EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 6, 265-274 (1981)

SLUMPING AND PLANAR SLIDING ON HILL-SLOPES IN RWANDA

JAN MOEYERSONS

Koninklijk Museum voor Midden-Afrika, B-1980, Teruuren, Belgium

ABSTRACT Field observations on Rwaza Hill near Butare (Rwanda) revealed the nearly omnipresence of small natural terracettes, developing as a result of superficial slips and accelerated creep movements. On the steepest slope under study, the humic A-horizon of the soil profile migrated downslope over a mean distance of 14.5 cm from October 1977 till October 1978.

A high hydraulic conductivity horizon, displaying low cohesion and consistency characteristics as compared with overlying and underlying material, is developed at the base of the sliding soil.

Planar slides, as well as rotational slips occur, the location of failures being determined by the presence of this plane of weakness.

Generation of basal interflow in an artificial soil column showed that the state of failure is preceded by localized rheological creep of 3 plastic viscous soil material, undergoing seepage pressure.

KEY WORDS Failure Flow Hydraulic conductivity Interflow Planar sliding Rheological creep Rotational slip Seepage pressure

INTRODUCTION

Soil conservation is a major problem for Rwanda since its dense population largely depends on agriculture. Large scale anti-erosion measurements have been taken by terracing and by the initiation of contour-like disposed trenches in order to disharm overland flow. However, surface wash still remains active in many places, sometimes even in an alarming way.

In addition, some localities are badly affected by slides, earth flows and other related processes. The actual situation around Butare in southern Rwanda is such that about one hill out of five displays large scars or cuts.

Miniature landslides and related mass movements also frequently affect hillslopes. Indeed, a prospection tour in 1977 and further investigations in 1978, as part of a research program of the Institut National de Recherche Scientifique (Butare) revealed a widespread occurrence on the apparently intact slopes of natural benches or terracettes (Figures 1 and 5). They are present on all slopes steeper than 5", with the exception of freshly cultivated plots where they are masked as a result of human activities. The terracettes follow more or less the contour lines, disappearing, bifurcating and reappearing laterally. Their width, of the order of 0.5-2.5 m, and the height of the steps depend largely on the general slope inclination: the steeper the slope the more the terraces are marked. It could be observed on Rwaza Hill near Butare that they often are separated by subvertical cracks, especially some time after heavy rains. This was a first indication that miniature sliding occurs upon this type of hummocky slopes.

It should be mentioned that soils in Rwanda are often very clayey. A clay content (< 2 micron) of 20 per cent is normal but records from diverse localities indicate often 35 per cent! These soils occur on valley sides, sometimes steeper than 40", and are exposed to beating rains, resulting in a yearly precipitation of over 1000 mm in most areas. Therefore, they are inherently unstable.

0197-9337/81/030265-10$01.00 @ 1981 by John Wiley & Sons, Ltd.

266 J. MOEYERSONS

Figure 1. Upslope view on the northeastern part of cross-section 3 of Rwaza Hill. Terracettes and iron plate 15. The 50 m distance between plate 10, on top of the hill and plate 15 had increased in 1 year by 4 cm at least

SOME OBSERVATIONS ON RWAZA HILL

A catena of topographical features On steep slopes such as that, shown on the western part of cross-section 3 (Figure 2), a downhill sequence

of topographic features, indicating different processes or different intensities of the same process can be seen (Figure 3). The top of the hill is governed by a phyllite rock outcrop, surrounded by a stoney soil with clay matrix. In the immediate vicinity of this outcrop, the first terracettes appear. Small exposures show that the soil becomes gradually less stony downslope, the coarse fragments becoming concentrated in a stone line, separating the red clayey subsoil from the overlying grey humic horizon. While the coarse stone line fragments disappear gradually, the terracettes become more accentuated. From about 50 m below the top of the hill cracks can be observed, as mentioned above. They interrupt the continuous grass cover. Finally, about 40 m above the valley bottom the arrangement of the terracettes becomes less regular as they become more widely spaced and form small and irregular dams between which the red subsbil appears, generally devoid of vegetation. This is illustrated in Figure 3 where the parallel distribution of benches gradually describe amphitheatre-like forms, announcing the downslope presence of the head of a gullied badland. It appears that interflowing water concentrated here to a seepage line (Bunting (1961)) pressuring benches to slide over the red subsoil. Hollows, cavities and pipes (Fletcher, Harris, Patterson and Chandler (1954))

NE CROSS-SECTION 2

SLUMPING AND PLANAR SLIDING

sw

267

C R o s s - S E c T m 1 sw

51 SLOR INCLHATCN OVER 10M OlSTANCE IN 1 5 0 m

PERCENTAGES

Figure 2. Three cross-sections at Rwaza Hill, with changes of the original 50 m distances (October 1977-October 1978)

.E . w.

