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Application of Geosynthetics in Embankment. Geosynthetics for ground improvement of Embankments. Various Functions of Geosynthetics. Sealing. Protection. Drainage. Filtration. Separation. Reinforcement. Important for Ground Improvement. Ground Improvement.
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Various Functions of Geosynthetics Sealing Protection Drainage Filtration Separation Reinforcement Important for Ground Improvement
Ground Improvement • Coastal Roads- Mumbai ,Vizag ,Cochin, Manglore , Kandla, Rann of Kutch etc • Peculiar Characteristics of Marine Land • Soft ground - which is susceptible to failure or cause excessive settlement when superstructure is constructed over it • • Marine soil mainly consists of - • Marine Saturated clays and fine silts ( alluvium), • Loose very fine sand and silts( especially when submerged)
PROBLEMS in EMBANKMENTS Problemsassociated with Construction of Embankments on soft ground • Construction Material: Soft Soil • Fine soil having very low undrained shear strength and plastic • Very High Compressibility and moisture content varies from 60-100 • Very low permeability (10-5mm/sec - 10-8 mm/sec), • Normally consolidated & insitu shear strength varies from 7-20 kPa. • the failure happens at an undrained condition within a short period after the embankment construction.
Increased construction activities in coastal Cities and harbors pose a challenging task to engineers to meet the needs of rapid development • Stability of an embankment in soft clay depends • on interaction of the following parameters. • Height of embankment • Base width of embankment • ‘Depth of soft clay • Shear strength of soft clay Failures can occur in embankemts of very low heights , say 1m to 1.5 m , when clay is having poor shear strength and/or Embankment is having very restricted base width
Failure modes Bearing Capacity /Foundation Extrusiion Rotational Failure Lateral Sliding Overall Failure
FAILURE PATTERNS • Failure patterns -Embankments on soft soils : • foundation soil failure • slide of embankment fill on underlying soft soil • excess of displacement
Geosynthetics reinforcement • A high strength and stiff reinforcement layer at the fill base can be used in order to improve embankment stability What can geosynthetics do? to increase the safety-factors against catastrophic failure to limit the magnitude of overall settlements to control differential settlements YES NO PARTIALLY
Geosynthetics in Embankments • More common and well known, well established solution • Used as an alternative to conventional system • Considered as cost-effective solution • Basic Application includes:
Solutions using Geosynthetics Basal reinforcement PVDs Basal reinforcement for Piled Embankments Stone Column
Anchorage block Embankment Reinforcement Drainage blanket (0.5 m crushed stone) GROUND IMPROVEMENT A. Concept of Basal Reinforcement
Working principle of Basal Reinforcement - • Basal reinforcement is used to form a geosynthetic raft & transfers the load to • the soil directly, or • With sand piles, or • With stone columns or piles, • It enables to maximize the economic benefits of piles/ stone columns installed in soft foundations by decreasing the diameter and increasing the spacing. Pile cap dimensions can be optimised wihen Basal reinforcement is utilised. • The reinforcement also counteracts the horizontal thrust of embankment fill and need for raking piles along the extremities of the foundation can be eliminated.
Contribution of Reinforcement for stability of Embankment By placing a stiff reinforcement layer at the embankment base, improved Safety Factors can be achieved for the following- 1. Rotation Stability of Embankment Fill 2. Sliding Stability of Embankment Fill 3. Overall Stability 4. Foundation Bearing/Extrusion Stability
Contribution of Reinforcement for stability of Embankment • The load in the reinforcement increases to a maximum during construction. • As the foundation consolidates the reinforcement carries less load • When the foundation has consolidated it carries all of the embankment load - the reinforcement is no longer required
End of construction Force in reinforcement Time Consolidation of soft soil CU of soft soil Time Contribution of Reinforcement for stability of Embankment
Application of Geo synthetic as basal reinforcement along with other techniques of ground improvement • • Basal Reinforcement + Piles – • Pile foundations are used to carry and transfer the load of the structure to the bearing ground located at more depth below ground surface. • So they are used in such cases where the foundation soil is not able to take the structure load at all and ground improvement is either not possible or is more expensive than pile foundations.
