Landslide Initiation in Saprolite and Colluvium Brasil

2007) 104–119www.elsevier.com/locate/geomorph

Geomorphology 87 (

Landslide initiation in saprolite and colluvium in southern Brazil:Field and laboratory observations

Willy A. Lacerda ⁎

Civil Engineering Program, COPPE – Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa,UFRJ – Federal University of Rio de Janeiro, Brazil

Received 25 September 2004; received in revised form 4 December 2005; accepted 23 March 2006Available online 30 November 2006

Abstract

The weathering of granitic and gneissic rocks in tropical regions can reach depths of more than 100 m. In southeast Brazil thereare situations where landslide initiation depends on the fluctuation of the groundwater level, on the impact of falling rocks and onintense rainfall, causing superficial slides. The fluctuation of groundwater induces cyclical variations of the pore water pressure,and consequently of the effective stresses. This variation causes cyclic expansion and contraction of the structure of the saproliticsoil, weakening the imbrication of grains and loss of the cementation that may exist. This could be called a “fatigue” phenomenon.The practical effect is the lowering of the Mohr shear strength envelope, and a sudden rupture of the soil at a lower groundwaterlevel than that which would be compatible with the intact soil strength properties, initiating a landslide.

Another situation arises during intense rains, when a rock slab or a rock block detaches from the uppermost parts of a slope.This occurs where steep rock outcrops exhibit relief joints or where residual blocks of granite roll down the slope, impacting thecompressible, saturated colluvial soil overlying the saprolitic soil. The sudden increase of pore pressure can liquefy the soil. Finally,another mechanism is that of the advance of a saturation front in a steep slope of unsaturated saprolitic soil, reaching a depth belowthe root zone. The loss of the cohesion due to suction, without the beneficial contribution of the roots to the shearing strength,causes a sudden slide. During extreme rain episodes literally hundreds of such superficial slides, reaching 1 to 3 m in depth, occurin a given basin. The concentrated runoff that flows along the surface of the thalweg of the basin carries this soil in a muddy state,and a debris flow ensues.© 2006 Elsevier B.V. All rights reserved.

Keywords: Landslides; Debris flow; Tropical soils; Saprolite; Colluvium

1. Introduction

In Fig. 1 the southeastern coast of Brazil is shown. Ashaded portion lies between a massive mountain range(The “Serra do Mar”) and the Atlantic Ocean, betweenparallels 20 °S and 30 °S. The basal rock is part of the

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coastal shield, and is composed mainly of gneiss andgranite, with intrusions of basic rocks (diabase dykes arecommon). From the highest elevation of the mountains(reaching little more than 2000 m in isolated peaks, withan average of 800 m) down to the coastal plains, sapro-litic and colluvial soils cover the rock mantle, until thealluvial plains are reached. It is along these slopes thatlandslide phenomena that affect engineering works areactive.

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105W.A. Lacerda / Geomorphology 87 (2007) 104–119

2. Soil nomenclature

Blight (1997) defines “residual soil” as the weatheredmaterial of the “in situ” rock, and which did not suffertransport since its formation. In Brazil it is common, ingeotechnical literature, to call saprolitic soil “residualsoil”, meaning that the igneous rock was decomposed “insitu” (Barata, 1969). A further distinction is made for thesuperficial part of the saprolitic soil, in general cementedby iron and aluminum oxides. It is usually called inBrazil “mature residual soil”, and the underlying sapro-litic soil, with no cementation, is referred to as “youngresidual soil”. Colluvial soil, being a superficial deposit,is also subject to this oxide forming process, and may bemistaken as “mature residual soil”. In this paper the termsaprolitic soil will be used meaning both young andmature residual soil. The term colluvium, in the presentpaper, refers to both practically intact soil material fromslides with little transport to completely remoldedmaterial of large and sudden landslides.

