Published on

International Journal of Engineering Research and Applications (IJERA) is an open access online peer reviewed international journal that publishes research and review articles in the fields of Computer Science, Neural Networks, Electrical Engineering, Software Engineering, Information Technology, Mechanical Engineering, Chemical Engineering, Plastic Engineering, Food Technology, Textile Engineering, Nano Technology & science, Power Electronics, Electronics & Communication Engineering, Computational mathematics, Image processing, Civil Engineering, Structural Engineering, Environmental Engineering, VLSI Testing & Low Power VLSI Design etc.

Published in: Technology
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide


  1. 1. Dr. K.V.Krishna Reddy, Prof. E.S.Rao / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1166-11691166 | P a g eEngineering of Weak GroundsDr. K.V.Krishna Reddy, Prof. E.S.Rao2ABSTRACTIn this paper an attempt is made here tohighlight the principles for improvement ofweak grounds. A few successful case histories inrelation to pre-compression, granular piles, soilreinforcement are brought out. One casehistory, which needs special attention, was thetreatment of an area filled with garbage makingit fit for an expressway.Key Words: Weak grounds Garbage fills Soilreinforcement1. IntroductionWith rapid industrialization andurbanization, foundation soils having higherbearing capacity to support heavier structures havebecome a necessity. In the absence of such grounds(areas), it is imperative that the foundation engineerhas no alternative but to look for methods ofstrengthening such areas.It is well understood that the solution for aparticular area does not lie in the versatility or thepopularity of a particular technique but one has toarrive at it on sound engineering principles. Thiswould require an understanding of the soil, the typeof the structure, economical aspects and timeconstraints at hand. Needless to say that the impacton environment on such a choice cannot beignored.2. Principles and techniquesThe earliest techniques adopted in thisdirection were the mechanical stabilisationtechniques. This technique widened in its scopethrough blending of aggregates, use of chemicals,bitumen, etc. Other methods which caught theattention of the Civil Engineer were those based onhydraulic principles adopting sand drains,preloading methods, de-watering, etc. Thisprogress towards ground improvement gainedfurther momentum with the introduction ofvibratory principles through vibro-floatation,blasting, etc. Some of the recent trends in thisscience are the introduction of the stone columnsfor improvement of bearing capacity and the use ofgeosynthetics, geo-membranes and geo-grids.These have found wide applications in the stabilityof slopes, drainage control, as separators, etc.2.1 Preloading Technique For Garbage FillsThe use of unconsolidated fills ofuncontrolled and mixed waste materials is a specialproblem because of the highly heterogeneousnature of materials and the lack of knowledge oftheir behaviour under loads. However, because ofthe pressure of urban development, the area aroundpopulated districts which were once used fordisposal of rubbish often require to be developedfor residential and industrial use.One such problem was encountered inMumbai, India for the design of an embankmentfor the Chembur-Mankhurd link road, a 1400mlong portion which passes over an area filled up bycity refuse for several decades. The portion of theroad, which passes over the fill is in embankment,the maximum height being 2.5m and the averagedesign height being about 1m. It is significant tonote that although the height is small, the problemof continued post-construction settlements provedto be a cause for serious concern especially in viewof its significance and utility.Undisturbed samples were collected frommid-depth of the refuse fill at different chainagesand laboratory tests were conducted. Some typicaltest results are presented in Table.Table 1. Laboratory test results on samples ofcity refuseSampleNo.Wetdensity(gm/cc)Drydensity(gm/cc)SpecificgravityOrganicmatter(percent)Plasticityindex1 1.56 1.01 2.34 16.1 Less than 62 1.21 0.55 2.32 20.6 Less than 63 1.06 0.26 1.76 49.0 Non-plastic4 1.18 0.77 1.97 37.5 Less than 65 1.40 1.02 2.35 17.0 Non-plastic6 1.25 1.25 2.48 5.2 Non-plasticAs it can be observed from the table, thelaboratory test results were very erratic. The grain-size distribution curve showed a large proportion ofsolid matter. Direct shear tests under drainedconditions gave inconsistent results. On the whole,the laboratory test results did not show anyuniformity or reliability. Only the consolidationtests conducted under different load intensities
