WHERE WILL THE NEXT SLIDE OCCUR?
TABLE OF CONTENTS
WHERE WILL THE NEXT SLIDE OCCUR?
TABLE OF CONTENTS
Appreciating the magnitude of the Black Mallet/Maynard Hill (BM/MH) problem and recognising its similarity with that of Boguis in 1998, the engineers of Engineering Construction and Management Consulting Ltd (ECMC Ltd.) were prompted to write this joint article. The Engineers have attempted to review all major issues relevant to the causes of the landslides and raised some instructive points on the way forward.
Most importantly, the need to correct some misconceptions and the myriad of recent uninformed and ill-advised statements warranted absolute objectivity. Thus, whereas some conclusive statements are made, the article demonstrates that no single factor is responsible for the slide and the economics of "quick fixes" must be evaluated against the background of geological and soils information available.
It was Mon Du Don in 1990, Bocage in 1992, Boguis in 1998 and now Black Mallet/Maynard Hill, in 1999. The frequency of movement of those massive land masses appear to be as relevant as the causes if not more important. The apparent tendency for persons, to quickly assume the causes of the recent earth movement at Black Mallet/Maynard Hill, has prompted this joint article.
As responsible engineers, conclusive opinions on engineering failures or events are predicated on sound and reliable supporting data. In the absence of data, opinions will be speculative base on a range of contributory factors. This article is aimed at exploring the range of possibilities that can influence landslides and is not intended to be prescriptive.
The history of the Black Mallet/Maynard Hill area has been gleaned from discussions with long standing residents while information on antecedent events have been obtained from various authoritative sources. Though not geologist by profession, some attention will be paid to the actual geological formation of the sites as a possible contributory factor. Finally, bearing in mind the cross-section of readers, an attempt is made to limit the use of heavy engineering jargon in this article. However, the layman's explanation is given when jargon is used to capture the more apposite interpretation in the article.
All slopes are susceptible to movements. However, some are more susceptible than others. The movements experienced by slopes may be near the surface or deep within. The movements depend on the forces and pressure that the slope is subjected to. Landslides are the result of slope movement, which may progress slowly over a long period, or may involve large displacements thereby threatening human lives. The failure of slopes resulting in landslides can occur in four main ways:
Geological structure deals with the manner in which the rock and soil is formed, for example, bedding(layers) cleavage, faults, joints, fissures in over consolidated clays (vertical or horizontal discontinuities in the rock or soil which may be uniform or non-uniform, a few inches or mile long). All influence the possibility of slides.
Ground water can affect a slope in many ways: reducing the resistance of soil to movement; creating forces through its flow and assisting other forces that destabilise slopes; acting as an agent of weathering and erosion to dissolve soluble rocks and minerals and causing erosion of the particles. However, groundwater is the only natural characteristic of the slope that can be altered economically, to improve slope stability. Further, stresses within the slope are generated by its self-weight, pressures and movement of groundwater, geological history or accelerations from earth tremors. The preceding clearly indicates that a plethora of factors can contribute to a single landslide. Additionally, natural slopes in which landslides occur have a geological history of instability.
The recent landslide, at Black Mallet/Maynard Hill may be the latest movement associated with continual slope erosion. Such slides are not immediately apparent at ground level, and its presence must be sought with the aid of careful subsurface investigation.
The geological history of St. Lucia suggests that a number of the inhabited slopes are old landslides that have settled for a period, and are likely to move in the future under the influence of the forces discussed earlier.
Geological formations
The Island of St. Lucia is of a volcanic origin and geologically, it is composed almost entirely of volcanic rocks of Tertiary and Quaternary age. The rocks vary from Basalts in the north, andesites in the central region and dacites in the south, where the youngest rocks are found. With specific reference to the north, the rocks are the most basic, usually basaltic in composition, highly folded and are the oldest on the Island (Saint Lucia Development Atlas; Central Secretariat, Organisation of American States, 1987).
In quaternary volcanic zones, volcanoes consisting of lava, welded tuff and scoria (fragments of lava) beds usually form very steep slopes. During eruptions, hydrothermal alterations take place coupled with volcano-induced earthquakes. Those characteristics, together with the heavy precipitation associated with volcanic regions, often lead to slope instability.
With the focus on the two most recent areas, Boguis and Black Mallet/Maynard Hill, interpretation of the landscape suggest that in geological times, these land masses were formed through slides off the mountain tops or lava flows. In the case of Black Mallet/Maynard Hill, it appears from the shear drop at the 'Parker Hill', a landslide occurred at that site many years ago.
At the higher elevations of Maynard Hill, near the top of one of the many flights of stairs, there are obvious signs of huge rock outcrops. As to whether these are simply huge boulders interbedded with soil or bedrock of the slope, one is not certain. The topography appears to be one evolving from a landslide. Similarly at Boguis, excavated test pits revealed that the site evolved from mud or lava flows interbedded with round boulders. Such characteristics are clear attributes of lands formed from a previous slide or material flowing off a mountain.
