GB2311518A - Grout to improve soil load-bearing capacity - Google Patents

Description

IMPROVEMENT OF SOIL LOAD BEARING CAPACITY This invention relates to the improvement of the load bearing capacity of soils by use of a combined soil stabilisation system in a dry grout treatment.

Increasingly civil engineering projects are requiring a method of considerably increasing the load bearing capacity of soils of poor engineering quality.

Also, many of the soils requiring strengthening also require contamination control to reduce the risks of contaminants from leaching. This is particularly so in industrialised areas where land reclamation is being undertaken.

In many parts of the world, roads are required to be constructed very cheaply. Costs can only be reduced if expensive base materials such as cement and stones can be reduced or eliminated from the construction.

Of the available methods of improving soil properties, mechanical stabilisation by compaction with suitable granular material is the most widely employed.

Mechanical improvements of soils using 10-15% of cement has been used but this does nothing for stabilisation of the soil at a colloidal level. Lime stabilisation is also used extensively for stabilisation of soil, but is best suited for soils with a very high clay content. However, the strengths achieved using lime stabilisation is usually less than 500kN/m2. The strength requirements currently being demanded from the construction industry world wide is > 3000kN/m2 (3MPa), which is not achievable using lime alone.

A method of hardening and stabilising peaty soils, which have a very high organic and water content, some humus and very little or no clay, is disclosed in Japanese patent specification SHO 51-139458. This specification discloses the use of cement containing 10% or greater of calcium chloride and, optionally, calcium oxide. The stabilisation is achieved by using 35% of the cement/calcium chloride mix with the soil.

This would be far to expensive for normal soils. In practical applications, strengths of only 20-30 kPa could be achieved using this approach, which requires spraying of the grout onto the peaty layer.

A method of improving the permeability for liquid grouts applied by injection is disclosed in Japanese patent specification SHO 51-6802. This is claimed to improve the permeability of soils to the injection of cement slurries.

Sodium silicate solutions are also widely used for chemical grouting by injection or spraying onto surfaces in conjunction with cement. When injected the silicate is mixed with a material that yields a reduced pH to form silica gel. A severe disadvantage of using silicates is that they cause colloidal particles to become negatively charged. The consequence of this is that stronger inter-colloidal repulsions develop which causes the particles to increase their inter-colloidal distances significantly.

This results in a considerable loss in compressive strength of the soil.

A severe limitation of all of the above grouting systems is that they only introduce some stiffness into the soil but in doing so increase the water content of the soil. This additional water causes further separation of the soil particles and hence the full benefit of the grout is not realised. The present invention has been made to overcome these problems.

According to one aspect of the invention there is provided a dry grout composition comprising a cationic flocculant, an inorganic coagulant and hydraulic lime.

Cationic polymers that are particularly preferred for the dry grout of the invention are those having the structure:

where m and n are in the range 1000 to 5000, preferably 2000 to 4000, and

whereRisC10-0andZhasthestwcture:

An advantage of the structure is that it imparts hydrophobicity to the particles onto which it adsorbs.

Sedimentation is, therefore, encouraged and water desorption occurs allowing greater compaction of the soil particles. The above polymer structures have nitrogen contents of between 0.4% to 0.8% and molecular weights of 2500 to 8000.

It is a further requirement of the discovery that the polymers are an integral part of the dry grout system. In practice, many of the soils and clays used for building upon require aggregate to give them strength due to the high natural moisture content (NMC). This invention makes use of this NMC should it be above the optimum moisture content (OMC) (which is the moisture content at which optimum density occurs).

In addition to the polymer, the dry grouting system includes divalent ions of calcium, magnesium with the chlorides of potassium and sodium which may be in the following ratios: Calcium Chloride 71.2% Magnesium Chloride 22.8% Potassium Chloride 4.4% Sodium Chloride 1.6% A soluble silicate may be included at a level predefined by the contaminants present in the soil and hydraulic lime.

Silicates can be used for controlling the leaching of heavy metal ions. However, the use of silicates of sodium or potassium at various Na,O: Sio2 ratios was found to have a very negative effect on coagulation of dispersed soil colloids by soluble divalent salts such as calcium chloride, magnesium chloride and barium chloride. Although sedimentation would occur, the resulting sediment was found to occupy approximately twice the volume of a non-silicate treatment. Even significant increases in the concentration of divalent salts would not overcome this effect. The inclusion of a cationic polymer, particularly of the structure of the polymers of Formula I and II, at concentrations of from 0.005% to 0.1% overcomes this dispersion effect.

