seismic monitoring of structural and geotechnical

MATERIALS FORUM VOLUME 33 - 2009 Edited by Dr Steve Galea, Associate Professor Wingkong and Professor Akira Mita © Institute of Materials Engineering Australasia Ltd

SEISMIC MONITORING OF STRUCTURAL AND GEOTECHNICAL INTEGRATED SYSTEMS G. Fabbrocino, C. Laorenza, C. Rainieri, F. Santucci de Magistris Structural and Geotechnical Dynamic Laboratory StreGa, University of Molise, Termoli (Cb), Italy

ABSTRACT Several applications of Structural Health Monitoring and several techniques exists in order to assess the health state of a civil engineering construction. Bridges and buildings are the structural typologies usually monitored: therefore, currently the dynamic behaviour of a number of superstructures is extensively studied. Geotechnical aspects, instead, are less investigated: in particular, the dynamic behaviour of a flexible retaining wall under seismic load conditions is currently not fully understood. The Structural Health Monitoring system of “Casa dello Studente” at University of Molise has been designed and is currently under construction in order to obtain a deeper knowledge of the dynamic behaviour also of geotechnical structures. The above mentioned SHM system is an example of cooperation of several different skills: geotechnical and structural engineers have worked together during all phases of design and installation of the system and a large effort for a full integration of geotechnical and structural models is currently under development. As regards geotechnical aspects, data coming from the SHM system, together with centrifuge tests and numerical models, will be used to increase the knowledge about the dynamic behaviour of the soil-retaining wall system in case of earthquake. On the other hand, data coming from the building of “Casa dello Studente” can be used for classical SHM applications. Anyway, the most important aspect is related to the possibility of combining structural and geotechnical knowledge and models and apply them in different fields. In the present paper, the main aspects of an integrated SHM system at University of Molise will be described, pointing out the targets which oriented its design and implementation. A specific sensor module, developed by PCB Piezotronics Inc. under the supervision of the workgroup of University of Molise, will be described together with the phases of installation within the instrumented piles. 1

institutions can work together in order to increase performance and reliability of such systems, whose promising perspectives seem to be almost clearly stated. Informations obtained from such systems could be useful for maintenance or structural safety evaluation of existing structures, rapid evaluation of conditions of damaged structures after an earthquake, estimation of residual life of structures, repair and retrofitting of structures, maintenance, management or rehabilitation of historical structures. As reported in [3, 4], reduction of down time and improvement in reliability enhance the productivity of the structure and the results of monitoring can be used to have a deeper insight in the structural behavior which is useful for design improvement of future structures. In order to get all these objectives, an effective Structural Health Monitoring system should be based on integration of several types of sensors in a modular architecture. Moreover, the advances in the field of Information Technology and communications assure data transmission also in critical conditions. In the present paper, the main aspects of an integrated SHM system under development at University of Molise (Italy) will be described, pointing out the targets which oriented its design and implementation. In fact, it is an interesting result from cooperation of different skills (structural, geotechnical, seismological). Data coming from the system will be useful for damage assessment of monitored structures, but also to study effects of earthquakes. In particular, specific studies about soilstructure interaction will be carried out starting from

INTRODUCTION

Structural Health Monitoring for civil structures is becoming increasingly popular in Europe and worldwide also because of the opportunities that it offers in the fields of construction management and maintenance. Main advantages related to the implementation of such techniques are: reduction of inspection costs; research resulting in the possibility of better understand behavior of structures under dynamic loads; seismic protection; real or near real-time observation of the structural response and of evolution of damage; possibility to develop post-earthquake scenarios and support rescue operations. Structural Health Monitoring (SHM) is defined as the use of in-situ, non-destructive sensing and analysis of structural characteristics in order to identify if a damage has occurred, define its location and estimate its severity, evaluate its consequences on the residual life of the structure [1]. Even if SHM is a relatively new paradigm in civil engineering, the assessment of the health state of a structure by tests and measurements is a common practice, so that evaluation and inspection guidelines are available since a long time [2]. SHM objectives are consistent with this practice but it takes advantage of the new technologies in sensing, instrumentation, communication and modeling in order to integrate them into an intelligent system. Thus, Structural Health Monitoring is a very multidisciplinary field, where a number of different skills (seismology, electronic and civil engineering, computer science) and 404

detection process by taking advantage of the recent advances in information technologies [7]. In this framework, identification of the modal parameters of the structures under operational conditions plays a primary role. Recently, some strategies have been set up in order to automate identification and tracking of modal parameters [8, 9, 10, 11, 12, 13] and allowing a full integration of modal identification within SHM systems. Reliable procedures are necessary also towards data reduction and transmission, in particular after an earthquake, when a limited communication bandwidth is available: wavelet-based approaches seems to be particularly promising in this field [14, 15]. However, real-time interpretation of data can fail due their poor quality and, in particular, in case of sensors failure: therefore, in case of automated applications, this verification must be conducted by the data processing system itself. Recently, some interesting approaches have been proposed in this field [16]. The most recent and innovative applications concern of possible interaction among earthquake early warning, structural health monitoring and structural control. However, unlike traditional seismic monitoring, an event driven monitoring system is not useful: continuous condition assessment and performance-based maintenance of civil infrastructures are necessary in order to assess the short-term impact due to earthquakes and the long-term deterioration process due to physical aging and routine operation. In this framework, a monitoring system can be used for disaster and emergency management, traffic control, damage evaluation, post-earthquake scenarios definition. The use of monitoring systems on underground pipeline systems may be considered as an example of postearthquake emergency management: damaged gas utilities, in fact, can cause secondary disasters and, as a consequence, serious losses. In this case, informations about abnormal pressure changes in gas pipelines can lead to an emergency shut-off. Similar controls can affect traffic, if informations about structural integrity of

