Keywords: diaphragm wall, building movement, instrumentation.

The United States Capitol Visitor Centre (USCVC) is one of the largest underground structures ever constructed in the Washington D.C. metropolitan area. The 55,750 m² structure is immediately adjacent to (610 mm) the existing United States Capitol Building and is entirely underground, with the main support of excavation and water cut off mechanism consisting of a 24 m high, 800 mm thick, and 640 linear meter diaphragm (slurry) wall supported by 623 to 1600 kN tiebacks. Due to the proximity of this wall to the existing United States Capitol Building, a significant instrumentation program consisting of inclinometers, vibrating wire piezometers, and building monitoring points was mandated for the project.

Instrument readings were manually collected on a weekly schedule from November, 2002 through January, 2005. Data was collected and reduced, analyzed and disseminated electronically to the respective parties weekly. This electronic transmission of the data expedited the transfer of information between the design and construction teams, which in turn allowed the parties to make rapid decisions with respect to excavation depths, locations, and techniques. Construction delays associated with the review of the instrumentation data were essentially eliminated.

Overall, the maximum deflection of the diaphragm walls adjacent to the existing structure was less than 7.6 mm prior to detensioning of the tiebacks, and was about half that estimated with the numerical models. This overestimation of the movement is most likely contributed to an underestimation of the strength and stiffness of the soil, the wall, and the lateral supports used in the design models and an overestimation of the building surcharge loads. The diaphragm wall generally exhibited a 'bulging' type of behaviour, with the maximum movement near the vertical midpoint of the wall and about 6.1 m below the foundations of the existing Capitol Building. A similar wall behaviour was also identified in the preconstruction numerical design models. However, the elevation of the maximum movement estimate in the models was near the bottom of the excavation. Toe movement was also observed at most measurement locations. The magnitude of the actual toe movement was much less than that estimated through the numerical models, suggesting the actual passive pressure was not fully mobilized.

Movement of the diaphragm wall was observed after de-tensioning of the tiebacks at almost all inclinometer locations. The magnitude of this movement was approximately equal to that estimated from elastic shortening of the concrete slab. The stressing of the upper level of tiebacks pulled the wall back into the soil at most locations adjacent to the existing structure, suggesting the surcharge load estimations of the existing Capitol Building, and subsequent tieback brace loads for the diaphragm wall, may have been over approximated in the design phase of the work.

A vast array of monitoring points were installed on the exterior and interior of the existing Capitol Building to monitor the movement of the building during the excavation, construction of the building, and de-stressing of the tiebacks. The data was analyzed to evaluate any vertical movement of the columns and footings. Some indication of the excavation activity was recorded within the existing building. However, at no time during the excavation, building construction, or tieback de-stressing did the building movement exceed the Threshold Values of 12.5 mm.

Differential settlement and shear and extension within the Capitol Building were measured through angular distortion and horizontal strain comparisons between each of the survey points located within and on the exterior of the building structure. The magnitude of this movement of maximum was well below the Threshold Value set for the project (1:1250), and fell within the 'Negligible' damage categories defined in references [1] and [2].

1

M. Boscardin, E. Cording, "Building Response to Excavation Induced Settlement", Journal of Geotechnical Engineering, ASCE, USA, 115, 1, 1-21, 1989. doi:10.1061/(ASCE)0733-9410(1989)115:1(1)

2

M. Son, E. Cording, "Estimation of Building Damage Due to Excavation-Induced Ground Movements", Journal of Geotechnical Engineering, ASCE, USA, 131, 2, 162-177, 2005. doi:10.1061/(ASCE)1090-0241(2005)131:2(162)

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