BACKGROUNDS

Up to the present, at least 140 water wells in the Village of Manggis, District of Puncu, Regency of Kediri have collapsed. Approximately 130 wells have been filled up and new ones have been bored in the same location. The process of the collapse has continually proceeded with water in 110 wells becoming murky, waiting for the wells to collapse.

Various theories and hypotheses have been presented by the specialists and disaster management experts to deal with the phenomena on the mass media and social media. Such theories and hypotheses should be answered so that the public feel more certain in preventing and mitigating for such phenomena.

Studies were conducted in the neighborhood units experiencing these phenomena, collapsed water wells in Dorok (240m asl), nanas (280m asl), Jambean (280 m asl), Manggis (260M asl) and Ringinbagus (330m asl). Dusun (Neighborhood) of Manggis and the other neighborhoods mentioned above are located on the southwester slope of Mount Kelud, which recently erupted in 2014.

IMPLEMENTATION

Participative geological, geophysical and social studies were conducted by the lecturers and students of UPN “Veteran” Yogyakarta’s Disaster Management Study Program and partners from Jangkar Kelut from 3rd to May 8th, 2017. Interviews were carried out with the owners of the collapsed water wells in the following dusun (neighborhoods): Dorok (42), Jambyean (17), Manggis (14) and Nanas (36) and in Ringinbagus (14) as well as the measurement of the geophysical formulation data in all those neighborhoods.

RESULTS OF THE STUDIES

Soil Water Surface (SWS) changes varied from one dusun (neighborhood) to another. The SWS in Dorok, whichwas originally 10-20 meters, increased by 515 meters and has now only 1.5-8 meters. Water in the wells in Candi Dorok, which was originally murky and dusty is now filled with water reaching approximately 40 centimeters high. The SWS in Jambeyan increased by 5-15 meters from its original point at 13-20 meters has now become 4-12 meters. In Manggis Neighborhood the original Soil Water Surface increased by 9-13 meters in the northern part and by 3 -18 meters in the southern part, from its original 23-37 meters. The soil water surface in the neighborhood of Nanas had increased by 14-19 meters to become 3- 14 meters and leaving only 2-12 meters from the ground. In Ringinbagus, wells with a depth of 27-29 meters increased the soil water surface by 16 meters.

The increased soil water surface in each neighborhood was associated with the aquifer existing in a depth of 5-13 meters and 17-72 meters. In Nanas Neighborhood the increase of soil water occurred at a depth of 8-14 meters and 32 -44 meters, in Ringinbagus at a depth of 25-35 meters and 48-59 meters. In Manggis, the increase occurred at a depth of 16-22 meters and in Jambeyan, at a depth of 12-27 meters.

The signs of the collapses were in the form of thundering sound and sound of smoothly flowing water, which were followed by the soil collapse. The waiting period of a collapse varied, from one day to one week, depending of the geological condition or structure. The process of the water becoming murky and the collapse in the neighborhood of Manggiswas considered to be fast, only one day.  This period varied in the different surrounding villages/areas. The collapse process in the neighborhood of Jambeyan was slow, taking up to 4 days, usually accompanied by a thundering sound and the water in the well becoming murky. The increasing soil water surface was coincided with the signs of the soil instability on the walls of the wells, which had been perceived by residents of the neighborhood of Ringinbagus for two months. One well had earlier collapsed in Jambeyan two years earlier.

The collapses may have form one single point on the well brim, a vertical or widening gap. In the neighborhood of Jambeyan the pattern of the collapse was widening, reaching a radius of 5 meters and causing the buildings on the surrounding grounds to crack. The wells located in the western sector tend to be deeper and the collapses seem to have swallowed the entire wells and spreading to the ground surface with a diameter reaching more than 2 meters. In Ringinbagus, the lower part of the well walls had collapsed, leaving a depth of 6-8 meters. Unlike the case in other villages, the well brims are commonly 2 meters deep and have posed no burdens.

For walls of a well reaching a depth of 50 meters are commonly composed on deposits of  alluvium, from the top to the bottom and followed by layers of loose sand from the volcanic materials deposited there, consisting of coarse sand to fine sand, these lithological components have caused the collapses to occur fast. The upper part of the walls of wells in the neighborhood of Ringinbagus and some in the neighborhood of Manggis are composed of 3-6 meters thick vertical calcite rock formation (breksipecal), so the collapses only occurred on the lower part of the walls and caused the water to b murky but have not made the wells collapse.

A close watch of the geological map and aerial image from Google Earth show vault straightness with northwest to southeast directions. These straight cuts have played a role in localizing the sectors of the collapse and speeding up the distribution of soil water.

CONCLUSION

From the results of the studies, it can be concluded that:

1. The well collapses have occurred as the result of the well walls collapse, due to the instability of the walls
2. The instability has been caused by the saturation of the materials forming the walls caused by the increasing soil water surface. The additional excessive water caused the walls to lose their stability.
3. The pattern of collapses varied from one area to another, depending on the geological structure of the walls.
4. In more detailed explanations, the pattern of collapses depends on the depth of the aquifers, the well brim structure, structure of the layers forming the well cavity and the rate of the changes in the soil water surface.

RECOMMENDATION

1.  Carry out landfill and soil pressing in the collapsed wells
2. To strengthen the stability of the well walls by building brick walls or cemented/concrete walls circumflexing the cavity of the wells
3. To make alternative drilled/bored water wells.

Eko Teguh Paripurno,

+62818260162 / parpurno@upnyk.ac.id

Magister program of Disaster Management, UPN “Veteran” Yogyakarta.