GB2289078A

GB2289078A - Improved soil nail installation

Info

Publication number: GB2289078A
Application number: GB9508961A
Authority: GB
Inventors: David A Greenwood; Leonard Threadgold
Original assignee: Cementation Piling and Foundations Ltd
Current assignee: Cementation Skanska Ltd
Priority date: 1994-05-03
Filing date: 1995-05-03
Publication date: 1995-11-08
Anticipated expiration: 2015-05-03
Also published as: HK1000212A1; GB9408713D0; GB2289078B; GB9508961D0

Abstract

A method of reinforcing a soft clay soil is disclosed, the method comprising the steps of boring at least one hole in the soil, introducing a nail bar 4 into the hole, and filling the space between the nail bar and the sides of the hole with lime 6. <IMAGE>

Description

IMPROVED SOIL NAIL INSTALLATION The present invention relates to the installation of soil nails as a soil reinforcement in construction work, and in particular, but not exclusively, to the reinforcement of soft clay soils.

Construction work on soft clay soils is problematic because the clays, which are generally naturally occurring poorly consolidated soils or poorly compacted fills, are susceptible to moisture ingress and swelling if not originally wet. In locations such as the slopes of cuttings or embankments, soft clay soils may creep progressively or may even collapse.

It is known to stiffen and strengthen soft clay soils by using lime columns. The soil is stiffened and achieves greater shear strength by boring holes, which are generally temporarily cased, and introducing quicklime compound into each hole as the casing is withdrawn. As the quick-lime slakes, it absorbs water from the pore spaces of the surrounding soil. Because of the low permeability of the clays, the water cannot be replaced quickly in existing hydraulic circumstances. The clay therefore shrinks and consolidates as it dries in an annulus surrounding the column. This creates tension cracks into which the lime may migrate, enabling longer term chemical reactions to occur in these cracks, thereby locally hardening the clay. In the process of slaking, the lime may expand in volume by between 50 to 100% while the surrounding soil shrinks as water is removed.The consolidated and stabilised soil does not easily soften again on exposure to moisture in the longer term. Such lime columns are generally 100 to 300mm in diameter, and are spaced 1 to 3m apart to cover the volume of soil requiring treatment. The stabilising effect of a lime column, however, is concentrated around the bore and diminishes radially. A bulk mass of soil penetrated by lime columns therefore has improved average shear resistance across potentially sliding surfaces, but the soil does not acquire any significant tensile strength.

It is also known to provide soil reinforcement, in particular to cut slopes, by using soil nails, which comprise 100 to 150mm diameter sub-horizontal bores into which grout is introduced along with a central steel bar of diameter typically around 20 to 30mm. The soil is bonded to the grout and the grout is bonded to the bar, thereby enabling the bar to restrict deformation of the soil by imparting tensile strength to the soil though shear bonding. By installing such soil nails generally 1 to 3m apart in a matrix as excavations proceed, it is possible to reinforce and stabilise a cut face in soil.

The known type of soil nail works best in overconsolidated soils which are dense, compact and relatively stiff, although not always enough to be stable. This is because the stress transferred by the soil to the bar is through bonding. A weak soil cannot transfer as much stress as a strong soil, and larger holes and closer spacing are therefore required for weak soils. The known type of soil nail is therefore impractical for use in soft clay soils.

According to the present invention, there is provided a method of reinforcing a soft clay soil, which method comprises the steps of boring at least one hole in the soil, introducing a nail bar into the hole, and filling the space between the nail bar and the sides of the hole with lime.

This method of reinforcement gives improved shear strength as well as a degree of tensile strength to the soil.

The holes may be bored or drilled at any appropriate angle, and are advantageously temporarily cased. The nail bar is preferably provided with centralising spacers, the spacers at each end of the nail bar advantageously being solid discs so as to resist longitudinal expansion of the lime at the ends of the nail bar. The lime is preferably quick-lime, although the precise nature and composition of the lime will generally be determined by installation constraints.

