WHERE WILL THE NEXT SLIDE OCCUR?
WHERE WILL THE NEXT SLIDE OCCUR?
THE BLACK MALLET/MAYNARD HILL
Historical Perspective
As a prerequisite for the preparation of any investigative report on the landslide at Black Mallet/ Maynard Hill, a site reconnaissance is absolutely necessary to obtain information from residents and background of the site. The site visits performed on October 16 and 25, 1999, involved inspecting of the impact area, estimating the extent of the slide and gathering data, particularly from residents in order to develop a proper perspective of the landslide.
The following observations which were reported are considered pertinent to the assessment of the slide:
A footpath between Black Mallet and Maynard Hill remains damp year round.
Cracking of houses at those two areas has always been a continual occurrence whether on old or new construction.
There have been sections of the Black Mallet road that has been continuously damp due to sub surface flows.
During the construction of one of the houses at the Black Mallet Corner, water had to be continually pumped from the foundation. This house was apparently located within the alignment of a natural drainage canal.
Consideration for proper drainage surface or sub-surface is not evident. Water from lands at the high elevation (Parker Hill) or generated locally simply seeped into the subsurface.
The entire community is aware of the continuous cracking of buildings and collapse of retaining walls even during construction. Hence cracking of structures has become an accepted phenomenon.
Though unscientific, there were mooting of the older persons years ago, anticipating the eventual downward movement of the Black Mallet land mass
Antecedent Events to the Landslide
Despite the acceptance that the Black Mallet/Maynard Hill lands has been in a state of motion for many years now, the sudden accelerated flow of the land had to have been brought about by one main cause or a combination of factors. Consequently, since it may be too early to conclude which factor or activity is the major contributor, a list of antecedent events are highlighted below.
An earthquake measuring 4.0 on the Richter Scale was recorded on July 11, 1999. Residents reported cracks in buildings, retaining walls and the ground. Moreover a series of tremors have been recorded within recent times. Below is a list of earthquakes for the year 1999 to date.
Date in 1999 |
Ritchter
Scale Magnitude |
February 19 |
3.9 |
April 06 |
3.9 |
May 29 |
2.9 |
June 08 |
5.0 |
July 03 |
4.5 |
July 11 |
4.0 |
October 03 |
2.5 |
The section of the recently constructed river wall, which is directly within the zone of failure of the slide, was erected during the months of August to September 1999.
Preliminary analysis of rainfall data for the thirty-year period (1970 - 1999), suggests that total rainfall for the months of June to September for the five- year block 1995 - 1999, is the highest (Figure 1 below). Additionally, the rainfall for June and July 1999, are both above the thirty-year average for those months.
Source:- Ministry of Communications, Works, Transport and Public Utilities
(Though the rainy season is from June to November, information for October
onwards is not yet available for 1999)
Development Planning Issues
Issues of planning are critical for the assessment of the causes of the Black Mallet/Maynard Hill slide. Those issues are of paramount importance when considered against the background of increased development on the hillside, particularly in recent years. Such development has resulted in increases in:
The Failure Scenarios
Information strongly suggests, that the MB/MH landmass has been in motion for a significant number of years (at least a generation and a half). With this as a given, a number of failure scenarios can be posited as follows;
The earthquake of July 11, 1999 apart from cracking houses, created huge cracks in the ground. Those cracks, which were generated from the dynamic (vibratory) loading of the landmass allowed for the ingress of increased amount of water into the substrata with the attendant consequences. With the wall providing a certain level of restraint, the mass has been prevented from entering the river. Further, it needs to be noted, that there were five reported earthquakes prior to that on July 11, 1999, with the one on June 08, 1999 being a massive 5.0 on the Richter Scale. Additionally, there was one on October 03, 1999; the day before the official report of the slide, measuring 2.5 on the Richter Scale. Six reported earthquakes (each with a magnitude above 2.5) in as many months can be considered as an active period.
