Reliable Tilt of Objects Subjected to Rectification and Located in Mining Areas

In Silesia, where tests described in this paper were carried out, hard coal is mining. As a result, the post-mining void is formed, which causes ground deformation in the shape of a subsidance through, additionally some discontinuous deformations [1, 2] and also ground vibrations [3]. The subsidance through is described by indicators of deformation of the mining area [4, 5, 6], which include not only depression but also horizontal deformations, land surface curvature and a change in the land slope (T_gór). As a result of T_gór, deflections T_bud of civil structures on the surface are observed. Deflection T_bud causes troublesome use of buildings [7], underestimates their value, and in extreme situations leads to exceeding limit states. Deflection of a building is usually considered as a process altering technical properties of buildings, including dynamic properties [8, 9] and reducing the comfort of their use [10, 11]. Deflection is often regarded as load while analysing serviceability limit states [7, 12] and ultimate limit states [13, 14, 15], the additional stress-strain measure of floor [16] is also taken into account. For the latest case combinations of load, including deflection as permanent load and seismic load as variable are particularly important [17, 18]. Under specific conditions, the deflection of buildings is eliminated (rectification process).

The most common method of rectification consists of elevating buildings by means of jacks built into their walls. In this method, the building is divided into two parts: one which is non-uniformly elevated and the other one remains on the ground and acts as a support for jacks. The value of deflection T_bud is required for the non-uniform elevation of buildings. This value is used to determine values u_obj of elevating the building corners. This paper reviews the methods of determining T_bud for buildings of different sizes: single-storey buildings, single-storey buildings with a usable attic, two-storey buildings, and 11-storey buildings. Moreover, specific situations were analysed, in which the elevation height was determined by a range of additional works performed during rectification. This paper describes analyses performed for eight buildings. Four of them have not been described in the literature, and data for the analysis of four buildings was taken from the reference literature.

The elevation height of single-storey one-piece buildings was analysed with reference to a wooden structure of a church (Fig. 1a). The building was built in 1737 in a cruciform layout, into which a rectangular with dimensions of 29.94 m × 17.43 m [19] could be inscribed. There was one nave extended by a three-sided chancel, a two-storey choir, and a bell tower. The vestry and the chapel are adhered to the chancel. The building is 12.65 m high from the ground level to the roof ridge (Fig. 1). Walls of the building were made of timber beams in the log wall construction, joined at corners by dovetailing. The building, which had already been repaired many times and translocated, is placed on the reinforced concrete slab and reinforced concrete ribs.

Figure 1.

Discontinuous deformation, in the form of linear level of ground variation, runs through the building which has affected its deformation. Measured differences in the floor indicated that its greatest slopes were near the chancel and were up to 35 mm/m. The floor near the tower was in a horizontal position. Wall verticality was measured at four levels from 0.5 m to 5.5 m above the surface area. The measurements indicated that deflections of walls considerably varied at the maximum deflection of the chancel up to 40 mm/m to the north. The average deflection of walls equal to 12 mm/m was regarded as a reliable deflection. Taking the average deflection as the reliable one was the consequence of accepting and conducting the technique of rectification and repair of the building. This technique was based on the nonuniform elevation of the timber superstructure and the performance of the new floor finish in a horizontal arrangement. Hence, the slope of the floor finish was not reliable for determining the elevation height as its slope was eliminated by performing a new floor finish. The elevation height determined from the wall deflection ranged from zero near the building entrance to 337 mm near the vestry. The building under rectification is shown in Fig. 1c, and after a removed deflection in Fig. 1d.

For single-storey buildings with a usable attic, the analysis is conducted for a residential building with a car garage within a dense plan, which can be fitted into the rectangle with a side length of 21.15 m × 12.89 m (Fig. 2). The building height is 7.9 m from the ground level to the roof ridge. The main entrance and the entrance to the garage are on its western side, and a patio and stairs to the garden are on the southern side of the building.

Figure 2.