3. Some characteristics of saprolitic and colluvialsoil in a tropical environment

Ab'Saber (2003) describes the main characteristics ofsaprolitic and colluvial soils in Southeastern Brazil:

• Deep weathering of crystalline rocks (up to 100 m ormore);

Fig. 1. The “Serra do Ma

• generalized occurrence of red yellow podzolic soils,or latossols, along the hill slopes, developed on col-luvial and eluvial deposits and on the top of roundedhills, and also on the saprolitic soil derived fromthese crystalline rocks;

• superposition of colluvial deposits, due to the cli-matic fluctuations of the Quaternary, often separatedby stone lines, burying paleo-thalwegs;

• presence of “sugar loaves” in areas where the tectonicjoint spacing is abnormally large, of the order ofhundreds of metres, and where the relief jointingsystem is more frequent;

• rivers and springs fed perennially by the phreaticwater;

• perched water tables along the humic layer of theforests.

These facts are also reported by Fookes et al. (2000),who, commenting on their geomorphological model forhot wet climate, state that the depth of weathering can bemany tens of metres, controlled by the distribution ofdiscontinuities in the rock mass. Also, duricrusts are for-med where groundwater concentrates soluble weatheringproducts (Thomas, 1994). Deeply weathered colluvialsoils are often difficult to distinguish from in-situ weathe-ring profiles. Laterites are hard subsurface deposit ofoxides of aluminum and iron found in tropical soils wherethe water table fluctuates with seasonal changes in preci-pitation. This process is called “laterization”, and when

r” region in Brazil.

Fig. 2. Formation or colluvium under a rock outcrop (Fookes, 1997).

106 W.A. Lacerda / Geomorphology 87 (2007) 104–119

the laterite is not present, the soil is called “lateritic soil”by some authors, and this includes both colluvial andsaprolitic soil. These definitions were discussed in theProgress Report of the Committee on tropical soils of theISSMFE (Committee on tropical soils of the ISSMFE,1985). Fig. 2 shows the formation of colluvium in atropical environment.

4. Stress–strain behavior of saprolitic and colluvialsoils

A feature that shows the distinct behavior of sapro-litic and colluvial soils is their stress–strain curves intriaxial tests, or stress–displacement curves in directshear tests, as shown in Fig. 3 (Lacerda and Silveira,1992). It can be seen that the saprolitic gneissic soil (a)exhibits peaks up to 600 kPa, whereas the shear stress ofcolluvial soil (b) is strain hardening.

Both colluvial and saprolitic soils near the surfacecan form cementation due to iron and aluminum oxideformation. In this case, a true cohesion exists, and thestress–strain behavior of both soils is similar. In Brazilthe geotechnical engineers use “mature residual soil”, or“lateritic soil” for this condition. Sometimes lateriticcolluvial soil exhibits peak and dilatancy at low normalstresses, under 100 kPa.

Sandroni and Maccarini (1981) discussed the in-fluence of feldspar, quartz and mica on the strengthenvelope of drained direct shear of saprolitic soils. Foramounts of mica above 30% there is a tendency forlower shear strength envelopes, with effective frictionangles below 20°, while for low mica content and highfeldspar/quartz content friction angles are above 25°,typically 30°.

5. Mechanisms of instabilization

Several authors (Barata, 1969; Costa Nunes, 1969;Wolle, 1985 and Lacerda, 1997, 1999, 2004) have dealtwith the causes of instabilization in tropical regions.

The main mechanisms of instabilization of saproliticand lateritic soils and rock slopes operative in the Serrado Mar range can be summarized as follows:

• Rupture by cyclic pore pressure in saprolitic soils• Loss of suction• Debris flows• Rupture induced by fall of rock slabs onto saturatedcolluvial or residual soils

• Rock slides along relief joints• Slide reactivation due to artesian aquifers• “Injection” of water from deeply seated permeablerock veins

• Obstacles to ground water flow

The main mechanisms of reactivation of colluviumslopes with a near surface ground water level are:

• Cuts made at the toe of the slope• Loads (usually road embankments) imposed fromtheir middle section up

• Elevation of the ground water table

6. Rupture of saprolitic soil by cyclic pore pressurevariation

The stress state of an element of soil in a slope hastwo components: the total stress state and the pore waterpressure (positive or negative) acting on it. The positive

Fig. 3. Normalized shear stress vs. displacement of residual (left column) and colluvial soils (right column) (Lacerda and Silveira, 1992).


pore pressure variation is due to the variation in theground water level. In this case, the total stress remainsconstant, for practical purposes. Failure can then occur ifthe pore pressure is such that the effective Mohr strengthenvelope is reached. This mechanism of failure of satu-rated saprolitic soil with one cycle of pore pressureincrease was called to attention by Brand (1981).