Basal Reinforcement+ Piles The pile and basal reinforcement technique allows structures to be constructed to required heights without any restraint on construction rate with proper control of post construction settlements
Basal Reinforcement + Stone columns • Basal Reinforcement + Stone columns– • The stone column technique, also known as vibro-replacement or vibro-displacement, is a ground improvement process where vertical columns of compacted aggregate are formed through the soils to be improved. • These columns result in increased vertical load carrying capacity and improved shear resistance in the soil mass. • For soils with appreciable silt or clay content that do not respond to deep vibratory compaction it is advantageous to create stiff reinforcing elements in the soil mass.
Basal Reinforcement + PVD • Basal Reinforcement + PVD – • Ground improvement technique using prefabricated vertical drains PVD) is also a suitable method to overcome the problem of excessive settlement in soft soils. • PVD’s band shaped (rectangular shaped) material consisting synthetic geo textile jacket surrounding inner plastic core. • Jackets are of non woven polyester or polypropylene geo textile. • Used in Embankment construction: • To accelerate settlements • To reduce time for consolidation • To avoid Post Construction Settlements
Principles of Ground Improvement Using PVD • Horizontal/Radial Consolidation • Use of preload (Pre-compression) • Resulting in • Advancement of the expected settlements and reduction in post construction settlements. • Reduction in time required for consolidation.
Functioning of Drain Functioning of Drain Functions of filter jacket- • To prevent soil particles and allow water to flow in • To provide the cover to internal drain flow i.e. function of filter jacket mainly filtration, to some extent separation Functions of the core- • To provide internal flow path for water along the drain • To provide support to filter jacket • To provide the resistance to longitudinal stretching as well as buckling of the drain
Advantages Advantages of Using Basal Reinforcement for Ground Improvement • Easy and Rapid construction is possible over weak soil • Improved load carrying capacity • Better drainage arrangement can be Provided • Cost effective if compared to conventional solution • Differential settlement can be controlled • Chemically inactive, non biodegradable material hence high durability
CASE STUDIES - Use of geo synthetic Basal Reinforcement for GROUND IMPROVEMENT WORKS
Project: ROB FOR CONNECTIVITY BETWEEN MUNDRA AND NH8A, MUNDRA , GUJRAT, INDIA Problem: - For ROB at Mundra in Gujarat, the solid approaches were retained using Reinforced Soil (RS) Walls. The shear properties of existing soil were weak which could have led to a major problem of bearing and global instability.
SOLUTION- In order to achieve the required global and bearing stability, ground improvement using basal reinforcement over piles and for lower heights basal reinforcement over stone columns was proposed.
Site Conditions Maximum Height of reinforced soil wall – 9 meter Soil up to 3.0 meter was clayee silt which is followed by sand and silt with traces of clay till 9.0 m. Ground water table was up to 1.5 meter. Basal Reinforcement selected : High Strength Geogrids having monoaxial array of Geosynthetics strips , which has a planar structure were used as basal reinforcement to improve the strength of the underlying soil.
Quality Aspects at site Site preparation Reinforcement storage Placement of Reinforcement’ Reinforcement Jointing End Anchorages Fill considerations
Reinforcement is temporarily secured in place with pins, as required by fill properties. Fill in the immediate contact with reinforcement is placed and spread in the longitudinal direction of reinforcement only.
The unidirectional ultimate strength of the mono-axial geo grid was 200kN/m. Stone columns were used to reduce the settlement of the approach road at higher heights. High strength geo synthetic used as basal reinforcement were effectively able to distribute the stress uniformly to the foundation soil , thereby decreasing the differential settlement. Maximum tensile load was calculated as the sum of the load needed to transfer the vertical embankment loading on the stone columns and the load needed to resist lateral sliding. The settlement of the soil in between the columns was also reduced considerably since the load was transmitted to columns through basal reinforcement..
Concluding Remarks • Construction of embankments on soft soil can be critical because of they have low strength & high compressibility. • Reinforcement material can be placed at foundation level to act as a Basal Reinforcement to improve stability of embankments on Soft Soil.
