Open Journal of Soil Science, 2012, 2, 320-332 http://dx.doi.org/10.4236/ojss.2012.23038 Published Online September 2012 (http://www.SciRP.org/journal/ojss) A Model Study on Accelerated Consolidation of Coir Reinforced Lateritic Lithomarge Soil Blends with Vertical Sand Drains for Pavement Foundations George Varghese, Hegde Ramakrishna, A. G. Nirmal Kumar, L. Durga Prashanth, G. Santosh Department of Civil Engineering, National Institute of Technology Karnataka, Surathkal, India. Email: [email protected]; [email protected] Received May 12th, 2012; revised June 10th, 2012; accepted June 24th, 2012 ABSTRACT Sub-grade soils of lateritic origin encountered in the construction of highway embankments in various regions of India, often comprise intrusions of soft lithomargic soils that result in large settlements during constructions, and differential settlements at later stages. This necessitates the use of appropriate soil improvement techniques to improve the load-carrying capacity of pavements. This work deals with accelerated consolidation of un-reinforced and coir-rein- forced lateritic lithomargic soil blends, provided with three vertical sand drains. The load-settlement characteristics were studied for various preloads ranging from 50 kg (0.0013 N/mm2) to 500 kg (0.013 N/mm2) on soil specimens pre- pared in circular ferrocement moulds. It was observed that at lower preloads up to 200 kg, across the blends, the relative increase in consolidation (Rct) for randomly reinforced soil with vertical drains was significantly higher than that of un-reinforced soil without vertical drains, with an average value of 124.8%. Also, the Rct for un-reinforced soil with vertical drains was quite higher than that of un-reinforced soil without vertical drains, with an average value of 103.9%. In the case of higher preloads, the Rct values for randomly reinforced soil with vertical drains were moderate with an average value of 30.88%, while the same for un-reinforced soil with vertical drains was about 20.4%. The aspect-ratio of coir fibers used was 1:275. Keywords: Vertical Sand Drains; Accelerated Consolidation; Settlement; Coir Reinforcement; Laterite; Shedi; Lithomarge 1. Introduction tively employed in the improvement of sub-grade strength by accelerating the drainage of soil-moisture content re- Construction and maintenance of pavements in water- sulting in enhanced consolidation. logged areas pose challenging problems to engineers. The defects in road sub-grades mainly arise due to poor The name laterite was initially suggested by Francis compaction and consolidation. This is of major concern Buchanan [1] to describe “ferruginous, vesicular, un- in road-works associated with submersible areas. Incor- stratified, and porous soil with yellow ochre’s due to porating the use of natural fibers in soil-stabilization, high iron content, occurring in Malabar, India”. In the along with the laying of natural fiber reinforced vertical district of Dakshina Kannada, and Udupi, laterite soil can sand drains can contribute towards strengthening of road be found to occur above underlying shedi soil (or fine sub-grades especially in the construction of highway em- silty soil). Laterite soil is quite stronger than shedi soil bankments. due to the higher content of oxides of iron, while shedi Differential settlement of sub-grade soils cause dam- soils are characterized by low bearing strengths under ages to pavements in many regions. The problem as- moist conditions. sumes critical importance in clayey and silty soil sub- In India, lithomargic clays are often found to occur at grades that have low permeability values. In such soils, depths of 1 - 3 meters below the top lateritic outcrops the consolidation and the resultant settlements occur over throughout the western coastal areas extending from a longer time-span. This work focuses on conducting Trivandrum to Mumbai, and also in the areas adjoining extensive studies on the consolidation characteristics of the Deccan Plateau. Shedi soil is the name given to the weak soil sub-grades. locally available whitish, pinkish or yellowish lithomar- Natural geo-textiles made of coir or jute can be effect- gic soils with high silt content, and low bearing strengths. Copyright © 2012 SciRes. OJSS A Model Study on Accelerated Consolidation of Coir Reinforced Lateritic Lithomarge Soil Blends with 321 Vertical Sand Drains for Pavement Foundations These soils are formed by weathering of soils in tropical forced L-S soil provided with 3 vertical drains regions, and contain hydrated alumina, primary silicates, In the case of tests on blended L-S soils, an optimum and kaolinite. Lithomargic soils comprise 50% - 90% fiber content of 1.0% of coir fiber was adopted for lateritic constituents, while soils with 25% - 50% laterite random reinforcement. content are known as lateritic lithomarges. The sand-drains were fabricated using sand randomly reinforced with 1% of coir fibers. Loosely-woven jute 2. Scope and Objectives of the Present