Pertemuan 04 GEOGRIDS/GEONETS

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Matakuliah S0522/ Aplikasi Geosintetik Dalam
Teknik Sipil Tahun Juli 2005 Versi 01/01

• Pertemuan 04GEOGRIDS/GEONETS

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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

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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
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• Reinforced Soil
• Embankments on Soft Foundations

• Lecture Outline.
• The problem.
• Analysis design.
• Requirements of geosynthetics.
• Types of geosynthetics.
• Examples / case studies.

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The Problem

• Construction of embankments on soft ground can
  lead to
• Deep seated foundation failure
• Excessive settlements

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Example of Failure
Failure by rotational movement of very soft mud
due to small differential loading
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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

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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

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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

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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

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Rotational Stability
Beware of ditches in this area!!!
Reinforcement Full width or localised areas

• Common form of analysis
• Simplified Bishop Method for circular surfaces
• Failure can be deep seated or shallow

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Lateral Sliding
Embankment fill
Horizontal movement of fill, driven by active
wedge
Reinforcement
Tr
Tr
Soft Clay Foundation
Reinforcement tension develops in the plane of
the reinforcement

• Resistance to lateral sliding determined from
  active driving force
• Geosynthetics/soil interface should be obtained
  from testing

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Foundation Extrusion
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)

• If soft soil thickness gt embankment base width, a
  bearing capacity analysis will be required
• If soft soil layer thickness lt than the
  embankment base foundation width extrusion occurs
  at the toe.

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Geocell Mattresses
..

• 7.

Example of a Geocell Mattress
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Use of a Slip Line Field

• Based on bearing capacity theory
• Tensile forces determined from stresses acting on
  the base of the mattress

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Serviceability Limit States
Geosynthetic

• Both ultimate limit state and serviceability to
  be considered

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Failure Surface

• Stiff Reinforcement may alter the failure surface
  shape
• Computer programs can consider non-circular
  failure surfaces

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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)

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Case History Ripon By-Pass
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View of Ripon By-Pass

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Load Transfer Platforms
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Load Transfer Platform Duffryn Link
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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

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Basal Reinforcement using Geotextiles

• Requirements of Geosynthetics

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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

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Woven or Knitted Geotextiles
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Extruded Geogrids
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Woven or Welded Geogrids
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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

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Lok Ma Chao Highway, Hong Kong
Embankment constructed over swamps and fishponds.
Soft clay foundations, 10 m deep, strength as
low as 3 kPa
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Lok Ma Chao Highway, Hong Kong
Geotextile avoided need for long preload or
replacement of soft soils
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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

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Second Severn Crossing
Soils cross section showing extensive deposits of
estuarine mud underlain by peat.
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Second Severn Crossing
Vibro concrete columns (VCCs) being formed using
a modified vibroflot
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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.
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Second Severn Crossing
Construction is quick, with placing of the
geogrid and the granular layer keeping pace with
the construction of the VCCs.
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Second Severn CrossingAerial View of Works
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Second Severn CrossingCompleted Bridge
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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.
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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
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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.
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WENT, Landfill Access Road, HK
Construction of the geocell mattress is rapid and
easily outpaces the infilling with granular fill
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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.