The use of cationic polymers for this purpose is not confined to dry grout systems, but can be applied to liquid grouting systems also.

According to another aspect of the invention there is provided a grout system comprising a silicate for controlling the leaching of heavy metal ions and a cationic polymer flocculant.

Unlike the limes of calcium oxide and calcium hydroxide, which require carbon dioxide absorption to form calcium carbonate for strength, hydraulic lime contains silicates which set under water. This is a distinct advantage when used in soils of moisture content above their OMC. This setting action imparts on the sedimented colloidal particles a cementitious effect giving the soil high mechanical strength.

Optionally, the composition may contain up to 5% Portland cement to increase the strength still further.

Preparation of the grouting composition is preferably effected by dry mixing of the powdered ingredients and spraying a solution of the polymers onto the powder during mixing. Spraying is preferably carried out using fogging nozzles which results in good distribution of the liquid throughout the powder.

This is important as the process is kept simple for construction in that the process is a one shot system which can, however, be tailored for individual sites.

Soil mixing can be done on sites to depths of up to 50 cm using for example a stabiliser or reclaimer machine. These machines are approximately the width of a car lane. They are fixed with rotating cylinders fitted with steel fingers which churn up the soil very vigorously. Mixing of powders into the soil is very rapid. Once mixed the soil can then be consolidated.

Alternatively, the soil may be excavated and taken to processing equipment, mixed and then returned to the site for compaction which is preferably done as quickly as possible after mixing.

Many old industrial sites requiring remediation have soils contaminated with cyanides. Soil washing and bio-remediation are currently the only two options available for this contaminant. However, it has been found that calcium hypochlorite destroys the cyanides in contaminated soils. Furthermore, the calcium hypochlorite can be included in the dry grout composite by substituting for some of the hydraulic lime or the calcium chloride. The presence of the alkaline materials in the grout neutralises the oxides so produced. The resulting calcium chloride assist in the coagulating of the soil colloids.

The following examples further illustrate the invention. In the Examples reference is made to the accompanying drawings which are graphs showing the strengths of soil samples containing varying amounts of composition according to the invention.

EXAMPLE I Composition A Hydraulic Lime 94.13 Cationic Polyacrylate (35%) 3.00 Calcium Chloride 2.04 Magnesium Chloride 0.65 Potassium Chloride 0.13 Sodium Chloride 0.05 Note: for ease of mixing the cationic polymer was diluted to 35% of its original concentration to reduce its viscosity.

As shown in Fig.1 the maximum strength achieved using 5% lime alone and without any composition of the invention was 436kPa which was a good result for this mode of stabilisation. Additions of 5% OPC (ordinary Portland cement) boosted the strength to 854 kPa (Fig.2). Both results are way below the 3MPa being demanded in the market place. Fig.3 shows the result obtained using 5% of Composition A and 5% OPC.

Figs.4 to 6 show the progressive gain in strength a Moscow clay yielded when treated with 3, 5 and 7% of Composition A, all with 5% OPC.

The mixes and preparation of the samples used in the tests, the results of which are shown in the drawings, was as follows: Fig.1 Test Mix Description: 5% Lime Stabilised Astrakhan Clay 7 days air dried.

Preparation: Compacted height (mm) 75.0 diameter (mm) 38.0 moisture content (%) bulk density (Mg/cu.m) 1.97 dry density (Mg/cu.m) Cell pressure (kPa) 0 Rate of Strain($/min) 0.50 Strain at failure (%) 0.93 Max. deviator stress (kPa) 476 Undrained strength (kPa) 238 Unconsolidated Undrained Triaxial Test Fig.2 Test Mix Description: 5% Lime Stabilised and 5% Cement Astrakhan Clay 7 days air dried.

Preparation: Compacted height (mm) 200.0 diameter (mm) 105.0 moisture content (%) bulk density (Mg/cu.m) 1.29 dry density (Mg/cu.m) 1.29 Cell pressure (kPa) 0 Rate of Strain (%/min) 0.50 Strain at failure (%) 0.70 Max. deviator stress (kPa) 854 Undrained strength (kPa) 427 Unconsolidated Undrained Triaxial Test Fig.3 Test Mix Description: Astrakhan Clay + 5% Composition A + 5% OPC 7 days air dried.

