Geosynthetics Engineering: in Theory and Practice

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328 email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay Module - 7 LECTURE - 36 Geosynthetics for steep slopes

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay OUTLINE

 Introduction  Limit equilibrium design methods for circular arc slope analysis  General guidelines for the design of reinforced soil slopes  Schemertmann’s simple sliding wedge method  Labhane’s method for slope stability  Jewell’s method for slope stability analysis  Back wrapping technique for reinforced soil slopes  Geosynthetics for in-situ slope stabilization  Texsol technique  Design of 300 meter high artificial mountain using geosynthetics

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Slopes may be manmade or natural. It may be unstable and failure may occur. In such a case, the conventional methods will be very expensive and sometimes it is very difficult to construct as per the desires of the owner.  Introduction of geosynthetics as primary and/or secondary reinforcements will enable to construct a stable slope over foundation soil. The primary geosynthetic reinforcement is placed horizontally to stabilize the steep slopes against potential global failure.  Sometimes, it is required to stabilize the face of the slope by providing relatively small and closely spaced secondary reinforcements, i.e., short edge strips or by wrapping the geosynthetic reinforcement at the face during fill placement and compaction.

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  It is needed to protect the slope from erosion. In such cases, geomeshes or geocells filled with soil can be used for vegetation. The most important is to provide the chimney drain and geotextile-wrap drainage pipe behind the reinforced soil zone to eliminate seepage forces.

 Christopher et al. (1990) reported that if the face inclination is less than 70o, it can be considered as reinforced soil slope. If the face inclination lies between 70o to 90o, it can be considered as reinforced soil wall.

 Various designs, guidelines, specifications and manuals have been provided by Cristopher et al. (1990), Berg (1993) as well as by Elias and Christopher (1997).

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Different types of arrangements of reinforcements:

(a) Equal length reinforcement with equal spacing

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay (b) Spacing equal, different in length (bottom minimum and top maximum)

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay (d) Equal spacing with one layer secondary reinforcement

(e) Equal spacing with two layers secondary reinforcement

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  The principal applications of reinforced soil slopes are reported by Tensar (1987):

 New slopes construction,  Widening of highway,  Alternative to retaining walls,  Repair of failed landslide slopes  Now a day, infrastructure, transportation, residential, commercials and industrializations are developing rapidly causing the requirements of land availability.

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay • Maximum land required for slope stability

(a) Unreinforced slope

• Saving of land

(b) Reinforced slope Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay (c) Widening of road by making the existing slope steeper with inclusion of reinforcement

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Instead of conventional retaining walls, geogrid reinforced soil slopes can be used

 Repairing of the failed landslide slopes can be possible with the inclusion of geogrid reinforcements

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Some typical reinforced soil slope sections: (a) Cross-section of a reinforced slope structure

(b) Geosynthetic reinforced soil slope over firm foundation with drainages

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Failure modes of reinforced soil slope (After Berg et al., 1989): (a) Internal failure - Failure surface passes through the reinforced zone

(b) External failure - Failure surface passes behind and underneath the reinforced soil zone

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay CIRCULAR ARC SLOPE ANALYSIS CONSIDERING COHESIVE SOIL ( = 0) - It is a limit equilibrium design method  Unreinforced Soil Slope:

M d  W e .x 1  W f .x 2

2 M r  C.R .

2 We.x1  Wf .x2  C.R .

W .x  W .x C  e 1 f 2 R 2 .  C F.O.S  f  u u C C

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay We = weight of failure zone at right side of the centre line x1 = distance between right side weight and the center line

Wf = weight of failure zone at left side of the centre line x2 = distance between left side weight and the center line R = radius of failure circle θ = angle by the failure arc at the center of the circle C = mobilized cohesion along the failure surface

f = shear resistance provided by the soil

Cu = undrained cohesion of soil

FOSu = factor of safety against slope failure

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Reinforced Soil Slope: For cohesive soil, shear strength does not rely on the normal force on shear plane. No slices are considered in the analysis. A) Single layer reinforcement Unreinforced case:

c.L.R Mr FSu = = W.X+q.d Md

Reinforced case:

Mr Mr Mr Thor Y FSr   Md Md

∆Mr= Thor x Y = Tincl x R

∆Mr = increased resisting moment due to reinforcement Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay c = undrained cohesion = 0.5 qu qu = unconfined compression strength of soil, R = radius of failure circle, L = length of the failure arc, W = Weight of the failure zone, X = moment arm to the center of gravity of failure zone q = surcharge load, d = distance between surcharge load and center line,

Le = required embedded length of the geosynthetic layer, Y = moment arm to the geosynthetic layer

Thor = allowable tensile strength of the geosynthetic layer.