Lacerda (1989) noted the fact that many saproliticslopes may fail during a rainy period of lesser intensitythan previous ones, which were withstood withoutfailure. The natural conclusion is that the soil elementhad been subjected to a higher pore pressure in thepast than that which caused failure in the present. Sincepore water pressures fluctuate seasonally, it could wellbe that the soil undergoes a weakening of the inter-granular bonds due to this cyclic behavior, which isgenerally below the critical level of pore pressures thatlead to failure. The basic assumption is that the cyclingof pore pressure below the one cycle failure porepressure value breaks the interlocking of the soilparticles, and it may fail with a lower failure envelope.

This phenomenon could be interpreted as a fatiguebehavior.

That is to say that the pore pressure cycling up to amaximum pore pressure that is less than that necessaryto fail the specimen in a triaxial stress path with mono-tonic increase of pore pressure. This cycling inducesaccumulated shear strains that help the breaking of thebonding or interlocking responsible for “true” (saturat-ed) effective cohesion, thus lowering the Mohr strengthenvelope, until the soil fails with a smaller pore pres-sure. The hypothesis that ϕ′ is constant during thisprocess is not unreasonable, and is adopted.

So, the soil may fail under an effective stress statebelow the failure envelope determined by shear strengthtests of intact soil specimens. This proposal was inde-pendently put forward by Eigenbrod et al. (1987) forcompacted soils.

Fig. 4 shows the typical situation in gneissic–graniticrocks, where the range of variation of piezometric levelin the lower aquifer of the saprolitic soil is shown. Fig. 5shows the failure by a monotonic increase of pore

Fig. 4. Typical variation of piezometric levels in residual soils from gneissic–granitic rocks (Lacerda, 1989).


pressure, with the total stresses maintained constant;Fig. 6 defines pore pressures uo, umax and uf. Porepressure uo is that corresponding to an equilibriumfriction angle, such as the residual effective frictionangle. Pore pressure uf is that necessary to fail the soilby a monotonic increase in pore pressure, and umax isthe maximum value of the applied cyclic pore pres-sure in the cyclic triaxial test. The ratio Uc= (umax−uo) /(uf−uo) gives an idea of the degree of intensity of porepressure cyclic loading. Uc=1 means rupture in just onecycle.

Fig. 5. Rupture by monotonic increase in pore water pressure (Brand,1981).

It is difficult to obtain field data of pore pressuresprior to a failure, since the exact location of the ruptureis often unpredictable, but a simulation of this mech-anism can be made in the laboratory. Drained triaxialtests with cyclic pore pressure (CPP tests) were per-formed (Fig. 7). Tests were carried out on undisturbedsamples of a saprolitic soil from the Soberbo Road Slidein Rio de Janeiro. The tests were performed using thefollowing procedure: (i) saturation by back pressure;(ii) consolidation under isotropic condition; (iii) increaseof deviator stress with controlled stress (anisotropic

Fig. 6. Definition of pore pressures uo, umax and uf.

Fig. 7. Cyclic pore pressure triaxial tests.

Fig. 9. Uc vs. number of cycles to failure.


consolidation) until a desired value of shear stress; (iv)application of cyclic pore water pressure. During theCPP tests, axial and volumetric deformations weremeasured. Santos Jr. et al. (1997) describe the testingprocedures in detail.

Lacerda and Santos Jr. (2000) obtained yield surfacesfor this granitic saprolitic soil in a saturated conditionfrom drained isotropic compression tests, which con-firmed tests by Lacerda and Silveira (1992) on the samesoil, and from tests with failure induced by increasingthe pore pressure while maintaining the shear stressconstant (Fig. 8).

The same yield surface was defined by both tests. Thecurve of the yield or limit states (Fig. 8) has the ap-proximate shape of an ellipse in a p–q diagram, as also

Fig. 8. Yield surface of granitic residual soil.

shown by Sandroni and Maccarini (1981) and Leroueil(1997).

