maisteratorbos: una formula teonimica venetica?

Suitable for forming a temporary cut-off barrier to exclude groundwater in most saturated and near-saturated soils by lowering the ground temperature to form a wall with contiguous ice cylinders. Some soils with moisture content as low as 10 per cent can be frozen satisfactorily.

It is mainly used to assist in the construction of shafts and tunnels, although ice walls have also been used as propped retaining walls around open excavations and as temporary underpinning (J S Harris, 1995).

Depth of treatment will be limited by the accuracy of drilling for the freeze tubes.

Refrigeration plant is costly to install but once the ground is frozen the system can be operated cost-effectively for long periods. Liquid nitrogen

Case: freezing in tunnel East Lancs Road

Pipe jacking 2m below a dual carriageway had to stop when running sand at the tunnel face threatened major subsidence. Liquid nitrogen was used to freeze an annulus >2m long which was safely excavated to allow jacking of

An introduction to geotechnical processes 146

as the refrigerant is only viable for short-term stabilisation.

Ground freezing is not recommended where:

• ground heave may occur during freezing, affecting adjacent structures

• thawing may leave voids in the soil, causing settlement under load and self-weight

• there are flooded cavities or insufficient porewater

• there are variable strata with different thermal conductivities and groundwater flow exists.

The thermal conductivity and heat capacity of the soil and the temperature of the refrigerant used govern the rate of freezing. The strength of the frozen ground depends mainly on the soil lithology, porosity and moisture content, affecting the volume of ice formed.

Ground investigation should therefore determine the following properties as part of the ground model before deciding on the use of artificial ground freezing:

• geological and hydrogeological conditions

• position of the water table and degree of fluctuation

the next pipe section. Freezing each section took 24h and the ground remained adequately frozen for 48h. No heave occured at the road surface during freezing but the water conten of the soil had increased on thawing—no adverse effects were observed.

Case: freezing around shaft in Edinburgh

In order to prevent blowing of the base, groundwater was excluded from the shaft by freezing silty sand around the shaft and into a clay aquiclude.

Groundwater control–exclusion methods 147

• flow of groundwater

• soil strength parameters

• soil water content and chemical composition

• thermal properties of the soil.

Design has to ensure that the thickness and strength of the frozen ground is adequate to prevent structural failure during construction. An intact cylindrical ice annulus in ground around a shaft or tunnel will be capable of resisting significant hoop stress (Auld and Harris, 1995).

Refrigerants—Brine (sodium or calcium chloride) at a temperature of −30 to −40°C is used if ground freezing is considered at the design stage of a project, such as sinking a deep mine shaft, to lower ground temperature to at least −5°C. A large refrigeration plant is required with reliable pumps circulating brine to the freeze tube system and back to the plant for as long as the ground has to remain frozen.

Liquid nitrogen (LN) at a temperature of −196°C is used as the refrigerant for short-term projects or when emergencies such as unforeseen running sand arise. Only LN is likely to be effective in adequately freezing porewater in cohesive soils. It is fast and efficient in freezing the ground, but as it is vented to the atmosphere during the process and not returned to the plant, it is expensive. The onsite plant is usually a vacuum-insulated pressure vessel with an evaporator to produce flow to the freeze tubes; no power connections are needed. Mobile tankers deliver LN to the pressure vessel from the manufacturer’s plant as needed, and once the soil is frozen only a limited amount of LN in the gas phase is needed to maintain the freeze due to the considerable initial sub-cooling adjacent to the tube.

Typical vertical freeze tube for LN

Length of freeze tubes for brine refrigerant is limited by the practicality of drilling precisely aligned adjacent holes. Vertical and horizontal freeze tubes

An introduction to geotechnical processes 148

Freeze tubes of the type illustrated here at 1m centres will produce interlocking ice cylinders, with time to freeze depending on the refrigerant. Two rows of tubes to form the barrier are usual for long-term freezing where structural strength and water exclusion are of equal importance. When freezing clayey soil, two rows are desirable to avoid problems with vertical cracks occurring between frozen cylinders. The development of the frozen cylinders is monitored by thermocouples run into probes between a selection of cylinders.