experimental results. Finally, closeness to a fault allows implementation and testing of site specific early warning strategies [5], so that integration between structural health monitoring and seismic early warning systems as a tool for seismic protection of strategic structures and infrastructures will be experienced. 2

CURRENT TRENDS IN SHM

A monitoring system consists of a variety of sensors to monitor the environment and the structural response to loads. A typical architecture of the monitoring systems is based on remote sensors wired directly to a centralized data acquisition system. However, the expensive nature of this architecture, due to high installation and maintenance costs associated with system wires [6], is causing replacement of wire-based systems with new low-cost wireless sensing units by spreading knowledge over the entire monitoring network. As a consequence, a larger effort is currently required in order to build effective data processing algorithms, in particular taking into account such a new architecture. Another relevant task is related to the strategies to be implemented to manage data and combine informations coming from a variety of sensors and, therefore, related to different physical variables. In the field of damage detection, a lot of algorithms has been proposed on the base of several different mechanical and physical principles. However, they can be classified into two main classes: a first group of techniques, the so-called “modal-based” algorithms, aims at tracking changes in structural response directly or indirectly related to the mechanical characteristics (such as natural frequencies, etc.) of the structure before and after damage. Conversely, the second approach is based on the post-processing of measurement data to detect anomalies from measurements (ARMAV modelling, wavelet decomposition, etc.). In both cases, the trend is in using methods able to automate the

Table 1. Relevant worldwide SHM systems

Country

Structure

Year 2004

N° of sensors N.A.

Seismic zone No

Canada

Pipelines

Denmark

Wind turbine

2002

N.A.

No

USA

Prestressed concrete pile

2008

8 (4 + 4)

No

USA

Golden Gate Bridge

200006

64 nodes

Yes

Wireless accelerometers

2006

8

Yes

GPS Antennas

2004

≥ 30

No

FOS, LVDT’s

2005

≤ 10

Yes

Accelerometers

2006

≤ 30

Yes

Accelerometers

China Sweden Portugal Italy

Donghai Bridge Gröndal Bridge Historical structures School of Engineering Tower

405

Sensors type

Main features

FOS FOS, MEMS accelerometers Accelerometers, Strain gauges

N.A. N.A. Embedded wireless sensors The largest wireless sensor network for SHM GPS-based SHM system Comparison FOSLVDT SHM of historical structures Automated OMA

infrastructures are available. Knowledge of still operable bridges can help decision makers to arrange a route to the disaster area for rescue personnel and goods. 3

has been designed and it is currently under implementation at StreGa Laboratory at the University of Molise. It takes advantage of different skills and it is a good chance to mix knowledge and models coming from different scientific areas but characterized by several common aspects. Data coming from the system under operational conditions will be processed and used to enhance numerical models and improve the current knowledge about flexible retaining walls. On the other hand, data recorded during seismic events are also crucial to have a deeper insight in the dynamic behaviour of such structures and in the soil-structure interaction during string motion events: in fact, these data will be useful to improve seismic design procedures for this kind of constructions. Linear and non-linear models and data processing techniques will be used to correctly interpretate the dynamic behaviour of the structure and its interaction with soil. Geotechnical and structural skills will act together to this aim. Currently, two piles belonging to the flexible retaining wall have been instrumented with embedded piezoelectric accelerometers. System design, sensor characteristics and installation phases will be described in detail in the following sections. Monitored piles have been chosen in order to avoid as much as possible boundary effects (Figure 1). The structural health monitoring system will be completed by installing a number of sensors on the building which will be constructed on the excavated side of the wall (Figure 1). Closeness between the two structures (Figure 2) suggests that a kinds of interaction: can exist. Thus, knowledge about structural behaviour can help in understanding measurement results obtained from the geotechnical sensors. In the following sections, after a review of typologies of retaining structures, some aspects related to structural and seismic design of flexible retaining walls will be discussed. They will be useful to better understand the idea at the base of design of the monitoring system and some structural changes necessary to assure that instrumented piles have the same strength and stiffness like the other close piles.

