Condition Monitoring of Urban Infrastructure: Effects of Ground Movement on Adjacent Structures

Underground space is an essential element critical to the solution of many problems associated with the emerging large urban clusters around the US and worldwide. Many of these urban clusters have developed initially as smaller, relatively independent entities which have grown into heavily interdependent clusters of entities. This interdependence has wide ranging implications related to transportation. Planners increasingly are finding that underground space is one of the few options available to solve the myriad of problems posed by these urban clusters. For example, in the San Francisco Bay Area several underground construction projects are in various phases of design and construction to eliminate important transportation bottle necks (e.g. MUNI central subway, Silicon Valley Rapid Transit, Trans Bay Terminal). Similar projects are underway in Seattle and New York. Damage to buildings adjacent to excavations is a major design consideration when constructing underground facilities in congested urban areas. As new infrastructure is constructed or existing infrastructure rehabilitated, the excavations required for tunnels or 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. Similarly, tunnel operations often times include provisions, such as compensation grouting, to keep minimize the ground deformations associated with tunneling. 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. The key issue in the structural evaluation is to define the level of ground movements that will prevent or minimize damage to the adjacent structures. This depends on the type of building that is being impacted by the operations, resulting in a wide range of possible allowable movements. In many projects, the allowable movements are set arbitrarily, and without consideration of the details of either the structures to be protected or the ground conditions. In past work funded by Infrastructure Technology Institute (ITI), the projects have focused on the predictions of the ground movements. This work with real time monitoring systems at a number of excavation sites in Chicago and Seattle allowed us to develop an adaptive management approach that can be used to predict ground deformations under a variety of ground and support conditions. A key aspect of the methodology is the incorporation of the real time monitoring as a means to help guide construction activities and to allow a quantitative approach to find key soil parameters based on field performance data that result in an accurate prediction of the ground movements caused by excavation. The objectives of this proposal are to collect and evaluate detailed ground and building movement data not normally collected during excavation monitoring to allow development of rational criteria for establishing allowable ground movements associated with excavations. In particular, it is proposed to monitor the ground movements caused by the excavation for the William Jones High School in Chicago and to evaluate the effects of these deformations on two adjacent structures founded on shallow foundations. To this latter end, it is proposed to monitor the movements of the two buildings most affected by the cut. This project provides the opportunity to evaluate in detail the effects of excavation-induced ground movements on the existing buildings. This data will be supplemented with building movements caused by excavation obtained by the PI at several other excavations in the Chicago area. It is likely that the damage levels will be very slight at these buildings, as they were at the other case studies, so that conclusions can be drawn regarding the relation between the deformations at the foundation level at an impacted structure and the initiation of damage. These magnitudes can be used as a basis for setting rationale criteria regarding allowable deformations. 2.0 Excavation for the William Jones High School The proposed structure is located at the southeast corner of State and Polk Streets south of the Loop in Chicago. The proposed excavation is approximately 100 ft by 400 ft in plan, will be 18 ft deep. The excavation will be made using bottom up techniques with a temporary lateral support system consisting of a sheet pile wall supported laterally by two levels of cross-lot bracing. The soil conditions generally consist of about 14 ft of urban fill overlying a sequence of glacially-derived clays. This stratigraphy is typical of those found in the downtown area of Chicago with the important exception at this location of the presence of a very soft clay stratum that underlies the excavation. Because of this soft clay, there is a potential of ground movements that may cause damage to adjacent buildings in spite of the relatively shallow cut. There is a narrow alley that separates the excavation and three buildings, two of which are founded on shallow foundations, at this side of the cut. Access through the alley must be maintained throughout construction. These buildings are 6 and 7 stories with one basement level. Beneath State Street to the west of the site, there are an existing subway, as well as electric, gas, sewer and water lines that will be impacted by the excavation. Along the south end of the site, there is an abandoned 8-ft-diameter city water tunnel located about 60 ft below ground surface. Because of the presence of the public utilities and existing buildings, the Board of Underground of the City of Chicago has dictated that surface settlement points be established and monitored around the site to monitor the ground response close to these utilities, and that inclinometers be placed around the property line to measure lateral movements within the subsurface to evaluate the effects of the excavation on the buildings and utilities. Hayward Baker, Inc., the excavation support subcontractors and designers of this system for the project, is our partner for this project. The matching funds for this project are derived from the excavation support system for the project, the excavation costs and the conventional instrumentation installed at the site, and the effort to collect the conventional performance data. The letter of support is appended to this proposal.