Proc. 2nd Int. Conference on Soil Structure Interaction in Urban Civil Engineering Zurich / March 2002
Effects of the expected settlements following excavation of a proposed underground subway tunnel in the historic centre of Florence
M. Severi & G. Vannucchi
Dipartimento di Ingegneria Civile – Università di Firenze Via di S. Marta, 3
50139 Firenze (I)
ABSTRACT: Florence is a city of average size (about 400,000 inhabitants), with narrow old streets that are quite unsuitable for modern surface vehicular traffic and with a building and monumental patrimony that is among the greatest in Europe. The city is at the centre of a metropolitan area populated by about 1,6 million in- habitants, and is one of the most visited tourist destinations in the world.
In order to contribute to solving traffic and mobility problems in its urban area, a group of researchers of the University of Florence carried out a feasibility study on a newly developed underground rapid-transport system, called “Micro-Metro (µM)”.
The aim of the geotechnical part of the feasibility study was to estimate the absolute and differential ground settlements following excavation of the tunnel, and to evaluate their effects on the building environment. Differ- ent excavation methods were investigated, and typologies and vulnerability of the buildings situated along the layout of the tunnel were examined. The main results obtained from the said study are described in this paper.
1 INTRODUCTION
The development requirements of a modern city seem to be in contrast with the need to preserve the cultural heritage. This is particularly true in cities of average size that have an urban fabric with a long his- tory, of which Florence is an example. Within this perspective, development of mobility in particular and, consequently, of the relative infrastructures must be planned so that these do not interfere with the his- toric urban structure. Instead, the aim must be to make always greater use of the underground space as an al- ternative to and in combination with the surface space. In city planning, the entire urban context has to be viewed as a single integrated surface/underground system. The latter must be not only the substrate of economic and settlement activities, but also a space to be occupied and managed, i.e. a resource to be ex- ploited. This all implies the need to have an overall knowledge of all aspects of the physical components of the system: in particular a detailed knowledge of the geological, hydrogeological, geotechnical, and ar- chaeological conditions.
The Micro-Metro (µM) is a rapid mass-transport system based on the technology of an underground, light and totally automated subway. The main techni- cal specifications of the project are the following:
- One-way tunnel with a small section (De = 5.5 m), located at an average depth of 18 m from ground level;
- Line with an overall length of 26 km and 22 transit stations, which assumes the form of a necklace that touches the three points in which the main inter- change stations are located (Figure 1);
- Trains with completely automated controls; a ca- pacity of 200 passengers for each train, consisting of 2 or more carriages; an asymmetrical section that im- proves accessibility and makes possible the realisation of a service and emergency platform along the entire layout of the tunnel;
- Travelling speed of 45 km/h; minimum times be- tween trains of 90 seconds; hourly potential limit of 8,000 passengers.
2 GROUND CONDITIONS 2.1 Geology
Most of the urban area of Florence is settled on the eastern borders of the wide valley that extends as far as Pistoia, known as the paleo–lake of the Florence- Pistoia basin (Capecchi et al., 1975).
The maximum depth of the bedrock in the Floren- tine urban area is about 50 m. Proceeding from bot- tom to top, the fluvio-lacustrine deposits generally consist of a thick layer of clay and silty clay locally interbedded with coarse to fine sand (typically 1 m thick) and fine gravels a few centimetres thick. The clay layer is followed by a layer of brown silts inter- bedded with medium to fine gravel with an abundant silty matrix. These sediments are the most recent ones, and represent the fluvial deposits of the Arno River and its tributaries.
For the feasibility study of the Florentine µM sys- tem, a computerised database was prepared including all the stratigraphies obtained from the data of about 1,000 boreholes.
2.2 Hidrogeology
The water table is located in the recent alluvial sediments of the Arno and its tributaries. These con- sist mainly of pebbles, gravel and sands with medium to high permeability. In the subsoil of the Florentine plain, there is a water-bearing stratum almost every- where: in fact, the water table is located at a depth of between 1 and 10 metres, differing according to both the zones and the seasons. Aquifer gravel is generally present at this depth, at least in the urban area of Flor- ence. Where the surface layer is very clayey silt with
very low permeability, the aquifer is confined. This occurs in the zone of Novoli (the N-W part of the lay- out): there, the aquifer is represented by lenses of dis- continuous clayey and silty gravel having a relatively low permeability.
The permeable levels in the fluvio-lacustrine suc- cession underlying the recent floods are also confined aquifers. The levels consist of gravel and sands that often have an abundant silty matrix.
The main water table is, however, the water- bearing stratum contained in the recent floods of the Arno and its tributaries. In short, the entire layout of the µM project is under the water table.
Figure 1. The layout of the µM transport system
micrometro (µM) Layout
Transit Station
Main Station
A
B
A B
A B
Figure 2. Northern and southern geological profiles along the µM line (Boccaletti and Pranzini, 2001)
2.3 Geotechnical properties
On the basis of a considerable number of bore- holes, laboratory and in situ tests carried out at differ- ent times, in different ways and with differing aims, it has been possible to determine with good approxima- tion the succession, thickness, and geotechnical prop- erties of the layers that constitute the subsoil of the city of Florence. The values for each geotechnical variable are included within a limited range. Further- more, assumption of the mean values of the geotech- nical properties is justified by the fact that the settle- ments due to tunnelling involve a considerable volume of soil and are, therefore, function of the mean – and not precise – values.
With reference to the longitudinal sections (Fig.
2), the stratigraphical succession is the following:
Layer 1 is formed of heterogeneous filling soils of a more or less recent epoch with fragments of bricks.
The layer has an extremely variable thickness (from 1 to 5-6 metres), and consists mainly of silts ranging from weakly sandy to not very clayey, often with pebbles, from medium dense to dense. In view of the depth of the tunnel, this layer is never involved in the excavation process; therefore, the mechanical proper- ties relative to it have been disregarded.
Layer 2 is formed of alluvial soils consisting mainly of well-graded gravel, sand and pebbles in a mainly silty-clayey matrix with intercalations of sandy silt to clayey silt, mostly sporadic and with a variable thickness. This constitutes the prevailing formation, and is present along the entire layout.
The permeability coefficient, which is calculated on the basis of well pumping tests, was found to be between 10-2 and 10-3 cm/sec.
The numerous Standard Penetrometric Tests (SPT) showed a dispersion of values due to the granu- lometric heterogeneity of the deposit. Moreover, by considering the values of the Standard Penetrometic resistance closest to the lower limit, which are less in- fluenced by the presence of gravel and pebbles, a rela- tive density for the soil is estimated of the order of 45% to 50% in the more surface level, between 10 and 15 m from g.l., and more than 60% below 15 me- tres.
The angle of shearing resistance, which was evaluated using De Mello’s correlation, is estimated to be equal to 32° to 33° in the first 10 m from g.l., and no lower than 35° at greater depths. The effective Young’s modulus values are between 30 and 60 MPa.
Layer 3 consists of formations of alluvial or flu- vio-lacustrine deposits, mainly made up of over- consolidated clay or clayey silt, at times with inclu- sions or intercalations of gravel and sand. The soil is consistent and homogeneous.
This type of soil is also encountered along much of the layout, with the exception of the tracts in which there is an emergence of the rocky substrate as in the zones of Piazza Beccaria, the left bank of the Arno (Oltrarno), and Piazza Repubblica.
Figure 3 provides a representation of the plasticity chart of the samples from the layer 3 (wL =40÷60; wP
=15÷35).
The undrained shear strength (cu), which was iden- tified with tests both on site and in the laboratory, is between 160 and 200 kPa.
Figure 3. Plasticity chart for the silty clay of Layer 3
Layer 4 Bedrock (flyschoide) is present only in certain parts of the µM layout. The components (marls, argillites, marly and sandy calcareous rock) appear with a very close stratification, with a general prevalence of the marly and argillitic components that are intercalated by centimetric levels of sandstone of marly calcareous rock. The rock shows a dense net- work of discontinuities. In particular, the upper part of the formation (10 to 15 m in several tracts) is very fractured and weak. This phenomenon most greatly involves marly and argilittic components which, while maintaining traces of their original structure, have taken on the consistency of a very stiff cohesive soil, but one that is no longer rocky. The degree of weak- ness was found to be accentuated in the arenaceous levels, while the calcareous component is not very weak.
The mechanical properties of the rock were inves- tigated by means of pressiometric tests, which in- volved the part below the weak layer, and by means of SPT tests within the framework of the weak layer.
3 EXCAVATION METHOD
The three priorities in function of which to operate the choice of the excavation method are the follow- ing:
1. Maximum safety for all operators inside the tun- nel;
2. Maximum safety for the buildings or other ele- ments on the surface;
3. Minimum open-front area of the tunnel in all cir- cumstances.
The mechanised excavation method with an earth pressure balance (EPB) shield was judged to be most suitable. A study of the methods and machinery util- ised for the underground subway systems in Rome, Milan, Lyons, London, Cairo, and above all Lille, where the dimensions of the cavity are similar to those foreseen for the Florence µM, has amply con- firmed this fact in practice.
As the EPB shield advances, soil is excavated through slots in a rotating cutter head and deposited in a spoils retaining area. Simultaneously, soil is re- moved from the enclosed spoils retaining area by a screw conveyor. The rates of soil excavation and re- moval are controlled by the operator of the shield. To- tal earth pressures are measured inside the spoils re- taining area during advancement of the shield. These measurements indicate the status of the soil volume
inside the spoils retaining area (Clough et al., 1983;
Finno and Clough, 1985).
4 METHODS OF ANALYSIS
The prediction of ground movements due to tunnel- ling is normally made using empirical relationships which are based on previous measurements obtained at several greenfield sites. Surface settlement data are available from many sites around the world. The pres- ence of buildings is assumed to have no effect on the surface settlement, and any damage parameters are calculated using the predicted greenfield movements.
It is recognized that a rational procedure must be ap- plied to assessing the risk of damage to buildings re- sulting from tunneling-induced ground movements.
An evaluation procedure following ‘preliminary’,
‘secondary’, and ‘detailed’ stages has been described by Burland (1995), and Mair and Taylor (1993). It first requires the prediction of ground movements and a preliminary evaluation of all the structures along the route of a tunneling project. Predictions of ground movements can be made by using numerical or em- pirical approaches. In this study, the empirical method was used.
The empirical design method predicts a transverse surface settlement profile (orthogonal to the direction of tunnel drive) in the form of an inverted Normal Gaussian distribution (Peck, 1969; Attewell and Woodman, 1982; O'Reilly and New, 1982; Rankin, 1988). The value of settlement δv above a single tun- nel excavation is shown in Figure 4, and can be ob- tained from:
( )
⎟⎟⎠
⎜⎜ ⎞
⎝
⎛
− ⋅
⋅
= 2
2 m
v 2 i
exp x
x δ
δ (1)
where:
x is the distance from the tunnel centre line in the transverse direction;
i is the distance from the centre line to the point of in- flexion of the curve;
δm is the maximum settlement in correspondence with the tunnel axis (at x=0);
δv is the settlement;
D is the diameter of the tunnel.
The value of i can also be obtained from the em- pirical relationship (Clough and Schmidt, 1981):
8 . 0
r 2 H r
i ⎟
⎠
⎜ ⎞
⎝
=⎛ (2)
where:
H and r are respectively the axis depth and the radius of the tunnel.
The volume of the settlement trough per metre length of tunnel is obtained by integrating equation (1) with respect to x to give:
m m
s 2 i 2.5 i
V = π ⋅ ⋅δ ≅ ⋅ ⋅δ (3) from which the tunnel volume loss VL, as a percentage of the theoretical excavated volume is obtained:
2 m s
s
L D
i 192 . 3 A
V V ⋅ ⋅δ
=
= (4)
To obtain a prediction of surface settlement above a tunnel diameter, D, the input values of VL must be es-
tablished. The volume of soil excavated will be greater than the theoretical volume of the tunnel as defined by its geometry. This is because ground is lost in the face and perimeter of the excavation during construction.
The equations are such that at the point of inflec- tion, the settlement profile switches from sagging to hogging mode (Fig. 4). In this point the horizontal movement is maximum, and the horizontal strain is zero. In the sagging region (x<i), the horizontal strains are compressive, whereas in the hogging re- gion (x>i) they are tensile.
Figure 4. Definitions for settlement profiles of Gaussian dis- tribution (Attewell, 1978)
A cumulative probability function is used to generate the profile of longitudinal settlement ahead of and be- hind the advancing tunnel face (Attewell and Wood- man, 1982).
