REINFORCED SOIL STRUCTURES

Transcription

1 GEOSYNTHETICS AND REINFORCED SOIL STRUCTURES SUSTAINABLE INFRASTRUCTURE DEVELOPMENT & NATURAL GEOSYNTETICS Dr. K. Rajagopal, Professor Dept. of Civil Engineering IIT Madras, Chennai, India

2 What is sustainability Literal meaning future generations should be able to do what we are doing today i.e. they should have access to similar natural resources, clean air, clean water, etc. This term may also include the aspects related to the economic opportunities for rural or tribal people

3 Geosynthetics & Sustainability Geosynthetics can help in all aspects of sustainability Reduces the requirement of natural construction materials il like aggregate, soil, etc. Promotes the use of locally available soils Carbon foot print of most highway projects can be reduced by use of geosynthetics Natural geosynthetics help in rural employment

4 Functions of a geotextile layer below road base

5 Advantage with thick coir geotextile layer

6 Letter Symbols for Different Functions of Geosynthetics B D E F P R S Barrier (fluid) Drainage Surficial Erosion Control Filtration Protection (of geomembranes) Reinforcement Separation

7 Flexible Break water unit made of beach sand filled bags in rope net gabions

8 Environmental Sustainability of Natural Geosynthetics One Hectare of jute plants can absorb about 15 metric tons of CO 2 from atmosphere and release 11 metric tons of O 2 into the atmosphere during their 100 days of growth Coir fibres are derived from the husk of coconut fruit by retting process mostly performed by semi-skilled manual labour

9 How long do the natural geosynthetics last in soil? 4 to 6 years depending on soil environment and type of material (coir, jute, hessian, etc.) How long do you need them in soil? Depends on application Examples of Limited need Soil erosion problems: Need protection for 1 or 2 seasons until vegetation takes root Pre consolidation: may be for 12 to 18 months

10 Different Types of Fibres Natural fibres can be of vegetable, animal or mineral origin. Vegetable fibres extracted from the fruits of the plant, are light and hairy (coir, cotton). Bast fibres are found in the stems of the plant providing the plant its strength. Usually they run across the entire length of the stem and are therefore quite long (hemp, jute,. Leaf fibres extracted from the leaves (pineapple, banana, Palmyra, Sisal) are generally rather rough and sturdy.

11 Other Types of fibres Mineral fibres (aspestos) Animal fibres (wool, silk)

12 Coir Fibre Fruit fibre separated from coconut during defibreing Fibre is extracted from cocunut husk either by retting or by mechanical crushing and decorticating Coir fibre has length of 5 to 30 cm and diameter of to mm

13 Indian coir industry Area under cultivation 5,100,000 hectares Annual production 12, 600 million nuts (largest in the world) Utilization of husk is around 35% Production of coir fibre 369,400 MT Employs nearly 590,000 workers (80% are women) Coir is exported to more than 80 countries

14 Basic composition of Lignocellulosic Fibres

15 Production of Coir fibre from coconut fruit Sarma (2011)

16 Coir rope nets and textiles Spinning and weaving to produce coir ropes and nets Indigenously developed d needle punch machine for coir geotextiles Sarma (2011)

17 PVDs made of coir & Jute Coir rope serves as the wick or the drainage medium Coir or Jute geotextile for the filter surrounding the drain PVD made of coir

18 Different coir products Sarma (2011)

19 Cocologsfor erosion control Sarma (2011)

20 Permeable form work for concrete Enables proper distribution of moisture to fresh concrete during curing process Sarma (2011)

21 Water absorption capacity of some natural materials Coir: up to almost 100% with 24 hour immersion in water Jute: 12 to 14% This property helps them provide moisture for the growth of plants in erosion control applications

22 Typical erosion on steep slopes

23 Coir mat used for vegetation growth to prevent surface erosion

24 Large landfill in India lined with coir mat to promote green finishi

25 Surface characteristics of Natural Geosynthetics Very rough compared to the synthetic geosynthetics Excellent surface interaction with surrounding soils Higher mobilisation of reinforcement force at lower strains

26 TEST PROGRAMME clay gravel clay Series - 1 Series - 2 gravel Rif Reinforcement layers clay clay Series - 3 Series - 4 Ramakrishna (1997), Rajagopal and Ramakrishna (1998)