L Rp 5

Figure 3. Schematic sketch of the western part of cross-section 3 with map of spurs in the resurgence belt

268 J . MOEYERSONS

Figure 4. Resurgence phenomena and downslope slid relict of terracette at the right (arrow): western part of cross-section 3, Rwaza Hill, October 1978

evidence the reappearance of seepage water, suspended above the red subsoil horizon (Figure 4). Finally, further downslope, a real badland exists as a result of runoff, following the progressive concentration of resurging and surface water.

Local variations exist around this general model. So, it could be observed some 100 m to the north of the western part of cross-section 3 that a ravine head had cut sufficiently high up the slope as to affect the belt where the terracettes still have a more regular distribution. In another locality, the topography of the belt of sliding terracettes shows the evident contours of an initial slide over a deep seated failure.

An artificial exposure in the terracettes

following facts: A trench (Figure 5 ) dug through the terracettes just above the resurgence belt (Figure 3) revealed the

1. The soil profile shows three superposed horizons. The upper one, defined as an Al-horizon where a certain accumulation of humic material takes place, has a clay content of 15 per cent. Apart from a continuous root bed going to a depth of about 10 cm, the homogenous structure is only interrupted by cracks as indicated on Figure 5 . The middle horizon sometimes forms a gradual transition to the lower subsoil horizon, sometimes appears as a very distinct feature. It is characterized by the presence of laminated patches of fine sediment, often situated below the cracks in the upper horizon as well as by the presence of closed, not interconnected cavities and hollows, the dimensions of which can vary between some millimetres and several centimetres. The bulk of this horizon is composed of aggregates, 2 to 10 cm in diameter, displaying a laminated arrangement. Measurements in situ, by means of a pocket penetrometer and a Thorvane shear device, revealed that the consistency as well as the rate of cohesion of the middle horizon are markedly low compared with the upper and lower horizons. The middle horizon does not reflect the terraced aspect of the topographical surface!

The underlying red subsoil horizon is characterized by its high clay content (31 per cent) and its homogeneous structure.

SLUMPING AND PLANAR SLIDING 269

I

ROTATIONAL SLIP

Figure 5 . Trench just above the resurgence belt, indicated on Figure 4. 1 . Humic A-horizon (clay content 15 per cent); 2. Middle horizon. A. Laminated patch, B. Aggregates in laminated disposition, C. Closed cavities; 3. Subsoil (clay content 31 per cent); 4. Cracks; 5 . Rotated soil mass; 6. Phyllite rock outcrop; kg/cm2: pocket penetrometer test; cm/year: downslope movement, measured

as explained in text

2.

3.

The wet patches on the wall of the trench can be subdivided into two groups. One group is associated with the subvertical cracks. They evidence overland flowing water infiltrating along the cracks till the middle horizon level. The wet patch in the left part of the wal1 indicates resurging water, coming out of the middle horizon where it is cut by the topography. Two types of mass movements are evidenced by the exposure Figure 5. First, rotational slip can be distinguished in the left part of Figure 5. The configuration of the slip surfaces has schematically been drawn above. The basal part of the major slip plane coincides with the middle horizon, forming a surface along which shearing takes place. Second, the configuration of the cracks in the right part of the exposure indicates a simple planar movement of the upper horizon over the lower subsoil, the middle horizon acting as a conveyor belt. This movement, whether it should be defined as slide or creep affects the upper horizon which is divided by subvertical cracks into separated earth floes. This movement seems to be related to Sharpe's (1938) slips on major slip surface or lubricated zone.

It should be noticed here that observations in 25 Young-pits (Young (1960)) distributed equally along the three cross-sections of Rwaza Hill, confirmed the existence of the here described middle horizon over most parts of the hill where terracettes are developed. It could not be decided whether rotational slips or planar slides are dominant.