The building foundation is made from reinforced concrete footing with a T-section having a height of 1.3 m, web width of ca. 0.35 m and variable width of the base. Exterior load-bearing walls are masonry walls made of hollow brick units, which were then insulated outside with a layer of foamed polystyrene and plastered. Their total thickness is 0.45 m. Internal load-bearing walls have a thickness from 0.25 m to 0.40 m and are made of hollow brick units. A wall separating the kitchen from the dining room is made from reinforced concrete. The thickness of the partition wall is 0.12 m. Reinforced concrete cores run through corners of load-bearing walls. The floor above the ground floor is cast-in-situ made of reinforced concrete with a thickness of 0.16 m and supported on walls through reinforced concrete ring beams. The roof structure is a timber system covered with roof tiles.

Independent measurements were taken for the position of individual elements with respect to the vertical plane. These elements included: corners of external and internal walls (measured deflections) and flooring on the ground floor and first floor (measured inclinations). They indicated that values of deflection (and inclination) of individual elements in the building significantly differed. The greatest mean deflection was found for the exterior walls. The resultant of their deflection, determined as the average of seven points from two directions was 26 mm/m. The smallest mean value of the resultant inclination, equal to 16 mm/m (the inclination components were 14 mm/m and 7 mm/m) was typical for the flooring in the usable attic. The resultant inclination of the patio flooring was 25 mm/m.

Differences in measured values of wall deflection and flooring inclination were mainly caused by performing these elements at different times while mining exploitation was in progress. Shear strains of the building did not have such a significant impact. The obtained results demonstrated that at first the building elevations were performed (it was the most deflected element – under the longest exposure to mining exploitation), and the floor in the first floor was the final element.

By removing the deflection, the base of the raised part was regarded as a plane. Thus, it required one reliable value of deflection. Floor inclination on the ground floor could not be considered as reliable value because the applied technology of rectification required new floors on the ground floor. Similarly, internal layers of exterior and internal walls required restoration. The floor in the usable attic was an element, which was not restored during rectification. Hence, its deflection was taken as the reliable value of deflection. This value was used to determine the elevation heights of the building corners, which were equal from u_obj = 48 mm to u_obj = 315 mm.

The building during rectification, which involved 40 jacks, is shown in Figure 3. The tearing plane of the building was 0.63 m above the level of the ground floor.

Figure 3.

The conducted tests on many two-storey buildings with basements and small size of floor plans showed that the deflection of walls and inclination of floors had the same values, directions, and senses. The building shown in Figure 4 is such an example. It has a rectangular floor plan with sides of 10.29 m and 9.55 m in length, and a height is 8.85 m from the floor in the basement to the roof ridge. The basement walls are made of concrete blocks. Their thickness on external walls is 0.40 m, and in internal walls is 0.28 m. The internal walls in the ground and first floor have a width of 0.24 m and are made of breeze blocks. The external walls on these floors are layered masonry walls with a thickness of 0.43 m. The internal layer is composed of hollow masonry units, and the external one is made of brick. All slabs are monolithic, made of reinforced concrete with a thickness of 0.14 m, supported on ring beams with section of 0.25 m /0.25 m. The roof slab is not ventilated, ans insulated with a layer of slag. Layers of asphalt on concrete levelling make the roof cover. The building was deflected to the northern east, and the resultant deflection was 95.2 mm/m. It was rectified with 31 jacks built into the basement walls, and the tearing plane was 1.15 m above the level of the basement floor. The north-eastern corner was raised the most, by 1312 mm (Fig. 5). Steel guides were installed inside to ensure the stability of the building during rectification. Moreover, the non-uniformly elevated south-eastern corner of the elevated part continuously affected the corner of the part left on the ground. In this way, friction forces counteracted relative displacements of the part to the horizontal direction.

Figure 4.

Figure 5.