Fig. 9 shows these results (Santos et al., 1997). For acyclic pore pressure ratio Uc of 67% failure is reachedafter 400 cycles (forUc=1 the soil fails in the first cycle).This is a possible mechanism that helps to explain someslope failures under low accumulated rainfall.

It is important to note that this “fatigue” behaviorapplies only to stress states below the yield line. Whenthe soil is in a stress state above the yield region thecementation bonds have already been broken, and thesoil behaves essentially as a granular material. At shal-low depths the soil stress state is within the yield region.These results indicate that this hypothesis seems to betrue for a saprolitic granitic soil involved in the largehistoric landslide at Soberbo Road.

7. Loss of suction

Loss of suction is a major instabilization process, andthe resulting unstable mass has been the source materialfor some of the severest debris flows, when it occurs inclosed basins.

Vegetation increases the shear strength of the super-ficial soil, which will show a cohesion intercept due tothe root reinforcement. In slopes where the phreaticlevel is low, these slopes are essentially stable. Never-theless, infiltration of rain water brings this superficialsoil to a near saturated state.

If the saturation front reaches a depth situated belowthe root zone the loss of apparent cohesion may causethe slope to fail (Fig. 10). Wolle and Hachich (1989)have shown this phenomenon as the cause of thousandsof landslides near the summit of the coastal range nearCubatão, Sao Paulo, during periods of extremely heavyrainfall. The accumulated debris from the landslides inthe flatland near the sea was the cause of the mud

Fig. 10. Shallow slide due to loss of suction.

Fig. 11. The Santa Genoveva Nursing Home accident: cross section(Barros et al., 1989).


covering the city of Caraguatatuba, near Santos in 1967(Jones, 1973).

8. Debris flows

When the rain intensity is higher than 70 mm/h, and ithappens after a period of extended rain (typically200 mm of accumulated rain in the last 7 days) there isthe danger of tens or thousands of shallow landslidesinitiating almost at the same time. Jones (1973), as wellas Barata (1969) and Costa Nunes (1969), describe theextreme events of 1966 and 1967 in this region. Locally,rainstorms can affect just a small region, and isolatedlandslides initiate debris flows. Sometimes the trigger ofa debris flow is a shock wave, produced by impactloading of rock or of a landslide that suddenly covers asaturated colluvium (Sassa, 1985; Barros et al., 1989).

9. Rupture induced by fall of rock slabs onto saturatedcolluvial soils

A debris flow of moderate proportions caused by theimpact loading of fragments of a large granite–gneissicrock slab is described by Barros et al. (1989), andLacerda (2004). They explain the triggering factor of thedebris flow as follows. A slab of appreciable dimensions(thickness from 2 to 4 m, length of more than 10 m)detached from a gneissic scarp of the Corcovado massifdue probably to cleft water pressures. The event tookplace in 1988, after 18 days of almost continuous rain(total of 840 mm), which were more than sufficient to

completely saturate the 5 m thick colluvium, composedof more than 30% of rock blocks, some of which morethan 4 m in diameter, at a 20° inclination. Sometimesgeologists in Brazil use the term “talus/colluvium” todistinguish this particular type of colluvium. Thisamount of rain was divided in three distinct periods bySantos Barros and Brandão (1992); the accumulatedrainfall in the last 3 days was 441 mm, with a peakhourly rainfall of 56 mm/hr in February 22, the day ofthe debris flow. The slab fell from elevation 525 m andwas broken on its first impact on the rock scarp justbelow, at elevation 400 m; the resulting flying blocks(with an energy that cut some high trees in half)impacted the colluvium at elevation 280 m. Figs. 11

Fig. 12. The Santa Genoveva Nursing Home accident: plan view(Barros et al., 1989).


and 12 show the section and the plan view of the site.The colluvium liquefied, and ran until a break in thedeclivity of the slope, in which there was a nursinghome, at elevation 180 m. Another debris slide occurredin an almost parallel paleo thalweg, also saturated, andwhich usually carried superficial water flows duringstorms. The two debris flows joined exactly at thenursing home, indicated by the black arrow, as can beseen in Fig. 12. About 53 people were killed in thisaccident alone. Figs. 13 and 14 show respectively the

Fig. 13. Aspect of the nursing home 1 day aft

final position of the debris of the landslide, on a break ofthe slope, where the buildings were situated.