Preparation: Compacted height (mm) 200.0 diameter (mm) 105.0 moisture content (%) 0 bulk density (Mg/cu.m) 1.80 dry density (Mg/cu.m) 1.80 Cell pressure (kPa) 0 Rate of Strain (%/min) 0.50 Strain at failure (%) 0.70 Max. deviator stress (kPa) 2727 Undrained strength (kPa) 1363 Unconsolidated Undrained Triaxial Test Fig.4 Test Mix Description: Moscow Clay + 3% Composition A + 5% OPC 7 days air dried.

Preparation: Compacted height (mm) 75.0 diameter (mm) 38.0 moisture content (%) bulk density (Mg/cu.m) 1.95 dry density (Mg/cu.m) Cell pressure (kPa) 0 Rate of Strain (%/min) 0.50 Strain at failure (%) 1.47 Max.deviator stress (kPa) 2098 Undrained strength (kPa) 1049 Unconsolidated Undrained Triaxial Test Fig.5 Test Mix Description: Moscow Clay + 5% Composition A + 5% OPC 7 days air dried.

Preparation: Compacted height (mm) 201.0 diameter (mm) 105.0 moisture content (%) 0 bulk density (Mg/cu.m) 2.05 dry density (Mg/cu.m) 2.05 Cell pressure (kPa) 0 Rate of Strain (%/min) 0.50 Strain at failure (%) 0.80 Max. deviator stress (kPa) 2881 Undrained strength (kPa) 1440 Unconsolidated Undrained Triaxial Test Fig.6 Test Mix Description: Moscow Clay + 7% Composition A + 5% OPC 7 days air dried.

Preparation: Compacted height (mm) 75.0 diameter (mm) 38.0 moisture content (%) bulk density (Mg/cu.m) 1.89 dry density (Mg/cu.m) Cell pressure (kPa) 0 Rate of Strain (%/min) 0.50 Strain at failure (8) 2.03 Max.deviator stress (kPa) 4385 Undrained strength (kpa) 2192 Unconsolidated Undrained Triaxial Test These clays used in the above tests were found to have the following characteristics: SOIL TYPE CLAY CLAY REGION ASTRAKHAN MOSCOW Natural Moisture % NMC 8.3 22.2 Optimum Moisture % OMC 12.0 14.5 Optimum Density (kg/m3) 1864 1984 ATTEBERG LIMITS Liquid Limit 23 37 Plastic Limit 12 17 Plasticity Index 11 20 From the results it is clear that Composition A can be used to treat soils having moisture contents well above the optimum moisture content. Also highly plastic clays can be treated to considerably raise their strength.

EXAMPLE II In view of the high plasticity of the Moscow clay it was used to compare the sedimentation rates of the cat ionic polyacrylamide of Formula I with the cationic silicone polymer of Formula II. The results were as follows: The test consisted of placing 50g of finely divided clay into a beaker, adding 150 cm3 demineralised water containing various percentages of the polymers.

The solutions were then stirred on a magnetic stirrer for 5 minutes and then left to settle. The times taken for the water to become free of suspended clay were then recorded.

POLYMER CONCENTRATION SEDIMENTATION TIME (min) % Polvacrylate Polysilicone Nil > 30 > 30 0.005 12 14 0.010 10 11 0.020 8 9 0.040 7 7 0.080 5 5 0.100 5 5 0.200 5 5 Thus at very low dilutions, the cationic polyacrylate causes a marginally faster sedimentation than the polysilicone cationic compound. At higher concentrations they have equal sedimentation effects.

However, a benefit of the cationic silicone polymer is the slightly hydrophobic effect it has on the clay cylinders treated with Composition A but in which the cationic polyacrylate has been replaced by the cat ionic silicone polymer, as the following results illustrate: Polymer Test Cylinder Weight Average % (0.2% Number Dry Wet Gain Acrylate 1 112.6 132.8 17.94 2 114.1 136.1 19.28 Silicone 1 108.3 126.7 16.99 2 111.7 132.0 18.17 Average Difference 1.11 EXAMPLE III A soil containing 0.05% mg/kg of cyanide was mixed with 7% of the following composition: Hydraulic Lime 90.3 Calcium Hypochlorite 5.0 Calcium Chloride 1.0 Magnesium Chloride 0.7 Cationic polyacrylate (35%) 3.0 The sample was then compacted and allowed to air dry for 7 days. After the 7 days curing, the sample was crushed into granules sufficient to pass through a 1 mm sieve. The sample was then leached using water acidified to pH 4.5 with acetic acid in a sealed container, with shaking, for 24 hours. Analysis of the resultant liquor failed to find any residual cyanide.