Groundwater flow causes difficulties in producing uniform cylinders around brine freezing tubes: a velocity >3m/day will require pre-grouting to reduce the flow.

Alternatively, dewatering to counter the flow may be effective provided the water table is not lowered at the freezing zone. If LN is used as the refrigerant, groundwater flowing at 30m/day can be frozen using at least a two-row barrier of tubes. Variations in the thermal properties of the strata to be frozen also cause problems with non-uniform ice cylinders, resulting in gaps in the ice wall. The much lower temperature of LN will mitigate these problems.

Estimated quantities of refrigerant required are based on private communication from for liquid nitrogen are limited to 30m. Horizontel freeze tubes are similar in disign, but baffles are needed to ensure refrigerant flow is not affected by inclination.

Problems in forming contiguses ice cylinders

The reduis of the frozen cylinder aroun the freeze tube will depend on the thermal conductvity, the moisture content of the soil and the refrigerant temperature. In order to achive an effective ice barrier each adjacent frozen cylinder must be contiguouse throughout its length.

Groundwater control–exclusion methods 149

British Oxygen Company, heat-transfer principles (J S Harris, 1995; Holden, 1997). The graph shows the approximate quantities of LN required for the initial freeze to lower the soil temperature from 10°C to 0°C at a radius of 610mm (W G Grant, 1968). Beyond this the growth rate of the ice decreases significantly. The tube diameter only marginally influences the quantity of LN required initially for the same frozen zone, but less LN is used when the tubes are in a liquid/gas series than when all tubes are full of liquid. The total LN requirement is dependent on the time the ground has to be kept frozen.

Installation—accurate drilling is a critical part of the process, requiring cased holes, usually by duplex drilling and to considerable depths for shaft sinking, into which the freeze tubes are inserted. It is therefore essential to survey the alignment of each hole so that additional tubes can be installed where gaps in the ice wall are likely.

It may be necessary to grout around the freeze tubes as the casing is withdrawn to avoid an aquifer discharging into unsaturated strata. If an ice annulus is to be formed around a tunnel prior to excavation, the same care must be taken with the horizontal drilling methods—directional drilling navigation may be appropriate.

Where vertical freeze tubes are necessary to stabilise soil for tunnelling, a technique called ‘rolling freeze’ has been successfully deployed (Baker and James, 1990). All the vertical freeze tubes are installed initially, but LN is circulated only to the particular section about to be excavated. Note that the steel tubes will cross the line of tunnelling and have to be cut out by miners.

Pipework for freezing deep shafts should provide for separate delivery of brine to each freeze tube off the header main. For LN systems, two or three freeze tubes may be connected in series off the main depending on the depth of the tubes; in this case it is desirable to be able to reverse the flow to ensure that the liquid phase is efficiently used initially to produce uniform cylinders.

For brine and short-term LN freezing it is not usually necessary to fit ‘cryogenic’

pipework: steel pipe with good tensile properties at −50°C is adequate for the tubes. All exposed pipes above groundwater should be fully lagged.

Quantity of LN to freeze cylinder of ground initially An introduction to geotechnical processes 150

Operation—time to freeze soil depends on the thermal properties of the soil, the temperature of the refrigerant and particularly on the diameter of the freeze tube. It will take several weeks to freeze a 1m diameter soil cylinder using brine in 75mm tubes, whereas with LN the time will be reduced to around 2 days in similar conditions;

increasing to 3 days using LN in a 50mm tube.

Frost heave may occur in some frozen soils, depending on permeability: free-draining soils are less likely to be affected by moisture expansion. In most cases, provided that the confining pressure around the frozen zone is greater than the expansion pressure, heave should not occur. Porewater migration under capillary action during the freezing of silts may increase the volume of water being frozen, leading to increased expansion and potentially unstable soil when the ice thaws. Freezing will permanently alter the structure of peaty soils.

Excavation of frozen soil will require the use of mechanical tools operating in very cold conditions. Construction techniques also have to cope with freezing conditions, but slip-forming of concrete against the ice wall has been successful (Collins and Deacon, 1972).