GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of Civil Engineering, IIT Bombay, Powai, Mumbai 400076, India. Tel.022-25767328 email: cejnm@civil.iitb.ac.in
Module - 8 LECTURE - 40 Geosynthetics for embankments on soft foundations
OUTLINE Introduction Design of basal reinforced embankment Placement of geosynthetics underneath embankment Construction of basal reinforced embankment Widening of existing roadway embankment
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 for embankments 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 time consuming. Stability of embankment over soft soil is a major problem due to the lower permeability of foundation soil. It takes a lot of time to consolidate after embankment loading. Improvement in the shearing resistance of foundation soil is not sufficient to improve the stability. Basal geosynthetic reinforcement is needed to be placed between the soft foundation soil and embankment fill to control the embankment stability. The basal reinforcement prevents shear failure, reduces differential settlement as well as improves the bearing capacity.
In case of basal reinforced embankment, the foundation soil consolidates due to embankment loading. The tensile force of reinforcement will decrease over time due to the effect of creep. The proper selection and adaptation of polymer materials will depend on type of polymeric materials and their creep factors i.e. low creep reinforcement or high creep reinforcement. Various authors have described the failure modes of basal 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 settlement of the embankments: Basal reinforcement beneath embankment Geosynthetic embankment with one layer of geosynthetic
Geosynthetic embankment with one layers with folded ends Geosynthetic embankment (two layers) with folded ends
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 piles
Geosynthetic embankment using enclosed stone column Geosynthetic embankment with PVD (Alternative to sand drains)
Modes of Potential Unsatisfactory behaviour of embankment failure (After Halliburton et al.1978 and Fowler,1981) Movement Geotextile Embankment Tension Cracks Foundation Sliding outward along Geotextile With Crest Slumping
Potential Failure Plane Mud Wave Embankment Geotextile Foundation Geotextile must be torn at this location Rotational Sliding / Slumping of Embankment (Foundation Failure)
Movement Mud Wave Embankment Mud Wave Foundation Geotextile Excessive Elongation of Geotextile. Embankment Sinking Foundation compress & displace
DESIGN OF BASAL REINFORCED EMBANKMENT The design is performed considering the ultimate limit state and serviceability limit state conditions (After Koerner et al., 1987; IFAI, 1990 and FHWA, 1998). A) Determine the engineering properties of foundation soil and embankment fill. Check the ground water table. B) Specify the required dimensions of the embankment, i.e. length of embankment, height of embankment, width of the crest and slope. C) Specify the external loading over the embankment.
D) Ultimate limit state 1. Local stability 2. Bearing capacity 3. Global or overall stability 4. Rupture 5. Lateral sliding or spreading 6. Pullout or anchorage 7. Foundation extrusion or squeezing E) Serviceability limit state 1. Reinforcement strain or elastic deformation 2. Foundation settlement
ULTIMATE LIMIT STATE Step 1: Local stability of embankment fill Embankment may fail due to the slip of slope within the embankment. Firstly, stability of the unreinforced embankment fill should be considered. Local stability of embankment fill
Bishop s (1955) circular slip analysis is considered to check the local stability of embankment fill. Check the stability of unreinforced embankment, FS tan tan ' 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 2 horizontal) = effective angle of shearing resistance of the fill = 30 Check stability of the embankment slope. Solution: tan ' tan30 FS FS 1. 15 tan tan26.6 (OK)
Step 2: Bearing capacity Bearing capacity failure occurs when the maximum stress exerted by the embankment fill over the foundation soil is greater than the bearing capacity of the foundation soil. Bearing Capacity failure
Geosynthetic is placed at the interface of foundation soil and embankment fill. The safety against bearing capacity can be checked by the conventional geotechnical theory, q ult = C f N c γ fill. H e q ult = Ultimate bearing capacity of the soil (kn/m 2 ), C f = Undrained shear strength of the foundation soil ( kpa), N C = Bearing capacity coefficient, (from Bonaparte et al., 1986) γ fill = Unit weight of the embankment fill( kn/m3), and H e = Height of the embankment (m), Allowable bearing capacity, q allow = q ult / FS Where, FS = Factor of safety = 1.5
Bearing capacity factor, N c (After Bonaparte et al., 1986) Rough Foundation B/H f 2 N c = 5.14 B/H f > 2 N c = 4.14 + 0.5B/H f B/H f 0.61 N c = 5.14 Smooth Foundation 0.61 < B/H f 2 N c = 5.64-0.52 B/H f B/H f > 2 N c = 3.5 + 0.25 B/H f H f = thickness of the foundation soil B = width of the embankment between midpoints of the side slopes
If the maximum embankment stress over foundation soil exceeds its bearing capacity, The bearing capacity of foundation soil is to be increased. In other way, the embankment stress can be reduced if the width of embankment can be increased by flattening its side slopes also resulting the increased bearing capacity coefficient (N c ).