When the holes are temporarily cased, the lime may be introduced at the mouth of the hole as the casing is withdrawn. Depending on the orientation of the hole, the lime, which will generally be in pellet or powder form, may simply be tipped into the hole, or be introduced by pneumatic means along a tremie pipe inserted alongside the nail, or by any other convenient means. In order to ensure proper packing, the lime may be compacted along the length of the hole by using a tubular ram surrounding the nail bar. This process may be repeated as the casing lengths are withdrawn, leaving the nail bar compacted in lime within the hole.

A top plate or centraliser may then be fitted to the end of the nail bar and any residual length of the hole may then be plugged with grout or rammed soil or similar suitable material.

The effect of the swelling lime as it slakes is to provide a frictional bond between the nail bar and the surrounding soil. The associated water absorption and chemical reactions generate an annular region of stiffened clay around the hole, thereby increasing the effective perimeter of the nail. This allows a greater degree of soil reinforcement per nail to be achieved, and therefore results in greater installation economy.

The nail bars are preferably ribbed in order to enhance bonding. The lime may be mixed with pulverised fuel ash if a permanent hardening is required.

Alternatively or in addition, the lime may be mixed with Ordinary Portland Cement, preferably in a proportion of 25 to 50% by dry weight to achieve early hardening.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made to the drawings, in which: FIGURES 1 to 4 show the installation of a soil nail according to the present invention; and FIGURE 5 shows cross-section I-I of Figure 4.

In order to reinforce a soft clay embankment 1, as shown in Figure 1, a hole 2 (with a diameter typically between 100 to 150mm) is drilled and lined with a casing 3. A nail bar 4 (with a diameter typically between 20 and 30mm), including an end centraliser 5, is then introduced into the hole 2 as shown in Figure 2, and lime 6 is then poured into the hole 2. Figure 3 shows the casing 3 being withdrawn as the lime 6 is compacted with a tubular rammer 7. Once the casing 3 has been completely withdrawn, a top centraliser 8 is fitted to the end of the nail bar 4, and the mouth of the hole 2 is packed with grout or similar material 9.

A number of installations as shown in Figure 4 are generally provided to stabilise and reinforce a mass of soil 1, and are typically spaced between 1.5 to 4m apart.

Figure 5 shows a section through a soil nail installed according to the present invention. The annular space between the nail bar 4 and the side of the hole 2 is packed with lime 6. The lime 6 absorbs water from the soil 1 as it slakes, which causes the lime to expand. This improves the frictional bond between the nail bar 4, the lime 6 and the soil 1.

Furthermore, the absorption by the lime 6 of water from the surrounding soil 1 consolidates and chemically stabilises an annular volume of soil 1 around the hole 2.

Claims

CLAIMS:

1. A method of reinforcing a soft clay soil comprising the steps of boring at least one hole in the soil, introducing a nail bar into the hole, and filling the space between the nail bar and the sides of the hole with lime.

2. A method according to claim 1, wherein the nail bar is provided with at least one centralising spacer.

3. A method according to claim 2, wherein at least one end of the nail bar is provided with a centralising spacer in the form of a solid disc.

4. A method according to any preceding claim, wherein the holes are temporarily cased.

5. A method according to claim 4, wherein the lime is introduced at the mouth of the hole as the temporary casing is withdrawn.

6. A method according to claim 5, wherein the lime is compacted along the length of the hole through the application of a tubular ram surrounding the nail bar as the temporary casing is withdrawn.

7. A method according to claims 4, 5 or 6, wherein an end plate or centralising spacer is fitted to the exposed end of the nail bar after the temporary casing has been substantially withdrawn.

8. A method according to any preceding claim, wherein any residual length of the hole is plugged with grout, rammed soil or a similar material or materials.

9. A method according to any preceding claim, wherein the nail bar is ribbed.

10. A method according to any preceding claim, wherein the lime is mixed with pulverised fuel ash.

11. A method according to any previous claim, wherein the lime is mixed with Ordinary Portland Cement.

12. A method according to claim 11, wherein the proportion of Ordinary Portland Cement in the mix is from 25 to 50% by dry weight.

13. A method of reinforcing a soft clay soil, substantially as hereinbefore described with reference to the accompanying drawings.