SHM SYSTEMS: A SHORT REVIEW

SHM systems have been applied to a variety of structures, such as building, bridges, pipelines [17], wind turbine blades [18]. A synthesis is reported in Table 1. SHM of bridges can provide a reduction in maintenance costs and confidence in the performance of the structure. Several applications of health monitoring to bridges are reported in the literature [19, 20, 21, 22]. The Donghai Bridge SHM system in China [21] is an interesting example of application of GPS antennas in structural monitoring: however, low sampling rates (10 Hz maximum) are currently available and, therefore, GPS is not yet suitable for a wide range of applications. In [23] a performance comparison of the Fiber Optic Sensors (FOS) and LVDT’s for SHM applications pointed out the effectiveness of FOS but also the high cost of the FOS-based monitoring system, which resulted more suitable for periodic than for continuous monitoring. Geotechnical applications of FOS are reported in [24], where such sensors have been used extensively in Geosynthetics and above all in micro piles for corrosion and damage detection purposes. However, a few applications of embedded sensors in piles are reported in the literature. Song and Zhou [25] have monitored steel reinforcement and soil stresses for static purposes. Szyniszewski et al. [26], instead, installed wireless sensors during casting of prestressed concrete piles in order to monitor stresses and accelerations during driving: however, their interest was focused only on preventing microcracking of piles during driving, thus extending life of such elements in a marine environment. Monitoring of buildings is desirable particularly in areas prone to earthquakes and strong winds, or for historical or heritage structures [27, 28, 29, 30]. In [31] an automatic data management system based on Matlab Web Server, with several buildings monitored at the same time, is described. The School of Engineering Tower SHM system in Naples is an example of Italian application in this field. It is an example of integration between structural monitoring and seismic early warning [32, 33]. 4

5

RETAINING WALLS: AN OVERVIEW

A retaining wall is any wall that retains material to maintain a change in elevation A large variety of type of soil-supporting structures are employed in civil engineering works. A short review of the main wall typologies is done here. Readers might refer for instance to [34, 35, 36] for details on this subject. The most common types of retaining walls are gravity concrete, cantilever T-type reinforced concrete, and cantilever and anchored sheet pile walls. Alternate types of retaining walls, including mechanically stabilized backfill and precast modular gravity walls, might also be employed. Counterfort and buttressed reinforced concrete walls are less commonly used.

INTEGRATED STRUCTURAL AND GEOTECHNICAL SHM SYSTEM: MOTIVATIONS AND APPROACH

Flexible retaining walls are a widespread geotechnical structure. Even if a number of design methods are already available, they have to be validated and improved. Real scale experimental data concerning soil-structure interaction, in particular in case of seismic events, cannot be easily found in technical literature. Thus, an integrated structural and geotechnical monitoring system 406

and precast modular gravity walls can be substantially more economical to construct than conventional walls [37]. However, a short life, serious consequences of failure, or high repair or replacement costs could offset a lower first cost. In addition, the design engineer must assure the overall adequacy of the design since the manufacturer of the wall may provide only that part of the design above the foundation. Embedded walls are constructed from contiguous or interlocking individual piles or diaphragm wall-panels to form a continuous structure. Embedded walls may be cantilever, anchored or propped. Cantilever walls derive their equilibrium from the lower embedded depth of the wall. They rely on the passive resistance of the soil in front of the lower part of the wall to provide stability. Anchored or propped walls derive their equilibrium partly from the embedded portion of the wall and partly from an anchorage or prop system which support the upper part of the wall. Braced sheet pile, consists of a row of vertical prestressed concrete sheet piles, backed by batter piles connected to the sheet piles by a cast-in-place horizontal concrete beam with shear connectors as required to resist the vertical component of load in the batter pile.

A gravity wall consists of mass concrete, generally without reinforcement. It is proportioned so that the resultant of the forces acting on any internal plane through the wall falls within, or close to, the kern of the section. A small tensile stress capacity is permissible for localized stresses due to extreme and temporary loading conditions. Gravity walls rely on their significant mass and geometrical dimensions for stability against sliding or overturning. Small or no contribution at all to stability is assumed to be provided by passive resistance of any soil acting on the face of the wall. A cantilever T-type reinforced concrete wall consists of a concrete stem and base slab which form an inverted T. The structural members are fully reinforced to resist applied moments and shears. The base is made as narrow as practicable, but must be wide enough to ensure that the wall does not slide, overturn, settle excessively, or exceed the bearing capacity of the foundation. The bottom of the base should be below the zone subject to freezing and thawing or other seasonal volume changes. The T-type wall is usually the most economical type of conventional wall and is widely used. Retaining walls using mechanically stabilized backfill

Figure 1. Schematic view of flexible retaining wall and of the monitored pile location 407

pile. The deflections at the head of the wall might be high. Well constructed anchor walls undergo less lateral deflection than braced walls and so provide a better control of backslope subsidence. Anchor installation only requires a small excavation to allow equipment access. However for braced wall installation there is often a requirement to excavate below the level of support. Anchored walls are always pre-stressed which essentially removes the slack from the system. The anchors will maintain their load throughout the excavation sequence unless creep occurs. The anchors also place the entire soil mass between the anchors and the wall in compression, thus creating a very large gravity wall. Propped walls may have one of more levels of prop in the upper part of the wall. They can be designed to have fixed or free earth support at the bottom and derive their stability from the props. They are common in cofferdams. For propped walls in the free earth condition the penetration of the piles should be such that the passive pressure in front of the piles will resist forward movement of the toes of the piles but will not prevent rotation. The piles are supported by ties at the top of the wall and the soil at the base of the wall. In fixed earth conditions further penetration of the pile is required to ensure that not only the passive pressures in front of the wall resist forward movement but also that the rotation of the toe is restrained by the passive pressures located near the toe at the rear of the wall. The above conditions also apply to anchored sheet pile walls. Soldier piles, also known as Berlin Walls, are constructed of wide flange steel H sections spaced about 2 - 3 m apart, driven prior to excavation. As the excavation proceeds, horizontal timber sheeting (lagging) is inserted behind the H pile flanges. The horizontal earth pressures are concentrated on the soldier piles because of their relative rigidity compared to the lagging. Soil movement and subsidence is minimised by maintaining the lagging in firm contact

