R. Chudley, Construction Technology, Longman

On completion of this lecture, you should be able to:

There are various processes for improving soil properties insitu either temporarily or permanently. The methods are fall into the following categories:

Ground water in a pervious soil stratum may be replenished directly from rain falling on the ground surface or be percolation from run offs, streams or nearby rivers.

The water pressures in pervious layers will usually change as a result in variations in seasonal rainfall or rivers and sea level in cases where there is direct connection between the pervious stratum and these sources.

The problems created when surface and ground water enter excavations are:

In some cases, the ground water conditions found during site investigation may change before or during site investigation. Such changes may be due to the construction of basements nearby, natural flooding or artificial causes, such as a burst water main.

The methods of ground water control may be divided into three broad groups:

1. pumping,
2. cut-off walling, and
3. special methods.

The choice of method depends mainly on site conditions and on the soil characteristics. These include:-

7. Dewatering Methods – Pumping

Pumping systems utilize:

1. sumps
2. wells
3. well-points  

Widely used in deep excavations for trench or basement. There are several major problems:-

The general solution is to dig sump at corner of excavation below formation level.

7.1.1 Open Sump

7.1.2 Jetted Sump

7.2 Pumping from wells

For depths 9m, use other methods. Use wells where wellpoints are not suitable.

7.2.1 Tube Well

1. Sink lined borehole, diameter 300 – 600 mm to depth required (i.e. below impermeable stratum as a rule)
2. Place smaller tube – ‘inner well lining’ – inside, having portion perforated at level to be dewatered. Lower end acts as sump.
3. Use plunger to ‘surge’ initial flow and wash out unwanted fines.
4. Connect pump to lining operate – submersible type is used.
5. Disconnect and withdraw both linings as annular space is filled, or in stages, or on completion.
6. Depth of well depends on depth of impermeable stratum. If far below excavation formation level wells can be spaced well apart to create draw-down curve just below formation level.

7.2.2 Horizontal Wells

Formation level at or slightly within impermeable stratum where vertical wells impracticable.

A. Method 1:

1. Sink vertical well outside excavation area to below proposed formation level.
2. Make horizontal borings in radial pattern from vertical well.
3. Results: water drains from upper surface of impermeable stratum into large well. Pump (submersible) operates there.

B. Method 2:

1. Lay 80mm PVC suction pipe (perforated) up to 6m depth around excavation area. Suction pipe is covered with nylon filter sleeve which prevents particles of soil from entering pipe (Laying is be special horizontal well-point placing machine: it digs trench, lays pipe, backfills in one operation at up to 160 m/hr).

2. Connect suction pipe to pump (Length of pipe per pump depends on soil conditions and pump capacity). Water flows to drainage channel formed in laying pipe.

7.2.3 Pumping from Well Points

It is used for non-cohesive soils, minimum grain size 0.1 mm. The wellpoint consists of a slotted or perforated pipe which is covered with a screen mesh. At the foot of this pipe is an orifice which permits jetting of the pipe into the ground during installation. A simple ball valve above the orifice prevents the entry of soil particles through the orifice when water is sucked in during the pumping operation.

The construction steps in the wellpoint system are:

1. a wellpoint is jetted into the ground;
2. the annular void is filled with filter media;
3. the wellpoints are connected to a header pipe by means of a riser;
4. the header pipe is connected to two suction pumps for pumping.

A. Multi-Stage Wellpoint Installations

It consists of the installation of wellpoints at two or more levels. It is also be used in shallow well pumping systems.

B. Shallow Well Systems

This system can handle greater volume of water compared with Wellpoint systems. The construction steps are:-

1. a cased well is bored,
2. a filter tube is placed in the borehole,
3. filter media are placed in the annular void and the casing withdrawn,
4. the filter if flushed and a suction pipe is lowered into the filter tube,
5. the suction pipe is connected to the header main, and
6. the header main is connected to a self-priming pump.

C. Deep Well System

The method of forming a deep well is similar to that of a shallow well, except that in a deep well, a submersible pump is used to pump out the water.

