLE VALUES OF KEY PARAMETERSPREDICTED BY NUMERICAL MODELLING

horizontal displacement at the top of the wall: 5 mm toward the excavation

change in vertical displacement at the top of the wall: - 5 mm opening of the key of the arch: negligible change in tilt angle: 72 (mean value along the wall height)

let us see the measured values

opening of the key of the arch

change in tilt angleinclinometer measurements