Matakuliah : S0522/ Aplikasi Geosintetik Dalam Teknik Sipil Tahun : Juli 2005 Versi : 01/01 Pertemuan 04GEOGRIDS/GEONETS
Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : • Mahasiswa mampu menghitung dan mendesain aplikasi pemakaian geogrid/geonet sesuai kondisi lapangan dan kebutuhan. C3 , C5 • Mahasiswa mampu membandingkan pemakaian geogrid/geonet sesuai dengan analisa desain bagi perbaikan tanah. C4
Outline Materi • Pendahuluan mengenai material Geogrids dan Geonets • Fungsi dan properties • Metoda desain dan pemilihan material • Perhitungan dan perencanaan • Metoda Konstruksi pemasangan • Analisa desain dengan dan tanpa material geosintetik Sebagian Materi ini diambil ari : No. 8 of 19Reinforced Soil – Embankments on Soft Foundations by Jim Paul Tensar International
Reinforced Soil Embankments on Soft Foundations Lecture Outline. The problem. • Analysis & design. • Requirements of geosynthetics. • Types of geosynthetics. • Examples / case studies.
The Problem • Construction of embankments on soft ground can lead to: • Deep seated foundation failure • Excessive settlements
Example of Failure Failure by rotational movement of very soft mud due to small differential loading
Solutions to the Problem • removal of soft material and replacement with quality fill • displacement of soft material • piling • staged construction • toe berms • elevated structure • lightweight fill • ground improvement • geosynthetics
Geosynthetics • Structurally sound solution at a lower cost • Faster than traditional methods • In its simplest form geosynthetics tie the potentially failing area back into the stable zone
Example of Basal Reinforcement Using Geogrids • High tensile strength required to maintain stability and prevent failure • As the foundation shear stress increases the required geosynthetic tensile force reduces
Analysis and Design • Established geotechnical and stability methods used • Soil parameters generally considered in total stress terms • Three main failure mechanisms considered • Rotational Stability • Lateral Sliding • Bearing Capacity
Embankment Fill Tr Tr Soft Clay Foundation Reinforcement tension develops as a vector of these forces Rotational Stability Common form of analysis: • Simplified Bishop Method for circular surfaces • Failure can be deep seated or shallow Beware of ditches in this area!!! Reinforcement: Full width or localised areas
Lateral Sliding Embankment fill • Resistance to lateral sliding determined from active driving force • Geosynthetics/soil interface should be obtained from testing Horizontal movement of fill, driven by active wedge Reinforcement Tr Tr Soft Clay Foundation Reinforcement tension develops in the plane of the reinforcement
Foundation Extrusion • If soft soil thickness > embankment base width, a bearing capacity analysis will be required • If soft soil layer thickness < than the embankment base foundation width extrusion occurs at the toe. Embankment fill Lateral extrusion of foundations due to settlement of fill Reinforcement Soft Clay Foundation The solution to this mode of failure is to reduce the settlement by making the base stiffer (Geocell mattress)
Geocell Mattresses .. 7. Example of a Geocell Mattress
Use of a Slip Line Field • Based on bearing capacity theory • Tensile forces determined from stresses acting on the base of the mattress
Embankment fill Settlement Soft Clay Foundation Serviceability Limit States • Both ultimate limit state and serviceability to be considered Geosynthetic
Failure Surface • Stiff Reinforcement may alter the failure surface shape • Computer programs can consider non-circular failure surfaces
Embankment Over Voids • Collapse due to abandoned mine workings or natural voids • Geosynthetics perform either short-term or long-term function • Case History: Ripon By-Pass (Paralink)
Load Transfer Platform: Duffryn Link
Requirements of Geosynthetics • High tensile strength, at least in roll direction • Ability to carry high loads for several years • Ability to transfer loads from soils to geosynthetic, by interlock or friction • High resistance to damage during installation • Durable against UV, chemical and biological effects
Basal Reinforcement using Geotextiles Requirements of Geosynthetics
Types of Geosynthetics • Woven or knitted geotextiles • Normally polyester Tult to 1250 kN/m • Extruded geogrids • Many uses, where ultra high strength not needed • Woven or welded geogrids • Mix of the properties of both geotextiles and extruded geogrids
Case Studies • High strength Geotextiles • Lok Ma Choa Highway, Hong Kong • Load Transfer Platform • Toll Plaza Second Severn Crossing • Geocell Foundation Mattress • Went Landfill Access Road, Hong Kong
Lok Ma Chao Highway, Hong Kong Embankment constructed over swamps and fishponds. Soft clay foundations, 10 m deep, strength as low as 3 kPa
Lok Ma Chao Highway, Hong Kong Geotextile avoided need for long preload or replacement of soft soils
Lok Ma Chao Highway, Hong Kong • Geosynthetic Band drains were installed to increase the rate of consolidation. Five sections were set up with instruments to monitor : • Geotextile strains • Settlement • Porewater pressure • Lateral displacements
Second Severn Crossing Soils cross section showing extensive deposits of estuarine mud underlain by peat.
Second Severn Crossing Vibro concrete columns (VCCs) being formed using a modified vibroflot
Second Severn Crossing Geosynthetic used to transfer loads from the embankment fill to the VCCs Full scale testing shows good load transfer at low imposed loads.
Second Severn Crossing Construction is quick, with placing of the geogrid and the granular layer keeping pace with the construction of the VCCs.
WENT, Landfill Access Road, HK The access road crosses an alluvial plain on an embankment which is up to 10m high The embankment is founded on soft clay 6m thick, strength 10kPa to 25kPa.
WENT, Landfill Access Road, HK The solution was to install band drains to increase the rate of consolidation, with a Geocell mattress as the foundation to the embankment
WENT, Landfill Access Road, HK The Geocell mattress consists of transverse grids with a triangular honeycomb structure formed by interlacing other sheets of uniaxial geogrid into the mesh.
WENT, Landfill Access Road, HK Construction of the geocell mattress is rapid and easily outpaces the infilling with granular fill
WENT, Landfill Access Road, HK Filling takes place from one edge, once an area is filled, delivery trucks can run on the surface of the geocell. Lateral extensions in the geocell were less than 1% and the mattress acted as a ‘stiff’ foundation.