27 60 Natural geotextile Polymer coated geotextile ) 40 ( strain (%) Ramakrishna (1997), Rajagopal and Ramakrishna (1998)

28 Fatigue strength th of coir geotextile til Ramakrishna (1997)

29 0 ate p essu e ( a) H = 150 mm 40 ( ) 80 Series Series - 2 Series - 3 Series - 4 comparison of performance of various systems Ramakrishna (1997), Rajagopal and Ramakrishna (1998)

30 plate pressure (kpa) p p ( ) ( ) 80 clay soil 120 h= Ramakrishna (1997), Rajagopal and Ramakrishna (1998) Figure?. Pressure-settlement response of system Pressure settlement data of unreinforced systems

31 plate pressure (kpa) ( ) 80 clay soil 120 h= Ramakrishna (1997), Rajagopal and Ramakrishna (1998) Data from Series III Tests Figure?. Pressure-settlement response with reinforcement layer at base of aggregate layer

32 plate pressure (kpa) ( ) 80 clay 120 h= Ramakrishna (1997), Rajagopal and Ramakrishna (1998) Test data from Series 4 tests Figure?. Pressure-settlement response with reinforcement layers at base and mid-depth

33 RESULTS Ultimate pressure on clay bed = 20 kpa Ultimate pressures (kpa) from plate load tests Thicknes unreinforced one layer of s of subbase layer reinforcement gravel layer (h) 100 mm two layers o reinforcement Ramakrishna (1997), Rajagopal and Ramakrishna (1998)

34 Ramakrishna (1997), Rajagopal and Ramakrishna (1998)

35 SOME OBSERVATIONS Coir geotextile at base of gravel layer did not improve the performance much. Reason is low friction between coir and soft clay However, this base layer will act as a good separator and filter layer in the long run. Additional coir geotextile at mid depth of gravel resulted in significant improvement. Good interface friction between coir and gravel and hence strength of geotextile is mobilised

36 Rao and Sreedhar (2012) on the use of coir geotextiles along with fly ash for pavements modulus friction factor Polymeric geotextile 52 (kn/m) 0.94 Coir geotextile

37 Test set-up Rao and Sreedhar (2012)

38 Bearing Capacity Improvement Ratio at settlement g p y p equal to 5% footing width Rao and Sreedhar (2012)

39 Bearing capacity improvement ratio at ultimate state Rao and Sreedhar (2012)

40 Comparison between Jute and other synthetics Properties Jute Polyester Polypropylene Specific gravity Tenacity, g/d 3 to 5 2 to to 5.5 Breaking strain % 0.8 to 2 7 to Elastic recovery, % 75 to to to 95 Moisture regain, At 125to % R.H and 27 C Effect of heat Does not Sticks at Softens at melt up to 180 C 180 C 154 C Sanyal (2011)

41 Jute Fibres Fineness essand dspinnability High initial strength Consistency in tenacity High rigidity Good roughness Low elongation at break Excellent drapability Mulch & creating of congenial mico climate for plant growth

42 Vegetation growth on a highway slope treated with g g g y p jute geotextile Sanyal (2011)

43 Sanyal (2011)

44 Stone column encased in bamboo grid Dutta et al. (2012)

45 120 Bamboo geogrid Tensile fo orce (kn/ /m) Strain Load-strain properties of bamboo grid Dutta et al. (2012)

46 0 Pressure (kpa) Settleme ent (mm) only clay ordinary stone column Full length polyester geogrid encasement Full length bamboo encasement Dutta et al. (2012)

47 0 Pressure (kpa) Settlement t (mm) bamboo encasement of 2 D length bamboo encasement of 3.5 D length full length bamboo encasement Dutta et al. (2012)

48 CONCLUSIONS Geosynthetics can help in sustainability in several ways. Natural geosynthetics can play an important role in engineered structures. Moisture holding capacity and surface roughness of natural geosynthetics are highly beneficial properties p for successful applications. Natural Geosynthetics can be applied in erosion control Natural Geosynthetics can be applied in erosion control, drainage, PVDs, etc. Their limited strength can be used in non-critical applications such as in low volume rural roads, pre-consolidation and erosion control works.

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