Tincl = tangential component of the allowable tensile strength Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay B) Multilayer reinforcements

Mr Mr FSr  Md

n Mr =  T i  Y i 1

 Minimum factor of safety can be obtained by varying the radius and coordinates of the centre of the circle. it is very difficult to solve manually the above equations.

 Now a day, many readymade computer programs are available to design the reinforced soil slope. Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay CIRCULAR ARC SLOPE ANALYSIS CONSIDERING COHESIONLESS SOIL (c = 0)

 Unreinforced Soil Slope (After Bishop,1955) :  For cohesionless soil, the slope analysis is dependent on the vertical stress over failure surface. The vertical stress will vary along the length of the failure arc.

 The failure zone is divided into equal width multiple slices to carry out the slope stability analysis

 Factor of safety can be defined as the ratio of sum of the resisting moments for all slices to the sum of disturbing moments.

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay W.tan .sec.R 1 (tan .tan / F.S. ) F.S.  u u Wsin.R

Unreinforced soil slope in limiting equilibrium condition W = weight of the slice,  = angle of shearing resistance α = slope of the tangent to the soil slice at slip circle R = radius of the failure circle Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Reinforced Soil Slope: Single layer reinforcement Additional resisting moment,

∆Mr = Tg .R cos θ

W.tan secα  Tg .cosθ 1+ tan tanα/F  F = r r  W.sinα

W.tan secα  Tg  F. r  W.sinα-   .secθ 1+ tan tanα/Fr  

Fr = required factor of safety = 1.5 (generally) θ = angle by the circle radius with the center line at the intersection of slip circle and the reinforcement

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay CIRCULAR ARC SLOPE ANALYSIS CONSIDERING (c - ) SOIL

Slip circle analysis using Modified Bishop’s method

i n i  n cbWub'i ( i  i i )tan'sec  i /(1tan'tan    i /) F    TYcYiR i (  )/ i1 i  1 F  i n  Wisin  i i1

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay F = Factor of safety, c' = effective cohesion of the soil at the base of the slice bi = width of the slice (i), Wi = weight of the slice (i), ui = pore water pressure on the base of the slice (i), ' = effective angle of friction at the base of slice (i),

αi = inclination of the base from the horizontal for slice (i),

Ti = mobilized tensile strength of geogrid (i)

Yc = the Y coordinate of the centre of the slip circle,

Yi = the Y coordinate of the geogrid (i), R = radius of the potential failure surface, n = total number of slices considered, i = slice number

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  In the analysis, the inter-slice forces are neglected.

 The factor of safety should be within 1.25 to 1.5.

The stability equation can be written as,

MD  MRS MRR

MD = Disturbing moment due to the weight of failure zone plus the surcharge,

MRS = Resisting moment due to the strength of the soil, and

MRR = Resisting moment due to the reinforcement

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Embedment or Anchorage Length (Le): PFS Le = 2Ci .  n .tan  i

P = pullout resistance,

Ci = interaction coefficient for pullout (Dimensionless)

n = normal stress on the geogrid.

i = peak angle of friction for reinforced soil, and FS = factor of safety against pullout failure

 Minimum required anchorage length beyond the potential slip circle = 1 m

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay CONSTRUCTION OF REINFORCED SOIL SLOPE  Prepare the site: Remove all grass and debris. Prepare a uniform sub-grade using roller or rubber-tired vehicle.

 Face construction:

. If the slope angle () is greater than the angle of friction () of soil, a wrapped-face is to be provided to prevent the erosion or sloughing.

. A form work is to be placed to support the steep slope face during construction.

. The lift thickness generally considered is of 500 mm to 650 mm. Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay Construction steps of reinforced slope:

a) Lift 1 plus reinforcement for lift 2

b) Lift 2 with face wrapped

c) Lift 2 completed

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Placement of geosynthetic:

. Higher strength is required along the width of the slope. Therefore, it is recommended to place the geosynthetic with machine direction along the width of the slope.

. It should not be wrinkled. To protect from the wind, the geosynthetics should be anchored with the aid of sand bags or pins.

. A minimum overlap of 150 mm should be provided while stitching two geotextiles. Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Placement of backfill and compaction:

. Place the backfill material over the geotextile and compact it with a rubber-tired vehicle for cohesive soil or using roller for granular soil.

. A minimum 150 mm thickness should be maintained between the wheel of the roller and the geotextile reinforcement.

. Always, lightweight compactor should be used near the face of the slope. Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay . The compaction should be achieved about 95 % of standard maximum dry density within  2% of optimum moisture content.

. Then, repeat the procedure with a next layer of geotextile and backfill, and so on.

. Vegetations are to be provided at the face of the slope to prevent soil erosion.

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay  Drainage layer:

A geotextile or geocomposite drainage layer has to be placed behind the reinforced soil zone with a drain outlet at the base of the reinforced zone to drain out the water.

Drainage in reinforced soil slope

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay Please let us hear from you

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Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328 email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay