WO1999014288A1

WO1999014288A1 - Clay soil reinforcement

Info

Publication number: WO1999014288A1
Application number: PCT/GB1998/002838
Authority: WO
Inventors: Gurmel Singh Ghataora; Gianmarco Piermaria DALLÁQUA
Original assignee: The University Of Birmingham
Priority date: 1997-09-18
Filing date: 1998-09-18
Publication date: 1999-03-25
Also published as: AU9173498A; GB9719761D0

Abstract

A clay soil is reinforced by intimately mixing it with randomly oriented polymer (e.g. polypropylene or polyester) fibres having an average length in the range of 6 to 18 mm; and compacting the resultant mixture. The fibres are preferably monofilaments rather than fibrillated and may be straight or crimped fibres and most preferably have a diameter of about 15 to 60 νm. The fibres are preferably coated with a dispersing agent to prevent them from aggregating whilst being mixed into the soil.

Description

CLAY SOIL REINFORCEMENT

This invention relates to the reinforcement of clay soils, for example, to enable their potential use in road construction, in the manufacture of landfill liners, and in the construction of reinforced embankments for roads and rail. The term "clay soil" as used herein includes any clays or clayey soils, i.e. cohesive soils as opposed to particulate soils such as sandy soils which lack cohesion.

It is well known to reinforce concrete with many types of internal reinforcement including steel rods (which may or may not be pre-stressed), mesh, rubber (such as automobile tyre pieces) and fibres. Discrete polymer fibres are conventionally used in concrete as additives to prevent surface cracking. However, very little research work has been undertaken with regard to the fibre reinforcement of clay or clayey soils, despite the fact that it has been known for centuries that clay reinforced with natural fibres (reeds, straw, animal dung) can be used as a building material and that vegetation having a fibrous root structure is capable of preventing or retarding erosion of soil.

O. B. Andersland et al, in "Shear Strength of Kaolinite/Fiber Soil Mixtures", C. R. Coll. Int. Renforcement des Sols, Paris 1979, pages 1 1 to 16 investigated the shear strength of kaolinite dry mixed with natural pulp fibres having a weighted average length of about 1.6 mm and a diameter of about 0.02mm in proportions of 16 and 40% by weight of fibre, followed by slurrying and consolidation to make samples for testing. There is no indication of how these natural fibres would mix or perform in a wet soil (as opposed to a slurried soil).

R. Craig et al, in "Fiber Reinforced Soil-Cement", Volume SP105 of Fiber Reinforced Concrete Properties and Applications edited by Shah and Batson, Vol. SP105, 1987 pages 1 19 to 139, disclose the use of four different types of fibre to reinforce soil-cement, the four different types of fibres being straight steel, hooked steel, polypropyelene and glass fibres. The fibres investigated were approximately one inch (25 mm) long and it was concluded that the glass fibre reinforcement was most effective in improving the strength properties of the soil-cement, although glass fibres were found to be detrimental to durability. Two different soil mixtures were utilised, one containing a sand aggregate and the other containing a clay sand aggregate. Only the glass fibre reinforcement was used in the clay sand aggregate and it was concluded that glass fibres may prove useful for improving compressive strength with this type of aggregate. It was also concluded that fibres do not significantly enhance either the compressive strength or the shear strength of soil-cement and that, therefore, the use of fibres to improve soil-cement road bases would probably not be justified.

As a result of an intensive investigation into the reinforcement of clay soils, it has been discovered that useful improvements in strength and durability can be obtained by the use of reinforcing fibres having a length within a relatively narrow range.

According to the present invention, there is provided a method of reinforcing a clay soil comprising the steps of intimately mixing the clay soil with randomly oriented polymer fibres having an average length in the range of 6 to 18 mm; and compacting the resultant mixture.

If the average length of the fibres is less than 6 mm, then an insufficient improvement in the strength of the clay soil is achieved. If the fibres have an average length of greater than 18 mm, then it is found that there is a tendency for such fibres to ball up during mixing with the result that the strength properties suffer. Preferably, the average length is about 12 mm.

The fibres are preferably monofilaments rather than fibrillated and may be straight or crimped fibres and preferably have a diameter of not more than about 100 μm, typically about 10 to 100 μm, more preferably about 15 to 60 μm. The fibres may be typically formed of polypropylene or polyester. The fibres are preferably coated with a dispersing agent to prevent them from aggregating whilst being mixed into the soil.

The amount of polymer fibres included may vary from 0J to 1 % by weight based on the total weight of the fibre reinforced clay soil, preferably 0.1 to 0.8% by weight for straight, long fibres or 0.1 to 0.4% by weight for crimped fibre.

