Algeria-Sliding Mass

1

9th ICSGE, Ain Shams University 10-12 April, 2001.

1. Geotechnical & Marine HCE Dept. of Dar Al-Handasah (Shair and Partners).

2. Ain Shams University –Department of Geology and Dar Al-Handasah Consultants.

Stability Assessment of Active Land Creep in Soft Clayey Marl

A Case Study, Algeria

Issam Ghalyini

1 and Samir Abdel Tawab

2

ABSTRACT

Creep problems of active land mass were observed along a stretch of Lakhdaria – Liaison Motorway

(Algeria). The moving mass is highly fractured and weathered Upper Cretaceous clayey marl. Detailed

geological mapping, geophysical (shallow seismic) survey and investigation boreholes were carried out to

assess and determine the factors controlling the instability of this active mass.

The collected data from geological, geophysical and geotechnical investigations are used for developing a

two-dimensional slope stability analysis model. The slope stability analyses as well as site observations are

used for developing alternative solutions for stabilizing the land mass creep.

Key words: Land mass creep, Seismic survey, slope stability analysis and stabilization.

INTRODUCTION

Land mass creep represents one of the commonly encountered problems in the clayey marl

interbeds of the Northern part of Algeria. These thick sedimentary deposits are defined as Cenomanian

flysh. The central part of the under construction motorway (Lakhdaria – Liaison) is suffering from

extensive widely open (more than 0.5m) tension cracks resulted from slow land mass movement. The extent

of the study area is about 300 m length and 150 m in width.

Topographically the area is about 300 m high above the sea level and down sloped towered the

south of about 270 m above the sea level. Several signs of the creep land forms are observed in the study

area and include curved widely opened tension cracks, concave topography at upper slope and convex

topography at the lower part of slope, tilted trees and damage of recently- constructed drainage ditches have

been observed along the underconstruction the Motorway.

The main objectives of this work are studying the geological and geotechnical conditions of the

creep mass and recommending optimum engineering solutions for stabilizing the creep land mass.

GEOLOGY OF THE SITE

The land creep area is covered by thick deposits of clayey marl interbeds. These deposits are

affected by low grade of metamorphism resulted from the Alpine tectonic orogeny. The clayey marls

(Flysh) interbeds are belonging to the shallow marine deposits of the lowermost part of the Upper

Cretaceous (Cenomanian) age. The topmost part (about 10 m thick) is highly weathered and fractured. The

competence and strength of this unit is lost due to the effect of the increase of their water content. The

lower part as observed from the recent Motorway excavation is mainly composed of stiff to hard clayey

marl with some tight joints and foliation. The water content is minimal as if compared with the overlaying

clayey marl layers. The main outcropping geological units and the areal distribution of the widely- open

tension cracks are shown in the site geological map Figure 1.

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Figure 1: Engineering geologic map of the land creep mass of study area (Djebahia -Algeria).

The trends of the widely- open tension cracks are N60W and N80W. The angle of dip ranges

between 80 & 70 toward the South. Some antithetic tension cracks (cracks dipping toward North) are

observed and forming structurally tilted grabens as indicated in the central part of the mapped area.

LAND CREEP PROBLEMS

Creep is defined as slow, more or less continuous, down- slope movement of mineral, rock and

soil particles under gravitational stresses (Bate and Jackson, 1987). Bloo, 1998 defines the creep as barely

perceptible and non-accelerating down slope movement of soil or rock materials. The material moving

down slope is called colluviums, whether it is derived from in-situ weathering or from transferred

sediments. Terzaghi, 1950 distinguishes between seasonal and continuous creep. Seasonal creep is caused

by temperature and moisture variation in the soil or rock within the surface layer, which takes place

primarily in clayey or silty soils. The continuous creep is caused by gravitational forces and occurs below

the shallow zone of seasonal variations. A variety of surface phenomena are usually attributed to creep.

Some of these phenomena are down slope tilted trees, walls and utilities as well as tension cracks.

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The creep movement can be divided into three stages (Mitchell, 1976). The primary stage

characterized by decreases of the movement rate with time. In the secondary stage the rate of movement is

relatively constant and may last for tens of years. The tertiary stage creep rate is increasing rapidly and

eventually leads to failure. Some examples of the famous documented creep rates are 100 mm and 260 mm

per year in argillaceous schists at Heizenberg, Switzerland (Haefeli, 1953), 300mm/yer in heavily over

consolidated clays along the coast of California of slope height 50 m and slope inclination 10o (Goud, 1960)

and 4 mm/yr creep movement for the entire surface layer of interlocking roots moves downhill at Puerto

Rico rainforest, (Lewis, 1974).

The active slow mass movement along the Djebahia area causes serious instability problems along

the excavated part of the Motorway, as shown in geologic map Figures 1 and some site photos Figure 2.

The roadside graded cuts (Benches and Berms), the drainage ditches and manholes are distorted and tilted.

The existing oil pipeline allocated at the northeast part of the area has a severe dislocation and oil leakage.

