GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · Step 3: Global /overall stability Global...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

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

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


Module - 8LECTURE - 40

Geosynthetics for embankments on soft foundations

Introduction

Design of basal reinforced embankment

Placement of geosynthetics underneath embankment

Construction of basal reinforced embankment

Widening of existing roadway embankment

OUTLINE


The embankment may fail due to Low Shearing resistance of the foundation soil or excessive

deformation. Low bearing capacity High differential settlement

The conventional methods to improve the foundation forembankments are, Excavate and remove the soft soil and replace with good

quality soil Densification by dynamic compaction or vibro- compaction. Densification by grouting, and Sand/ stone/ lime column


The conventional methods are very expensive and timeconsuming.

Stability of embankment over soft soil is a major problemdue to the lower permeability of foundation soil. It takes a lotof time to consolidate after embankment loading.

Improvement in the shearing resistance of foundation soilis not sufficient to improve the stability.

Basal geosynthetic reinforcement is needed to be placedbetween the soft foundation soil and embankment fill tocontrol the embankment stability.

The basal reinforcement prevents shear failure, reducesdifferential settlement as well as improves the bearingcapacity.


In case of basal reinforced embankment, the foundationsoil consolidates due to embankment loading.

The tensile force of reinforcement will decrease overtime due to the effect of creep.

The proper selection and adaptation of polymer materialswill depend on type of polymeric materials and their creepfactors i.e. low creep reinforcement or high creepreinforcement.

Various authors have described the failure modes ofbasal reinforced embankment on soft foundation soil [i.e.Halliburton et al. (1978 a&b); Christoper and Holtz (1985);Terzaghi and Peck (1967) and Koerner (1990)].


Different types of systems to control stability and settlementof the embankments:

Basal reinforcement beneath embankment

Geosynthetic embankment with one layer of geosyntheticProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Geosynthetic embankment with one layers with folded ends

Geosynthetic embankment (two layers) with folded endsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Geosynthetic embankment with berm with one layer or more layers without and with folded ends

Reinforced embankment application (After Bonaparte and Christopher, 1987)


Geosynthetic embankment with geocell

Geosynthetic embankment with vertical pilesProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Geosynthetic embankment using enclosed stone column

Geosynthetic embankment with PVD (Alternative to sand drains)


Movement

Sliding outward along Geotextile With Crest Slumping

Tension CracksEmbankmentGeotextile

Foundation

Modes of Potential Unsatisfactory behaviour of embankment failure (After Halliburton et al.1978 and Fowler ,1981)


Geotextile

Potential Failure PlaneMud Wave

Geotextile must be torn at this location

Rotational Sliding / Slumping of Embankment(Foundation Failure)

Foundation

Embankment


Movement

Mud Wave

Excessive Elongation of Geotextile. Embankment Sinking Foundation compress & displace

Foundation

Embankment

Geotextile

Mud Wave


DESIGN OF BASAL REINFORCED EMBANKMENT

The design is performed considering the ultimate limit stateand serviceability limit state conditions (After Koerner et al.,1987; IFAI, 1990 and FHWA, 1998).

A) Determine the engineering properties of foundation soiland embankment fill. Check the ground water table.

B) Specify the required dimensions of the embankment, i.e.length of embankment, height of embankment, width ofthe crest and slope.

C) Specify the external loading over the embankment.


D) Ultimate limit state

1. Local stability2. Bearing capacity3. Global or overall stability4. Rupture5. Lateral sliding or spreading6. Pullout or anchorage7. Foundation extrusion or squeezing

E) Serviceability limit state

1. Reinforcement strain or elastic deformation2. Foundation settlement


ULTIMATE LIMIT STATE

Step 1: Local stability of embankment fill

Embankment may fail due to the slip of slope withinthe embankment. Firstly, stability of the unreinforcedembankment fill should be considered.

Local stability of embankment fillProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Bishop’s (1955) circular slip analysis is considered tocheck the local stability of embankment fill. Check thestability of unreinforced embankment,

tan

'tanFS

FS = factor of safety,

= effective angle of shearing resistance of the fill (degree),

β = slope angle (degree)


Example:

Embankment slope (β) = 26.6 degrees (1 vertical to 2horizontal)

= effective angle of shearing resistance of the fill = 30°

Check stability of the embankment slope.

Solution:

tan

'tanFS 15.16.26tan

30tanFS

(OK)


Step 2: Bearing capacity

Bearing capacity failure occurs when the maximumstress exerted by the embankment fill over thefoundation soil is greater than the bearing capacity of thefoundation soil.

Bearing Capacity failure


Geosynthetic is placed at the interface of foundation soil andembankment fill. The safety against bearing capacity can bechecked by the conventional geotechnical theory,

qult = Cf Nc ≥ γfill. He

qult = Ultimate bearing capacity of the soil (kN/m2),Cf = Undrained shear strength of the foundation soil ( kPa),NC = Bearing capacity coefficient, (from Bonaparte et al., 1986)γfill = Unit weight of the embankment fill( kN/m3), andHe = Height of the embankment (m),

Allowable bearing capacity, qallow = qult/ FS

Where, FS = Factor of safety = 1.5


Bearing capacity factor, Nc (After Bonaparte et al., 1986)

Hf = thickness of the foundationsoil

B = width of the embankmentbetween midpoints of the sideslopes

Rough

Foundation

B/Hf ≤ 2 Nc= 5.14

B/Hf > 2 Nc= 4.14 + 0.5B/Hf

Smooth

Foundation

B/Hf ≤ 0.61 Nc = 5.14

0.61 < B/Hf ≤ 2 Nc = 5.64 - 0.52 B/Hf

B/Hf > 2 Nc = 3.5 + 0.25 B/Hf


If the maximum embankment stress over foundation soilexceeds its bearing capacity,

The bearing capacity of foundation soil is to beincreased.

