Investigations on Accelerated Consolidation of Coir Reinforced Laterite, Lithomargic Clay and Blended Soils with Vertical Sand Drains for Pavement Foundations

Abstract

Pavement layers built over the sub-grades are designed to transmit loads to the soil layers below, keeping the deformations within limits even under adverse climatic and loading conditions. Sub-grades on embankments need to provide structural stability to support trafficloads. Soil obtained from borrow pits, transported, and laid on embankments for highway construction, have very low CBR values in the range of 1-2%. In the conventional practice of road construction, the consolidation of soil layers take 1-2 years to materialize after the soil is compacted. In this context, the use of natural fibers such as coir in providing vertical sand drains and in soil-reinforcement is expected to accelerate the process of consolidation by permitting pore-water pressures to be easily dissipated when subjected to overburden pressures, which will prevent further subsidence of lateritic sub-grades. The use of vertical drains, accelerate the radial drainage and enhance consolidation, by reducing the length of the drainage paths. However, it is found that more investigations need to be performed on the use of vertical drains for coir-fiber reinforced lateritic soils of the peninsular regions of India with special reference to the District of Dakshina Kannada. Research in this direction is expected to generate information on improving the efficiency of vertical drains, and will have a profound influence in the field of highway construction especially in tropical regions where coir is abundantly available. Laterite and Lithomargic (Shedi) soil samples used in this study were collected from a site close to National Institute of Technology Karnataka, located in the district of Dakshina Kannada, India. Tests for basic properties and CBR, were performed as per specifications of Bureau of Indian Standards (BIS). Investigations on consolidation were performed for samples of laterite soils, shedi soils and laterite blended with shedi (lithomargic) soils, using circular test moulds of ferrocement (of 70 cm. internal diameter, and 85 cm. internal height). Tests for consolidation were performed on the un-reinforced soil samples with and without the use of vertical sand drains. Similar tests were performed on randomly reinforced soil samples also, to assess the effect of the use of vertical drains. The randomly reinforced soil samples were prepared for optimal coir-fiber content by weight of soil, determined based on CBR studies. Each soil sample was first subjected to a preload of 50 kg (1.2x10-3 N/mm2) at the top of the cylindrical test mould, and the settlement was studied. When the settlement rate reduced to lesser than 0.02 mm per hour, the next increment of preload was applied. The procedure was repeated until the settlement readings were taken for a final preload of 500kg (11.6 x10-3 N/mm2). In the above studies, locally available river-sand passing through 4.75 mm IS sieve was used for fabricating the vertical drains. The aspect-ratio of coir fiber used was 1: 275. In this study, the use of sand randomly distributed with 1.0 % coir fiber was used. The coir reinforcement imparts lateral stability to the vertical drain, while enhancing the drainage properties. River sand of same characteristics was used in the preparation of the top and bottom layers of the cylindrical test mould to ensure uniform loading and drainage for the tests for settlement and consolidation. While analyzing the results for 100%L+0%S, it was seen that the soil attained stability at around the 121st minute after application of the pre-loads for UR (un-reinforced soil), UR-VD (un-reinforced soil with vertical drains), and RR-VD (randomly-reinforced soil with vertical drains). Using the 121st minute as the datum, it was observed that the effect ofproviding vertical drains alone was not significant when compared to the rate of settlement, for the entire range of pre-loads from 50 kg (1.2x10-3N/mm2) to 500kg (11.6x10-3N/mm2). Also, in the case of 100%L+0%S UR soils, it was observed that the maximum value of Cv of 2.0825 cm2/s was found to occur at a higher pressure range of 3.5 to 4.6 x10-3 N/mm2. In the case of UR-VD soil conditions, the maximum value of Cv of 2.2683 cm2/s was found to occur at a lower pressure range of 2.3 to 3.5 x10-3 N/mm2. Also, in the case of RR-VD soil conditions, the maximum value of Cv of 2.2882 cm2/s was found to occur at a lower pressure range of 2.3 to 3.5 x10-3 N/mm2. This indicates that the consolidation occurs at a faster rate at lower pressure ranges for reinforced soils. While analyzing the results for 0%L+100%S, it was seen that the soil attained stability at around the 225th minute after application of the pre-loads for UR, UR-VD, and RRVD soils. Using the 225th minute as the datum, it was observed that the effect of providing vertical drains was significant considering the rate of settlement for pre-loads ranging from 50kg (1.2x10-3N/mm2) to 250kg (5.8x10-3N/mm2). However, for higher pressures varying from 300kg (7x10-3 N/mm2) to 500kg (11.6x10-3N/mm2), further settlement was not found to be significant. For UR-VD soils, the relative increase in the settlement when compared to that of UR soils, ranged between 40.61% and 294.55%, with an average increase of 176.6% for the preload ranging between 50kg (1.2x10-3 N/mm2) and 250kg (5.8 x10-3 N/mm2). This is very significant from the practical point of view. But for pre-loads higher than 250kg, the effect of providing vertical sand drains alone (as in UR-VD soils) was not significant as it was found to vary between 4.01% and 13.06% only, with an average increase of 7.67%. Also the case of 0%L+100%S RR-VD soils, there was an additional increase of 32.6% in the settlement when compared to that of UR-VD soils for pre-load ranging from 50kg (1.2x10-3N/mm2) to 250kg (5.8x10-3N/mm2). For higher pre-loads, an increase of 10.76% was observed. Thus, it can be concluded that for 0%L+100%S RR-VD soils, there is a significant increase in settlement due to random reinforcement with coir fibres when coupled with the use of vertical drains. In the case of 0%L+100%S UR soils, it was observed that the maximum value of Cv of 1.0821 cm2/s was found to occur at a higher pressure range of 3.5 to 4.6 x10-3 N/mm2. In the case of UR-VD soil conditions, the maximum value of Cv of 1.3661 cm2/s was found to occur at a lower pressure range of 2.3 to 3.5 x10-3 N/mm2. Also, in the case of RR-VD soil conditions, the maximum value of Cv of 1.8277 cm2/s was found to occur at a lower pressure range of 2.3 to 3.5 x10-3 N/mm2. This indicates that the consolidation occurs at a faster rate at lower pressure ranges for reinforced soils. Similar studies were made on 75%L+25%S soils, 50%L+50%S and 25%L+75%S soils. In the above study, it was observed that in the case of lithomargic soils there was a significant increase in settlement at lower preload pressures. Reinforced soils of this category also displayed very high settlements, indicating that softer soils can be effectively consolidated using vertical drains and random reinforcements using natural fibers. In the case of pure lateritic soils, the use of vertical drains alone was not found to be effective. However, for other lateritic blends, the use of vertical drains significantly contributed to the settlement.