This type of wall has been used for coastal flood walls. It is ideal for wet areas because no excavation or dewatering is required to construct the wall. The disadvantage is that it is more indeterminate than other wall types. Steel sheet pile walls are constructed by driving steel sheets into a slope or excavation. Their most common use is within temporary deep excavations. They are considered to be most economical where retention of higher earth pressures of soft soils is required. They have an important advantage in that they can be driven to depths below the excavation bottom and so provide a control to heaving in soft clays or piping in saturated sands. This is not possible with the soldier pile which is also a more permeable structure. However sheet piles are more costly and less adaptable to hard driving conditions particularly where boulders or irregular rock surfaces occur. Easy driving conditions are experienced in clays, sands, and clay-sand mixture due to the comparatively small displacement of soil. However they may permit large movements in weak soils and also effective de-watering is often required since they do not provide a watertight boundary. Seepage commonly occurs through the interlocks and this can be sufficient enough to cause consolidation of organic soils and soft silty clays, (compressible materials). For sandy soils ravelling will not occur if the interlocks are tight, but driving sheet piles into loose sand can cause subsidence. Cantilever sheet pile walls are mainly used for temporary excavations of moderate depth. Because of the large earth pressures and deflections that may develop they are rarely used to retain excavations greater than a depth of 5 m. However even this may be excessive where soft or loose soils occur in front of the wall. Stiffer cantilever walls, of concrete or steel including diaphragm walls and heavy composite walls, may be satisfactory to heights of 12 m providing the ground is string enough. The required penetration depth is high because the support is totally derived from the passive pressure exerted on the embedded portion of the

Figure 2. Scheme of the flexible retaining wall and of “Casa dello Studente” building foundations 408

method. The retaining wall considered in this research is an embedded wall, cantilever sheet pile type made of two set of contiguous piles disposed along two lines (Figure 1). A top beam connects all the piles.

with the soil. Bored piles are used when a soil replacement rather than a soil displacement method of piling is required and also when there is a need to minimise vibration. They are unsuitable where the ground water level on the retained side is high. The best application is for cohesive soils. The advantage of the bored pile is that only one pile need be bored at a time. Therefore when working close to a foundation only a short length of the foundation need be exposed to any risk at a given time. It is also easier to overcome ground obstructions than with sheet piling or diaphragm walls. Also bored piles are able to penetrate moderately hard bedrock materials more easily than other methods Close bored or contiguous piles are constructed in a line with a clear spacing between the piles of 75 to 100 mm. Therefore they cannot be used as water retaining structures. Their main use is in clay soils where water inflows are not a problem. However they have also been used to retain dry granular materials or fills. Where water is not a problem the spacing of the piles can be adjusted so long as the gap between piles is such as to prevent soil collapse between them. In water bearing granular soils loses are likely to occur in the gaps between the piles. This can be prevented by providing a seal between adjacent piles. Secant piles are constructed so that there is an intersection of one pile with another. The usual practice is to construct alternative piles along the line of the wall leaving a clear space of a little under the diameter of the required intermediate piles. The exact spacing is determined by the construction tolerances which can be achieved. These initially placed piles do not have to be constructed to the same depth as the intermediate piles which follow, depending on the way in which the wall has been designed and reinforced. Although the piles can be use to form a continuous watertight wall, it is dependant upon the control of tolerance for plan position and boring direction. A lack of intersection quickly makes the wall non-water tight. Finally, diaphragm walls provide a water tight barrier and are constructed with a minimum backslope subsidence. They formed from reinforced concrete and are constructed as normal cast-in-place walls with support which become part of the main structure. The slurry trench method is commonly used which involves the excavation of alternating panels along the proposed wall using bentonite slurry to prevent the sides of the excavation collapsing. Diaphragm walls can be considered to be impervious and therefore the dewatering of granular soils is often neglected. However care must be taken to ensure that there are no openings or joints since they may result in sudden loss of soil. Diaphragm walls of shallow depths are often left unsupported since they are classed as semi rigid structures. However for deeper excavations support is required to restrict lateral deflections. Diaphragm walls are ideal for soft clays and loose sands below the water table where there is a need to control lateral movements. However they are relatively costly. They are also unsuited to strong soils conditions where penetration is slow and difficult due to the use of the slurry trench