Also the water reservoir and pumping station, which are located on the top of the scarp face, was also

suffering from progressive movements under the effect of the developing wide tension cracks, which could

lead to complete collapse. The construction of the motorway has been sustained until the proposed remedial

measures were implemented.

Figure 2: Some site photos showing the creep features (A and B- Trees tilting and ground

cracks, C and B, Creep scarp 5m displacement and extension cracks).

EXPLORATORY WORKS

A site investigation program was carefully studied and elaborated. The program was focused on

defining the areal extent of the creeped mass, position and nature of the active sliding surface and the

causative factors controlling the mass movement. The investigation program included the following

activities:

1. Detailed geological mapping: A geological map of scale 1:2000 was prepared for the land creep

area. All outcropping geological units and the signs of the creep movement such as tension cracks

are mapped as illustrated in the geological map in Figure 1.

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2. Shallow refraction seismic profiles: Six profiles of shallow seismic refraction survey have been

carried out along the central and eastern part of the Djebahia area. The total length of the seismic

profiles is 1365 m.

3. Site investigation boreholes: The study area has been investigated by five rotary-drilling

boreholes with depths ranging between 15 m and 21 m.. The core samples are geologically

identified and tested in laboratory to determine the geotechnical parameters of the clayey marl

interbeds.

The topographic survey, pattern of the shallow seismic profiles and the locations of the drilled 5

boreholes are shown in Figure 3.

Figure 3: Topographic map and site investigation program layout (Topographic profiles, seismic

profiles and Boreholes) of study area.

DATA EVALUATION AND GEOTECHNICAL PARAMETERS

On the basis of the geological mapping, geophysical seismic survey and site investigation

boreholes it was possible to define the necessary parameters and conditions of the creeped mass. The

geological mapping defined clearly the boundary of the active area, which is about 4500 m2. The active

mass boundary is characterized by widely opened, inclined and relatively deep (3-5 m) tension cracks. The

trend of these cracks is parallel to the creeped mass (NW-SE). The geological mapping has detected the slip

surface between the potentially active creep rock units (highly altered and fractured clayey marl) and the

underlaying stable units of layered stiff slightly altered clayey marl.

The seismic refraction velocity layers of the land creep area obtained from the seismic refraction

profiles (Figures 4) indicated that, the rock units of the creeped mass could be divided into two main units,

based on their seismic velocities:

1- Upper Unit:

This unit extends allover the surveyed area with an average thickness of about five meters. This

surfecial unit has a low velocity values ranging between 330 m/s and 950 m/s.

2- Lower Unit:

This unit extends allover the surveyed area with a relatively big thickness. This lower unit is

characterized by high seismic velocity values, which range between 1600 m/s and 2500 m/s.

3- Intermediate Lenses:

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Some local intermediate clayey marl lenses are also encountered with an average seismic velocity

of 1000 m/s.

Figure 4: Seismic velocity layers along the creep land mass (seismic profiles A-A,B-B and C-C are

trending WNW-ESE).

The seismic refraction velocity layers are shown in Figures 4 is confirmed with the

geological measured data. The highly altered surfecial unit corresponds to the low velocity range

(330 m/s to 950 m/s). The slightly weathered stiff clayey marl unit corresponds to the high

velocity range (1600 m/s to 2500 m/s). The geo-seismic cross-section taken in the direction of

land mass movement was used for predicting the depth of the slip surface (Figure 5). Based on the

geo-seismic cross-section, the slip surface is expected to pass through the base of the highly

altered low velocity surfecial clayey marl unit, with an average depth of 5 m.

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Figure 5: Geoseimic cross-section parallel to the direction of land creep mass movements.

The site investigation boreholes revealed that the top layer is about 6 m thickness. It mainly

composed of sandy clayey marl with iron oxides and ferruginous staining along the clayey marl fissures.

The lower layer up to the end of the boreholes is stiff to hard clayey marl. A highly deformed (foliated and

sheared) clayey marl layer of 0.5 m thickness is encountered at depth 5 m in borehole No.6 and No.2. The

laboratory measured geotechnical parameters of the Djebhia creeped mass can be summarized as follows:

The average bulk unit weight = 20 KN/m3

The average water content = 11%

The average degree of saturation = 80%

The average liquid limit = 42%

The average of plasticity Index = 22%

The average unconfined compressive strength = 350 Kpa (6.0 – 9.0 m depth)

Undrained and drained triaxial test parameters:

Undrained condition cohesion (c) = 30 Kpa

Friction angle () = 11o

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Drained condition cohesion (c’) = 15 Kpa

Friction angle () = 15o

The highly weathering condition of the upper clayey marl unit (about 5 m thickness) and the water

leakage from the top slope water reservoir and pumping station are considered the main factors causing the

triggering and activation of the Dejebahia Land mass creep. Also the leaked and percolated water through

the existing widely opened cracks is severely decreases the shearing strength parameters of the surfecial

clayey marl units.

STABILITY ANALYSIS

Stability analyses for the Dejebahia land creep area have been carried out using the residual shear

strength parameters. In general the determination of residual shear strength parameters along the sliding

plan using the laboratory test is difficult. The soil anisotropy and disturbance of the extracted samples

represent the main factors of laboratory measured residual shear strength parameters uncertainty.

To obtain a realistic evaluation of the creeped area shear strength parameters a slope stability back

analysis was carried out. Three critical sections along the creeped mass were analysed using Bishop

simplified methods (slope software version 9R, Borin, 1998). Based on the geological and geophysical data

the soil model used in the analysis consists of a top layer of soft clay having a thickness ranges between 3m

and 6m and underlaying layer of stiff to hard clay.

The adopted shear strength parameter of the hard clay is cohesion value of 75 kN/m2. The shear

strength parameters of the soft layer which results to a minimum factor of safety equal to one were

determined. These parameters are considered as the average parameters causing the unsuitability (land mass

creep). The results of stability analysis at a minimum Factor of Safety are giving in table no.1.

Table No.1: Shear Strength Parameters at Minimum factor of Safety.

Cohesion

C kN/m2

Friction Angle

Factor of Safety

F

1 20 0.987

2 19 0.994

3 18 0.994

4 17 0.989

5 16 0.980

From this table we can conclude that the shear strength parameters that have triggered the land creep are as

follows:

Cohesion strength = 1 to 5 KN/m2

Friction angle = 16o to 20

o

The slope stability results of the three selected profiles (S1-1, S2-2 & S3-3) by using the triggered

shear strength parameter. These back analyses revealed that the existing ground surface geometry is unsafe

under the critical shear strength parameters (=20 correspond to a ratio between the undrained shear

strength, Su and the effective overburden soil pressure, Po).

STABILIZATION MEASURES

Stability analyses were carried out for the three selected profiles S1-1, S2-2 and S3-3 of the land mass

creep area (profiles geographic locations are shown in Figure 3). The back-calculated shear strength

parameters are cohesion = 2 kN/m2 and friction angle = 19

o which correspond to a factor of safety equal 1.0

(failure condition). A series of slope stability runs by using Bishop’s simplified method (Horizontal

interslice forces) have been carried out at different graded slope geometry of the creeped mass.

The slope stability runs of the proposed ground surface geometry (3H: 1V slope angle, 3 m berm

width and 6 m height of each berm) give reasonable factors of safety. The stability run of the western side

profile S1-1 gives a factor of safety of 1.35 and 1.13 at static and seismic condition respectively. The results

of the central and eastern profiles S2-2 & S3-3 give a factor of safety values equal 1.0 at natural ground

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condition and 1.3 after using protection line of concrete piles. The illustrative sketches of the stability

analyses runs are given in Figures 6, 7 and 8.

Figure 6: Slope stability model and proposed protection measures at section S1-S1.


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In addition to slope regrading and concrete piles, a system of deep trenches not less than 5m depth

filled with granular material is also recommended to drain the infiltrated ground water within the land

creeped area.

CONCLUSIONS

The combination of geological mapping, geophysical survey and site investigation boreholes is

important tools for studying and evaluating the land mass creep.

The deep weathering (about 5 m depth), water leakage and low strength parameters represent the

main causative factors of land mass movement.

The average depth of the slip surface is 5 m as indicated from the shallow refraction velocity layer

profiles.

Slope stability analyses revealed that the recommended safest slope geometry is as follows:

Slope angle = 3H: 1V

Berm width = 3 m

Berm height = 6 m

A deep drainage system (5.0m deep trenches) is recommended for the whole creeped area.

Reinforced bored concrete piles are recommended to support the stretch of the oil pipeline passing

through the creep area.

ACKNOWLEDGEMENT

The authors would like to thank Dar Al-Handasah Shair and Partners and Mr. Issam Ghalayini

(Department Director of Geotechnical – Marine and Heavy Structures) for the encouragement and

permission of publishing this work.

REFERENCES

Bates, R.L. and Jackson, J.A. (1987), Glossary of Geology, American Geological Institute, Third

Edition, 787 p.

Bloom, A.L. (1998), Geomorphology a Systematic Analysis of Late Cenozoic Landforms, Prentice

Hill, 481 p.

Borin, D.L. (1998), Slope Stability and Reinforced Soil Analysis and Design Program, Version

9R, Geosolve.

Could, J.P. (1960), A study of Shear Failure in Certain Tertiary, Marine Sediments, ASCE

Research Conf. On Shear Strength of Cohesive soils, Colorado, pp. 615-641.

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Haefeli, R. (1953), Creep problems in Soil, Snow and Ice, Proc. 3rd

Int. Conf. On Soil Mechanics

and Foundation Engineering, 3, pp. 238-251.

Lewis, L. A. (1974), Slow Movement of Earth under Tropical Rain Forest Conditions, Geology,

V.2, pp. 9-10.

Mitchell, J.K. (1976), Fundamental of Soil behavior, John Wiley and Sons, Inc. New York.

Terzaghi, K. (1950), Mechanism of Land Slides, Geological Society of American, Engineering

Geology (Berkley) Volume, pp. 83-123.

Algeria-Sliding Mass

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