  2. 2. Dr. K.V.Krishna Reddy, Prof. E.S.Rao / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1166-11691167 | P a g egave time-settlement curves, which followed atrend similar to those of peats. There were no well-defined stages of primary or secondaryconsolidation as are normally obtained in the caseof inorganic clays.Realizing that the laboratoryinvestigations do not provide any reliable basis forpredicting the magnitude and the rate of settlementin the field as far as rubbish fills are concerned, afield test embankment was planned that couldprovide a meaningful and realistic basis forpredicting both the magnitude as well as the rate ofsettlement of the prototype embankment. Thus, anembankment 60m long and 26m wide at top wasraised in the refuse dump area to a height of 2.5mfor a length of 45m and to a height of 3.7 m for theremaining length with side slopes of 1:2. Twodifferent heights of embankment were adoptedwithin the length of embankment so that hebehaviour of test embankment could be seen underdifferent load intensities. The embankment wasfully instrumented with settlement platforms,displacement stakes and the Swedish GeotechnicalInstitute settlement gauge. The embankment wasbuilt up of moorum, the locally available granularmaterial under strict quality control.A study of the observational data over aperiod of one year indicated that about 50% of thesettlement took place almost instantaneously uponplacement of load. Another 28% of the totalsettlement occurred over a period of the next 3months. Yet another 12% occurred over a period ofthe next 3 months and finally, about 10% of thetotal settlement took place over the remainingperiod of 6 months. The observations continued forabout 1 1/2 years. There was no significant heavein any of the displacement stakes. There was alsono noticeable rebound observed upon removal ofload.It would, therefore, seem that thesettlement characteristics of a rubbish fill are, byand large, similar to those of peats. It appears thatthere is no reason to believe that the rubbish fillwould by itself constitute a serious source ofcontinuing significant settlements, beyond a periodof 1 year by which time more than 90% of totalsettlement occurs. The simple technique ofpreloading can be effectively used for treatingrubbish fills in comparison to the use of lightweight materials, replacement methods, etc. whichmay prove to be prohibitively costly.2.2. Granular Piles In Soft Marine ClaysThe Outer Harbour Project inVisakapatnam has about 2m. of sand at the groundlevel underlain by 10 to 15m of soft marine claysupported by weathered rock below. When theHarbour authorities were confronted with theproblem of stacking iron ore to a height of about10m to export it to other countries in an area whereit was possible only to stack up to 3m, the need forground improvement arose. The marine clay hadundrained shear strength of about 15kN/sq.m.Since ground improvement was to be taken up on aturn-key basis, the authorities finally decided toemploy vertical drains either in the form sanddrains or sand wicks, after studying the feasibilityof several such techniques.In order to understand the relative efficacyof the two, three test areas of size 34.14X 21.95mwere selected. One of these areas was treated withvertical sand drains 38.1cm in diameter spaced at2.1m centre to centre using the open mandreltechnique.Another area was treated with sand-wicks(jute bags filled with coarse sand under pressure),of 6.3cm diameter placed at 1.2m centre to centre.The third area was used as a control panel and wasleft untreated.All three test areas were loaded withgravel and sand bags to an intensity of 88 kN/sq.m. The loaded areas were instrumented withsettlement gauges, heave stakes, inclinometers, andobservations were made for a period of about oneyear. Field vane shear tests were also conductedregularly to measure the gain in strength and theresults indicated that there was considerableimprovement satisfactory to the objectives of theproject.The area treated with sand drains alwaysrecorded maximum settlement at given time periodcompared to sand wicks, thereby establishing theirsuperiority in terms of effecting rapid artificialconsolidation and gain in strength. The untreatedarea settled about 20% less but caused excessivelateral heave, indicating plastic flow. It was laterdecided to recommend gradual increase of heightof iron ore stacks to benefit from the gradual gainin strength. Both sand drains and sand wicksproved successful, and sand drains, in particular,showed a better rate of gain in strength, over sand-wicks. This is probably due to the reduction ineffective diameter due to intrusion of clay particlesaffecting sand-wicks more than the sand drains.2.3. Drainage as A Measure Of Land SlideControlLandslides are not uncommon in thefoothills of Himalayas and are a constant source oftrouble for road communication. A large majorityof these landslides are surficial in nature and areactivated by heavy rainfall. These are classified asslumps, rotational slides, debris falls etc,.