Step 3: Global /overall stability Global stability or overall stability Global stability analysis provides the required strength of the basal reinforcement. Reinforcement provides the additional restoring moment - Foundation soil is fine-grained cohesive soil and in undrained condition, - Overall stability analysis is carried out using undrained shear strength parameters of the foundation soil.
FS = Restoring Moment (M R )/ Disturbing moment (M D ) FS = (τ s. L. R + T g. Y)/ (W. x) τ s = Shear stress = c (cohesive soil), L = Arc length, R = Radius of failure circle W = Weigh of failure zone, X = Distance between the origin and the C.G. of weigh of failure zone, T g = 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 can calculate the factor of safety as follows: Case 1: Without reinforcement FS (cele cf L f )R (WX WX ) e e f f R = Radius of failure circle, c e = Cohesion of embankment soil, c f = Cohesion of foundation soil, L e = Length of the failure arc for embankment, L f = Length of the failure arc for foundation, W e = Weight of the failure zone for embankment, W f = Weight of the failure zone for foundation, X e = Moment arm to centre of gravity of failure zone in embankment, X f = Moment arm to centre of gravity of failure zone in foundation
Case 2: With reinforcement FS (c e L e c (W X e f L e f )R W X f n i 1 f ) T Y i i Slope stability with multilayer reinforcements T i = Allowable reinforcement strength, Y i = Moment arm to the i th layer reinforcement, n = No. of reinforcement layers
If there is no geosynthetic layer at the interface of foundation soil and embankment fill, rotational failure of the embankment is catastrophic. On the other hand, if geosynthetic layer is introduced between the foundation soil and embankment fill, the failure is not catastrophic or less catastrophic because of the large deformation of geosynthetic reinforcement. It is very rigorous to determine the factor of safety by hand calculation. However, many software are available in the market to find out the factor of safety against global stability. For cohesive fill, tensile strength (T g ) of geosynthetic is according to 2% strain and for cohesionless fill, it should be according to 5%-10% strain. If the fill soil is peat, the strain limit is 2%-0% (FHWA, 1988).
Step 4: Check for Rupture/Tearing failure Reinforcement fails in tension and embankment slides over the foundation soil.
Forces at the vertical edge section of the embankment: P fill = Active earth pressure acting at the vertical face = Total driving force = 0.5 k a γ e H e 2 k a = co-efficient of active earth pressure, γ e = unit weight of embankment fill, and H e = height of embankment
Shear force at the bottom of embankment = (C a + vt tan f ) x L s = C a x L s ( f = 0, as the foundation soil is completely saturated) Let, tension in reinforcement = T g Total resisting force = T g + C a x L s Tg Ca.Ls FSrupture 2 0.5 k H a e e Minimum factor of safety against rupture = 1.5 Determine T g from the above equation. Now, we have to consider the reduction factors.
Check: (T g ) required = R.F. x T g < T allowable T allowable = Tensile strength from global stability analysis In longitudinal direction, we can provide the geosynthetic with tensile strength (T g ) required Seam strength (T g ) 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 (P fill ) = active earth pressure force = 0.5 k a γ e H e 2 Total resisting force (R g ) = W s. tan e + C a. L s = 0.5. γ e. L s. H e. tan e (C a = 0 for granular soil) W s = weigh of the sliding side slope, e = friction angle between reinforcement and embankment fill
Factor of safety against sliding = R g / P fill > 2 (Safe) Step 6: Check for pullout strength Rotational failure also occurs in embankment. It is required to check for pull-out strength of geosynthetic. (T g ) design = τ top L e + τ bottom L e
τ top = shear stress on the top of geosynthetic = σ v tan δ e, τ bottom = shear stress at the bottom of geosynthetic = C a, σ v = vertical stress = γ e H e, L e = Embedded length of geosynthetic beyond the slip line Therefore, (T g ) design = σ v tan δ e L e + C a L e = γ e H e C i tan e L e + C a L e Generally, C i = Interaction coefficient between geotextile and embankment fill = 0.7, and C a = 40 % of the undrained cohesion (C u ) of foundation soil
Step 7: Required elastic strength of the geotextile E reqd T reqd f ε f = strain in geosynthetic (Considering 5% strain, ε f = 0.05), T reqd = required tensile strength of the geotextile
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Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai, Mumbai 400076, India. Tel.022-25767328 email: cejnm@civil.iitb.ac.in