270 J. MOEYERSONS

The rate of downslope transport: indications and first measurements In October 1977, Rwaza Hill near Butare was instrumented along three cross-sections (Figure 2) in order

to measure superficial soil loss. More than 2000 nails, 18 cm long, were used as erosion pins. They were inserted, their head level with the ground surface, at a distance of 100, or 200, or 300 cm, measured in downslope direction. Depending on circumstances, every cross-section carried 3 , 4 or 5 of such nail lines, the distance being 1 m between every line. The southernmost nail line of every cross-section was equipped at distances of 50 cm (*l cm) by a numbered iron plate, fixed on the ground by stakes.

Measurements in September and October 1978 revealed the following facts: Terracettes in the middle and upper part of the three cross-sections are rapidly smoothened as a result of deposition upon the benches and erosion of the spurs by overland flow. This is illustrated by Table I. The rate of flattening is so fast that the terracettes should have been entirely obliterated after some years. Since this is not the case a rapid formation of the terracettes is implied. In addition, it was observed on the northeastern part of cross-section 2 that strong and rather well spread overland flow can result in a parallel retreat of the micro-scarps, giving rise to terracettes which had lost their initial relation with the subvertical cracks. This process leads to a fast removal of the entire upper soil horizon with formation of initial badlands, where, as a result, seepage water reappears at the surface. Slope-wise measured distances between nails changed considerably. This appeared most clearly on lines where nails were placed at 1 m distances in 1977. The 1978 records show distances varying between 1.07 and 0.96 m. This shows undoubtedly that soil floes move more or less as individual blocks. Finally, a general trend in transport could be deduced from the changes of distance between the numbered plates mentioned higher. Figure 2 shows that distances between the plates on the middle and upper slope sections did increase while distances on the lower slopes decreased calculations showed that on the southwestern part of cross-section 3 a mean downslope movement of k14.5 cm/year takes place just above the resurgence belt. This rate of movement is thought to include both soil sliding movements over the subsoil and slumping along deep in the weathered bedrock seated failures. No changes were recorded on the eastern part of cross-section 2. This could be expected because terracettes are not present on the upper and middle part of the slope while terracettes on the lower part are highly degraded by runoff wash, indicating slope stability there. Observations have shown that the valley side locally is sustained by an indurated terrace deposit more than 3 m thick.

AN EXPERIMENT RELATED TO BASAL INTERFLOW, CREEP AND SLIDE

Purpose and experimental set up In relation to the phenomena, described here, an experiment was carried out in order to study the

eventual downslope movements of a soil block, affected by seepage pressure exerted by basal interflow.

Table I. Epitome from observations of nail positions along part of cross- section 3 (Rwaza) data in millimetres; +: head of nail covered; -: head of nail above surface; (B) : position of nail on bench; (S) : position of nail on

spur ~ ~~

Cross-section 3 line 1 line 2 line 3

row 11/41 +3 (B) -5 ( S ) -8 6) 11/43 +7 (B) -7 6) +2 (B) 11/45 +3 (B) +3 (B) -10 (S)

11/49 +75 (B) +13 (B) +3 (B) 1211 +62 (B) -15 (S) -10 (S)

11/47 +2 (B) 0 (B) -15 (S)

SLUMPING AND PLANAR SLIDING 27 1

Soil material, collected in the upper horizon at Rwaza was used to form an artificial soil block of 65 cm long, 20 cm wide and 25 cm high, resting in an iron box, 70 cm long, 20 cm wide and 15 cm high, provided by 0.5 cm high anti-slip plates in the bottom (Figure 6). A thin wire was inserted vertically through the soil column in three places, in order to visualize and to measure eventual vertical differentiations in the expected downslope movement. An excavation accident after the experiment made data of wire 1 unavailable.

The contours of the 1.51 g/cm3 bulk density soil column were projected on a screen during the experiments. The box was inclined over 20". Two perforations with a diameter of 1.2 cm were provided in the downslope end of the box, 2 cm above the bottom.