5.1 Design of retaining walls for earthquake loadings Earthquakes might cause permanent deformations of retaining structures and even failures. In some cases, these deformations originated significant damages with disastrous physical and economic consequences. For gravity walls, the dynamic earth pressures acting on the wall can be evaluated by using the Mononobe-Okabe method, while Newmark rigid sliding block scheme is suitable to predict the displacements after the shaking, as demonstrated by several experimental tests. Instead, this simplified approach is not very useful for embedded retaining walls for various reasons and then, there is room for innovative approaches in design of such structures. Here, for sake of simplicity, reference is made only to flexible walls. Readers might refer for instance to [38] for a large overview of design method of retaining walls under static and seismic actions. 5.1.1 Design of embedded retaining walls with limit equilibrium methods In this procedure, the wall is assumed rigid, the soil has a rigid-perfectly plastic behaviour and the pressures deriving form the interaction depend on the expected movements of the wall. The kinematical mechanism is affected from the constraints applied on the wall. Generally, the free embedded cantilever walls are distinguished from the anchored or multi-anchored walls. Here, only the former are considered. First, a short recall of the static methods is reported and then the seismic actions are included. 5.1.2 Static design of free embedded walls As specified before, the stability of a cantilever wall is guaranteed from the passive resistance of the soil in which the wall is embedded. In the limit equilibrium methods the wall movement that conducts to limit conditions is constituted by a rigid rotation around a point O placed near to the bottom of the wall. The theoretical earth pressures distributions on the wall are plotted in Figure 3. To eliminate stresses discontinuities in correspondence of the rotation point and to obtain a simplified shape of the pressures distributions, different simplifications and assumptions were proposed in literature. (Figure 4 and Figure 5). In a first case, the net pressure distribution is simplified by a rectilinear shape. It is assumed that the passive resistance below the dredge level is fully mobilized. The rotation point coincides with the zero net pressure point. At the bottom of the wall the soil strengths, active and passive, are mobilized and the net pressure assumes the values reported in Figure 4. A second method assumes that the net pressure 409

distribution below the point of rotation can substituted with the net force R applied at a distance z’ = 0.2d’ from the bottom of the wall.

Padfield and Mair [39] assert that reasonable values of the soil-wall friction for the calculation of the earth pressure coefficients are δA = 2/3 Φ ' and δP = 1/2 Φ '. 5.1.3 Seismic design of free embedded walls

KA γ

H

h

In the EuroCode 8 Part 5 [42] is described a simplified pseudostatic approach to analyze the safety conditions of retaining walls. The seismic increments of earth pressures may be computed with the Mononobe-Okabe M-O method. Its application for rigid structures is more prompt than for embedded walls for which the stability is mainly due to the passive resistance of the soil in the embedded portion. As for the Coulomb theory in static conditions, the M-O theory gives very high values for passive earth pressure coefficient when the soil-wall friction is considered. For this reason, the evaluation of passive pressure should be conducted assuming zero soil-wall friction. In the pseudostatic analyses, the seismic actions can be represented by a set of horizontal and vertical static forces equal to the product of the gravity forces and a seismic coefficient. For non-gravity walls, the effects of vertical acceleration can be neglected. In the absence of specific studies, the horizontal seismic coefficient kh can be taken as:

z'

d d'

KP γ

KA γ

O

KP γ

KA γ d

KP γ (h+d)

Figure 3. Earth pressures distributions assumed in limit equilibrium method

h

KA γ

d

d'

(KP - KA) γ

z'

kh =

S ag r g

(1)

[K P (h+d) - KA d] γ

where S is the soil factor that depends to the seismic zone and considering the local amplification due to the stratified subsoil and to the topographic effects, ag is the reference peak ground acceleration on type A ground, g is the gravity acceleration and the factor r is a function of the displacement that the wall can accept. For non gravity walls, the prescribed value is r = 1 [42]. Furthermore, for walls not higher than 10m, the seismic coefficient can be assumed constant along the height. The point of application of the force due to the dynamic earth pressures should be taken at mid-height of the wall, in the absence of a more detailed study taking into account of the relative stiffness, the type of movements and the relative mass of the retaining structure. Assuming that the position of the point of rotation O near to the bottom of the wall is the same of the static condition, the application of the Blum method to search the seismic limit equilibrium of a free embedded wall can be conducted adopting the loading system represented in Figure 6. The earth pressure thrusts have the following expressions:

Figure 4. Simplified earth pressures distributions: Full Method

h

KA γ

d d'

KP γ

0.2 d'

R

Figure 5. Simplified earth pressures distributions: Blum Method

The main problem for the design of embedded walls is then the right choice of the earth pressure coefficients KA and KP when the soil-wall friction δ would be considered. It is well-recognized that the Coulomb theory provides unrealistic values of the passive earth pressure coefficient when δ > Φ'/2. Different suggestions can be found in the literature [39, 40, 41]. Since knowledge on this field is limited, in the current practice is commonly adopted δA = 2/3 Φ ' for the active case and δP = 0, for the passive case. In this manner, passive resistance of soil on the dredge side of reinforced concrete walls, realized with piles or diaphragm, is largely underestimated.

1 2 K A γ (h + d') 2 1 2 ∆S AE = (K AE − K A )γ (h + d ') 2

SA =

410

(2)

1 K P γd' 2 2 1 ∆S PE = (K PE − K P )γd ' 2 2

acceleration, that is incorporated into the soil factor S, but that can be better evaluated through a site response analysis. For many structures, including embedded retaining walls, there may be reasons to question the assumption that the structure should be designed assuming a constant peak acceleration. The validity of the two assumptions (spatial and temporal invariance) will be examined separately for clarity. Figure 7 shows a M-O active wedge which interacts with a vertically propagating harmonic shear wave of frequency f and velocity VS, characterized by a wavelength λ = VS/f larger than the height of the wedge H. In this case, the variation of the acceleration along the height of the wedge is small, inertial forces (per unit mass) are about constant and the motion of each horizontal element is approximately in phase.

