GB1603517A - Method for speeding the consolidation of hydraulic clays fills - Google Patents

Description

(54) A METHOD FOR SPEEDING THE CONSOLIDATION OF HYDRAULIC CLAY FILLS (71) We, JOHN LAING SERVICES LIMITED, a British Company, of 133-139 Page Street, London NW7 2ER do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the reclamation of land from the sea particularly by hydraulic means, for use for example as industrial estates and airports. Fill material which is applied to an area to be reclaimed and which contains clay and/or silt, for example being composed of fine particles such as silt with clay, has a low permeability, and thus takes a long time to consolidate. Such fill material when pumped and particularly when placed below water initially contains a high volume of voids.It is therefore very weak, and highly compressible. In order to pump clay, it must have a low shear strength ( < 10 kNim2). Construction cannot commence on the site until the material has undergone the majority of consolidation compression under its own self-weight.

The mechanism of consolidation has been well known to geotechnologists for many years. (See for example - Terzaghi K. (1923) "Die Berechnung der Durchlassigkeitsziffer des Tones aus dem Verlauf der hydrodynamischen Spannungserscheinungen" Akademic der Wissenschafen in Wien. Sitzungsberichte Mathematisch - naturwissenschaftliche Klasse, pt. IIa, Vol. 132 No. 3/4, pp.

105-124).

It can be calculated that for a day based fill material with a coefficient of consolidation value of the order of 1.7 m2/year, a hydraulic fill of 10m depth and great lateral extent will take at least 13 years to undergo 90% of its final consolidation settlement, and before construction may safely commence.

The rate at which clay under load consolidates is known to be a function of the square of the furthest distance that water must travel to a free drainage boundary to allow the dissipation of excess pore water pressures due to loading. Thus the inclusion of one prefabricated consolidation blanket layer at the midheight of the fill will reduce the time for consolidation to one quarter of the time required without the layer. If the time for 9Oo self- consolidation of the untreated fill were 13 years, the inclusion of three equispaced layers would reduce- the time by a factor of 16 times to about 10 months, giving a time saving of about 12 years.

A method of reclaiming land in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figures 1 and 2 show vertical sections through a region to be reclaimed, Figure 3 shows a diagrammatic cross section through a consolidation blanket, and Figure 4 is a perspective sketch of a spacer tor use In a consolidation blanket.

Referring to Figure 1, there is shown a section through a region of sea covered land which is to be reclaimed. The original level of the sea bed is indicated at 1. Upon the sea bed 1, there have been constructed bund walls 2 enclosing an area 3 to be reclaimed.

A prefabricated consolidation blanket 4 constituted by filter layers separated by a spacer is weighted and laid beneath the water level 5 over the base 6 of the enclosed area 3 and the sides 7 of the bund walls 2.

The construction of the particular blanket 4 will be described in detail with reference to Figure 3. Hydraulic fill material 8 having a water content which is preferably a minimum compatible with transportation, is pumped, as indicated at 9, into the area 3.

The surface of the fill is roughly levelled when a volume of fill has been placed calculated to give the correct vertical inter val between consolidation blankets. A further layer of consolidation blanket is laid on the levelled surface and the sides 7 of the bund walls 2 and then more fill material is pumped on top of this further layer. The process is continued until the situation shown in Figure 2 is reached in which the fill material has reached a level 11 with alternating layers of fill material 12, 13, 14 and 15 and consolidation blanket layers 16, 17 and 18.

It will be appreciated that the hydraulic fill material can be conveyed by means other than pumping, for example tipping and depositing from barges.

However, the invention is concerned with the speeding of consolidation compression, i.e. that part of compression of the fill which is retarded by the need for the expulsion of water to occur.

Referring to Figure 3, there is shown a consolidation blanket which is composed of two types of element (i) a spacer 20, which allow the water leaving the soil to be carried away along its length, with as little pressure differential as possible.

(ii) fabric layers 21, 22 which are connected or bonded on either side of the spacer 20 and which allow the passage of water into the region 23 defined by the spacer while preventing the soil surrounding the blanket from entering and blocking the spacer region 23.

Associated with the method is the problem of preventing the fill from entering the spacer region of the blanket. When tipping is employed this problem is more pronounced since more material would be likely to disperse in water, and perhaps enter the spacer region at some distance from the point of tipping. However, tipping of the fill is possible.

The spacer region 23 must have sufficient capacity to allow the volume of water leaving the soil to escape freely. In addition the spacer 20 must be of low compressibility across its thickness so that the pressure of the fill causing consolidation does not restrict the outflow of the water along the spacer region 23, that is the spacer 20 should not collapse under the weight of fill above it.

The water entering the spacer regions 23 flows to the edges of the consolidation blankets after having passed up the sides of the bunds and is dispersed over the top of the bunds. The spacer, which in the preferred embodiment is of non-degradable plastic, is subjected to compression tests during manufacture to determine its safe load capacity.

In order to prevent the build-up of pressure in the spacer region 23, there must be sufficient free area of cross section to carry the water away. Consolidation theory for a particular soil will give the rate of volume of water expected at any time.