It is within the scope of the present invention for the method to be applied to clay soils stabilized with a stabilizer such as hydrated lime or cement, e.g. Portland cement.

When stabilizer is added, it is preferably added in an amount of up to about 10% by weight, more particularly 4 to 8% by weight, most preferably about 6 % by weight.

In the process of the present invention, to achieve maximum compaction, water is included. The amount of water to be included depends upon the nature of the clay soil to be reinforced. Simple tests can be conducted to ascertain the amount of water to be added to give the optimum dry solids density following compaction for any particular type of clay soil. For example, for kaolinite, the optimum water content (i.e. the water content required to give maximum compaction density) is about 23% by weight. It is found that, when fibres are incorporated in accordance with the present invention, an improvement in strength can be achieved in a range either side of the optimum water content, although rather better results are achieved on the dry side of optimum. It is believed that this is as a result of the lubricity imparted to the mixture by the fibres. Thus, it is advantageous for the water content of the mix to be in the range of about 19 to 24% more preferably about 21 to 23% in the case of kaolinite and clay soils having similar optimum water contents.

It is preferred to mix the fibres with the water to form a slurry which is then mixed with the clay soil.

The advantages achievable by the present invention will become apparent from the following experimental results:-

A series of test cylinders were produced each having a length of 100 mm and a diameter of 50 mm by dispersing fibres in water to form a slurry which is then mixed with kaolinite and, in some samples, hydrated lime or cement, followed by moulding and curing. In these experiments, the kaolinite used was a grade of china clay known as Speswhite produced by ECC International Ltd; the fibres were of the type sold by Fibrin (Humberside) Ltd and had the properties indicated in Table 1 below; the hydrated lime was GPR calcium hydroxide as supplied by BDH Ltd; and the cement was Ordinary Portland Cement, class 42.5N produced by Rugby Cement. The various proportions of water, fibre, kaolinite, cement and lime, together with the type of fibre employed, are as indicated in Tables 2A and 2B. The results obtained are as indicated in Table 3. The unconfined compression strength (UCS) was determined according to British Standard BS 1377:1990: Part 7. The unstabilised samples were prepared in accordance with BS 1924: Part 2: 1990. The dry density- water content relationship was determined according to BS 1377: 1990: Part 4 test 13 BS "Heavy". As can be seen from these results, useful improvements in strength can be achieved by following the teachings of the present invention.

Table 4 shows the relative gains in UCS for fibre-containing kaolinite over plain kaolinite at a water content of 23%. These results demonstrate that significant improvements in UCS are obtained for kaolinite containing the fibres of the present invention over a wide range of fibre content.

1. Supplied coated in a dispersing agent which was non-clay reactive Table 2A. Composition of test cylinders (non-stabiliseded kaolinite)

Table 2B. Composition of test cylinders (stabilised kaolinite)

1 . p.b.w = parts by weight

Table 3. Test results for the cylinders of Tables 2A and 2B

Table 4. Effect of fibre content on UCS relative to plain kaolinite at 23% water content

Claims

1. A method of reinforcing a clay soil comprising the steps of intimately mixing the clay soil with randomly oriented polymer fibres having an average length in the range of 6 to 18 mm; and compacting the resultant mixture.

2. A method as claimed in claim 1 , wherein the average length of the fibres is about 12 mm.

3. A method as claimed in claim 1 or 2, wherein the fibres are monofilaments.

4. A method as claimed in any preceding claim, wherein the fibres have a diameter of not more than about 100 ╬╝m.

5. A method as claimed in any preceding claim, wherein the fibres have a diameter of about 10 to 100 ╬╝m.

6. A method as claimed in any preceding claim, wherein the fibres have a diameter of about 15 to 60 ╬╝m.

7. A method as claimed in any preceding claim, wherein the fibres are formed of polypropylene or polyester.

8. A method as claimed in any preceding claim, wherein the fibres are coated with a dispersing agent before being mixed into the soil.

9. A method as claimed in any preceding claim, wherein the amount of polymer fibres included is from 0J to 1 % by weight based on the total weight of the fibre reinforced clay soil.

10. A method as claimed in any preceding claim, further including the step of stabilizing the clay soil with a stabilizer.

1 1. A method as claimed in any preceding claim, further including the step of adding water as a compaction aid.

12. A method as claimed in claim 1 1 , wherein the water is added by mixing the fibres with the water to form a slurry which is then mixed with the clay soil.