ENGINEERING GEOLOGY OF ARTIFICIAL ROCK SLOPES ON THE LADY YOUNG ROAD,
by
Cassandra T. Rogers, Engineering Geologist
Ministry of Energy and Natural Resources
1.0 INTRODUCTION
The Lady Young Road is a two-lane roadway coursing for seven (7) Kilometers
from the St. Ann's Roundabout in Port of Spain to the Morvant Junction, in the western region of the Northern Range.
(Fig. 1) It was constructed to provide access between the East-west Corridor and the valleys north of Port of Spain.
The roadway is cut in steeply dipping, low grade metamorphic rocks. Within recent times numerous landslips
have developed in the artificial rock slopes. Many of these are only a few cubic metres in size, but some
are large enough to have interrupted the free flow of vehicular and pedestrian traffic. This paper outlines
the characteristics of slope movement observed on the upper slopes of the Lady Young Road and accounts for their
occurrence.
2.0 TOPOGRAPHY
Topographically the Lady Young Road lies on the southern slopes of the
Northern Range. From the St. Ann's Roundabout, the road follows a northeast-southwest trending ridge for
approximately three (3) kilometres uphill. From here, at 200 metres high, it winds downhill through
hilly terrain to the St. Francois Valley Road, thence to the Morvant Junction. Dips of slopes vary from 750
to near vertical. Slopes are as low as 3 metres and as high as 30 metres.
3.0 GEOLOGY
Two formations outcrop along the Lady Young Road - the Chancellor Formation (Lower Cretaceous) and the Laventille Formation (Upper Cretaceous). These constitute the upper section of a sequence of low grade, regionally metamorphosed rocks - phyllites, quartzites, quartzitic sandstones and recrystallised limestones, which characterise the geology of the Northern Range. The entire Mesozoic sequence has been folded into an anticline overturned to the north (Potter, 1973).
On the Lady Young Road, the Chancellor Formation outcrops from the St.
Ann's Roundabout to approximately 50 metres east of the Lookout (Fig. 1). in this section the formation consists
of well-bedded calcareous phyllites and minor quartzites. The entire exposure is either partially decomposed
or deeply weathered. Phyllite beds are either intact or occur as friable layers exhibiting distinctive purple
and redbrown hues. The beds vary in thickness up to 0.5 metres and possess a well-developed cleavage oriented
parallel to bedding.Thin clay layers often exist between phyllite beds.
Some bedding surfaces are slickensided. Quartzite appears as thin beds and as irregular veins.
Opposite the St. Francois Valley Road Junction, the contact between the two formations is defined as an unconformity. From this location southward, the Laventille beds appear as deeply weathered phyllites and slates also exhibiting bedding plane foliation. Recrystallised limestones and some quartzite occur in minor quantities. Much of the outcrop is stained with an orange brown colour, the result of subaerial weathering.
Beds of the Chancellor and Laventille Formations dip steeply southward. Dip angles vary from 580 to 720 in the Chancellor beds and to as much as 790 in the Laventille beds. Outcrops of both formations show examples of jointing and local folding. Dominant joint directions are north-northwest to south-southeast. Many joints are infilled with clay. Fold axes plunge southward.
4.0 CHARACTERISTICS OF SLOPE MOVEMENT Three types of mass movement have
been identified:-
4.1 Movement of surface material (Talus)
4.2 Movement of loosened blocks (Rockfalls)
4.3 Movement of intact rock along predetermined planes (Slides)
4.1 MOVEMENT OF SURFACE MATERIAL(TALUS)
Fragments of weathered phyllite and quartzite move downslope gradually
and accumulate as talus at the slope toe. This type of movement is the most common of all observed types.
The volume of material which moves at any time is generally small (a few cubic metres). However, other such
unstable material has been observed to move downslope subsequently and to add to the volume of the fallen mass.
The displaced debris represents loose fragments which have weathered from the surface layers of the parent material
and have remained unstably positioned on the slope.
4.2 MOVEMENT OF LOOSENED BLOCKS(ROCKFI LL )
Quartzite blocks exposed on the surface of the slope become detached
from the rock mass and tumble downslope at a rapid rate. An example of this behaviour was observed at an
abandoned quarry site, located roughly 300 metres from the St. Francois Valley Road Junction. A highly weathered
quartzite boulder, approximately eight (8) cubic metres in size, rolled from a height of 10 metres onto the roadway,
where it disintegrated into smaller fragments of varying sizes. This type of movement is uncommon and appears
to be restricted to the Laventille beds.
