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SAND CUSHION

soft clay deposits in northern Egypt are characterized with their low bearing capacity

and high settlement. Sand cushion is considered one of the techniques used to improve

the performance of such soil. An experimental plane strain model of single strip footing

was constructed to evaluate the performance of foundation overlying soft clay soil using

reinforced sand cushion. The feasibility of using waste tire as an alternative

reinforcement material in earth work was introduced and a comparison between its

effect with that for popular reinforcing materials such as geotextile and geogrid is

presented in this work. Waste tire materials employed were shreds and sheets to

represent randomly reinforced and regular sheet reinforcement respectively. The

parameters investigated in this study includ, the variation of sand cushion thickness,

position of randomly reinforced zone and depth of reinforced zone. Test results

indicated that the use of waste tire as a reinforcing material in sand cushion inhance the

performance of soft clay soil. The ultimate load increased with increasing the thicknessof sand cushion. The beneficial effect of randomly reinforced layer with shreds tire

decreased with the increase of sand cushion thickness. Effect of reinforced zone

positions are much pronounced in case of upper third reinforced zone of sand cushion

thickness.

1. Introduction

Many different improvement techniques were used to enhance the performance of

foundation overlying weak soil. These techniques include chemical stabilization,

compaction and replacement of weak soil and reinforcement with different types of

geosynthetic materials beneath foundation. The use of reinforced sand cushion

technique has become popular due to the availability of durable and strong

reinforcement materials and it is now become a successful and reliable technique.

Several types of reinforcing materials were used in sand cushion to enhance the

performance of foundation behaviour over soft soil. The main types of reinforcing

materials are natural, metallic and geosynthetic (such as geotextile and geogrid)

materials. In most studies geosynthetic materials were used to reinforce sand cushion.

The use of such materials is a money consuming and the cost of the project runs high.

Nowadays researchers directed their attention to use waste materials as an alternative

material in civil engineering applications. Two major benefits of using such materials

were achieved, reducing the cost of soil improvement techniques and keeping the

environment clear and safe from these materials. Shredded waste tires are now being

used as subgrade reinforcement for constructing roads over soft soils, as an additive to

Page 2asphalt, as a substitute for leachate collection stone in land fills and as sound barriers

[1,2]. Large earth work projects using recycled tires such as those encountered in

highway construction are an ideal application for shredded tires because there is

potential to use vast quantities of tires while improving or maintaining performance of

the earth structure. Another large earth work projects over soft soils have been

constructed in Minnesota 52000 tires have been used, Pennsylvania and Vermont 2700

m3 of tires have been used. Significant performance has been documented in each of

these projects. [3,4]. Also Edil and Bosscher [4] demonstrated that using waste tire in

earth work does not adverse effect on the ground water quality.The main objective of the study described herein is to evaluate the performance of

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footing overlying soft clay soil using sand cushion reinforced with waste tire. The

parameters investigated include, the variation of sand cushion thickness, position of

randomly reinforced zone and depth of reinforced zone.

2. Materials and testing preparation

Three different materials were used in this research : soft clayey soil, sand and waste

tire . Spesific gravity, liquid limit, plastic limit and shrinkage limit for the clayey soil

are 2.74, 62.4, 28.1 and 47.39 respectively. Its specific gravity, uniformity coefficient,

D50 and coefficient of curvature are 2.65, 2.85, 0.5 mm and 0.97 respectively.

Maximum dry density and optimum moisture content determined from the modified

compaction test were 1.81 gm/cm3 and 9.3%.

Waste tire material used in this research was a employed in two features, shreds and

sheet layers. Unit weight of waste tire ranged from 0.87 to 0.99 gm/cm3 and specific

gravity is equal to 1.11. Hassona et al. [5] demonstrated that, shreds tire size of 55 mm

and 3% shreds tire content were considered the optimum size and content to increase

the resistance of sand to external loads. Therefore, herein shreds size of 55 mm and

3% shreds tire content were adopted to reinforce the sand cushion. Modified

compaction tests was conducted on sand-tire mixture. Its maximum dry density and

optimum moisture content were found to be 1.78 gm/cm3 and 10.3% respectively.

The components of the model tests consists of loading system, loading foot,

deformation measuring devices, compaction tools and the technique of surface

deformation measurements which as well as sample preparations are all described in

much details by Ali [ 6 ]. Sand cushion was placed with different thickness of 30, 60, 90

and 120 mm or in other words 0.5B, B, 1.5B and 2B, where B is the width of the

loading foot.

4. Testing programme

The testing programme of this research involves the determination of the difference

between the behaviour of sand cushion with and without different reinforcing materialsfor identical tests at various test conditions. Four series of laboratory model tests were

carried out to study the following parameters: four different thickness of sand cushion,

H = 0.5B, B, 1.5B and 2B where B is the width of footing, three different reinforced

zone positions which are upper third, middle third and lower third of sand cushion

thickness sand cushion, five different reinforced zone depths of upper third, upper two-

third, lower third, lower two-third and whole-reinforced depth of sand cushion were

used to study the effect of reinforced zone depths and its location relative to sand

cushion surface.