The geotechnical engineers design foundations and other structures on the soil after investigation of the type of soil, its characteristics and its extent. If the soil is good at shallow depth below the ground surface, shallow foundation such as footings and rafts, are generally most economical. However if the soil just below the ground surface is not good but a strong stratum exist at a great depth, deep foundations, such as piles, wells and caissons are required. Deep foundations are quite expensive and are cost effective only in the where the structure to be supported is quite heavy and huge. Sometimes the soil conditions are very poor even at greater depth and it is not practical to construct even deep foundation. In such cases various methods of soil stabilization and reinforcement technique is adopted. The objective is to improve the characteristics at site and make soil capable of carrying load and to increase the shear strength decrease the compressibility of the soil.

Tested materials

Three clayey soils with different plasticity indexes used in the present experimental testes were obtained from the three parts of Iran named as (soil A, soil B, soil C) They are defined as high plasticity soils (CH) according to the Unified Soil Classification System.

The grain-size distribution and engineering properties of the collected soils are presented in table 2. The polypropylene fibers are shown in fig.5, fig 6.The rubber

fibers used in this study were obtained from polymer west materials. The scrap tire rubber fibers were supplied by local recapping Track Tyres producer in Qazvin city of Iran.

Curing procedure

In order to cure the stabilized peat soil samples with cement and polypropylene fibers, air curing technique has been used. In this technique, the stabilized peat soil samples for CBR tests were kept in normal room temperature of 30±2°C and relative humidity of 80±5% without any addition of water from outside. This technique is used to strengthen the stabilized peat soil samples by gradual moisture content reduction, instead of the usual water curing technique or moist curing method which has been a common practice in the past for stabilized peat soil mixed with cement . The principle of using this air curing method for strengthening stabilized peat is that, peat soil has very high natural water content and when mixed with cement has sufficient water for curing or hydration process to take place and does not need more water (submerging the samples in water) during the curing process. The technique used for curing samples will cause the stabilized peat soil samples to gradually lose moisture content during the curing period and become dry and thereby gain strength.

2.1.5 Cement dosages

For CBR (un-soaked and soaked) tests, each sample consists of peat soil at its natural water content added with 15, 25, 30, 40 and 50% cement by weight of wet soil, with and without polypropylene fibers as an additive. The amount of polypropylene fibers used for the stabilized CBR soil samples was based on the result obtained from CBR tests to be carried out to determine the optimum percentage by weight of the wet peat soil samples.

Percentage of polypropylene fibers

 The usual dosage recommended for cement mixes varies from 0.6-0.9 kg m3. In this study, in order to find the optimum percentage of fiber content for the stabilized peat soil that would provide the maximum strength, peat soil samples at their natural water content were mixed with different percentages of cement and polypropylene fibers and were cured in air for a period of 90 days and then CBR test was performed on them. The samples examined for this purpose were prepared by adding 5, 15 and 25% cement and 0.1, 0.15, 0.2 and 0.5% polypropylene fibers. The sample which showed the maximum value of CBR after 90 days of curing was chosen as the optimum percentage of polypropylene fibers for further evaluation of strength of the stabilized peat soil.

CBR test procedure for soaked condition

According to AASHTO T193-63 and ASTM D1883-73, the soaking period of CBR samples for normal soil is 96 h or 4 days (Bowles, 1978). For this study, in-order to investigate the CBR values of the soaked stabilized peat soil, a set of CBR samples prepared with different dosages of cement and polypropylene fibers (15, 25, 40 and 50% cement with 0.15% of polypropylene fibers) to soil at its natural water content were cured in air for 90 days and then soaked in water for a period of 5 weeks. During these five weeks of soaking period, the soil samples were weighed periodically for possible weight increase due to increased saturation. When the samples attained a constant weight and no further increase in weight was observed, it was assumed that the samples became completely saturated. The samples were weighed every day for the first 2 weeks, every 2 days during the next 1 week and every 5days for the last 2 weeks.