SP =

(3)

h

in which the earth pressure coefficients with the subscript E are referred to the seismic conditions while those without the subscript E are the static coefficients.

∆S AE

d d'

SA

∆S PE

SP R

0.2 d'

a(z,t)

H

Figure 6. Earth pressures on a free embedded wall subjected to seismic loadings according to EC8-5 pseudostatic analysis

S

The moment equilibrium of the forces around the point O provides a simple relationship for the limit depth of embedment:

d=

1.2h 3K PE − K P 3 −1 3K AE − K A

(4)

Figure 7. Mononobe-Okabe wedge interacting with harmonic wave characterized by large wavelength

a(z,t)

S

λ

H

If the seismic horizontal coefficient kh = 0 (static conditions), the seismic earth pressure coefficients are equal to the corresponding static values. The EC8-5 indications on the soil-wall friction conduct to a very conservative design of the depth of embedment, underestimating the soil passive resistance. The use of the Blum method with the seismic passive earth pressure coefficient given by the lower-bound limit method proposed by Lancellotta [43] allows establishing more reasonable depths of embedment for cantilever walls.

Figure 8. Mononobe-Okabe wedge interacting with harmonic wave characterized by small wavelength

5.1.4 The New Italian Building Code

In Figure 8 a case is depicted in which, either because VS is smaller (the soil is more deformable) or f is larger, λ is small if compared to H. In this case, at a given time t, different horizontal wedge elements are subjected to different inertial forces, and their motion is out of phase. Therefore, at each t the assumption of spatial invariance of the acceleration is no longer valid, and, at each t, the resultant inertial force on the wedge must lead to a smaller resultant force SAE than that predicted with the M-O analysis. Steedman and Zeng [46] have proposed a method for evaluating the effect of spatial variability of the inertial forces on the values of SAE, maintaining the hypothesis that the wedge is subjected to a harmonic wave.

The new Italian Building Code [44] introduced some innovations on the seismic design of embedded walls to eliminate some discrepancies existing on the application of the pseudostatic analyses for embedded walls (see for instance [45]). The pseudostatic analysis of an embedded retaining wall should be carried out assuming that the soil interacting with the wall is subjected to a value of the horizontal acceleration which is: • constant in space and time (this is implicit in a pseudostatic analysis); • equal to the peak acceleration expected at the soil surface. Deformability of the soil can produce amplification of 411

It should be clear that coefficient r in equation (1) depends on the displacements that the structure can accept with no loss of strength. That is, it may be acceptable that over a small temporal period during an earthquake the acceleration could be higher than a critical value producing limit conditions, provided that this will lead to acceptable displacements and that these displacements do not produce any strength degradation. This is equivalent to state that the behaviour of the structure should be ductile, i.e. that strength should not drop as the displacements increase.

kh = α ⋅ β ⋅

Sa g

(5)

g

where α ≤ 1 and β ≤ 1 are factors for the deformability of the soil that interacts with the wall and for the capability of the structure to accept displacements without losses of strength, respectively. Their values are reported in Figure 9 and Figure 10. The points of application of the forces due to the dynamic earth pressures can be assumed to be the same of the static earth thrusts, if the wall can accept displacements.

1.2 Ground type A 1.0 B

0.8

α

1.0 C 0.6

D

0.8

β

0.4

0.6

0.2

0.4

5

10

15

20

25

30

35

40

45

50

0.2

H (m)

Figure 9. Diagram for evaluation of deformability factor α (NTC, 2008)

0.1

0.2

0.3

us (m)

Figure 10. Diagram for evaluation of displacement factor β (NTC, 2008)

To account these aspects, in the latest Italian Building Code NTC two coefficients were introduced. In the absence of specific studies, the seismic horizontal coefficient kh can be estimated with the relationship:

Instead they should be taken at mid-height of the wall, in the absence of more detailed studies, accounting for

Figure 11. Scheme of developed sensor module (courtesy of PCB Piezotronics Inc.)

412

the relative stiffness, the type of movements and the relative mass of the retaining structure. From the short note reported above it might then concluded that even though retaining walls are well widespread, design methods need to be validated and improved, especially when dealing with flexible retaining structures. 6

inside of the enclosure. Design drawings of sensor module are shown in Figure 11. A picture of the prototype of the sensor module is, instead, shown in Figure12. A 1-1/2 NPT conduit hub, which has a gasket that seals against the outside of the enclosure, and a 11/2 NPT x 4” straight nipple have been used to connect pipes, for cable routing, to the enclosure (Figure 13). Each enclosure has been equipped with a pipe for cable routing during installation.