10. Rock slides along relief joints

The upper part of rock outcrops is sometimes steeplyinclined, and the relief joints are subjected to waterpressures due to rain infiltration. This can lead to thedetachment of individual blocks or slabs. When theseblocks fall on the scarp below, they may either initiate adebris flow, as discussed in Section 7, or, if the region isforested, and the soil below is not saturated, can accu-mulate on top of saprolitic or colluvial masses (Fig. 15),and become part of it. In tropical regions it is rare toobserve true talus deposits, which are more common indry, or temperate regions with little or no saprolitic soilformation.

11. Slide reactivation due to artesian aquifers

Saprolitic soils in the saturated state are often sepa-rated from the upper colluvial cover by a clayey soil,which can be an oldmature residual soil that was coveredby the colluvium, or a layer with a concentration ofelluviated, fine material from the more impermeablecolluvium. In this case, the aquifer of the saprolitic soilcan be confined, and semi-artesian or even artesian porepressures can develop in the lower stratum if the rate ofinfiltration from higher elevations is high. The exampleof a slide due to this cause is that of Urubu's Hill in thecity of Rio de Janeiro, which occurred on April 8, 1966.Several previous slides had been reported by the

er the landslide (Photo by Ary Maciel).

Fig. 14. Aspect of the region washed through by the landslide. Gneissic rock appears where a 5 m thick colluvium/talus existed. At right and far left,note the intact colluvium (Photo by the Author in 1989).


residents of the densely occupied area. These slideshappened 30 years before, and it is logical to assume thateven when the area was uninhabited they may have alsooccurred, but had been unnoticed. The slide involved the

Fig. 15. Large fallen rock block, with more than 10 m in maximumdimension, in a forested slope (Tijuca forest, Rio de Janeiro). Photo bythe Author.

subsidence of the upper part of the slope, forming a scarpwith 4.5 m, cutting a house in half. At the foot of theslope the ground suffered a heave of 1m, and the soil andthe walls of the houses at this region were cracked. Allresidents had time to evacuate their houses, because theslide was not sudden. This accident is reported byMoreira (1974). Fig. 16 shows the plan of the site. Rotarydrill borings with Denison sampling, piezometers and 4inclinometers were installed. Sounding S-54, at the sideof inclinometer I-4, showed that the subsoil consisted ofa colluvium/talus up to 17 m thick resting on a clayey

Fig. 16. Urubu's Hill plan (adapted from Moreira, 1974).

Fig. 17. Cross section A–A′ showing failure surface and piezometric lines (adapted from Moreira, 1974).


laterized residual soil 4 m thick. Below this mature soil,“young” residual soil, or saprolitic soil was encountereduntil a depth of 42m. From 42 to 47m the rock, a biotite–gneiss was very fractured, with schistosity inclined20 degrees to the horizontal; from 47 to 50 m, wheresound gneiss was finally reached. In the colluvium,weathered boulders of gneiss were encountered, onewith 4.2 m in diameter, another with 2.7 m, as shown incross section A–A′, Fig. 17. This profile is typical ofmany colluvium deposits in southeastern Brazil.

From the Denison “undisturbed” samples, classifica-tion, drained direct shear and isotropically consolidatedundrained triaxial tests were performed. These resultsare shown in Tables 1 and 2. It can be noticed that thesaprolitic soil effective friction angle is similar to thepeak friction angle, despite the 40% amount of clay.Lacerda and Fonseca (2003) have shown that this occursin lateritic soils, which behave as granular materials.The base rock is migmatite gneiss, and the existence of aquarry at the other side of the hill permitted the visualinspection of the sound rock.

Stability analyses carried out with the piezometriclines indicated in section A–A′ were performed. For aSafety Factor of 1.0 the contact between the colluvium

Table 1Average from several classification tests in each layer (Moreira, 1974)

Depth (m) Soil Atterberglimits

Grain sizeanalysis

WL WP PI Clay Silt Sand

8–16 Colluvium matrix 42 22 21 33 17 501–15 Decomposed boulders – – NP 13 11 7217–18.4 Residual, clayey 58 34 24 40 25 3522–38 Saprolitic 48 32 16 14 11 75

and the clayey residual soil or the contact between theclayey residual soil and the saprolitic soil had to be in aresidual shear strength condition with c′ equal to zeroand ϕ′res equal to 30°. The inclinometric data showed amovement occurring along the thickness of the clayeysoil, with no movement in the saprolitic soil. The collu-vium moved as a block, accompanying the movement atthe contact with the intermediate, clayey layer.

Fig. 18 shows the 25-day accumulated rainfall for thesummer of 1966 (January to March). It can be seen thatthe rupture occurred in a period in which the accu-mulated rainfall was of the order of 350 mm, while amaximum of almost 600 mm was registered in January.It is postulated that during this time lag (the slide oc-curred in April 8) the pore pressure of the artesianaquifer reached a critical level. A similar situation wasalso reported by Jiao and Malone (2000).

12. “Injection” ofwater fromdeeply seated permeablerock veins

Water recharge by means of concealed springs con-nected to water bearing fractures in the underlying rockcan alter significantly the flow pattern in its neighbor-hood, and a suitably located piezometer would show

Table 2Shear strength tests (Moreira, 1974)

Soil Test c′ (kPa) ϕ′ c′res (kPa) ϕ′res

Colluvium DST*, drained 24 29 – –Clayey residual soil CIU** 29 30 – –Saprolitic soil CIU** 19 32 5 30

*DST – direct shear test; submerged; **CIU – isotropicallyconsolidated triaxial test, saturated.

Fig. 18. 25-day accumulated rainfall in the summer of 1966.


artesianism. Local artesianism can initiate landslides inupper colluvial layers, as the Author has seen in Brazil.

In order to simulate this situation Borges andLacerda (1986) made Finite Element analyses of aslope with an initially low water table, and thenapplied a source with a piezometric pressure just aboveground level, as Fig. 19a) and b) from Lacerda (1999),

Fig. 19. Influence of a hidden spring on the f

shows. The arrow indicates the direction of flow. Thewater level is significantly altered to a position closeto the slope surface. If a cut were made in this slopein the dry season, it would eventually fail during thewet season, and, even without a cut, the slope wouldcertainly show signs of instability. The observation ofsome slides in natural slopes just after a very heavyrainy period shows springs of water near the crown ofthe slide.

13. Obstacles to ground water flow

The remnants of the of Soberbo Road debris flowin the city of Rio de Janeiro, in 1967, have beenextensively studied (Soares et al., 1988; Lacerda andSchilling, 1992). The material deposited after the flowconstitutes a colluvium, which moves continuously (inreality, intermittently, according to the elevation of theground water table). Fig. 20 shows a partial plan ofthe slide. Section C–C1 along zones C, B and D isshown in Fig. 21. The two upper branches of thiscomplex slide present a colluvial layer on top of theresidual soil. The thickness of the colluvium liesbetween 6 and 10 m. The water table is close to thesurface, except at two zones, when it appears actuallyat the surface. At these points impermeable diabase

low pattern of a slope (Lacerda, 1999).

Fig. 20. Map of the Soberbo Road slide (Lacerda and Schilling, 1992).


dikes were found while perforating long (80 m)horizontal drains. The dykes are shown in Fig. 21,which also show the position of piezometers,inclinometers and superficial marks. The accumulatedmovement of the superficial marks can be seen toincrease as the diabase dykes are approached, as the

arrows in Fig. 20 show. Two piezometers and a waterlevel indicator were installed at most of the soundings,and they are shown as positions A, B and C. Thearrows in this figure indicate the direction ofmovement of the flow lines. They are seen to bendupwards near the diabase dikes. The position of the

Fig. 21. Section along the Soberbo Road Landslide showing two families of diabase dykes (Lacerda, 1999).


more impermeable dikes influences the flow lines.Thus, artesian pressures can be observed just beforethe dykes, as shown. In the Soberbo Road caseartesianism was indeed observed, the water level of

Fig. 22. Displacements measured by inclinometers in the region

the deepest piezometer just at the upper contact withthe dyke rising more than 1 m above the groundelevation. Of course, the ground was very wet, withrivulets of water springing at the surface.

between the diabase dykes (Lacerda and Avelar, 2003).