Further, preliminary calculations of the gradients of both Boguis and Black Mallet/Maynard Hill, suggests average slopes of 23 and 25 percent, with the higher elevations being 35 and 45 percent respectively. These, gradients are considered steep and extremely susceptible to movement
Subscribing to the speculations that the landslide at Black Mallet/Maynard Hill was caused by the river works or by any other single factor would be ignoring the geologic formations and characteristics from which most parts of the Island evolved. In fact, the geologic map of the Island suggests that the affected region is in a zone of basalt agglomerates (masses of lava embedded in dust and ash); a composition, which suggests the existence of un-welded lava flows. In such zones, failures are frequently induced by seismic activity and aggravated by the high pore water pressure.
Finally, it should be noted that "mountain-forming resulting in increased erosion potential, is one of the most important factors in landslide evolution" (Landslides in Japan - www.tuat.ac.jp). Thus, the issue of the location of the next slide is of paramount importance
The Boguis Landslide
To place the BM/MH landslide in proper perspective it is considered prudent to reference a similar occurrence at Boguis in 1998. Some factors considered pertinent to the occurrence of the landslide are highlighted below;
The cracks were first observed after an earthquake in September 1998, the exact date of which could not be confirmed. However, some additional information on earthquakes reported during the said month is shown in Table II below.
Date in 1999 |
Ritchter Scale Magnitude |
September 5 |
3.9 |
September 11 |
3.6 |
September 18 |
4.7 |
September 24 |
4.0 |
Test pits excavated within the landslide revealed the existence of boulders interbedded with clays of a high moisture content, highly sticky and plastic.
Drainage capacity of the soil was very low.
Site reconnaissance revealed the existence of a water supply service connection to a household being repaired and extended over a twenty-year period; suggesting the prolonged but relatively slow movement of the site.
Progressive and eventual dramatic failure of a health centre.
The Black Mallet/Maynard Hill Slide
The Black Mallet/Maynard Hill landslide was first officially reported on October 04, 1999. Visits to the site suggest that the land mass movement extend over an area of approximately 4.0 acres. Though not scientifically monitored by the authors, photographs of October 16, 1999 and October 25, 1999 provide some perspective of the acceleration of the movement. To be more definitive, the slide has moved at a much quicker rate than the one at Boguis and can be estimated as moving 30 inches in a week.
Apart from the several houses that have cracked and have been demolished, there are huge cracks in the ground and 60 feet of recently constructed river wall is severely damaged and on the verge of collapse. The situation is approaching catastrophic levels and an urgent solution is required. However, it is worth repeating that a workable and probably cost effective landslide repair cannot be attempted until the "sensitive factors" of the geological attributes of the site are fully evaluated. As a consequence, speed is of the essence and efficiency of drilling rigs, are just as relevant. The fact that exploratory soil investigation has been going on for over 15 days without significant preliminary results is disturbing and needs to be addressed quickly.
Considering the issues raised in the section on Science of Landslides, and the intimation of some residents that increased cracking and movement was observed subsequent to the earthquake of July 11, 1999, the effect of earth tremors within unstable slopes should influence the engineering analysis of the slide.
A seismic (earthquake) disturbance would transmit a seismic wave through the ground and would lead to some dynamic loading of the soil. This action would augment the shear stresses in the soil and quickly reduce the void ratio of soil giving rise to very high pore pressures. Essentially, the net result of the earthquake is a rapid reduction in the friction that exists between the soil particles and the eventual landslide. This is not to state that the cause of the landslide was absolutely the result of the earthquake of July 11, 1999. However, it is worthy to note that the smallest earthquake required to trigger slope failures similar to that at BM/MH is of the magnitude of 4.0 (Keefer, 1984) on the Richter Scale; that of July 11, 1999 was 4.0 and on June 08, 1999 one of 5.0 was recorded.
On the issue of the ongoing river works, before any definitive statement could be made about the relationship between the wall and the slide, soil investigations must identify the toe of the sliding mass (bottom end) and the location of the slip circle. Thus, an absolute imperative, is the proper evaluation of geological attributes of the soil, its strength characteristics and stratigraphy.
Excessive rainfall during the periods of July (July and August were above the 30-year period) to October would have only acted as a lubricant for the movement of the slide. Should the assumption be made that cracks existed in the ground subsequent to the earthquake of July 11, 1999, then it would be correct to state that the surface runoff entered the substrata and enhanced the downward flow of the landmass. Additionally, the intimations by residents that the recent "Parker Hill Development", has resulted in increased indiscriminate runoff, also needs investigation and verification. Furthermore, water is the nemesis of landslides; all other things being equal, the wetter a slope is, the less stable it becomes (Landslides.usgs.gov/Sacred/index.html)