In other way, the embankment stress can be reduced ifthe width of embankment can be increased by flatteningits side slopes also resulting the increased bearingcapacity coefficient (Nc).


Step 3: Global /overall stability

Global stability or overall stability

- Foundation soil is fine-grained cohesive soil and inundrained condition,- Overall stability analysis is carried out using undrainedshear strength parameters of the foundation soil.

Global stability analysis provides the required strength of the basal reinforcement.

Reinforcement provides the additional restoring moment


FS = Restoring Moment (MR)/ Disturbing moment (MD)

FS = (τs. L. R + Tg . Y)/ (W . x)

τs = Shear stress = c (cohesive soil),L = Arc length,R = Radius of failure circleW = Weigh of failure zone, X = Distance between the origin and the C.G. of weigh of failure zone,Tg = Tensile strength of the basal reinforcement, and Y = Vertical moment arm of basal reinforcement layer at the base of embankment


If the embankment soil differs with foundation soil, we cancalculate the factor of safety as follows:

Case 1: Without reinforcement

Le = Length of the failure arc for embankment,Lf = Length of the failure arc for foundation,We = Weight of the failure zone for embankment,Wf = Weight of the failure zone for foundation,Xe = Moment arm to centre of gravity of failure zone inembankment,Xf = Moment arm to centre of gravity of failure zone infoundation

)XWXW(R)LcLc(FSffee

ffee

R = Radius of failure circle,ce = Cohesion of embankment soil,cf = Cohesion of foundation soil,


Case 2: With reinforcement

Ti = Allowable reinforcement strength,Yi = Moment arm to the ith layer reinforcement,

n = No. of reinforcement layers

Slope stability with multilayer reinforcements

)XWXW(

YTR)LcLc(FS

ffee

n

1iiiffee


If there is no geosynthetic layer at the interface offoundation soil and embankment fill, rotational failure of theembankment is catastrophic.

On the other hand, if geosynthetic layer is introducedbetween the foundation soil and embankment fill, the failureis not catastrophic or less catastrophic because of the largedeformation of geosynthetic reinforcement.

It is very rigorous to determine the factor of safety by handcalculation. However, many software are available in themarket to find out the factor of safety against global stability.

For cohesive fill, tensile strength (Tg) of geosynthetic isaccording to 2% strain and for cohesionless fill, it should beaccording to 5%-10% strain. If the fill soil is peat, the strainlimit is 2%-0% (FHWA, 1988).


Step 4: Check for Rupture/Tearing failure

Reinforcement fails in tension and embankment slidesover the foundation soil.


Forces at the vertical edge section of the embankment:

Pfill = Active earth pressure acting at the vertical face= Total driving force = 0.5 kaγeHe

2

ka = co-efficient of active earth pressure,γe = unit weight of embankment fill, andHe = height of embankment


Shear force at the bottom of embankment= (Ca + vt tan f) x Ls= Ca x Ls

Let, tension in reinforcement = Tg

Total resisting force = Tg + Ca x Ls

H k 0.5L.CT

FS 2eea

sagrupture

Minimum factor of safety against rupture = 1.5

(f = 0, as the foundation soil is completely saturated)

Determine Tg from the above equation. Now, we have toconsider the reduction factors.


Check:

(Tg)required = R.F. x Tg < Tallowable

Tallowable = Tensile strength from global stability analysis

In longitudinal direction, we can provide the geosyntheticwith tensile strength ≥ (Tg)required

Seam strength ≥ (Tg)required


Step 5: Check for lateral sliding failure

Embankment slides over the reinforcement after formation of crack in the embankment


Force diagram for lateral sliding:

Total driving force (Pfill) = active earth pressure force = 0.5 ka γe He

2

Total resisting force (Rg) = Ws. tan e + Ca. Ls= 0.5. γe. Ls. He. tan e (Ca = 0 for granular soil)

Ws = weigh of the sliding side slope, e = friction angle between reinforcement and embankment fill


Factor of safety against sliding = Rg / Pfill > 2 (Safe)

Step 6: Check for pullout strength

Rotational failure also occurs in embankment. It isrequired to check for pull-out strength of geosynthetic.

(Tg)design= τtop Le + τbottom Le Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

τtop = shear stress on the top of geosynthetic = σv tan δe,τbottom = shear stress at the bottom of geosynthetic = Ca,

σv = vertical stress = γe He,

Le = Embedded length of geosynthetic beyond the slip line

Therefore,

(Tg)design = σv tan δe Le + Ca Le = γe He Ci tan e Le + Ca Le

Generally,

Ci = Interaction coefficient between geotextile andembankment fill = 0.7, andCa = 40 % of the undrained cohesion (Cu) of foundation soil


Step 7: Required elastic strength of the geotextile

f

reqdreqd

TE

εf = strain in geosynthetic (Considering 5% strain, εf = 0.05),

Treqd = required tensile strength of the geotextile


Please let us hear from you

Any question?


Prof. J. N. Mandal

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


GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · Step 3: Global /overall stability Global...

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Transcript of GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE · Step 3: Global /overall stability Global...