Procedure During the experiment, periods of basal interflow did alternate with periods of drying. Basal interflow

was generated by lateral percolation from the upper part of the box where the water level could be maintained, and also by vertical infiltration from the surface during periods of artificial rain. Sometimes only lateral percolation was created, while in other cases a combination of lateral percolation and vertical infiltration was used. Arrows on Figure 6 indicate the assumed combined percolation movement. Artificial rain was produced by a newly developed rain simulation device. During the experiment, drops with a mean diameter of 2 mm were used, describing an oblique trajectory of 70" striking the surface of the inclined soil column at right angle. In order to obtain a high infiltration capacity of the artificial soil and in an attempt to reduce splash destruction and ablation of the ground surface, the drop velocity was kept at 7.5 m/s.

I . I

Figure 6. The artificial soil block during and after experiment. Explanation in text

272 I. MOEYERSONS

Drying was realized by the use of spot lamps, increasing the ground temperature to about 55°C. Table I1 summarizes the time schedule as applicated during the experiment and includes some results.

Observations and results Basal interflow discharge D varied markedly as a function of several factors. High values were recorded

after drying period I1 (Table 11) and also during periods of artificial rain. In some cases the D-value was nearly equivalent to the pure additional result of (discharge of interflow before rain + the calculated supply by rain). In other cases, D during rain was markedly lower than would be expected.

Data from the screen on which the soil block contours were projected indicate a clear cyclic movement. The resultant vertical velocity distribution is given by wires 2 and 3 (Figure 6). Wire 3 gives a velocity distribution with two peaks: a peak at the interflow level and a peak near to the surface. Wire 2 shows only the velocity peak near to the surface but a marked increase of velocity at about 9 cm above the bottom of the box is also present.

DISCUSSION OF FIELD OBSERVATIONS AND EXPERIMENTAL RESULTS

Interpow at Rwaza Hill: a discontinuous water layer Throughflow (Kirkby (1969)) is an important feature on Rwaza Hill. It occurs as interflow at the base of

the humic upper soil horizon where an horizon of high hydraulic conductivity exists. The presence of laminated patches, considered as filled cavities, together with the higher mentioned resurgence phenomena indicate appreciable momentary but localized discharges. Interflow discharge is thought to vary sensibly from place to place, as a result of concentrated water supply along infiltration lines such as subvertical cracks in the upper soil horizon, and suction of the generated interflowing water by the clayey subsoil and the basal part of the upper soil horizon. In this respect, the interflowing water layer should be considered as a discontinuous body, with very local discharge characteristics. Only heavy rains of long duration, leading to a complete water saturation of the soil could generate a continuous layer of interflowing water, leading to storm peaks as mentioned by Whipkey (1965). The discontinuity of the interflowing water layer at Rwaza Hill can be proven by a simple mathematical calculation of the seepage pressure. According to de Beer (1971), seepage pressure for the western part of cross-section 3 of Rwaza Hill can roughly be formulated as:

9, = 0.5 density of water (1) where 0.5 equals approximately the hydraulic gradient.

Supposing a continuous interflowing water body, seepage pressure on a hypothetical surface perpendi- cular to the stream lines in the high hydraulic conductivity horizon equals its mean 300 g/cm2 measured cohesion after a distance from the hill crest of

300 g/cm2 0.5 g/cm3 -

- 600 cm

As the weight of the overburden of the high hydraulic conductivity horizon can be neglected, this calculation contradicts the situation in the field where the slope is devoid of its upper soil mantle at a distance of 160 m from the summit, where the slope inclination steepens to 40".

Types of failure and pre-failure conditions In situ measurements by means of a pocket penetrometer (Figure 5 ) and a Thorvane shear device indicate

potential failures in the high hydraulic conductivity layer. Curved failures, leading to rotational slip, partly follow this weak horizon, while rectilinear failures, resulting in planar sliding, are entirely developed within this horizon. In fact, linear failures are only visible in the upper soil horizon, while cavities and torsive structures in the high hydraulic conductivity layer indicate more complex movements there.

The morphological downhill sequence, described for Rwaza Hill, as well as the measurements along the iron numbered plates indicate an increasing downslope movement from the top of the hill to the resurgence belt. Thus, while sliding prevails more downslope, creep might be dominant near to the hill top.