  3. 3. Dr. K.V.Krishna Reddy, Prof. E.S.Rao / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1166-11691168 | P a g eThe foothills of Himalayas consist mostly rocks ofsedimentary origin- shales and slates. Schists andgneisses of metamorphic nature are also plenty.Shales give a deceptive picture of high strength indry state while they cause mudflows in the wetseason. It becomes, therefore imperative that thoseof the regions associated with the highways in thisregion have to be protected from the influence ofwater.A case history of a major landslide about15 km south of Gangtok in Sikkim which had beena source of trouble for several decades is taken upin this paper to highlight how planned drainagealone could tame a massive landslide. Thelandslide is in fested with foliated biotite schistswith well-defined planes of cleavage. The areafalls under a zone of severe earth-quake intensityand had undergone intense tectonic activity. Thetype of the soil in the region is predominantlyclayey silt and the thickness of mantle of soiloverlying the rock was of the order of 4 to 5m.Heavy rainfall of about 3000mm causes constantdisintegration of the rock and scanty vegetationleads to easy entry of water. Tension cracks couldalso be observed on the uphill side allowing freeingress of water further aggravating the problemand landsliding.The slope uphill of the road is inclinedabout 400and the downhill slope is flatter andmeets the river deep below. Toe erosion due to theriver is ruled out in this area. The road, which wasbuilt in 1957 with retaining structures, had to berebuilt almost every year due to landslides. Theroad also sank several times and was even washedaway. A new road is cut into the hill each time atdifferent levels to keep the traffic through.Investigation into the cause of the failure of this hillslope was attributed to the process of continueddisintegration of foliated schists under heavyrainfall. Lack of vegetative cover also contributedconsiderably to this accelerated disintegration.It was therefore decided to construct tworows of lined surface drains horizontally 25m apartwithin the 70m high hill slope to trap the surfacerunoff. These drains extended far away from theslide area on both sides and were connected toculverts at the road level. Lined road side drainswere provided on the uphill side of the roadsufficient enough to carry the surface runoff awayform the slide area. Deep-rooted grass and otherplants were raised in consultation with the localforest authorities over the denuded slope. Thetension cracks above the slide area were filled upwith local soil in order that they did not serve asreceptors to rainwater which otherwise would seepthrough and accelerate downward movement of thesoil mass. Further, the slope above and betweeneach row of drain is levelled so that rain is waterled into these drains without causing ponding.There were initial problems for thestability and alignment of drains built on loosesloping fill but coupled with vegetative cover andgradual stability, the surface drains gavesatisfactory results by reducing the frequency andmagnitude of landslides. In a couple of seasons theslope was fairly stable without any cause forconcern.It was thus possible to control a majorlandslide, which was a source of perennial troublethrough a network of drains (De-watering).Conventional methods of landslide control likeearth retaining structures, etc would have proved tobe costly without any guarantee for their ownstability. With the advent of geo-membranes, soilnailing techniques etc., it is probable that such ofthese problems could be controlled moreeffectively and with in the time constraints.2.4. Reinforcing Strips To Strength AnApproach EmbankmentRatnam, 1988 presented a casehistory of an approach embankment of a bridge in abuilt up area which required minimum spaceoccupancy. Considering reinforced earth techniqueas the best suitable solution to overcome thisdifficulty, it was tried on the prototypeembankment. The embankment was about 4m high,10.5 m wide and 60m long over an approach rampat a slope of 1 in 15. The sides were to be verticalfaces supplemented by reinforcement andinterlocking.The basic skin elements used were G.I.semi-elliptical strips of size 240x25x0.1 cm andR.C.C cruciform panels fabricated specially for thispurpose with interlocking edges of overall size90x60x8 cm. The embankment was raised usinggranular moorum with usual compaction control.The reinforcing strips were connected by bolts andnuts. The semi-elliptical skin elements weremoulded to the shape through a specially fabricateddevise. The following measurements were made.1. Lateral deformations of the reinforcedearth walls2. Lateral earth pressure on the panelsthrough diaphragm type earth pressure cells.3. Variation of tensile stress along thereinforcing strips using electrical resistance typestrain gauges.The lateral deformations were observed to be within acceptable limits. They could still be corrected infuture by providing stiffer and superiorreinforcement elements. The lateral earth pressure
  4. 4. Dr. K.V.Krishna Reddy, Prof. E.S.Rao / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1166-11691169 | P a g eon the wall was as predicted and could also betaken care by providing heavier panels in thebottom reaches and reduce deflections. The tensilestresses in the strips were recorded from the digitalstrain indicator at different heights of fill of theembankment. The tensile stresses varied from thepanel end continuously decreasing to zero at theend.The reinforcement technique certainlysatisfied the immediate objectives of the project byoccupying the minimum area in a heavilycongested locality. It would have otherwiserequired elaborate planning for control of traffic inaddition to displacement of residential complexesin the vicinity.3. CONCLUSIONS1. In order to use garbage fills as a good foundationmaterial, pre-compression serves as aneconomical and foolproof solution.2. Among the several granular piles practiced allover, sand drains appear to function moreefficiently because of the larger diameter and lesssusceptibility to contamination by clay particles.3. Surficial landslides can be controlledsatisfactorily through a carefully planneddrainage network.4. Soil reinforcement technique for the approachesto flyovers and overpasses in built-up areas ofthe cities will greatly help in economizing space.4. AcknowledgementAt the outset the authors would thank theHead, CED at Vasavi Engineering College for theirvaluable encouragement.References[1]. Nararajan.T.K. and Rao.E.S. (1977)“Geotechnical Engineering andEnvironmental Control”,9thICSMFE, Tokyo.[2]. CRRI (1971), Report on “FoundationAnalysis of the Ore-Handlig Yard, OuterHarbour Project, Visakhapatnam.[3]. Natarajan. T.K., Bandari.R.K., Rao.E.S.,Ajaib Singh, (1982) “A Major Landslidein Sikkim, Analysis, Correction andEfficacy of Protective Measures”,International Conference on Landslides,New Delhi.[4]. Naresh.D.N., Ratnam.M.V. andSubramanyam.G.,(1988), “AnExperimental Study on Performance ofa Prototype Reinforced EarthEmbankment”, Proceedings of theInternational Geotechnical Symposium onTheory and Practice of EarthReinfocement, Japan.