For multi-storey buildings testing the reliable deflection and determining the elevation height were conducted for an 11-storey residential building with a basement, whose floor plan can be fitted into the rectangle with a side length of 19.7 m and 17.3 m (Fig. 6a). The building foundation is made from reinforced concrete footing with a thickness of 0.40 m and various widths of: 1.00 m below the external walls, 1.2 m below the internal walls, and 2.55 m below the system of two load-bearing internal walls (walls along axes 3, 4 and walls along axes 5, 6). The basement has monolithic load-bearing walls made of reinforced concrete with a thickness of 0.30 m for external walls and from 0.15 m to 0.2 m for internal walls. External load-bearing walls in floors over ground level had a thickness of 0.3 m, and the thickness of internal walls was 0.15 m. They were built in 1975 in the slipform construction technique from concrete with pumice powder used in Silesia at that time. All corners of internal walls had core reinforcement connected with reinforcement of ring beams and header beams in external walls. All floors were in the form of continuous reinforced concrete slabs with a thickness of 0.12 m and with two-way reinforcement. Cantilevered slabs extending from ring beams of external walls form balconies. Walls in an elevator shaft are made of reinforced concrete, have a thickness of 0.15 m, and were performed in the slipform construction technique. Stair runs are made of reinforced concrete slabs, spanned between beams hidden in the step width. The roof structure is made of breeze hollow core slabs placed on openwork walls with a slope into the building. Asphalt layers on roof cement were used as the roof cover.

The building was vertically deflected. Hence, deflections of external walls, deflections of the elevation core, and inclinations of floors in some flats in the building were measured at the levels corresponding to consecutive floors. The results of measured deflections of floors, and inclinations of wall edges indicated considerable differences between particular storeys and flats. It was caused by the fact that some floor slabs in flats were levelled while the building was deflected. And deflection of external edges of the building was different at particular storeys, which mainly resulted from the applied technology of their construction and effects disclosed during construction. The best conformity was found for measured deflections of the elevation core. Therefore, the deflection of this element was measured using an optical plummet by the technique of offset from the measured section to the core walls. Deflection was measured from each core wall with simultaneous measurement of the level height, at which the measurement was taken. The measurements were taken at six levels, in planes parallel to the horizontal axes of the core. Results from the independent measurements of the core deflection in two elevator shafts are illustrated in detail in Figure 6a. The average deflection of the core was 22.3 mm/m to the longer side of the building and 10.3 mm/m to the shorter side of the building. The obtained results demonstrate that the structure of the elevator shaft was performed in the most precise way and did not undergo any alterations during the building use. Thus, the deflection of this element was considered to be reliable for determining the values required for rectification of the building. In that way, the value of non-uniform elevation of the building was determined. The southern corner of the building was elevated to the highest value u_obj = 592. The building during rectification performed with 89 jacks is shown in Figure 7.

Figure 6.

Figure 7.

Designing of elevation height u_obj of single-block buildings often requires the location of adjacent buildings. This issue was illustrated using an example of the complex of two structurally independent buildings marked as A and B, located within a distance of 3.2 m from each other (Fig. 8). The building A has a basement, two floors over ground level and a rectangular plan with a length of sides equal to 19.7 m and 12.6 m. The foundation is made from reinforced concrete footing. The basement walls are made of concrete, while the external walls have a brick loading coat. The above-grade walls are masonry walls made of brick and hollow masonry units, with a thickness from 0.42 m to 0.52 m. Floors and stairs are made of reinforced concrete, supported on walls and beams. The entrance is on the eastern side of the building, and what is important, it runs through the patio. The reliable resultant of the building deflection to the east was determined as the average of deflection of its corners in two directions and was equal to 37.2 mm/m.

Figure 8.

The building B has a rectangular floor plan with dimensions of 18.6 m × 14.1 m, and the reliable resultant of deflection was 26.7 mm/m. The building has the same structural solutions as the building A but has two entrances. The main entrance is on the southern part of the building, and, what is important for the analysis, runs through the patio. Ten steps lead to the patio which functionally connects the buildings A and B.

As the reliable deflections of the buildings A and B differed, they required independent designs to eliminate deflection for each of them. As presented above, the main entrances into both the building A and B are on the side of the patio. As a result of rectification, the entrance into the building A should be at the same level as into the building B. Therefore, the following procedure was applied for determining u_obj. At first, the rectification of the block A was designed by elevating its corners. Values u_obj, resulting from the reliable deflection, corresponding to the three mentioned corners were 17 mm, 463 mm, and 690 mm. None value u_obj of any displacement of the corner was equal to zero, which meant that the horizontal axis of the building rotation was beyond the floor plan. Such an assumed position of the axis was necessary to ensure that a rectified part of the building and the part remaining on the ground did not get caught during a non-uniform elevation.

