The Consequences of Non-Uniform Founding of Concrete Tank in Weak Wet Subsoil

Concrete liquid tanks seem to be rigid, heavy and insensitive to differential settlement. The practice shows, however, that tanks founded in the layered subsoil may be subjected to differential settlement, particularly if one on the layers is of low strength and low stiffness. The non-uniform distribution of stress transferred onto the ground strongly contributes to the increase in differential settlement, for example, stress under foundations of columns and walls are different from stress under bottom slab and stress under full chamber is different from stress under empty one in the multi-chamber tank. The problem increases if the underground water is present.

Some examples of differentially settled tanks are described for cylindrical ^{[5, 12]} as well as for prismatic tanks [2, 3, 9]. In the tank described below (Figs. 1 and 2), one more reason intensifying the differential settlement is present. The foundation is non-uniform: a part of the tank is founded on piles, whereas the rest of it is founded directly on the bottom slab.

Figure 1

Figure 2

The differential settlement may cause cracks, opening of the expansion gaps and even tilt or rotation of whole tank or its parts. Such damages are unwanted because the water tanks should be watertight. The decision of the repair procedure should be undertaken based on the information whether the settlement is finished or if the tank is stable. The examination of cracked water tank stability performed for that purpose is described in the following sections.

The troubles concerning the tank foundation occurred during its construction yet. The evidence of these troubles constitutes the handwritten remarks made with pencil on the original design drawing (Fig. 8b). In the chamber No. 1 next to the transverse north-east wall, the circles were drawn and described as follows: ‘sequence of additional pile holes to determine peat line. Some may not be needed’ (notation ‘1’ in Fig. 8b). There was, most likely, the program of additional geotechnical testing. In the town archives, there is information that ‘the tank was built without tentative tests, and as the consequence the tank settled demonstrating cracks’ [8]. The next hand-drawn element nearby the longitudinal partition wall (notation ‘2’ in Fig. 8b) is described as ‘additional piles’.

Figure 8

The above-mentioned handwritten remarks were made probably before concreting of the continuous walls foundation of chamber No. 2 (partition and outer wall) as well as the spot foundations and after finding out the peat in the excavation. In the original drawing of tank strengthening (Fig. 8a), the piles located on the both sides of the longitudinal walls making a pair and four piles crosswise connected under some spot foundations are present. It is not known what type of piles was designed. It can be supposed that there were timber piles, similar to that in the adjacent pumping station. Additional strengthening was executed probably after finishing the tank construction. It was written in the town archives that ‘there was a necessity to strengthen of the foundation with concrete piles’ [8]. In Figure 8a and 8c, the design sketch of this strengthening is quoted [1]. The piles were designed not under whole tank but only in the ABCD region – under chamber No. 2 and under small part of chamber No. 1. The piles were located nearby the corners of spot foundations and a new bottom slab was supported on them (notation ‘3’ in Fig. 8a and 8c).

Finally, on the basis of the analysis of Figure 8, it can be supposed that there are two types of piles under tank: piles made during construction under some foundations (probably timber piles) and concrete piles nearby the corners of some spot foundations supported the bottom slab. The tank owner and the experts preparing the previous technical assessments were not aware of existence of these piles.

The crack pattern indicates the reason of cracks, which is the non-uniform tank settlement. In Figure 9, the crack pattern is compared with the region, where piles were added. The cracks in roof slab and walls indicate the ‘break’ of tank as a consequence of non-uniform settlement and separation the part with piles from part without piles. This is confirmed by the crack in the bottom slab in chamber No. 2, which is almost in line with the piles region border and then runs along the edge of foundation of longitudinal partition wall.

Figure 9

In order to assess the stability of the tank, in 2012, the monitoring of concrete cracks was started, based on the results obtained from first measurement.