D. Vacuum Wells

In vacuum wells, a vacuum artificially increases the water flow towards the wells or wellpoints. All the pumping systems described so far are effective only in gravels and sands. If more than 10% silt is present, it is necessary to increase the water flow towards the wells.

Method:-

• Jet and sand in;
• Seal top 1m of hole with clay – vacuum is created in sand filter around wellpoint;
• Only free water is removed;
• Sudden shock can cause pronounced disturbance;
• Limitations – 6m maximum lift. Over 5m use multi-stage wellpoints;
• Preparatory work – from platform for each tier of wellpoints around excavation of platforms on which header pipe is situated.

E. Electro-Osmosis

• It is used in cohesive soils, where vacuum pumping is ineffective. It is because soil particles carry negative electrical charge, attracting + positively charged (hydrogen) ends of H₂O water molecules.

• Electro-osmosis has been found to be most successful in uniform beds of fine silts. However, the method can be very expensive and is therefore not commonly used.

 8. Dewatering – Water Exclusion Techniques

8.1 Freezing Methods

The principle of ground freezing is to change the water in the soil into a solid wall of ice. This wall of ice is completely impermeable.

Method:

Steel freeze pipes are inserted into soil at approximately 1m centres around site to be excavated. Pipes above ground level are insulated.Brine is pumped through system at –15 to –25°C, using calcium chloride or magnesium chloride cooled by refrigeration plant nearby – usually trailer-mounted.

Applications:

Problems:

8.2 Compressed Air

This method has been extensively used in the construction of the MTR in Hong Kong. It is used in caisson sinking and tunnel driving in waterlogged ground. Air pressure up to 350 KN/m² is possible which allows working at depths up to 35m below water table, using 1m³ of fresh air per person in working chamber.

The major disadvantage of using compressed air is the health risk to workers working in such chambers.

8.3 Grouting Methods

It is used where pumping is likely to be uneconomic – e.g. permeable soils, or variable ground (especially harder rocks) where boring of wells and wellpoints very costly. It is grouted by injecting rock or soil through pipes or holes in ground, using fluids which seal or reduce permeability of ground on setting.

The major problem is the vulnerability of existing u/ground structures; - cracks may be penetrated. The choice of media depends of soil particle sizes or sizes of fissures in rock.

8.3.1 Cement Grouting

It is suitable for very permeable coarse materials.

Method:

• Holes bored around excavation.
• Cement grout injected, starting thin and increasing viscosity by reducing water-cement ratio – e.g. neat cement and water; 4 parts sand, 1 cement; or PFA 1, cement 1, water 2, by wt.
• Secondary holes bored and injected mid-way between original holes to ensure complete grouting.

8.3.2 Bentonite Grouting

Bentonite adds very little strength to the soil. This system is used where soil particles are too small for cement grouting, especially alluvial soils beneath dam structures to create permanently impermeable layer.

When bentonite coagulates, it forms an impermeable gel. Bentonite may also be mixed with Portland cement or soluble silicates to form a permanent barrier.

8.3.3 Chemical Groutings

It is used in sandy soils of medium to coarse grading. Liquids are used which gel by reaction between base substance and hardener.

One Shot Process

In this process, two chemicals are mixed together prior to injection. One chemical is the base and the other the hardener or catalyst. When the two chemicals are mixed, a reaction takes place and a gel or solid is formed. In this process, the gel time should be sufficiently delayed to allow for the full penetration of the chemicals before gelling occurs. The time may be accurately controlled by varying the proportions of the two chemicals.

Two Shot Process

In this process, the first chemical, normally sodium silicate, is injected into the ground. The second chemical, normally calcium chloride, is then injected. An immediate reaction occurs resulting in the formation of a tough, insoluble ‘silica gel’.

Chemical grouting strengthens the soil and reduces its permeability.

Advantages over other methods:

8.3.4 Resin Grouting

Resin grouts have a low viscosity and are formed by adding a catalyst or hardener to a base solution. It is used in sandy soils of fine grading, and silts. The choice of materials depends on the chemical content of ground water.

8.3.5 Bituminous Grouting

It is used in fine sandy soils – e.g. cut-off walls beneath dams, etc. where no strengthening is required. It is not suitable for under-pinning.