Seismic response of bridges with cellular foundations ... Seismic response of bridges with cellular

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  • Seismic response of bridges with cellular

    foundations built in soft clay

    Juan M. Mayoral, Miguel P. Romo & Francisco A. Flores Institute of Engineering, UNAM, Mexico City, Mexico ABSTRACT A new foundation system based on structural cells has been used for supporting the columns of the elevated section of the 12

    th Metro Line, in Mexico City. This study consists on a numerical simulation employing a three-dimensional finite

    element model developed with the program SASSI2000, of one of the critical supports located in the so-called Lake zone, known by its difficult subsoil conditions. Clays in this region present low shear strength and high compressibility. The system response was computed for a typical seismic scenario, such as that prevailing at the zone, which assumes a potential Mw=8.2 seismic event. The cellular foundation is comprised of a rigid slab structurally tied to perimeter concrete walls, also structurally connected to each other. The computed upper deck acceleration is less than a half of that obtained directly from the recommended response spectra compiled in the Mexico City building code. RESUMEN Un nuevo sistema de cimentacin basado en celdas estructuradas va a ser empleado para soportar las columnas del tramo elevado de la Lnea 12 del Metro de la Ciudad de Mxico. El estudio aqu presentado, consiste en la simulacin numrica, usando un modelo tridimensional de elementos finitos desarrollado con el programa SASSI2000, de uno de los apoyos ms crticos localizado en la denominada zona de Lago conocida por sus difciles condiciones del subsuelo. Las arcillas en esta regin presentan una baja resistencia al esfuerzo cortante y alta compresibilidad. La respuesta del sistema fue calculada para un escenario tpico, como el que prevalece en la zona, asumiendo un evento ssmico de magnitud Mw = 8.2. La celda estructurada est conformada por una losa rgida conectada estructuralmente a muros perimetrales de concreto tambin conectados entre s. La acceleracin mxima calculada en la parte superior del tramo elevado es menor que la mitad de la obtenida directamente del espectro de diseo propuesto por el reglamento de construcciones de la Ciudad de Mxico. 1 INTRODUCTION A new 28.4 km long metro line is under construction in the southeast part of Mexico City, the so-called 12

    th Line.

    In particular, 13 km of this project will be elevated, and will cross two of the most difficult subsoil conditions in the city, the lake zone and the transition zone (Figure 1). A new foundation alternative (Romo et al., 2010) based on a skirted type foundation, consisting on a mat structurally tied to peripheral walls was used to support the columns of the elevated section (Figure 2a). This work presents the seismic performance evaluation of one of the most critical supports, hereafter referred as cl-34, which is located in the typical soft clay deposits found at the city. The seismic soil-structure interaction analyses were conducted using 3-D finite element models, considering a seismic environment characterized by a major seismic event, with moment magnitude, Mw, of 8.2. The response of the structure was obtained in terms of accelerations, displacements and response spectra. Transfer functions between free field and both foundation and structure were also computed. The high stiffness of both the column and the cellular foundation reduce the maximum accelerations obtained in the upper deck. 2 SUPPORT ANALYZED Figure 2 shows support cl-34 in elevation and plan view. Figure 2c depicts a longitudinal view of the elevated section. This consists of an upper deck resting on top of 30 m long beams, which are, in turn, supported by

    columns. Both beams and columns are pres-stress and made of high strength concrete. The columns have a hollow transversal section, and are structurally connected to the cellular foundation. The upper part of this foundation consists of a rigid square concrete mat 6.5 x 6.5 m

    2, with 1.7 m of thickness. This slab is rigidly

    attached to perimeter structural concrete walls 0.60 m thick. The embedment depth of this foundation is 15 m (Figure 2a). 3 SUBSOIL CONDITIONS To characterize the geotechnical subsoil conditions found at the site, one cone penetration test, CPT, and one standard penetration test, SPT, combined with selective undisturbed sampling were conducted. The studied site is mostly comprised by a very soft to medium clay deposits interbedded with thin sand lenses. Figure 3 shows the soil profile, the variation of tip penetration resistance with depth, SPT values, and rock core recovery percentage. Based on the field investigation and laboratory testing results, it was found that the soil profile is comprised by a sandy silt layer 3 m thick. Underlying this stratum, there is a soft to medium 25 m clay layer interbedded with silty sand lenses. Within this layer, from 3 to 7 m of depth, there is a high plasticity clay layer. At 6.5 m, water

    content, n, is 400 %, liquid limit, L, is 450 % and plasticity index, PI, is 172 %. Below 7 m and up to 21 m,

    n, varies from 50 to 200 % and the undrained shear

  • strength, su, goes from 21 kPa to 40 kPa. From 21 m to 28 m of depth, there is a low plasticity sandy clay layer,

    exhibiting a (%) of about 30. The preconsolidation

    pressure, p, is 10 kPa for both 6.5 m and 9.5 m of

    depth, and for 14.5 m, p, is 196 kPa. The coefficient of volumetric compressibility, mv, ranges from 0.162 cm

    2/kg

    to 0.047 cm2/kg.

