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Evaluating Excavation Support Systems to Protect Adjacent Structures
| Content Provider | Semantic Scholar |
|---|---|
| Author | Finno, Richard J. |
| Copyright Year | 2010 |
| Abstract | This paper presents an overview of methods that can be used to predict damage to buildings as a result of excavation-induced ground movements and describes an adaptive management approach for predicting, monitoring, and controlling excavation-induced ground movements. Successful updating of performance predictions depends equally on reasonable numerical simulations of performance, the type of monitoring data used as observations, and the optimization techniques used to minimize the difference between predictions and observed performance. This paper summarizes each of these factors and emphasizes their interdependence. Applications of these techniques from case studies are presented to illustrate the capabilities of this approach. Examples are given to show how optimized parameter based on data obtained at early stages of excavation can be used to predict performance at latter stages, and how these optimized parameters can be applied to other excavations in similar geologic conditions. INTRODUCTION Damage to buildings adjacent to excavations can be a major design consideration when constructing facilities in congested urban areas. As new buildings are constructed, the excavations required for basements affect nearby existing buildings, especially those founded on shallow foundations. Often excavation support system design must prevent any damage to adjacent structures or balance the cost of a stiffer support system with the cost of repairing damage to the affected structures. In either case, it is necessary to predict the ground movements that will induce damage to a structure. Practically speaking, a designer is attempting to limit/prevent damage to either the architectural details of a building, which occurs prior to structural damage, or to load bearing walls. To evaluate damage potential in buildings affected by ground movements resulting from deep excavations, one must first predict the magnitude and distribution of ground movements caused by the excavation. This may be done using empirical or finite element methods, depending on the importance of the building, budget considerations, and design phase of the investigation. After locating the affected building in relation to the expected ground movements, one then evaluates the impact of these movements on the building. The main two sources of uncertainties in this analysis are the structural evaluation of the affected building and the movement prediction. This paper summarizes damage evaluation methods and describes an adaptive management approach for predicting, monitoring and controlling ground movements. This approach can be thought of as an “automated” observational approach (Peck 1969). This methodology is a useful design tool in that decisions regarding trigger levels and responses can be thoroughly evaluated during design. Proceedings CIGMAT-2010 Conference & Exhibition 2 CRITERIA TO EVALUATE EXCAVATION-INDUCED DAMAGE Selected criteria that are applicable to evaluate excavation-induced damage are summarized in Table 1, wherein the relevant parameter and its limiting value are shown. Note that the parameter used to relate structural movements at the foundation level to damage depends on the method. Deep beam methods are more general than empirical methods (e.g., Skempton and McDonald 1956) which are applicable to damage of structures based on settlements arising from the weight of the structure. Table 1. Selected damage criteria for excavation-induced damage to buildings Reference Type of method Limiting parameter Applicability Burland and Wroth (1975) Deep beam model of building Δ /(L εcrit) Load bearing wall (E/G = 2.6), framed structures (E/G = 12.5), and masonry building (E/G = 0.5) with no lateral strain Boscardin and Cording (1989) Extended deep beam model β, εh L/H = 1 and assumption horizontal ground and building strains are equal Son and Cording (2005) Semiempirical Average strain Masonry structures; need relative soil/structure stiffness; use average strain in distorting part of structure Finno et al (2005) Laminate beam model Δ /(L εcrit) Load bearing walls, framed structures, masonry buildings, need bending and shear stiffness of components of walls and floors Boone (1996) Detailed analysis of structure crack width general procedure that considers bending and shear stiffness of building sections, distribution of ground movements, slip between foundation and grade and building configuration The following terms are related to the limiting parameters in Table 1, and are illustrated in Figure 1. Differential settlement between two points, i and j, is δij. The distance between two points i and j is lij. Distortion between two points, i and j, is defined as δij/lij. A concave-up deformation is commonly called “sagging,” while a concave-down deformation is termed “hogging.” An inflection point separates two modes of deformation. The length of a particular mode of deformation, bounded