4.3 MOVEMENT OF INTACT ROCK ALONG A PREDETERMINED PLANE (SLIDES)
These are by far the most dominant, result in the largest slides and offer the greatest potential danger. The moving material consists of a mixture of slabs of weathered phyllites, quartzite blocks, weathered rock fragments and residual soil. The material is translated downslope en masse along a discontinuity, such as a bedding plane or cleavage surface. Because of the very weathered nature of the rock, the unstable material tends to break up at the slope toe. Such slides are invariably hundreds of cubic metres in dimension and, where they occur, invariably block the roadway. Observation of these landslides have indicated that failure occurs either along one plane of weakness or between two planes and their line of intersection. In the latter case the unstable mass is confined in a wedge. In both cases the failure surface dips onto the slope face. Typical examples of this type of movement were observed in an exposure opposite the Lookout. Here phyllite beds are folded into a syncline, the axis of which
plunges 500 southwest onto the slope face. The limbs of the syncline dips 680 south and 62' north. In January 1981, approximately five hundred (500) cubic metres of weathered material were displaced downslope along the axis of the syncline. The entire roadway was covered with slide debris. Subsequently, in November 1982 and July 1983, two similarly large slides developed at the site, when bedding planes on the northern limb of the syncline failed in shear.
5.0 DISCUSSION
Mass movement and slope instability along the Lady Young Road is attributed to a number of factors, all of which influence the safety factor for the rock slopes. These are listed below:-
5.1 DEGREE OF WEATHERING
The deeply weathered nature of the dominantly phyllite rock is a major factor affecting its stability. These rock slopes have been subjected to mechanical and chemical alteration over time. This has led to decomposition of minerals and to the destruction of diagenetic bonds in the phyllite. In tropical climates, these processes tend to operate more rapidly and more deeply. Such degradation has resulted in a deterioration in the strength of the material forming the slope and subsequently, in the increased susceptibility of the material to slide.
5.2 PRESENCE OF DISCONTINUITIES
Unstable conditions resulting from weathering are compounded by the presence of numerous rock discontinuities. Phyllite is inherently weak because of the tendency of the rock-forming minerals to align themselves in a particular direction, thus defining a cleavage. Cleavages, like bedding planes and joints, are natural planes of weakness. There is therefore, a greater propensity for sliding to occur along these surfaces. This sliding potential is enhanced in the phyllite as the rock sequence is thinly bedded and as the cleavage is well-developed. Also, because cleavage is parallel to bedding, the number of potential sliding surfaces in the direction normal to the strike of the beds is increased. This accounts for the repeated occurrence of planar sliding in the metamorphics.
Structurally weak zones, such as areas of smallscale tight folding, also
contribute to unstable conditions. The dominant cause of slope instability however, is due to the existing relationship
between the orientation and inclination of the slope and the bedding, foliation and joint directions. The
almost
vertical slope face encourages gravity sliding by increasing the driving
force causing downslope movement. Where such steep slopes occur in conjunction with beds which intersect
the slope face, the tendency is for the material above the bedding plane to move down and out of the slope.
Mass movement is also facilitated by joint surfaces which are oriented roughly parallel to the strike of the beds. The effect of these is to break up the rock mass into individual blocks elongated parallel to bed strike. Since many of the joints have clay infillings, the frictional resistance between joint surfaces is at a minimum. Separation between blocks and subsequent failure results.
The susceptibility to sliding is also increased by the loss of cohesion between bedding and cleavage surfaces, due to progressive weathering. Once the shear strength of the discontinuity cannot support the overlying beds, planar failure occurs.
5.3 WATER CONDITIONS
The observation that slope movement on the Lady Young Road invariably follows a heavy period of rainfall confirms the view that water is a major factor controlling the stability of these slopes. The numerous fissures in the rock, both those inherent in the rock and those caused by loosening of the rock structure as a result of its decomposition, give to the rock a high permeability. This facilitates rapid infiltration. A lack of a vegetation cover on these slopes also promotes infiltration. Infiltration is often to appreciable depth, since these meso-and microstructures extend deep into the slope. Water entering small cracks, joints, bedding and cleavage surfaces contributes to, deep chemical alteration of the slope material and encourages even deeper infiltration.
In addition, water causes swelling of clay beds and lubricates sliding surfaces. More importantly ingress of water triggers slope movement as it causes a rapid build-up of pore and joint water pressure in the weathered rock and residual soil. These lead to general weakening of the rock structure and subsequently, to failure.
5.4 HUMAN FACTORS
Squatter development on the crests and upper slopes of hills above the Lady Young Road has led to indiscriminate deforestation of slopes. During periods of heavy rainfall, surface runoff is considerable increased. Large volumes of water originating upslope enters the lower slopes, triggering sliding. Also, vibrations resulting from a continuous traffic flow, and from heavy machinery related to quarry activities further upslope, may have contributed to sliding by causing a rearrangement of mineral grains and increased shear stressing of the rock mass.
5.5 SUMMARY
Three types of movement have been recognised in metamorphic rock slopes along the Lady Young Road. These are :- slow movement of talus material, rockfalls and planar sliding. Slope failure is largely controlled by the unfavourable orientation of rock discontinuities with respect to the slope face. This accounts for the abundance of planar slides in the metamorphics. The weathered nature of the rock also promotes unstable conditions. Infiltration of water and human activities contribute to slope instability by triggering movement downslope.
REFERENCES
Potter, H. C. , 1973, The overturned anticline of the Northern Range of Trinidad near Port of Spain, Jour. Geol. Soc. Lond., Vol. 129, pp 133 138.
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