Having looked at examples from Jacks Hill and Liguanea Ridge, it can be seen that there needs to be greater attention paid to the threat of slope failure, not just in the area of study, but in the entire country. Large-scale and damaging landslide events are not uncommon reports in the Jamaican news media, particularly associated with heavy rainfall events. Every slide has environmental impacts, but where human lives and infrastructure are at risk, the threat posed by slope failure takes on a whole new light, as a hazard is translated into a potential disaster. There is a need to minimize the social and economic effects of landslides discussed in Chapter 4, as far as possible.
In areas that are prone to slope failure, such as Jacks Hill, several reactions to the threat are possible. These range from ignoring the threat, as we have seen done by many respondents, to avoiding the hazard-prone area entirely. In areas already developed and populated, where neither option is feasible, a mid-point between these two extreme reactions would likely prove to be the most practical, and hence, have the best result. Employing landslide control measures to lessen the damaging effects of slides in an area prone to failure is known as mitigation. Similarly, measures applied to reduce the risk of slope failure are known as preventative methods, and either type may be structural or legislatory in form. (For convenience, both mitigative and preventative measures are grouped together in this project.)
Typical examples of structural methods, which we may also refer to as geo-technical, include structures such as retaining walls, gabion baskets, buttresses, rock bolts and cribs, as well as processes such as excavation, grouting and shotcreting, to name a few. Not all these methods are feasible in the Jacks Hill / Liguanea Ridge area because of the prevailing geological conditions. For example, when dealing with non-cohesive material, rock bolts and shotcreting are not options. Another factor is the expense of these structures and procedures, most of which are unaffordable to the average developing nation.
5.1 Structural or Geo-technical Measures
There is a stark contrast between techniques employed in Jacks Hill and Liguanea Ridge. Geo-technical methods used in the Jacks Hill area mainly take the form of retaining walls. Thirty-eight per cent (38%) of questionnaire respondents had already constructed some type of retaining wall to protect their property. Twenty-three per cent (23%) opted for the addition of vegetation to unstable slopes surrounding their property, perhaps as a cheaper, more natural, or more aesthetically appealing alternative to a retaining wall. Terracing techniques were employed by 9.5 per cent of respondents in Jacks Hill, to lessen the driving force behind slope failures. However, the remainder of respondents, approximately one third of those interviewed, had not yet made any changes to their properties to reduce the risk or damage cause by landslides, but some had similar ideas for the future.
In Liguanea Ridge, there was no observed or reported construction of retaining walls or use of reforestation techniques. Instead, the householders have chosen to employ cheaper and more ‘make-shift’ or informal methods of mitigation and prevention. While terracing has been done in the area, it was at a very small scale (that is, only a few, small terraces) and was therefore, not very effective. Residents here have dug trenches or gutters to channel the flow of mud, debris and floodwater down slope, away from their homes. An alternative for some members of the community is to place zinc sheets along the slope at the back of their homes to prevent the land from slipping down. The effectiveness of this is highly questionable.
Some of the other residents have made an effort to clean out nearby drainage channels on a regular basis so that floodwater will not accumulate in the area to increase the risk of slope failure. Also, old tyres have been used on a relatively wide scale in the squatter settlements to keep the bare slope in place. However, as plates 8A and 8B show, this too, was ineffective.
5.2 Natural Hazard Legislation
Natural hazard legislation has to do with the laws passed in an area, governing the development of hazard prone sites. This may take the form of land use policies and building specifications. Land use policies give a guideline as to areas suitable for development of various types, and those which are not. Developers may be banned from unsuitable sites or, alternatively, allowed to proceed with a development project on the condition that necessary changes be made to the site, as specified. In Jamaica, the Town Planning Department (T.P.D.) is guided in land use zoning decisions (with respect to natural hazards) by characteristics such as slope categories, proximity to streams and gullies, and past reports of flooding in the area.
Building specifications are the technical engineering standards which must be met during the construction process. Such standards have been used in other areas of the globe with similar problems. For example, in Los Angeles there has been a significant reduction in landslide damage (Cooke, 1984) which can be attributed to zoning and building ordinances that have existed since Octoner 1952 (Fleming et al., 1979). Similarly, in Tasmania, landslide susceptibility analyses have been carried out for the purpose of informing the public and restricting development in certain areas (Stevenson & Sloane, 1980). This too, has been quite successful. However, there are no true building specifications for the Jacks Hill area or Liguanea Ridge, but there are “unwritten rules” employed by local engineering geologists for construction in parts of the area.
These include: Always build retaining walls at the front and back of the property. Never construct on fill (i.e. loose, unconsolidated material). Always inform the client that the structure is NOT ‘landslide-proof’. This not only reduces the likelihood of slope failure and its associated damage, but also encourages homeowners to take additional (non-structural) precautions.
In Jamaica, several agencies work in conjunction with the T.P.D. to provide advice on planning the location of developments in Jacks Hill (Samuels, 1998). These include the Town & Country Planning Authority (T.C.P.A.), the Geological Survey Department, the Natural Resources Conservation Authority (N.R.C.A.), and the Office of Disaster Preparedness and Emergency Management (O.D.P.E.M.). The Town Planning Department does not have specific guidelines for development in hazard prone areas, but works with the comments of these agencies to determine the suitability of a site for development and measures needed, if any, to increase the level of hazard safety there. Unfortunately, the work of these agencies does not apply to the Liguanea Ridge area because of the area’s illegal status.
5.3 Other Recommendations
There are a number of measures which can, and should, be taken in the Liguanea Ridge area that would improve the conditions of the low-income settlements there, and have the additional effect of reducing the likelihood of the occurrence of devastating landslides.
5.3.1 Squatter upgrading
Squatter settlement upgrading in the communities of Liguanea Ridge would help to improve the sub-standard conditions of the area in terms of improved construction techniques and building materials, better organization of the community, and the installation of legitimate slope-stabilization structures. These improvements would help to reduce the risk, and the resultant damage that could be caused by landslides. Along a similar line of thought is the provision of secure tenure for the squatters, that is, giving them legal title to the land they already occupy. This has been seen in other areas to encourage residents to invest more of their money into construction, repairs, maintenance, and self-upgrading of their homes and community. This may also lead to the construction of more sound buildings and greater organization of the communities, with the spin-off effects of reducing the risk of severe landslide damage. The problem with these concepts, other than being quite costly, is that they are surrounded with political, economic and social controversy, and many argue that it will only encourage more squatters in the city.
5.3.2 Education
An aspect of mitigation and prevention that is frequently overlooked is that of education. It is the author’s opinion that educating the public about areas at risk and the various options available to the residents of those areas to improve their present situation is possibly the cheapest and a highly effective method in reducing risk. Fieldwork for this study uncovered the fact that a significant number of persons in the hazard-prone area, of both high and low income backgrounds, are unaware of the threat to which they are exposed, or the magnitude of it. Informing these residents, through the media for example, many cause them to take mitigative and preventative actions.
5.3.3 Landslide susceptibility mapping
One of the most effective means of providing the public and government agencies with useful information for landslide planning and mitigation, is the landslide susceptibility map. This is a map illustrating zones of varying degrees of susceptibility to slope failure (Maharaj, 1995) based on previous landslide activity in the area. The purpose of this map is to guide property owners, planners and future developers, in the location and protection of infrastructure, by predicting the likelihood of slope failure (Cooke & Doornkamp, 1994) The judgement of susceptibility is based on a slope classification scheme such as Crosier, 1984 (See Appendix 4).
Because the Kingston Metropolitan Area (K.M.A.), which includes upper St. Andrew, is particularly at risk to slope failure, a landslide susceptibility study of the area was carried out (Ahmad & McCalpin, In press). This included the preparation of landslide susceptibility maps, which, it is hoped, will aid in local prevention legislation.
5.3.4 Slope monitoring
Other recommendations include slope monitoring. There are a number of geomorphological signs which are indicative of ensuing slope failure. The development of concentric or parallel cracks in the crown of a slope can be monitored, and any increases in their width is a common measure of impending slope failure. Similarly, subsidence, sagging or upheaval in any part of a slope needs to be monitored. This may be done at the surface by repeated conventional surveying or the use of tiltmeters to record changes in slope inclination. Subsurface methods involve the use of inclinometers, rock noise instruments or geophysical techniques.
Slope monitoring is effective in aiding the prediction of slope failures, but can be very expensive to carry out. However, if the public, particularly the residents of high-risk areas, is informed about the ways of identifying if they are at risk (Ahmad, 1998a), and the actions they should take, the benefits in terms of loss reduction, may be the same.
Back to main page
Chapter 6 (Conclusion)