The fabric layers 21, 22 should of course be non-degradable during the period of consolidation, but may be made of woven or non-woven material or felted. A woven fabric layer 21, 22 will normally consist of a woven polypropylene tape and nylon strand fabric. This gives a stable hole configuration during handling. The hole size is uniform and known. A non-woven fabric layer will normally consist of a fabric of random fibres of polypropylene or polypropylene coated with nylon. The random array of fibres may be bonded by resin, or heat bonded in the case of nylon coated polypropylene, to provide a stable hole configuration. However, the holes vary in size from place to place, are generally very much smaller than in a woven fabric, and the fabric is essentially three dimensional.This sort of fabric will filter finer particles than a woven fabric, but has lower permeability at the soil/filter interface. Thus one might use a non-woven layer on the top of the blanket if tipping were envisaged.

The fabric layers 21, 22 must be capable of allowing water to enter the spacer region 23, and for this purpose they must have the highest open area possible consistent with the prevention of the plastic flow of the surrounding clay into the spacer region 23.

Therefore the ratio of the open area of the fabric layers to the total area of the fabric layers cannot exceed a value calculated to prevent this occurring. In addition, if the clay becomes dispersed upon entering the water, it may sometimes be necessary to use a non-woven filter layer on the upper surface of the spacer 20 to prevent penetration of the spacer region 23.

Referring to Figure 4, there is shown a spacer 20 which is folded in a concertina form to provide spacer regions 23 and which has cut-out portions 24 and 25 along its vertices 26 and 27 to allow water to pass freely into the region 23.

Although the description has been made with reference to a particular example, it will be appreciated that variations and modifications can be made within the scope of the invention.

For example, it will be appreciated that filling may commence before the bund walls reach their final height.

In a further embodiment of the invention the consolidation blankets 4 are carried through the bund walls instead of being laid up the sides of these walls.

The spacer and filter assembly could take other forms than that described. It could, for example, be made by using one of the well-known processes for making welded mesh of plastics filament material in one operation, as opposed to weaving the fila ments. Such processes are ideally suited for making the whole assembly in one operation, the spacer material being extruded between two layers of welded mesh plastics filament material, as the said two layers are extruded.

WHAT WE CLAIM IS: 1. A method of reclaiming land from water which includes the steps of enclosing the land to be reclaimed by walls, laying over the area to be reclaimed and on the inner side of one of the said walls a first consolidation blanket consisting of a porous spacer layer capable of conveying water to an edge of the blanket sandwiched between two layers of material capable of filtering the water, placing a first layer of fill material on top of the blanket, laying a second consolidation blanket on the first layer and continuing placing further alternate layers of fill material and consolidation blankets on top of one another until a required height has been reached.

2. A method as claimed in claim 1 including the steps of carrying a blanket through one of the walls.

3. A consolidation blanket when used in the method claimed in any one of the preceding claims wherein the material capable of filtering water is a fabric material which is non degradable at least during the period of consolidation.

4. A consolidation blanket as claimed in claim 3 wherein material capable of filtering water consists of a woven polypropylene tape and nylon strand fabric.

5. A consolidation blanket as claimed in claim 3 wherein material capable of filtering water consists of a random array of fibres of plastics material bonded together.

6. A consolidation blanket as claimed in any one of claims 3-5 wherein the spacer layer consists of a perforated sheet plastics material folded in a concertina fashion.

7. A method of reclaiming land from water as claimed in claim 1 substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

8. A prefabricated consolidation blanket as claimed in claim 3 substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. ments. Such processes are ideally suited for making the whole assembly in one operation, the spacer material being extruded between two layers of welded mesh plastics filament material, as the said two layers are extruded. WHAT WE CLAIM IS:

1. A method of reclaiming land from water which includes the steps of enclosing the land to be reclaimed by walls, laying over the area to be reclaimed and on the inner side of one of the said walls a first consolidation blanket consisting of a porous spacer layer capable of conveying water to an edge of the blanket sandwiched between two layers of material capable of filtering the water, placing a first layer of fill material on top of the blanket, laying a second consolidation blanket on the first layer and continuing placing further alternate layers of fill material and consolidation blankets on top of one another until a required height has been reached.

2. A method as claimed in claim 1 including the steps of carrying a blanket through one of the walls.

3. A consolidation blanket when used in the method claimed in any one of the preceding claims wherein the material capable of filtering water is a fabric material which is non degradable at least during the period of consolidation.

4. A consolidation blanket as claimed in claim 3 wherein material capable of filtering water consists of a woven polypropylene tape and nylon strand fabric.

5. A consolidation blanket as claimed in claim 3 wherein material capable of filtering water consists of a random array of fibres of plastics material bonded together.

6. A consolidation blanket as claimed in any one of claims 3-5 wherein the spacer layer consists of a perforated sheet plastics material folded in a concertina fashion.

7. A method of reclaiming land from water as claimed in claim 1 substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

8. A prefabricated consolidation blanket as claimed in claim 3 substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.