Having been formed from a landslide or lava flow, the slope materials within the landmass have weakened over time through the infiltration of water, long-term weathering and other processes. The landmass simply reached a stage where the pore water pressure had attained its critical limit and the shear strength of the soil, its minimum value necessary to maintain the stability of the slope. This being brought about by the indiscriminate and illegal development and construction, the downward movement of the slope was enhanced.
A combination of the two above scenarios.
To use a "quick fix" in an effort to satisfy the stakeholders may provide a temporary (or even permanent) solution for this specific MB/MH landslide. However, the issue is not simply about this particular area but to critically investigate the disaster and to develop policies and strategies, which would reduce the occurrence of such events on other hazardous slopes in St. Lucia. Further, large landslides can be formed by an aggregate of smaller features within that sliding mass. Thus concentrating on repairs simply at the bottom of the slide may not restrain the constituent slides from further movement.
Further Investigation of Landslide
The investigation of the slide must be thorough. Any "quick fixes" should only be done after results of preliminary soil investigation have been obtained and should be taken as a temporary solution. The fact that for years now, residents have intimated the "coming down" of the hill, though not a scientific, warrants in-depth site reconnaissance. The current soil investigation must be properly directed and hastened. It is necessary to source additional boring rigs so as to provide the information at a quicker rate, and in that regard Government should solicit the assistance of neighboring Caribbean territories.
In terms of a specific brief as to what format the investigation should take, we suggest the following;
Mitigative Measures
The nature of the BM/MH slide and the catastrophic consequences likely, if adequate mitigation measures are not undertaken mandate that there be a clear understanding of geological factors promoting instability before beginning engineering analysis or repair. So many times sites with a high risk of landslides have been the scene of repeated repair attempts within a few years of each other.
Engineering experience world-wide suggests that many landslide repair attempts are made without benefit or full understanding of the geometry and hydrologic regimen of the affected sites. More importantly, the uninformed implementation of traditional engineering repair schemes, may not serve to adequately mitigate all manners and forms of future slope instability.
Some of the more relevant issues include:
A number of retention structures have been successfully used to repair and mitigate landslides, namely; gravity structures (rubble wall, gabion basket), cantilever reinforced concrete walls (piles being used for the foundation), flexible bulk head walls, retaining structures using anchors, or a combination of any of the above.
Gravity structures:- These depend on their mass to resist the weight of the slide. A gravity wall may be bulky but provides a possible solution.
Cantilever retention structures :- These came into use with the advent of pile driving but this is a very expensive option.
Retained structures:- These employ tension elements such as rock bolts and ground anchors for its resisting strength. The cost and feasibility of such structures is almost wholly dependent on drill rig access, drillability of the ground and depth of bedrock.
Flexible retention structures :- These deflect in order to shed imposed loads. Such deflection reduces wall loads by allowing the ground mass to mobilise its shear strength
It is necessary, as a matter of urgency, to develop statewide landslide hazard mapping. We are aware that efforts have commenced in this direction, however progress is too slow and the information developed to date has not been disseminated or informing the existing planning process. There are also attempts to use Geographic Information Systems, a very useful planning tool. There is need to move these processes along in keeping with the physical planning challenges that are mounting, as more slopes are developed.
The hazard mapping process requires a team of experienced geologists to focus on particularly hazardous areas and where landslides are most likely to occur. The Ministry of Planning should head this initiative by providing policy direction since the consequences have implication across all economic sectors. More importantly the Ministry needs to coordinate information generated and develop strategies for widespread dissemination to all stakeholders. In this way, hazard information will inform future planning initiatives across all sectors.
Additionally the information generated from this hazard mapping process should also be incorporated into the long awaited building code. It is also another basis to review the present land use planning strategy and the physical planning requirements of the Development Control Authority.
This initiative should commence immediately in tandem with more effective monitoring of unplanned developments in high-risk areas. In furtherance of the way forward, we suggest the following;