THE EMBEDDED SENSORS

Taking into account the previously mentioned uncertainties in flexible retaining wall design, two contiguous piles, one for each line constituting the flexible retaining wall, have been instrumented with some embedded accelerometers. The singularity of application and a number of issues directly related to sensor embedment required design of a specific enclosure for the manufacturer. As a result, a new sensor module for embedded applications has born from cooperation among technicians and scientists of University of Molise and engineers of the factory. Each sensor module consists of two seismic, high sensitivity (10 V/g) ceramic shear ICP accelerometers model 393B12 by PCB Piezotronics Inc., placed in two orthogonal directions and encapsulated in a stainless steel enclosure which assures impermeability and protection against concrete pressure. Sensor bandwidth goes from 0.15 Hz to 1 kHz, with a broadband resolution of 8µg rms. Measurement range is 0.5g pk. For its features, this sensor is suitable for application both in operational conditions and under extreme events such as earthquakes. Moreover, they have an overload limit (shock) of 5000 g: therefore, even if specific procedures for concrete casting have been adopted, through a pipe progressively raised in order to avoid direct impact of concrete against sensor enclosure, the high shock limit has been fundamental in order to assure effectiveness of sensors, which are buried in concrete and, therefore, not repairable, in operational conditions.

(a)

(b) Figure 13. Conduit hub (a) and straight nipple (b) for pipe connection to module

7

DESIGN OF INSTRUMENTED PILES

Instrumented piles had to show similar characteristics with respect to the adjacent ones, in order to assure significance to the present study and avoid singularity in the overall behaviour of the structure. For this reason, due to the not negligible dimensions of sensor modules which caused some changes in pile geometry, specific computations and additional reinforcement have been provided in order to assure that the instrumented piles had similar strength and stiffness with respect to the nominal characteristics of the adjacent piles. Three sensor modules have been placed in each pile: positions have been chosen in order to be as far as possible from the computed locations of the center of rotation in both the building and operational phases. Additional two sensors have been placed on top of each pile, into a box over the top beams which connects all piles. A schematic view of instrumented piles is shown in Figure 14. Dimensions of sensor modules have required design of an additional reinforcement to be placed around them: in fact, where a module is located, due to its dimensions, pile section can be considered no more circular but it becomes an hollow section whose

Figure 12. Prototype of embedded sensor module

Sensors in each enclosure have been encapsulated through a hard non-conductive epoxy resin in order to assure rigidity to the walls of the enclosure, which has not to suffer any damage during casting operation or for concrete pressure. It assure also waterproofing of the 413

exterior diameter is 800 mm and whose interior diameter is 300 mm, that is the size of instrumentation. The additional reinforcement has been computed so that

the resulting section has similar strength and stiffness to those ones in the rest of the retaining wall.

(b)

(a)

(c)

(d) Figure 14. Scheme of monitored piles (a); details of head of piles and sensor housing (b), (c); layout of intermediate enclosures (d) 414

stiffness of piles, moments of inertia for the circular and the hollow section have been computed and compared. The effects of the four lattices and of the additional longitudinal reinforcement have been taken into account (Table 2): an increment of 0.9% of the moment of inertia has been obtained for the circular section with lattices with respect to the typical circular section; an increment of 0.6% has been, instead, obtained for the moment of inertia of the hollow section with respect to that one of the typical section.

The additional reinforcement consists of a longitudinal reinforcement made by 8 Φ14 bars and stirrups made by Φ10 bars placed at a distance of 200 mm each other. As said before, the additional reinforcement has been placed just around sensor modules and extended at both sides for the anchorage of longitudinal bars. The additional reinforcement has been connected to the typical one by mean of four lattices of the standard type Baustrada, 8/10/6, h = 125 mm (Figure 14a-d). However, they cause a negligible variation in strength and stiffness as proved by computations. In Figure 15, in fact, the strength domains of the typical pile section without additional reinforcement, of the pile section when lattices are present, and of the hollow section are reported: the maximum strength variation has been estimated in +5% for the hollow section with respect to the typical section and in +3% for the section plus the four lattices with respect to the typical section. Such values can be assumed in the limit of dispersion of strength. A similar computation has been carried out considering the shear stress: also in this case the increase in strength for the hollow section is lower than 5% with respect to the typical section of the piles. Shear strength for the circular and the hollow sections have been computed according to [47, 48]. As regards the effects of the embedded sensors on the

Table 2. Changes in moment of inertia of instrumented piles.

Section Circular (typical) Circular + 4 lattices Hollow

Moment of inertia [cm4]

Scatter with respect to circular section [%]

2.945.584

/

2.971.369

+ 0.9%

2.962.194

+ 0.6%

It is clear, therefore, that negligible variations in terms of strength and stiffness have been produced by the installation of sensors within the pile. The additional reinforcement, the presence of the sensors and of pipes for cable routing, and, finally,

Magenta: Circular + lattices Black: Circular Green: Hollow section

Figure 15. Comparison of flexural strength of the modified sections of the pile (1 t ≈ 10 kN).