Study fabric stitched at the seams was used in the installa- tion of sand-drains of 10 cm diameter. In order to attain higher bearing strengths in soft soils, it The results were compared with the same for 50% may be required to resort to accelerated consolidation laterite + 50% shedi soil samples [3], and 100% shedi during the construction stage by applying incremental [4] obtained in similar studies. loads on suitably prepared sub-grades in stages. The pore-water pressure that builds up under the overburden 3. Literature Review pressure and surcharge loads, can be released with the use of natural reinforcements especially on weak soils. Studies on the stability of loaded footings on reinforced Additionally, the use of vertical drains will shorten the soil using natural “iko” vegetable fibers, reported by Ak- length of the horizontal drainage-paths, and accelerate inumusura and Akinbolade [5] indicated that a significant the consolidation process. The enhanced dissipation of improvement in soil strength could be achieved. pore-water pressure due to the presence of natural fiber Investigations on using vertical drains made of natural reinforcements and the overburden pressures, will result fiber such as jute and coir for soil improvement simulat- a further acceleration of the consolidation process in lat- ing field conditions, was reported by Lee et al. [6]. The eritic and shedi (L-S) soil sub-grades of embankments. studies revealed that the axial filter permeability of fiber The natural degradation of natural fibers after a few years drains was higher than 10-5 m/sec for consolidation pres- [2] will not affect the strength and stability of L-S soil sures of up to 400 kN/m2. sub-grades due to the reason that the soil layers would Similar investigations in the field of application of have attained consolidation in the meantime. coir-fibers [7,8] highlighted the advantages in the use of The present work deals with investigations on load- natural fibers for stabilization of embankments associ- settlement characteristics due to accelerated consolida- ated with the use of fiber-reinforced vertical drains. tion of various blends of laterite and lithomargic (shedi) Stapelfeldt [9] reported that preloading and the use of soils, with and without the use of reinforcements of ran- vertical drains, can enhance the shear strength of the soil, domly dispersed coir-fibers, and with and without the use resulting in reduced soil compressibility, and permeabi- of vertical sand drains randomly reinforced with coir. lity prior to construction, preventing differential settle- This study also incorporates an assessment of the im- ments. provement in the California Bearing Ratio (CBR) values Comparative studies on soft clay soils of Bangkok by of L-S soil, when randomly mixed with coir for soaked Bergado et al. [10] revealed the use of compacted granu- and un-soaked conditions. lar piles (CGP), and prefabricated vertical drains (PVD). The objectives of the present study include the fol- The studies indicated that in the case of soils improved lowing: with PVDs, the settlement rates were higher by 30% - To prepare blended laterite-shedi (L-S) soil compris- 35% when compared to soils improved using CGPs. ing 25% laterite and 75% shedi soil, and to perform It was also noticed by Gosavi et al. [11] that the inclu- basic laboratory investigations such as grain size sion of randomly distributed fibers of jute and coir, in analysis, Atterberg’s limits, CBR tests, tests for stan- black cotton soils, increased the CBR values of clayey dard and modified compaction tests for L-S soil sam- sands (SW) and sandy-silt (SM) by about 96%. ples as specified by Indian Standard (IS) codes. Terzaghi [12] provides the basic theory for such one- To measure the load-settlement properties and con- dimensional consolidation for saturated conditions and solidation of fully saturated confined and un-rein- also provides the strain formulation. The relationship be- forced L-S soil without the use of vertical drains for tween the final settlement (Sf) and the settlement (St) at soil samples drained at the top and bottom. time t was expressed as: To measure the load-settlement properties and con- SUS (1) solidation of fully saturated confined and un-rein- tvf forced L-S soil with the use of 3 vertical drains. The expression for Uv, the average degree of consoli- To measure the load-settlement properties and con- dation at depth z at any instant t was given by Terzaghi solidation of fully saturated confined randomly rein- [13] as, Copyright © 2012 SciRes. OJSS 322 A Model Study on Accelerated Consolidation of Coir Reinforced Lateritic Lithomarge Soil Blends with Vertical Sand Drains for Pavement Foundations 22 5. Determination of Optimal Fiber Content UMv=1 2 exp MTv (2) m0 for Soil Samples Using CBR Tests where T = time factor (non dimensional) = (C t)/H2; C v v v CBR tests were conducted as per IS 2720: Part VII (1983) = coefficient of vertical consolidation (m2/s) = k/ (γ m ); w v [16].