SLUMPING AND PLANAR SLIDING 273

to 2 mm .......................... interflow: 202

Table 11. Time schedule as followed during experiment: Is (simulated rainfall intensity) in mmjh.; interflow: discharge in cm3/min; position 1 , 2 . . . indicated on Figure 5

. . . . . . . . . . . . . . . . . . . . . interflow: 158

Yearjmonthjday G.M.T. Application Result

’79-1-3 0: 9.30 h 10.1 1 h 12.09 h 16.46 h

’79-2-6: 8.50 h 9.00 h

11.34 h 13.00 h 13.14 h

13.20 h 13.23 h 14.15 h 15.22 h 15.27 h 15.50 h 16.43 h

’79-2-27: 11.00 h

11.25 h

12.20 h 12.22 h 13.03 h 13.15 h 13.38 h

‘79-3-2:

‘79-3-9:

9.00 h 10.19 h 13.50 h 14.00 h 14.14 h 15.27 h

12.15 h 13.00 h 13.40 h 13.55 h 14.06 h 14.12 h 14.24 h

’79-3-12: 14.00 h

Following this appreciation, the experiment, as carried out, reflects a situation not too far from the hill top where a pre-shearing situation exists. It is thought that the experimental results show the preamble to the development of a shearing surface. Hereby a sudden increase of creep velocity (wire 2) or even a velocity peak (wire 3) occurs at the level where failure can be expected, as indicated on Figure 5 . This indicates movement of a plastic viscous soil material, called rheological creep, differing in nature with creep

274 J . MOEYERSONS

movement occurring near to the top of a soil as described by Kirkby (1967). It should also be mentioned here that the measurements along wire 3 confirm the interpretation by Lewis (1974, 1976) of identical velocity distributions, recorded at Puerto Rico, where the basal peak was considered as a throughflow effect.

The volumetric problem related to planar sliding on R w a z a Hill It is not clear how planar diding, of a fissurated soil mantle can give rise to a stepped surface in the context

of the data available. It is a working hypothesis that backward tilting of the floes occurs during their downslope movement.

The origin of the high hydraulic conductivity horizon It is the author’s belief that the origin and development of the horizon of high hydraulic conductivity still

remains to be explained its situation in the soil profile, at the base of the more permeable humic A-horizon overlying the less permeable subsoil does not indicate the causal relation, Indeed, besides the hypothesis that interflow could originate in preliminary differentiated soil horizons, the possibility remains that soil differentiation could be the result of a process whereby initial overland flow buries itself gradually deeper into the soil as a result of outwash of fine particles at its base.

ACKNOWLEDGEMENTS

The author wishes to express his gratitude to the Rwandese government which kindly permitted this research program.

This study could only be realized with the aid of Director Mr. C. Rugamba and the entire Staff of the ‘Institut National de Recherche Scientifique’ at Butare.

I am grateful to Prof. J. de Ploey for the discussion preliminary to the production of this article. Finally I wish to thank participants of the 2nd Benelux Colloquium of Geomorphological Processes for

their comments.

REFERENCES

Bunting, B. T. (1961). ‘The role of seepage moisture in soil formation, slope development and stream initiation’, American Journal of

De Beer, .Grondmechanica. Dee1 I : Inleidende begrippen, Standaard Wetenschappelijke Uitgeverij, Antwerpen, Leuven, Utrecht. Fletcher, J . E.. Harris, K . , Patterson, H. B., and Chandler, V. N. (1954). ‘Piping’, Transacfionsof the American Geophysical Union, 35,

Science, 259, 503-518.

. - 259-262.

Kirkby, M. J . (1967). ‘Measurement and theory of soil creep’, Journal of Geology, 75,360-374. Kirkby, M. J . (1969). ‘Infiltration, throughflow and overland flow’ in Chorey, R. J., Water, Earth and Man, Methuen, London,

Lewis, L. A. (1974). ‘Slow movements of earth under tropical rainforest conditions’, Geology, 2,9-10. Lewis, L. A. (1976). ‘Soil movements in the tropics-a general model’, Zeitschrift f u r Geomorphologie Supple, 25, 132-144. Sharpe, C. F. S. (1938). Landslides and Related Phenomena, New York. Whipkey, R. Z. (1965). ‘Subsurface stormflow from forested slopes’, International Association of Scientific Hydrology Bulletin, 10,

Young, A. (1960). ‘Soil movement by denudational processes on slopes’, Nature, 188, 120-122.

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