by either the ends of a building or inflection points of the settlement profile, is L. The average slope, m, of a specific mode of deformation is defined as δkl/Lkl, where the subscripts k and l are boundaries of the mode of deformation. This slope differs from the distortion, δij/lij, which is the ratio for two adjacent points. The relative settlement of each mode, Δ, is the maximum deviation from the average slope of a particular deformation mode. The deflection ratio, Δ/L, is the ratio of the relative settlement to the length of the deflected part. Rigid body rotation of the building, ω, is the tilt of the building and causes no stresses or strains in the building. Angular distortion, βij, is the difference between distortion, δij/lij, and rigid body rotation, ω. Proceedings CIGMAT-2010 Conference & Exhibition 3 Figure 1. Quantities used to define limiting parameters for damage criteria The critical tensile strain, εcrit., is that at which cracking becomes evident. Tensile strains, εt, can be caused by bending, εb, diagonal tension due to shear, εd , or horizontal extension, εh, caused by lateral extension of the building due to lateral movement in the soil mass below the footings. Critical strains that cause failure in common building materials vary widely as a function of material and mode of deformation (Boone 1996). Burland and Wroth (1975) modeled a building as a deep isotropic beam to relate strains in the building to the imposed deformations, as illustrated in Figure 2. They suggested that for the sagging type deformations shown in the figure, the neutral axis is located at the middle of the beam. For hogging type deformations, they assumed the foundation and soil provide significant restraint to deformations, effectively moving the neutral axis to its bottom. They presented equations for limiting Δ/L in terms of maximum bending strain and maximum diagonal tensile strain for a linear elastic beam with a Poisson‟s ratio, ν, of 0.3 (implying a Young‟s modulus/shear modulus ratio, E/G, of 2.6) subjected to a point load with the neutral axis at either the center or bottom of the beam. A building not adequately represented by an isotropic elastic beam is characterized by different E/G ratios. They postulated that for buildings with significant tensile restraint, or very flexible in shear (i.e. frame buildings), an E/G ratio of 12.5 would be appropriate. However, for buildings that have little or no tensile restraint (i.e. traditional masonry buildings), they recommended that the E/G ratio should be 0.5. Figure 2. Deep beam idealization of building (after Burland and Wroth 1975) Proceedings CIGMAT-2010 Conference & Exhibition 4 Boscardin and Cording (1989) extended this deep beam model to include horizontal extension strains, εh, caused by lateral ground movements. They presented a chart relating β and εh to levels of damage for buildings with brick, load-bearing walls and an L/H ratio of 1 undergoing a hogging deformation with the neutral axis at the bottom. Similar to Burland and Wroth (1975), the building is idealized as a linear elastic beam with υ equal to 0.3. Direct transfer of horizontal ground strain to the structure is assumed in this approach, which may or may not be reasonable depending on the structure. For example, modern frame structures with floors that act as diaphragms do not move laterally with the ground (e.g. Geddes 1977,1991; Finno et al. 2002) for deformations normally associated with adjacent excavations. Son and Cording (2005) extended the Boscardin and Cording approach in a semiempirical manner. Resulting criteria are applicable to masonry buildings. They proposed use of a damage criterion based on the average state of strain within the distorting portion of a building. Their revised criterion is independent of E/G, L/H and the position of the neutral axis of the wall. They explicitly considered the shear stiffness of the walls on the distortions imposed by the ground settlements. They used model test and results of numerical simulations as well as case studies of building damage and distortion to calibrate the model. They noted that cracking in masonry walls significantly reduced effective wall stiffness. There is considerable overlap in categories of damage as a function of their parameters. Finno et al. (2005) extended the Burland and Wroth (1975) equations to allow explicit input of E/G and location of the neutral axis, resulting in equations that relate limiting Δ/L to bending strain at the top, εb(top), and bottom of a beam, εb(bottom), and the maximum diagonal tensile strain, εd(average). Figure 3 shows the effects of different E/G ratios on the conditions required for initial cracking. The kink in a curve represents the limit between shear critical and bending critical geometries of a beam. These results show that the limiting deflection ratio that causes cracks varies over wide limits, implying that structural details of a building must be considered when establishing criteria. However, it is difficult to select the beam charac |
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| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