The crack width was measured with eight crack gauges allowing to assess the changes in crack width as well as displacement along the crack (Fig. 10). The location of the gauges is shown in Figure 11. The measurement results are shown here as well. An analysis of the data from Figure 11 indicates that the crack measured with gauge No. 4 in the chamber No. 1 is immovable. The remaining cracks in this chamber show little changes in width (up to 0.2 mm). They also show very little displacement along crack (up to 0.05 mm), except the gauge No. 6 fixed next to longitudinal partition wall in the region with piles. This gauge was found to get displaced along a crack of 0.85 mm between 2016 and 2017. The crack width in chamber No. 2 changed to a maximum of 0.30 mm. The displacement along crack was equal up to 0.25 mm, except the crack in the transverse south-west wall measured with gauge No. 1 (0.85 mm in 2016).

Figure 10

Figure 11

The results of crack width measurement may be recapitulated as follows. The cracks, particularly cracks locating next to boundary of piles region, are not stabilised. Most intensive movement was observed in 2016. It is difficult, however, to compare their values to the measurement of underground water level, because piezometric tests were stopped in this year.

At the same time, the vertical displacement was measured. Nine controlled points were laid out on the upper surface of the tank roof slab along the axes of longitudinal walls. Their arrangement is shown in Figure 11. Owing to the fact that the tank was covered with embankments, special structures constituting the control points were designed. They were fixed to the roof slab and protruded over the embankment (Fig. 12). This enabled a direct measurement of control points without roof slabs uncover.

Figure 12

Moreover, the control levelling network based on benchmarks fixed to the walls of surrounding buildings was set up (Fig. 13). Vertical displacement was calculated in the local height system (H_RP1 = 100.000 m), as relative displacement between the first and next elevations in several surveying epochs [6]. Elevations of each controlled point were determined by direct levelling based on the benchmarks of confirmed stability. The precise digital level instrument named Trimble DiNi was used. In each surveying epochs, benchmarks No. 1 and No. 2 only were designated as stable ones.

Figure 13

The five-year levellings revealed movements of some controlled points; however, their displacement did not exceed 1.6 mm. Nevertheless, the movement of controlled points located along the axis of outer longitudinal wall of Chamber No. 2 (point No. 3, No. 6 and No. 9) was confirmed. Such conclusion was derived based on the accuracy of the displacement estimation (in various epochs of measurements, levelling accuracy ranged from 0.2 to 0.7 mm, depending on the measurement conditions) and confirmed by the ratio of the displacement (dH) to the error σdH-dH/σdH ∈2.1;5.7 $\left( {{\sigma }_{\text{d}}}H \right)\text{-}\left| {\text{d}H}/{{{\sigma }_{{\text{d}H}}}}\; \right|\in \left( 2.1;5.7 \right)$The control points along the longitudinal partition wall (point No. 5 and No. 8) as well as control points along the outer wall of the Chamber No. 1 (point No. 1, No. 4 and No. 7) showed the movement considered as negligible (dH/σ_dH < 2.1). This observation connected with small absolute displacement |dH|<0.5m$\left| \text{d}H \right|<0.5\text{m}$proves that these points are stable. Nonzero values of their displacement may be considered as the effect of the measurement inaccuracy, however, being within the limits of the average error of a single observation.

The movement of point No. 2 cannot be clearly interpreted. Its observation was interrupted in 2014 with the damage of the measuring mark. The re-start of the measuring in the subsequent epochs showed, however, that point No. 2 goes down to −1.0 mm. This suggested that this point also belongs to the zone subjected to the settlement.

The results of the changes in vertical confirmed displacements are shown in Figures 11 and 14.

Figure 14

The results of tank vertical movement estimation may be recapitulated as follows. There is a trend of the vertical displacement of some controlled points in relation to the initial position. There are minor but clear changes. The changes in position of point No. 3, No. 6 and No. 9, located along the axis of the outer longitudinal wall, were revealed in all the measurements.

The five-year crack width measurement and vertical tank displacement surveying show that part of tank is not stable yet. The part of tank supported on the piles is subjected to minor but clear settlement. This is manifested by vertical displacement of walls in chamber No. 2 and changes in crack width.

The analysis of the presented case study leads to the formulation of the following conclusions: