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Report on Schwallenburgh Sinkhole - Sept 23/07

Schwallenburgh, St. Ann

Andreas Haiduk


Acknowledgement

The members of the Jamaica Caving Organization (JCO) wish to express their gratitude to Bouygues Travaux Publics in particular Mr. Baradi who was instrumental in allowing JCO to enter the compound and to conduct the survey and Mr. Foulard and his team on site who provided the necessary background information and heavy equipment allowing JCO members to enter the shaft system without complications.


Entrance to Schwallenburgh Sinkhole

Background

Bouygues Travaux Publics (BTP) has been contracted to construct Highway 2000, a major national road project in Jamaica. The company is presently carrying out earthworks for the Mt. Rosser leg of the highway between St. Catherine and St. Ann. A sinkhole was detected in the Schwallenburgh area of St. Ann (coordinates: N 18 degrees 13.375', W 077 degrees 04.930', Elevation 1762 ft) in the immediate vicinity of the highway alignment and Mr. Haiduk in his capacity as Chief Hydrologist of the Water Resources Authority has been contacted with a view to determine which management strategy could be employed to a) safeguard the functionality of the sinkhole and b) to not compromise the structural integrity of the highway. Mr. Haiduk as a voluntary member of the Jamaica Caving Organization (JCO) has offered to assess the extent of the sinkhole and the available volume of the shaft by carrying out a survey. A compact disc with photos and video clips has been prepared and forms part of this report.


Entrance to Schwallenburgh Sinkhole

Sinkholes as Hydrologic Features

Sinkholes form part of the natural hydrology. These holes mainly found in karstic limestone rocks are generally located at the lowest point of depressions and serve as natural outlets for surface runoff generated by rain falling over sub catchments (sub watersheds).


Management Issues

Sinkholes have a naturally limited absorption capacity which is governed by the permeability of the underlying rock and its degree of karstification. The limitation in absorption capacity could be such that water enters the sinkhole but fills the hole and forms a small pond until all water is drained into the underlying aquifer. Compromising the absorption capacity by infilling or blocking the sinkhole can have negative implications on the natural hydrology and can lead to flooding. Leaving the sinkhole open and unprotected could expose persons to the danger of falling into the hole. The sinkhole could also lead to a cave system or shaft which could be the habitat of cave dwellers eg bats. Management strategies need to take the issues into consideration.


Objective of Survey

The survey sought to establish the following

  • determine extent of shaft by carrying out a physical survey

  • determine whether the shaft is a habitat for cave dwellers

  • advise BTP about a possible strategy and last but not least

  • expose locals to the existence of a nationally recognized organization with persons having the expertise to enter subterranean features.



Andreas Haiduk in Schwallenburgh Sinkhole

Report

On September 23, 2007 JCO representatives, Messrs. Jan Pauel and Andreas Haiduk met with Mr. Foulard / Project Director of BTP and site engineers at the site in Schwallenburgh. This area is under the control of the bauxite mining company and normally not accessible to the public. The sinkhole is located at the bottom of a major depression which most likely represents a mined out pit.

The depression has an estimated diameter at its rim of at least 300m and the estimated depth from the rim of the depression to the entrance of the sinkhole is 50 m. Parts of the sinkhole according to site engineers have already been blocked with boulders. However, a 3 m diameter hole was left unblocked. This hole served as the entrance into the shaft. A first visual inspection revealed a vertical shaft of about 10 m depth with an opening to a second shaft with undetermined depth. The shaft could only be accessed via descending gear as the walls were not considered suitable for scrambling. The undetermined depth led us to the conclusion that the largest available rope with a length of 100m and a thickness of 13 mm rope would be the best option. No trees were in the vicinity to allow for anchoring the rope. BTP provided a piece of machinery with an extendable boom which allowed us to affix the rope to a steel cage which was welded to the boom and to extend the boom over the sinkhole.


Bottom Sump of Schwallenburgh Sinkhole Mr. Pauel and Mr. Haiduk descended into the first shaft at about 1000 hours. The depth of the first shaft (S # 1) has been determined to be approximately 10m with an average diameter of 4 m. From S # 1 an approximately 1.5 m wide and 6 m high opening allowed access to a second vertical shaft (S # 2). The depth of S # 2 measured from the bottom of S # 1 has been determined 27 m with a similar diameter of 4 m. The overall depth of the S # 2 is 33 m as the top of the opening represents the ceiling of S # 2.

An opening in the bottom section of the S # 2 allows entry to a third shaft/chamber ( S # 3). The opening is very small (height about 0.8 m, width 0.6 m) and one has to scramble to get into the S # 3. This S # 3 serves as the point where surface runoff seems to percolate into the underground. There was evidence of a recent rainfall event with water still pooling and a large amount of fine saturated sediment making accessing the chamber difficult served as further evidence that this chamber is the final point of the shaft system. The height of S # 3 is estimated to be 6 m and the diameter about three meters.

The team exited the shaft system after about 45 minutes.


Cave Biota

There was no evidence of cave biota neither permanent nor transitory with the exception of a solitary cave cricket.

Geology

The limestone rock within the shaft has a low degree of karstification as the walls of the shafts are smooth and do not display the typical sharpness of highly karstified rock.


Volume of the Shaft System

The volume of the shaft system is determined based on the cylinder formula: V = /4 . d2 . h

Table 1: Volume Determination








Drawing of Schwallenburgh Sinkhole - Andreas Haiduk, JCO

Management Strategy

It is proposed to keep the sinkhole open but to fill the shaft system with boulders. The approach to the system would be protected with a geotextile and a boulder berm to avoid fine sediment to enter the system. This proposed strategy will allow the sinkhole to retain its function as a receptor for surface runoff. The filling with boulders will prevent the sinkhole against collapse and prevent possible structural damage to the road infrastructure.


Conclusion

Based on the survey conducted filling the shaft system with boulders will have no negative implications on the natural hydrology and will not impact on any cave biota. As there were no horizontal shafts or other chambers detected filling this shaft system will provide a suitable and sustainable engineering solution.


Recommended Filling Procedure

It is assumed that the contractor uses a purely mechanical filling procedure that is dumping boulders into the shaft system via tipper trucks. It is to be noted that S # 3 will remain unblocked as the small access hole between S # 2 and S # 3 will not allow boulders to move into S # 3. In case S # 3 will collapse it is unlikely to cause significant topographic changes at the surface due to the small extent of this chamber. In fact leaving it boulder free allows for undisturbed percolation.

When filling through S # 1 it is recommended to use smaller boulders as these would fit through the 1.5 m wide opening leading to S # 2. It is unlikely that the top section of S # 2 will be filled as the opening between S # 1 and S # 2 narrows from approximately 1.5 m at the bottom to approximately 0.5 m at the top of the opening.

Two approaches could be used. Either the contractor tries to open S # 2 from the top and fills the shafts separately or the opening between S # 1 and S # 2 be mechanically widened so that rocks dumped into S # 1 will roll into S # 2 through the opening.


Final Recommendation

The JCO is interested in providing its service to BTP whenever the need arises. As has been shown JCO members can be called upon at a short notice. As the JCO strives to inventorize all caves and other subterranean features any sightings of these features even if they are not within the immediate alignment of the highway can be reported to the members of the JCO residing in Jamaica directly at fatboy@cwjamaica.com (Jan Pauel), ahaiduk@wra.gov.jm (Andreas Haiduk) or to the JCO admin@jamaicancaves.org.


Prepared by: Andreas Haiduk

September 25, 2007