    Zone I

    Zone II

    Zone IIIa

    0 1 2.5 5 10 km

    Scale

    -99.20 -99.15 -99.10 -99.05 -99.00

    19.25

    19.30

    19.35

    19.40

    19.45

    LA

    TIT

    UD

    E

    LONGITUDE

    INTERNATIONAL

    AIRPORT

    VIADUCTO

    REFO

    RMA

    INS

    UR

    GE

    NT

    ES

    CIRCUITOINTERIOR

    PER

    IFR

    ICO

    AV. TLHUAC

    TLA

    LPAN

    PR

    OL. D

    IV. D

    EL N

    OR

    TE

    MIXCOAC

    ATLALILCO

    TALLERES TLHUAC

    Subway stationTunnel

    Underground box

    Shallow boxElevated section

    Zone IIIb

    Zone IIIc

    Zone IIId

    This zone will be considered as II (Transition)

    Norms for foundation design

    This region are no sufficiently investigated,therefore the proposed zoning is only indicative

    when using the complementary Technical

    STUDIED SITE

    MEZICALTZINGOERMITA

    12 LINE METRO

    CENA

    CUIG

    CUIP

    CUMV

    Seismological stations

    Figure 1. Project location and geotechnical zoning in Mexico City 3.1 Shear wave velocity distribution Due to the lack of shear wave velocity, Vs, field measurements, these were estimated with empirical correlations proposed by several authors. Equation 1 recommended by Seed et al. (1981) was used to estimate Vs for sands.

    60161 N = V s [1]

    Where: Vs is the shear wave velocity in m/s, N1(60) is the number of blows counts corrected by energy and overburden pressure, expressed in terms of the confining

    stress, , in Eq. 2.

    1(60) 60

    10

    'N N

    [2]

    The shear wave velocity distribution with depth for clays was estimated using the expression proposed by Ovando and Romo (1991), in terms of the tip penetration resistance, qc, measured with CPT.

    N

    q = V

    skh

    cs [3]

    Where: Vs is the shear wave velocity in m/s, qc is the

    tip cone penetration resistance in t/m2; s is the unit

    weight of the soil, in t/m3; Nkh and are parameters that

    depend on the soil type. For this study, Nkh=9.9 and

    =26.4 (Ovando and Romo, 1991).

    Column

    Beam

    2.26.5

    6.5

    (b)

    2.2

    3.2 Column

    1.1

    30 m

    Column

    Rigid

    slab

    Structural walls

    Beam

    3.2 m

    30 m

    0.60 0.60

    Structural

    walls

    Rigid slab

    Dimensions

    in meter(a)

    Dep

    th, z

    (m)

    0

    5

    10

    15

    (c)

    1.2

    01.7

    08.7

    4

    10.50

    2.5

    012.1

    0

    Figure 2.Support analyzed: (a) elevation, (b) plan view and (c) longitudinal view of the elevated section

    0 5 10 15 20 25

    Cone tip penetration resistance,

    qc (kg/cm

    2)

    0

    2

    4

    6

    8

    10

    12

    14

    16

    18

    20

    22

    24

    26

    28

    30

    32

    34

    36

    38

    40

    0 10 20 30 40 50

    Depth, z (m)

    50/20

    50/20

    50/25

    G.S.

    G. S.: Ground surface

    Fill: Sandy silt

    50/15

    Blow counts, NSPT

    50/15

    Dense to very dense silty sand

    with gravels

    Standard penetration test, SPT

    Rock core recovery

    Cone penetration test

    Water

    table

    High plasticity clay with silty

    sand intercalations

    Soft to medium clay with interbedded

    lenses of medium dense sand and

    sandy silt

    Low plasticity sandy clay with

    lenses of dense silty sand

    Very weak rock

    Low plasticity sandy silt

    0 20 40 60 80 100 % recovery

    15 m

    Figure 3. Subsurface conditions at the studied site Figure 4 presents the shear wave velocity profile estimated with expressions 1 to 3. The solid line is the representative average of Vs considered for analysis. The

  • shear wave velocity of clayey soils ranges from 40 to 120m/s and for granular materials goes from 230 and 430 m/s.

    0

    5

    10

    15

    20

    25

    30

    35

    40

    0 100 200 300 400 500

    ClaySand

    Dep

    th, z(m

    )

    Shear wave velocity, Vs (m/s)