415

installation of three inclinometers (Figure 14) made concrete casting more difficult. A pipe with a diameter of 120 mm has been used for casting: it has been raised during casting operations but being careful that its end was always under the surface of concrete. The large amount of reinforcement near sensors positions and the use of a pipe for concrete casting characterized by a reduced diameter required adequate studies about concrete properties. Concrete workability and fluidity were crucial for this application: thus, a Self-Compacting Concrete (SCC) and has been designed in order to obtain Rck = 30 MPa, which was the design value of concrete strength of the adjacent piles. Adoption of a self-compacting concrete made casting possible even in these particular conditions, without segregation phenomena. 8

order to verify verticality of pile reinforcement and of lattices after their introduction in the hole and before concrete casting. This assured a proper installation of sensors, with pile axis and normal to the retaining wall surface as measurement directions. In fact, computation of deflection of the system made by the four lattices and the additional reinforcement near the enclosures during the installation phase has shown that deformations are within the elastic limit of steel and, therefore, no permanent strain was expected after raised the system. Slope measurements will be periodically carried out throughout the life of the structure for static monitoring purposes. Moreover, displacements of the head of the piles will be monitored during excavation process using topographic methods. A sample record from an embedded sensor after concrete casting is shown in Figure 18.

SENSOR INSTALLATION 9

Sensor enclosure has been connected to the additional reinforcement by mean of a steel plate welded to the longitudinal bars. Four bolts have been used to fix the enclosure over the plate. The main issue in the mounting phase was related to sensor alignment. In order to assure it with very low tolerances, connection between sensor module and plate has been obtained by mean of four slot on the enclosure and by using three stud nut and a bolt in order to fix the enclosure at each point (Figure 16). The slots allowed rotations in the measurement plane of sensor module while the bolts allowed rotation along the pile axis, translations in the measurement plane and rotation with respect to the plane orthogonal to the latter and to the pile axis. By using three straight lines as references (Figure 17a) and checking parallelism of the walls of the enclosures, a precise alignment of sensors has been obtained. Proper orientation of sensors in the hole has been obtained by tracking some reference straight lines on the top of the adjacent piles and by checking parallelism between them and measurement directions, reproduced on the top of the instrumented pile reinforcement.

CURRENT ANALYSIS CAPABILITY AND FUTURE RESEARCH DIRECTIONS

At completion, the SHM system will combine different skills and models (mainly structural and geotechnical) and data coming from the flexible retaining wall, and from the structure and foundations of “Casa dello Studente” building will be processed and used to create a database of measurements and processed data able to deeply enhance knowledge about the dynamic behaviour of structural and geotechnical systems (namely, flexible retaining walls and foundations) and about soil-structure interaction. Operability of the SHM system also in the case of extreme events such as earthquakes will be assured by adopting particular hardware solutions and redundant transmission systems. Measurements will be stored locally on a MySQL database and continuously processed in order to achieve a substantial data reduction. Processing results will be permanently stored on the database, and a remote access to such data will be assured for remote assessment of the health state of the monitored systems. Raw measurements, instead, will be periodically deleted if no meaningful events (earthquakes) occur. Currently, a number of data processing procedure are already available and extensively applied [8, 49, 50, 51], but they are mainly referred to structural dynamics. They allow a continuous automated identification of modal parameters, whose variations could be in some way related to presence of damage. However, a number of models and data processing procedures is under development and test within the research group of the Structural and Geotechnical Dynamic Lab StreGa at University of Molise. Numerical simulations of the system are under both in the static and dynamic field. When experimental data are considered, new data processing procedures can be easily implemented and integrated into the structural health monitoring system thanks to the capabilities and versatility of LabView environment [52]. Home-made software allows easy and fast integration of new data processing algorithms, or the updating of the existing ones. Moreover, the

Figure 18. Embedded sensor record

Some slope measurements have been carried out in 416

Ciro Visone who has in charge the dynamic numerical simulations of the system. Mr. Marco Santone for his work on the field together with people from StreGa Lab. A final acknowledgement to Caparelli Impianti s.r.l. that plays the difficult role of the contractor involved in a seismic monitoring research project.

monitoring system can be easily expanded thanks to the presence of the remote database which works as information collector.

CONCLUSION Several worldwide applications of Structural Health Monitoring in civil engineering are reported in the literature and several techniques exists in order to assess the health state of a structure. Bridges and buildings are the structural typologies usually monitored: therefore, currently the dynamic behaviour of superstructures is extensively studied. Geotechnical aspects, instead, are less investigated: in particular, the dynamic behaviour of a flexible retaining wall under seismic load conditions is currently not fully understood. The Structural Health Monitoring system of “Casa dello Studente” at University of Molise has been designed and is currently under construction in order to obtain a deeper knowledge of the dynamic behaviour also of geotechnical structures. It is an example of cooperation of several different skills: geotechnical and structural engineers have worked together during all phases of design and installation of the system and a large effort for a full integration of geotechnical and structural models is currently under development. As regards geotechnical aspects, data coming from the SHM system, together with centrifuge tests and numerical models, will be used to increase the knowledge about the dynamic behaviour of the soilretaining wall system in case of earthquake. On the other hand, data coming from the building of “Casa dello Studente” will be used for classical SHM applications and for studies in the field of soil-structure interaction. Anyway, the most important aspect is related to the possibility of mixing structural and geotechnical skills and models and apply them in different fields. In this paper, the main aspects of design and implementation of the integrated SHM system developed at University of Molise have been illustrated. A specific sensor module, developed by PCB Piezotronics Inc. under the supervision of the workgroup of University of Molise, has been described. Since it is embedded into the piles, a specific design of the instrumented piles has been necessary: the main ideas underlying structural design of instrumented piles, and the procedure and phases for installation of sensors within the instrumented piles have been extensively reviewed.

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