Figure 5. Profile of longitudinal settlement
Combining the longitudinal and transverse equations can lead to predictions of the complete picture of sur- face settlement above a tunnel excavation.
To obtain predicted surface settlement profiles, the most likely volume losses must be estimated. On the basis of the data present in the literature, a conserva- tive estimate of the value sought is Vs /VT ≅ 1.5-2%.
The geometric characteristics of the tunnel useful in calculating the basin of the settlements are shown in Table 1.
Table 1.
Tunnel diameter D = 2r 5.5 m Excavation volume Vt 23.76 m3/m
Table 2.
Depth. Settlement parameters (m) δm(mm) B(m) β ρ -15 25.7 13.89 0.0028 3.7 ⋅ 10-4 -20 19.9 17.95 0.0017 1.7 ⋅ 10-4 -25 16.4 21.78 0.0011 0.9 ⋅ 10-4 -30 14.0 25.45 0.0008 0.6 ⋅ 10-4 In Table 2 are indicated the characteristic values cal- culated for the maximum settlement δm, the semi-
width of influence B, the maximum angular distortion β, and the maximum curvature ρ of the curves of the settlements relative to the transverse direction.
The green field settlement profiles produced by tun- neling at different depths are represented in Figure 6.
-50 -40 -30 -20 -10 0 10 20 30 40 50
0 5
10
15 20
25 30
(m)
(mm)
Depth –15 m Depth –20 m Depth –25 m Depth –30 m
Figure 6. Settlement trough profiles for a tunnel at different design depths
4.1.1.1 Adjacent tunnels
A separate case in evaluating surface effects con- sists of the three interchange stations (Porta a Prato, Repubblica, and Beccaria), in correspondence with which the line converges from two directions in the two travelling directions.
The movements and strains associated with the excavation of tunnels always create a disturbance zone for the soil around the cavity. Width of the plas- tic zone is mainly influenced by the sensitivity of the excavation soil, the diameter of the excavation, and the depth. The disturbance zones are not superim- posed if the distance from the centre lines of the tun- nels is greater than 2D, for tunnels in sensitive clay with a z/R ratio of greater than 3.
Figura 7: Settlement profile for parallel tunnels (Attewell, 1978)
The settlement associated with each tunnel is thus independent, and superimposition of the effects can be used to evaluate the total basin of the settlements due to both tunnels. In particular, it is possible to use the Peck-Fujita method, adding the contributions of each in order to obtain the overall profile of the set- tlements.
There is no superimposition of the zones of plasti- cised soil in the case of the three main interchange
stations, since the distance between the axes of the tunnels is always greater than 35 m.
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
10
20
30
(m)
(mm)
Figure 8: Settlement basin in the vicinity of the Beccaria station (projected depth –15 m from g.l.).
-50 -40 -30 -20 -10 0 10 20 30 40 50
0
10
20
30
(m)
(mm)
Figure 9: Settlement basin in the vicinity of the Porta al Prato and Repubblica stations (projected depth –20 m from g.l.).
4.2 Potential building damage assessment
The risk of damage to buildings with reference to the project parameters of the layout of the µM was analysed.
In consideration of the structure of the urban fab- ric of the city of Florence, as well as of its (sub)division into sectors or zones with different his- torical-architectural values, greater care was applied in estimating the effects induced in the fabric of the historic centre, inside of which many of the buildings and monuments whose integrity must be absolutely guaranteed are concentrated.
The following definitions for the assessment of risk of damage to buildings due to tunnelling and ex- cavation were used (Burland and Wroth, 1974):
- Rotation or slope θ is the change in gradient of a line joining two reference points (e.g. AB in Fig- ure 10).
- The angular strain α is the positive for upward concavity (sagging) and negative for downward concavity (hogging).
- Relative deflection ∆ is the displacement of a point relative to the line connecting two reference points on either size.
- Deflection ratio (sagging ratio or hogging ratio) is denoted by ∆/L, where L is the distance between the two reference points defining ∆.
- Tilt ω describes the rigid body rotation of the structure or a well-defined part of it.
- Relative rotation (angular distortion) β is the rota- tion of the line joining two points, relative to tilt ω.
Figure 10. Building damage parameters (Burland and Wroth, 1974)
The prevailing building typology consists of multi-storey masonry buildings (4 floors, on an aver- age).
