Strengthening and Crack Elimination of Historical Structures on Undermined Territory

Floods and undermining have an analogous destructive effects on building construction and decline their lifetime. Load from constrained strain of historical concrete and masonry structures. Design of the FEM model for solving interaction between foundation and subsoil. Solution of soil – structure interaction on flooded and undermined areas. Influence of groundwater level changes on settlement and tensile stresses of structures. Settlement of structure with changes of mechanically physical properties of subsoil. Arise a cracks in masonry and foundation structures. Elimination of cracks and tensile stresses by prestress and grouting. Key-Words: Undermined areas, historical masonry church, soil – structure interaction, prestressed foundation, slide joints, crack elimination 1 Deformation load Deformation load generally causes major internal forces within the building structures. Below are most frequent deformation impacts: temperature creeping and shrinking undermining flooding prestress Fluctuation of daily/yearly temperatures influences all building structures. Under normal conditions, if limit dimension of expansion units are followed, such fluctuation does not cause cracks to arise. Major impacts result from undermining and uneven subsidence caused by water-flooded underlying rock. 1.1. Undermining Impacts. When extracting a seam by means of a reasonably wide working (working face), changes also occur in geostatic/tectonic state of stress within neighbouring rock massif. The changes are accompanied with deformation and shift of rock from the overlying rock into the mined-out area. In case of long mining workings such as adits or roads, the impacts are not in fact evident beginning from the low depth thanks to arch action of the rock. In case of space mining workings, a subsidence through will appear after a certain time, depending on the excavation depth, geological structure of the overlying rock, seam thickness, and excavation method. The depth and layout of the subsidence trough depend mostly on the depth (h) and thickness (m) of the seam to be extracted, and limit angle () for surface extraction. In the Ostrava Karviná coal basin, the limit angle is about 65 degrees. The volume of the subsidence trough depends also on the extraction method which is expressed by the extraction factor (a), being 0.8 to 0.9 if collapse extraction is used. The subsidence through depth (s) is greater with greater seam thickness and lower seam depth under the surface. The subsidence trough consists of an internal quiet part, and boundary parts. The depth of the internal quiet part is almost identical as the subsidence depth (s), while the boundary parts are of vital importance for the proposed protective measures taken for the ground building. In order to describe the landscape deformation intensity in the subsidence trough boundary parts, the mining industry uses following geometrical quantities s subsidence [mm], v horizontal shift [mm], i inclination [rad], R radius of bending [km]  horizontal relative deformation [-]. Latest Trends in Sustainable and Green Development ISBN: 978-1-61804-132-6 125 The inclination (i) and horizontal shift (v) have peaks in the subsidence line inflection point (s) above the working face edge. The horizontal relative deformation () and the landscape bending ( = 1/Rreach the maximum at about + 0,4 r from the working face edge. Most dangerous for the ground buildings are the horizontal relative landscape deformations (They are positive, if above the subsidence line inflection point (landscape elongation), or negative, if under the subsidence line inflection point (landscape compression). Table given in ČSN 73 0039 classifies the sites among groups I. to V., considering the expected landscape deformation intensity caused by the underground mining. In practical calculations, the landscape relative deformation ranges between 3 3 10 . 7 10 . 1 − − ≤ ≤ bu ε Taking into account the current extraction intensity in the Czech Republic, typical deformation values are about the down limit, corresponding thus to the sites of group IV. or V. Figure 1. Subsidence Contours. 1.2. Flooding Impacts. Major and destructive floods in 1997 and 2002 resulted in many static faults in buildings throughout the Czech Republic. Some structures were directly destroyed by water flow or drifted objects, while many other buildings were damaged by uneven subsidence after a rather long time following the floods. Most frequent reasons for the subsidence include long-term changes in the groundwater table and changes in mechanical/physical properties of the cohesive soil. In case of the non-cohesion soil which makes up generally good foundation soil, fine grain particles erode, as the groundwater flow is higher. The consequences include occurrence of cavities and/or loss of contact between the foundation and underlying rock. The uneven subsidence results in stress changes in the foundation and the above ground buildings. Such changes can exceed the ultimate tensile strength of masonry, especially in old and historic buildings. As more and more subsidence occurred in the floodaffected buildings, the faults have been similar to those typical of the buildings which are located in undermined areas. It is however very problematic to forecast such subsidence, as the floods are of a random character. Figure 2. Typical Damage to Undermining-Affected Wall.