While determining the height of elevating corners of the building B, two demands had to be satisfied: eliminate deflection of the building and ensure the position of entrances to buildings A and B at one level. Hence, the following method of determining the height of non-uniform elevation of the building B was applied. At first, the height was determined, to which the sill of the entrance door to the building B should be elevated. That value resulted from the reliable deflection of the building A and was equal to 910 mm.

Then, the height was determined, by which the vertical position of that point could be changed in case of eliminating deflection of only the building B. That value was determined as 380 mm. The difference 910–380 was a value required for the uniform elevation of the building B. Finally, the values u_obj for individual corners of the building B were a sum of values corresponding to the non-uniform elevation of the building and its uniform elevation by 380 mm. The values u_obj of elevating three marked corners of the building B were: 543 mm, 965 mm, and 1164 mm (Fig. 8).

Elimination of deflection of the complex of buildings in accordance with the above method would require considerably high values of elevating the corners of the building B. In the final phase of elevating, the southeastern corner of that building was positioned higher by o u_obj = 1164 mm. The photo in Figure 9b presents this situation. It shows three temporary supports used to eliminate deflection, which were fixed near the southeastern corner of the building B. In the final phase of rectification, each of these supports is composed of a stack of 14 steel elements with a height of 72.5 mm and a jack extended to 128 mm in this phase. The elevation height was equal to the sum of a stack of elements and the last extension of the jack (14·74 + 128 = 1164 mm), whereas the total length of the support was enlarged by the height of the jack and was equal to 1754 mm.

Figure 9.

If rectification is accompanied by repair works conducted for the building part on the ground, or the works resulting from the employed technology of repair works that are to be done after rectification, then the elevation height u_obj has to be corrected. These situations were described with reference to three buildings.

The first building is a historic building from 1902, which now serves as a museum [20]. This building with a partial basement has two floors over the ground level, with a timber structure, and a rectangular floor plan with a length of sides equal to 10.3 m and 10.7 m (Fig. 10a). The deflection to the south was 29 mm/m, and to the west was 10 mm/m. The stone wall base and stone foundation of that building (Fig. 10b) were in very poor condition and required replacement. Therefore, repair works were designed and performed. They consisted of rectification of the building part over ground level and additional uniform elevation by 400 mm for the time of foundation replacement. The maximum height u_obj resulting from the non-uniform elevation was 357 mm. But due to work required to be conducted under the building the height was increased to 757 mm. The works were performed in the following sequence. After rectification and uniform elevation of the building by an additional 400 mm, its weight was transmitted to the new system of supports, which transmitted load directly to the ground beyond the foundation area (Fig. 10c), and jacks were removed from the wall axes. Then, the old stone foundation in very poor condition was removed. They were replaced with new reinforced concrete footing and foundation walls made of concrete blocks. When these works were completed, the elevated part was uniformly lowered by 400 mm and placed on new foundation walls and footing, which was its new permanent position.

Figure 10.

A similar procedure was conducted for a historic bell tower from 1520, which was deflected and in very poor condition [21]. The construction was 21.47 m high and had a rectangular floor plan of 7.80 m× 7.86 m. It was a timber frame structure with four main corner posts, angle braces in two-way direction and horizontal members at five levels (Fig. 11a). Before repair works, the corner posts with 340/340 mm section and braces (section from 170/170 mm to 200/200 mm) were fixed in the footing beam of 350/350 mm section placed at the level of the floor. The whole construction was based on a stone foundation made of irregular boulders with a diameter of up to 1.2 m placed to a depth of 1.5 m below the ground level. There are four bells in the tower. The oldest one named as “Słowo Pańskie” [Word of the Lord] with a weight of 200 kg dates from 1536. Bells robbed during the war in 1942 were replaced with three new bronze bells: “św. Jan Chrzciciel” [St. John the Baptist] with a weight of 600 kg, “Matka Boska Częstochowska” [The Holy Mary of Częstochowa] with a weight of 400 kg, and “św. Józef” [St. Joseph] with a weight of 500 kg.

Figure 11.

The building had many defects before repair works began. The most serious defect was very intensive corrosion of footing beams, corner posts, and angle braces to the height of 0.5 m above the level of the footing beam. Moreover, due to the non-uniform depression of the ground below the tower bell, the stone foundation went apart and did not provide the stable support for the construction. The value and direction of its deflection were determined from the measured deflection of walls at the level of +1.0 m. The average deflection to the south was 51 mm/m, and to the west was 10 mm/m (Fig. 11b). If works were only limited to the rectification of the tower bell, then the maximum value u_obj would be 479 mm.