Fig. 24. Sequence of elevation of the water level inside the flume(Lacerda and Avelar, 2003).


The local stability of the colluvium is decreased justabove the obstacle to flow which the diabase dykesrepresent. Indeed, superficial horizontal movementswere larger at this location, as already discussed, and asuccession of cracks and the inclination and displace-ment of small trees and inclinometers showed the signsof this instability (Fig. 22).

In order to clarify the behavior of colluvial massesunder seepage obstructed by barriers, such as the dia-base dikes, a flume was constructed (Avelar, 2003), andtests performed with different inclinations of the box,filled with clean sand, under different inclinations. Thesand used in the tests was a washed, fine beach sandconsisting predominantly of quartz grains. The grainsize was between 0.149 and 0.420 mm (passing # 40sieve, retained in the # 100 sieve), and its specificgravity was 2647. The friction angles varied from 32 to35.5 degrees under a confining pressure of 25 kPa, insaturated–drained triaxial tests, with void ratios between0.75 and 0.85 (near the loose state). The permeabilityvaried between 2 and 3×10−2 cm/s for this range ofvoid ratios. The dry density was in the range between14.2 and 15.4 kN/m3.

The dry sand was loosely placed with the help ofspades, with the box resting horizontally. After thisloose placing the wand was wetted with a water hoseunder low pressure, and its surface was straightened outwith a steel ruler. The end result was a plane surface,with a depth of sand of 15 cm. The barrier consisted of apiece of wood with a height of 10 cm, firmly glued to thebottom and sides of the box.

Fig. 23 shows the Flume set up (Lacerda and Avelar,2003). Water reservoirs at the top and bottom of the boxare maintained with a constant water level, andobservations are made during the steady-state seepagepattern established. Electric and standpipe piezometers

Fig. 23. Flume set up (Lacerda and Avelar, 2003).Fig. 25. Slide of the soil mass above the barrier (dyke) (Lacerda andAvelar, 2003).


monitored the advance of saturation. The displacementof the sand mass was measured at surface and withcolored vertical dark-stained sand “piles”, simulatinginclinometers.

The procedure was to raise the box to the desired angleand then begin the process of water percolation, through ahose connected to the upper reservoir, maintaining aconstant water level. The phreatic line rose slowly, and thefirst readings of the piezometers were in those nearest thebarrier. Fig. 24 illustrates this sequence, together with theobserved displacement of the sand mass. Tests were runwith the inclination of the box varying from 10° to 30°, in5° intervals. For tests with box inclinations between 20°and 30° there was a clear formation of the shear zone justbefore the barrier. Fig. 24 shows the shear zone, seen indetail in the photograph of Fig. 25. The barrier forces theshear zone to curve upward, corroborating field evidenceof near circular slides at this location. The shear zoneexpands as the movement progresses.

14. Conclusions and remarks

1) Slides in shallow residual soils are generally sudden,due to their brittle stress–strain response, at low stresslevels. Deep seated slides in saturated saprolitic soilsmay not be sudden, because the stress strain behaviorat confining stresses larger than about 1000 kPa isgenerally strain hardening.

2) Saturated colluvium deposits are common in tropicalareas. Their brittle behavior is limited to confiningstresses lower than 50 kPa. They present a strainhardening stress–strain behavior, whichmeans that theyslide progressively, subject to pore pressure variation.

3) Many processes can destabilize colluvium:

• Elevation of ground water level in perched aquifers;• Cuts made at the toe of long colluvium slopes;• Embankments or loads applied to their middle or topsections;

• Impact of loads (rock falls or slides);• Effect of impermeable dykes or formations at somepoint along the slope.

4) Some slides may occur after the cessation of rainfall,due to the delay in the recharge of deep artesian aquifers.

Acknowledgements

The PRONEX program (from the National ResearchCouncil - CNPq) has provided funds for the flume tests.The author acknowledges Mr. Luiz de França, for thehelp in Figs. 1 and 5, 6, 7, 8, 9.

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Landslide Initiation in Saprolite and Colluvium Brasil