EXAMPLE IV When investigating the effects of sodium and potassium silicates of various Na2O:SiO2 ratios on sedimentation, there was found both positive and negative effects. Addition of a silicate to an aqueous dispersion of clay colloids caused a rapid sedimentation to yield very clear water. However, the resultant sediment was about 50% of dense sedimented particles and about 50% sediment having a very woolly appearance. The latter was very voluminous and not dense. In total volume, clay colloids sedimented with silicates occupy approximately twice the volume of sediments that have been sedimented in the absence of silicates.

The following table shows the relationship between sodium metasilicate pentahydrate and a solution of cat ionic polyacrylamide on sedimentation of a silt from Holland: Weight Silt Silicate Polvmer Comments 20 0.2 0.1 Good compact sediment 20 0.2 0.2 Good compact sediment 20 0.2 0.4 Slight bulking 20 0.2 0.6 Moderate bulking 20 0.4 0.2 Good compact bulking 20 0.6 0.2 Slight bulking 20 0.8 0.2 Moderate bulking 20 1.0 0.2 Considerable bulking 20 0.8 0.4 Good compact sediment 20 1.0 0.5 Good compact sediment Notes: 1) Polymer solution contained 7% active material 2) Sodium metasilicate .5H2O contains 35.8% SiO32- From these results the solids ratio of Sio32- polymer could be calculated and yielded the following results: Weight (q) Ratio Silicate Polvmer Silicate Polymer Sedimentation 0.2 0.1 10:1 Good 0.2 0.2 5:1 Good 0.2 0.4 2.5:1 Poor 0.2 0.6 1.7:1 Poor 0.4 0.2 20:1 Good 0.6 0.2 30:1 Poor 0.8 0.2 40:1 Poor 1.0 0.2 50:1 Poor 0.8 0.4 20:1 Good 1.0 0.5 10:1 Good Thus good sedimentation occurred when the ratio of SiO32- to polymer was of the order 5:1-20:1. Below or above these ratios, poor sedimentation, i.e. bulking, occurred. Hence, between and at the ratios of 5:1 to 20:1 the charge densities on the particles must be reasonably balanced. Outside these ratios imbalances occur that lead to strong particle - particle repulsions resulting in bulking of the sediment.

Claims (16)

1. A dry grout composition comprising a cationic flocculant, an inorganic coagulant and hydraulic lime.

2. A dry grout according to claim 1 wherein said cationic flocculant comprises a polymer having the structure:

where m and n are in the range 1000 to 5000.

3. A dry grout according to claim 1 or claim 2 wherein said cat ionic flocculant comprises a polymer having the structure:

where R is C10- c2o and Z has the structure:

4. A dry grout composition according to claim 2 wherein m and n are in the region 2000 to 4000.

5. A dry grout composition according to any one of claims 2 to 4 which comprises a nitrogen content in the range 0.4% to 0.8% and molecular weights of 2500 to 8000.

6. A dry grout composition according to any one of claims 1 to 5 wherein soil composition includes divalent ions of calcium, magnesium with the chlorides of potassium and sodium.

7. A dry grout composition according to claim 5 wherein divalent ions of calcium and magnesium are in the form of calcium chloride and magnesium chloride respectively with the chlorides of potassium and sodium are present in the following ratios: Calcium Chloride 71.2% Magnesium Chloride 22.8% Potassium Chloride 4.4% Sodium Chloride 1.6%

8. A dry grout composition according to any one of claims 1 to 7 further including a soluble silicate at a level predefined by contaminants present in the soil and hydraulic lime.

9. A dry grout composition according to claim 2 or claim 3 wherein said cationic polymer is present at a concentration of 0.005% to 0.1%.

10. A grout system comprising a silicate for controlling the leaching of heavy metal ions and a cationic polymer flocculant.

11. A grout system according to claim 10 further including up to 5% Portland cement.

12. A method of preparing a dry grout composition according to claim 1 comprising dry mixing of powdered ingredients and spraying a solution of cationic polymers onto the powder during mixing.

13. A method according to claim 12 wherein the spraying is carried out using fogging nozzles.

14. A dry grout composition according to claim 1 substantially as hereinbefore described with reference to any one of Examples 1 to 4.

15. A dry grout system according to claim 9 substantially as hereinbefore described with reference to any one of Examples 1 to 4.

16. A method of preparing a dry grout composition according to claim 1 substantially as hereinbefore described with reference to any one of Examples 1 to 6.