pertemuan 04 geogrids/geonets

Matakuliah : S0522/ Aplikasi Geosintetik Dalam Teknik Sipil

Tahun : Juli 2005

Versi : 01/01

Pertemuan 04

GEOGRIDS/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 PaulTensar International

Lecture Outline.The problem.

• Analysis & design.

• Requirements of geosynthetics.

• Types of geosynthetics.

• Examples / case studies.

Reinforced SoilEmbankments on Soft Foundations

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

• 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

Solutions to the Problem

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

Rotational Stability

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

Reinforcement: Full width or localised areas

Soft Clay Foundation

Embankment Fill

Reinforcement tension develops as a vector of these forces

TrTr

Beware of ditches in this area!!!

Lateral Sliding

• Resistance to lateral sliding determined from active driving force

• Geosynthetics/soil interface should be obtained from testing

Reinforcement

Soft Clay Foundation

Embankment fill

Reinforcement tension develops in the plane of the reinforcement

TrTr

Horizontal movement of fill, driven by active wedge

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.

Soft Clay Foundation

Embankment fillLateral extrusion of foundations due to settlement of fill

The solution to this mode of failure is to reduce the settlement by making the base stiffer (Geocell mattress)

ReinforcementReinforcement

7.

Geocell Mattresses..

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

Soft Clay Foundation

Embankment fillSettlement

Serviceability Limit States

• Both ultimate limit state and serviceability to be considered

Geosynthetic

• Stiff Reinforcement may alter the failure surface shape

• Computer programs can consider non-circular failure surfaces

Failure Surface

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)

Case History: Ripon By-Pass

View of Ripon By-Pass

Load Transfer Platforms

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

Woven or Knitted Geotextiles

Extruded Geogrids

Woven or Welded 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.

Second Severn CrossingAerial View of Works

Second Severn CrossingCompleted Bridge

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.