But rectification was also accompanied by repair works of the tower, during which foundation piles under the construction and reinforced concrete cap beam supported on piles were performed (Fig. 11c), and all footing beams and fragments of corroded posts and braces were replaced. Due to technological reasons, the tower bell was elevated by an additional 63 mm. The maximum height, to which the southeastern corner was elevated was 542 mm. The elevation height u_obj of other corners is presented in Fig 11b. When the tower was in its permanent position, it was restored, particularly formworks were replaced (Fig. 12b).

Figure 12.

When the construction is elevated also with a foundation, jacks are supported on stacks of elements pressed into the ground, which are prepared specifically for this purpose. Due to formal reasons, these stacks are a part of the building remaining in the ground. Then, the height u_obj has to be enlarged to fill entirely with concrete the void between the elevated part and the ground (with stacks in the ground). Here we can refer to an example of determining the elevation height for a construction, other than a building – reinforced concrete slabs, on which the water tanks were based [22].

The steel tank has a cylindrical shape with an inner diameter of 12.221 m and a height of 8.520 m (Fig. 13 a,b). Its shell is made of a 3 mm thick metal sheet and reinforced with corner brackets placed at four levels. The roof framing is composed of steel bars made of cold-formed steel supported on the reinforced edge of the shell and two columns placed inside the tank. A three-layer panel with a polyurethane core is used as the roofing material. Inside the tank, there are water supply systems and components of the fire-fighting system used to pump out water. Tightness is ensured by PVC film of 1.5 mm in thickness, which is inside the tank.

Figure 13.

The reinforced concrete foundation slab, on which the tank is based, has an octagonal shape and a side length of 5.413 m. The slab has a thickness of 300 mm. However, the thickness changes gently at the edges up to 600 mm and the reinforced concrete beam formed around the slab has a (b/h) section of 800 mm /600 mm. A layer of 70 mm thick lean concrete is placed under the slab. The 2-way top and bottom reinforcement of the slab is composed of rebars with a diameter of 12 mm and a spacing of 150 mm. The bottom reinforcement of the beam around the slab contains six rebars having a diameter of 16 mm and stirrups made of bars with a diameter of 8 mm and a spacing of 250 mm. The weight of an empty tank with the foundation slab is 1347 kN.

The tank was deflected by 17.8 mm/m in the southeast direction. Consequently, it required the rectification. For that purpose 16 stacks of concrete blocks were pressed into the ground under reinforced concrete beams which were part of the foundation Then, 16 hydraulic jacks were placed on these supports. These jacks took the weight of the tank. At first, they were used to elevate uniformly the tank and the foundation by 200 mm, which was required to effectively fill with concrete the void between the elevated part and the ground. Then, the rectification began with non-uniform elevation. It means the whole tank was rotated around the axis 1. The minimum elevation u_obj was 200 mm, and the maximum was 431 mm. The space between the slab base (Fig. 13 c) and the ground formed as the result of rectification was filled with concrete.

On the basis of the reviewed literature and own research, the following conclusions were drawn. Wall deflection was the reliable value for single-storey buildings. For single-storey buildings with usable attic, deflection of the attic floor was considered as the reliable measure. The deflection of elevator shafts from vertical should be taken into account when the elevation height of multi-storey buildings is determined. If such buildings are not equipped with elevators, deflection of the non-deformed structural elements from vertical or horizontal is regarded as appropriate. For buildings with wall structures and stiff floors, a change in floor inclination is equal to changes in wall deflection.

Two situations should be differentiated for the complex of buildings. If each segment is an independent construction, particularly without any passage between the segments, then the reliable deflection of each segment should be determined separately from the deflection of a single-segment building. If the buildings are connected through passages, rectification should not lead to mutual vertical displacement of functionally connected buildings. The elevation height of rectified buildings also depends on the applied technologies of works. If such works include building a new foundation or filling the void under the foundation, then the rectified buildings have to be elevated by an additional value to perform the intended works.