RESULTS

Correlation coefficients

There is a low correlation between the water level and the crack width gauge displacements. These low correlation coefficients are expected since the crack metres measure the relative movement between the vertical cantilever concrete blocks. If two blocks were to be displaced equally in the downstream direction due to the rise in the water level, this would be recorded as zero displacement by the crack metres since the blocks would not have moved relative to each other. Therefore, the rise and fall of the water level does not necessarily cause relative movement of the blocks hence there cannot be a strong correlation between the water level and the displacements of the concrete blocks.

The non-parametric correlation analysis proved that there is a significant relationship between the water level and the geodetic survey and the tilt metre displacements.

Graphical analysis

Crack metre graphs

Crack metre displacement correlation matrix

Geodetic survey displacement correlation matrix

Tilt metre displacement correlation matrix

Crack metre displacement correlation matrix

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Water level vs time graph

Displacement vs time graph

Water level vs time graph

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The water level trend is cyclical and this shows different seasons of the year. During the winter season, the water level of the dam is generally higher than in the summer season. The centre moving average (CMA) was then plotted on the water level vs time graph and it shows there is a general increase in the water level of the dam over the years from 2002 to 2010.

The graphs show that there are similarities in the plots of each type of displacement for different metres. The graphs show a distinct change in the performance of the dam in 2008 (season ref # 25 and year ref # 6) when the dam’s displacements suddenly increase drastically. This might have been caused by the replacement of the crack metres on the dam or the readings could have started being recorded by new staff. These reasons could result in inconsistencies in the readings and could explain the sudden change in the recorded figures.

Geodetic survey graphs

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The graphs show us that during winter the displacements are smaller than those that occur during the summer season. During summer, the water level of the dam is lower and the dam moves towards the upstream direction and it displaces more than during the winter season when the dam water level is high and the dam moves in the downstream direction.

The graphs also show that the displacements shift in clusters as time moves on. With reference to the summer displacement plots, as time progresses from the year 1985 to 2010, the displacements transition and move more and more towards the upper left corner of the graphs. The plots also show two distinct regions of plotted points. This suggests permanent deformation of the dam.

This is also seen on the winter season graphs. When the dam is full, as time progresses the displacements are never the same magnitude despite the fact that the water level is constant. In other words, when the water level is the same and the dam is full, we expect the dam to displace by the same magnitude in the same direction. This is however not the case in the graphs shown above, the same water level has different displacements which is unexpected since the dam is expected to behave the same way under the same hydrostatic load. This shows that the dam is becoming permanently deformed as the years progress. The same plots were also done for the displacements in the y and z directions. These graphs agreed with the x-displacement graphs and showed similar results.

Tilt metre graphs

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The graphs support the previous conclusions made from the geodetic metre graphs. The graphs show a transition of the clusters on the scatter plots. For the summer and winter plots, the points move towards the upper left corners of the graphs as the years progress. These plots show permanent deformation of the dam and they also show that between 2003 and 2004, the water levels were low as shown in the previous sections. The spring graphs are much clearer. When the dam is full, though the water level is at the same level, the dam’s displacements are of very different magnitudes. This shows permanent deformation of the dam as explained above.

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TANYARADZWA USHAMBA

RESULTS