During the construction of soil-shell objects, large deformations of the shell, which is made of corrugated metal sheet, occur. This enables geodetic techniques to be used when monitoring such objects. On this basis, displacements of selected points of the shell are determined, and it is then possible to obtain bending moments, as shown in this paper. Based on measurements using strain gauges in the circumferential band of the shell, internal forces in steel are estimated. The algorithm given in the paper enables the impact of soil on the shell in the examined objects to be analysed. The proposed method of analysing forces in the contact layer becomes especially useful when the static conditions of the model of a shell, which is considered as a bar submerged unilaterally in the soil medium, are met. The paper indicates the possibility of using both measuring techniques. Calculations include a smaller share of axial forces on the contact impact during the laying phase of the backfill. The paper provides examples of the analysis of built shells and record-breaking objects, with an assessment of the effectiveness of the proposed algorithm. Good mapping of contact forces, which were calculated on the basis of bending moments, was indicated even when there was not a dense grid of measuring points. An important advantage of the algorithm involves reduction of the circumferential band that is separated from the soil-shell system to the bar, which is an element resulting from the division of the structure into subsystems.

#### Keywords

- soil-steel structures
- corrugated metal sheets
- monitoring of the construction phase
- strain gauge and geodetic measurements

Soil-shell objects that are made of corrugated metal sheet are characterised, among others, by the fact that bearings are not used in them [1, 23]. Therefore, the support conditions of the shell are inherently unstable, which means that the freedom of rotation is not fully preserved (therefore, the occurrence of moments) and, at the same time, displacements are possible. In practice, the design (computational) scheme is sought, i.e. articulated and non-sliding supports in the facility under construction [1, 2]. The paper considers the construction phase of soil-shell objects when the largest internal forces and displacements are created in the shell [11, 13, 17, 33].

During the construction of soil-shell structure, shell deformations and internal forces are by nature a random phenomenon. Soil impact, which results from its different levels on both sides of the shell and the backfill compaction, is usually not symmetrical. Few-day work breaks are also important [12, 25, 26, 28]. Thus, the function of soil interaction on a shell can be diversified [3, 10, 34]. The use of a strain gauge measuring base, which is located on the shell, gives the opportunity to observe changes in the internal forces in the corrugated metal sheet of the object [6, 10], deformations [11, 24] and the interactions between the backfill and the shell [12, 13]. In order to monitor shells with large spans, strain gauges and geodetic measuring techniques are used simultaneously. In such analyses of soil-shell structures, both measuring techniques are used in order to validate computational algorithms, as is the case in the paper [4, 6, 7, 8, 9, 18, 20, 25, 26, 27, 28, 29].

A characteristic feature of soil-shell objects as a subgroup of integrated structures [15], unlike in classic bridges, is the adoption of soil backfill as the basic material of the object and also the high impact of the road surface as load-bearing elements [15, 16]. A special principle concerning the hybrid bridge model [15] is division of the structure into subsystems [11, 12, 13, 14, 16]. In the computational model of the object given in Figure 1, the shell is distinguished as the basic subsystem and backfill with layers of the road surface of the bridge structure as the second subsystem. The interaction of both subsystems takes place in the contact layer – it is included in the form of a static condition of reciprocity of soil interaction on the shell. At the same time, displacement compliance is not taken into account, i.e. slipping in the contact layer between these subsystems is permissible. This is an important advantage of the algorithm, which reflects the work principle of such constructions.

The paper presents the results of measurements on a real object, i.e. obtained in a 3D system, and also those obtained from the static scheme of the 2D circumferential band, as in Figure 2, i.e. from the computational model. The possibility of reducing the model from 3D to 2D space results from the assumption of longitudinal symmetry of the structure and also analysis of soil interactions on the shell. The author uses the possibility of changing the 3D system into the perimeter section of an object. Therefore, the main advantage of the solution proposed in the paper is the analysis of a selected (examined) fragment of the structure of an object in the form of a bar with the shape of a parabola under the influence of soil, as in Figure 2. In the 2D model, the static scheme of the circumferential band of the shell is taken into account. The results of strain gauge measurements encode the shell deformations, which are obtained from geodetic measurements (as in Fig. 2a), and also the associated internal forces calculated on the basis of results of the strain gauge measurements (as in Fig. 2b). In the paper, the effects of displacements and internal forces are treated as the result of soil interaction on the shell during the construction phase of an object.

The basic example used in the paper is an object in Ostróda [12], which is the largest such structure in Poland. The geometrical characteristics of the shell, as can be seen in Figure 4, involve its shape in the form of circle sections with a curvature radius _{n}

The author determined the internal forces in the shell using a strain gauge measuring base. On the selected circumferential band of the shell, strain gauges were glued onto the corrugated sheet surface that was accessible from the inside, as can be seen in Figure 4b. In each measuring section, the sensors were arranged in pairs – at the apex and on the corrugation valley – and located in the direction of the circumferential band of the shell, as can be seen in Figure 4b. The twin sensor system and adoption of the principle of plain cross-sections enable deformations in the inertia axis of the cross-section of the corrugated metal sheet to be determined, and therefore, also the circumferential axial forces from the following relationship to be designated:

Equation (1) contains the geometry of the corrugated metal sheet, as can be seen in Figure 4b. In order to determine the change in curvature _{D}_{g}

Figures 5 and 6 show diagrams of the internal forces in the shell, which were obtained from equations (1) and (2), when stiffness ^{2} for the width of a band _{g} determines the thickness of the backfill, were selected as in Figure 2a. During the construction phase, the backfill is laid with mechanical compaction in layers that are 30-cm thick. During technical breaks, measurements of the shell deformations were carried out using selected monitoring techniques. When _{g} =

In the case of a support zone, extreme values of axial forces appear when the ground is placed above the shell (_{g} > _{n} = 6.32 m occurs. With an increase in backfill thickness, the moment of shell fixation in the support increases significantly, as in Figure 5. Moreover, the radial direction of the pressure force

Figure 4a shows the distribution of

When two adjacent sections are considered, as is the case presented in Figure 4a, the global radius of curvature, which is given in Figures 1–3, results from the geometrical dependencies as in the relationship

Therefore, the length of the examined section, as a sector of a circle, is equal to

With a significant radius

When the chord length is

The share of components derived from both internal forces

Two selected construction phases were adopted as characteristic ones. A distinguished special situation is the one in which the backfill reaches the shell’s crown, i.e. _{g} = _{g} = 10.4 m. On comparison of these diagrams, it is observed that the components derived from bending and compression are equivalent when _{g} < _{g} >

In the section of the shell where it is free of soil influence, i.e. when _{g} < _{g} =

In order to discretise the structure in this computational model, bar elements were used, as in Figures 1–4. They enable the function of the influence of internal forces, which are convenient for the calculations in the case of loads that change their position, to be created. The effectiveness of this model was verified in [14]. The computational algorithm uses the functions of the influence of bending moments

In the plain model of the 2D circumferential band of the 2D shell, the interaction of _{k} shell is related to the influence function _{k}, as in the equation:

With the same distributed force

The paper proposes the solution of a continuous system, as in equation (10), as a transformation into a discrete system, in which the arch is divided into sections with uniformly distributed loads (as in Fig. 9a). Therefore, equation (10) is transformed into a system of equations with the form

Pressure forces, which are distributed along the sections of the circumferential band of the shell, are summarised in vector

Thus, the terms of vector

In the system of equations (12), the elements of matrix

The graphic interpretation of equation (15) is shown in Figure 9c, where the points

In Figure 9b, an example of using the function in the form of a polyline is considered. In the scheme shown in Figure 9d, two adjacent sections with a common point _{k}(s) and

While adopting the analysed quantity _{j} from these two equations, the following equation can be formed:

Therefore, the terms of matrix

A very important solution in this approach is the separation from loads that occur in equations (15) and (19). This enables a system of equations as in (12), with a vector of load _{j}, which is given in Figure 9a and b, is completely different – these are the values in the points of division into sections, and not constant values in the entire section with the central point as in Figure 9c and d. The terms of matrix

From the formulation of the system of equations (12), it is possible to solve this system (when matrix _{j} values that are included in vector _{j} values are obtained from the system of equations (12). They do not correspond to the forms of function

The system of equations (12) can be used in an inverse algorithm when the interactions _{j} from equation (13) and the bending moments
_{j} values that were obtained from the Coulomb model (for the shell considered as a rigid wall), and the process is considered complete when the terms of vector

It plays the role of a criterion for assessing the convergence of the iterative process.

The advantage of the iterative solution algorithm is that vectors

Due to this, it is possible to maintain the original number of points at which the _{j} forces are determined, as in Figure 4, and listed in vector

The narrowed calculation system (equations) can now be presented in the form

As a result of iteration, a polyline is obtained as in Figure 9b, with terms _{j}, which form vector

The effectiveness of the iterative algorithm is illustrated using the previously analysed example of the object in Ostróda [6], which has the numbering of measurement points as in Figure 3. Figure 5 compares diagrams of the bending moments that form vector _{g} determines its thickness as in Figure 2a and also when _{g} = _{g} = 10.4 m.

The diagrams show support moments and the very significant influence of the shell’s corner geometry in the form of a reduced radius of curvature (_{n} = 6.12 m). In the case when the layer above the shell was 1.3-m thick, i.e. with a backfill thickness _{g} = 10.4 m, the soil pressure on the key _{9} ≈ 9 kPa was obtained. This is a visible effect of the formation of a vault inside the soil because this value does not result from the weight of the soil above the shell, as the volumetric weight is equal to 20.4 kN/m^{3}. In literature, such a situation is known as

When the pressure _{j} is considered as in Figure 10, bending moments are obtained from equation (20) and are summarised in vector _{it} in equation (21). On this basis, the diagrams presented in Figure 11 were made. These diagrams were related to the results shown in Figure 5. The bending moments that were obtained from strain gauge measurements form two branches with a common point in the key. Good convergence of these diagrams indicates the effectiveness of the estimation that was obtained from the iterative algorithm. It is important that the diagrams were obtained by adopting large simplifications and then calculated with completely different assumptions: equation (5) and algorithm in equation (20). The compared diagrams were obtained from a simplified 2D model and from a 3D object that is being built. The computational example assumes symmetry of the shell’s deformations.

The paper intentionally omitted the analysis of tangential forces

These forces change the sign (direction of action) in the area of the crown, and in addition, are usually small during the construction phase. In the support area, and in the change in geometry in the corner of the shell, the course of these forces is complex, and they are therefore omitted in Figure 12.

When striving to obtain a greater accuracy of contact interactions, the geometry of the corrugated metal sheet must be taken into account, as in Figure 1b. Therefore, the calculated values of _{x}, in circumferential direction, and ε_{y}, as the bending effect in the perpendicular direction, are sometimes obtained. Therefore, if the shell’s geometry was a surface with a constant thickness, interpretation of the analysis results would be easier.

In the case of using the results of strain gauges, equations (1)–(5), or the iterative algorithm with the results given earlier in the paper, are used to determine the contact interactions. During the construction phase of embankments, and thus also during the laying of the backfill next to the shell, geodetic measurements are carried out as a technological requirement. From such measurements, coordinates of the observed points on the shell are obtained, as in Figure 3. After relating them to the initial phase, information on changes in the shell’s deformation is obtained. Therefore, during the construction, results are obtained in the form of vertical and horizontal components of displacements. From them, displacements are obtained in a radial direction

The measurement results from the construction of the Shumal Bridge facility in Ras Al Khajmah near Dubai are presented below [30]. This facility currently has a record span. The geometrical characteristics of its shell, as can be seen in Figure 4, involve its shape in the form of circular sections with a radius of curvature _{n} = 8.39 m. This shell has a span

Two characteristic phases of construction were considered in the paper: when the level of the backfill is close to the key with _{g} = _{g}). This technical procedure is usually carried out in the case of an inclined overburden layer [31]. This causes a disturbance of the shell’s deformation symmetry, and in particular, in the lateral part of the shell, which is visible on the

Radial displacements that are obtained from geodetic measurements can be used to calculate bending moments from differential relationship [6]

In this equation, there are constant parameters of the analysed object

There is a small effect of curvature change in the shell’s key area

Figure 14 compares the bending moment diagrams, which were calculated from equation (27) based on displacements and relationships in equation (2), with the bending moments obtained based on strain gauge measurements. Their similarity can especially be seen in the shape and ordinate values of these diagrams. Therefore, the results of geodetic measurements and the bending moments calculated on their basis can be used to estimate the soil pressure according to the iterative algorithm presented earlier.

In the case of this object, a significant number of points (as in Fig. 3), with limited accuracy of geodetic measurements (1 mm), cause additional waves of the _{n}. Unlike with the object in Ostróda, the presented example has many computational difficulties.

The lack of symmetry of the shell’s deformation is very important. It causes the necessity to create an influence function for the entire arc (and not only for half the arc, as in Fig. 8). Moreover, symmetrical soil pressure on the left and right of the shell cannot be assumed, as can be seen in Figures 7 and 10. Therefore, simplifications of the model of the object in Ostróda cannot be used. With regards to these difficulties, Figure 15 presents the diagrams of pressure that was calculated from equation (5) and obtained using strain gauges. However, despite the asymmetrical deformation of the shell, the bending moment diagrams given in Figure 14 and the pressure diagrams given in Figure 15 are similar to the symmetrical system.

The general basic advantage of the presented algorithm of the analysis of internal forces in the corrugated metal sheet in a soil-shell object is the use of division of the structure into subsystems. This enables the circumferential band of the shell, which is separated from the object, to be analysed, and also leads to a very large reduction of the structure’s model. The use of the results of strain gauge measurements in a 3D object, in combination with the influence functions of internal forces from the 2D model, gives the opportunity to assess the contact forces between subsystems. The proposed model does not take into account the displacement compatibility, i.e. slipping in the contact layer between these subsystems is permitted. This is a very important advantage of the algorithm, which reflects the principle of operation of these structures.

Good mapping of contact forces, which is shown using the examples, occurs when bending moments with a small proportion of axial forces are dominant. Therefore, the algorithm presented in the paper is adapted to monitor the construction phase, i.e. the period in which the greatest internal forces and deformations occur in the shell. The possibility of using classical geodesy devices during the construction of embankments, instead of specialist apparatus and the measurement base of strain gauges, is of significant importance. In the iterative algorithm proposed in the paper, it is possible to limit the number of measuring points without reducing the mesh of contact points. However, the algorithm is sensitive to measurement errors. Therefore, when there are measurement inaccuracies, correct results cannot be obtained.

The diagrams of internal forces and displacements, as well as the contact interactions that are presented in the paper, are characteristic for shells immersed in a soil medium. The pressure diagrams show the formation of a vault in the key area, which is called “

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