This structural typology is the most vulnerable as regards the relative deflection and the deflection ratio.
The maximum value of the deflection ratio without damage for multi-storey masonry buildings founded on stiff clay and with a ratio between length and height L/H ≤ 3 is (∆/L)adm = 0.0003 (Polshin and To- kar, 1957). For this reason, this particular structural typology was considered for a preliminary assessment of the risk of damage.
The safety factors for the serviceability limit state, as defined by the ratio between the allowable value of the deflection ratio and the actual value, SF = (∆/L)adm/(∆/L) = 0.0003/(∆/L), were calculated.
The analysis was carried out by referring – along the layout of the µM – to the ground levels for esti- mating the surface settlements, and to the foundation depths of –1.5m and –3.5m from ground level for es- timating the effects on the buildings without and with a basement, respectively.
For both foundation depths, buildings oriented in the orthogonal direction to the tunnel axis and with a length LABi (i = 1, 2, 3, 4) equal to B, 3B/4, B/2 and B/4, were considered.
The point of extremity A of the straight-line seg- ment LABi (perfectly rigid foundations) has co- ordinates (B, f1(B)), where :
⎟⎟⎠
⎞
⎜⎜⎝
⎛−
⋅
= 2
2 m
1 2i
exp x )
x (
f δ (5)
A scheme representing the transversal settlement profile to the foundation depth, the 4 straight-line ABi
segments considered, and the corresponding relative deflections ∆, is shown in Figure 11
0 5 10 15 20
10 5 0 5 10 15 20 25 30 35
(m)
(mm)
Foundation curves
Transversal settlement profile B3
B2 B1
B4
B
∆(x) functions
A
Figure 11. Relative deflections for the project depths of the line.
From the data in Table 3, it is possible to identify the parts of the µM layout that are most exposed to the risk of damage. The most important result is the fact that, at the design depth of –20.00 m from ground level, the effects of the construction of tunnels and excavation on the building environment are negligi- ble. This is of fundamental importance, because the tunnel is designed to occupy this depth for almost its entire length. All the more so, the tunnel depths of – 25.00 and –30.00 m from g.l. do not represent any danger for the buildings overhead. Instead, at the tun- nel design depth of –15.00 m from g.l., which is reached only in correspondence with the Beccaria sta- tion, where there are masonry buildings erected in dif- ferent periods, several values of the safety factor are less than one.
The deflection decreases upon moving away from the square. This corresponds to the increase in the depth of the tunnel that gradually continues in the di- rection of the four stations connected with “Beccaria”.
The possible settlement effects on surfaces can be dis- regarded already at a distance of 50 to 100 m from the station (Figure 12).
Table 3: Safety factors for serviceability limit state of multi- storey masonry buildings on the µM layout
LAB = B 3B/4 B/2 B/4 Tunnel
depth (m)
Found.
depth (m)
SF SF SF SF -1.5 1.17 0.71 0.85 1.99 -15
-3.5 0.87 0.53 0.63 1.48 -1.5 2.06 1.26 1.50 3.51 -20
-3.5 1.68 1.03 1.23 2.86 -1.5 3.49 2.13 2.54 5.93 -25
-3.5 2.69 1.64 1.96 4.57 -1.5 4.36 2.67 3.18 7.42 -30
-3.5 3.86 2.35 2.81 6.56
5 CONCLUSIONS
The only part of the µM layout which may present some problems from the point of view of the risk of damage to buildings can be identified in the vicinity of the Beccaria station, in correspondence with which there is the minimum coverage of the entire layout of the µM line. However, the depth chosen for this sta-
tion derives directly from considerations of excava- tion costs that are linked to the presence of the rocky substrate at depths lower than –16.00 m from g.l.
Consequently, in a more advanced phase of the pro- ject, it is to be hoped that a detailed “costs / benefits”
comparative study will be made, in relation to both the effects caused and to the specific excavation tech- nique chosen. On the basis of these evaluations a lower design depth cannot be excluded.
In conclusion, in consideration of the studies car- ried out, the µM layout can be designed so as to guar- antee a realisation that is completely safe for its entire length, as regards both the excavation and the build- ing environment. This condition must be considered to be of primary importance in the design of every underground work in an urban context of inestimable value – namely, that of the city of Florence.
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