Exclusion methods

The aim of groundwater control by exclusion is to prevent groundwater from entering the working area. The methods used can be grouped into three broad categories:

1 Methods where a very low permeability discrete wall or barrier is phys-ically inserted or constructed in the ground (e.g. sheet-piling, diaphragm walls).

2 Methods which reduce the permeability of the in situ ground (e.g.

grouting methods, artificial ground freezing).

3 Methods which use a fluid pressure in confined chambers such as tunnels to counter balance groundwater pressures (e.g. compressed air, earth pressure balance tunnel boring machines).

Techniques used to exclude groundwater are listed in Table 5.1, which is based on information from Preene et al. (2000). Details of the various meth-ods can be found in Bell and Mitchell (1986) or the further reading listed at the end of this chapter.

One of the most common applications of the exclusion method involves forming a notionally impermeable physical cut-off wall or barrier around the perimeter of the excavation to prevent groundwater from entering the working area. Typically, the cut-off is vertical and penetrates down to a Figure 5.1 Drains for collection of surface water run-off (after Somerville 1986).

(a) Open ditch, (b) agricultural drain.

Table 5.1 Principal methods for groundwater control by exclusion

Method Typical applications Notes

Displacement barriers

Steel sheet-piling Open excavations in most May be installed to form permanent soils, but obstructions such cut-off, or used as temporary as boulders or timber baulks cut-off with piles removed at the may impede installation end of construction. Rapid

installa-tion. Can support the sides of the excavation with suitable propping.

Seal may not be perfect, especially if obstructions present.Vibration and noise of driving may be unacceptable on some sites, but

‘silent’ methods are available.

Relatively cheap

Vibrated beam Open excavations in silts Permanent. A vibrating H-pile is wall and sands.Will not support driven into the ground and then

the soil removed. As it is removed, grout is injected through nozzles at the toe

Slurry trench Open excavations in silts, Permanent.The slurry trench forms cut-off wall sands and gravels up to a a low permeability curtain wall using bentonite permeability of about around the excavation. Quickly or native clay 5
10^⫺3m/s installed and relatively cheap, but

cost increases rapidly with depth Structural Side walls of excavations Permanent. Support the sides of concrete and shafts in most soils and the excavation and often form the diaphragm walls weak rocks, but presence sidewalls of the finished

construc-of boulders may cause tion. Can be keyed into rock.

problems Minimum noise and vibration. High cost may make method uneconomi-cal unless walls can be incorpo-rated into permanent structure Secant As diaphragm walls, but As diaphragm walls, but more likely (interlocking) penetration through to be economic for temporary and contiguous boulders may be costly works use. Sealing between bored piles and difficult contiguous piles can be difficult, and

additional grouting or sealing of joints may be ncessary Injection barriers

Jet grouting Open excavations in most Permanent.Typically forms a series soils and very weak rocks of overlapping columns of

Table 5.1 Continued

Method Typical applications Notes

soil/grout mixture. Inclined holes possible. Can be messy and create large volumes of slurry. Risk of ground heave if not carried out with care. Relatively expensive

Mix-in-place Open excavations in most Permanent. Overlapping columns columns soils and very weak rocks formed by in-situ mixing of soil and

injected grout using auger-based equipment. Produces little spoil. Less flexible than jet grouting. Relatively expensive

Injection grouting Tunnels and shafts in gravels Permanent.The grout fills the pore using and coarse sands, and spaces, preventing the flow of cementitious fissured rocks water through the soil. Equipment

grouts is simple and can be used in

confined spaces. A comparatively thick zone needs to be treated to ensure a continuous barrier is formed. Multiple stages of treatment may be needed

Injection grouting Tunnels and shafts in medium As cementitious grouting, but mate-using chemical sands (chemical grouts), fine rials(chemicals and resin) can be and solution sands and silts (resin grouts) expensive. Silty soils are difficult

(acrylic) grouts and treatment may be incomplete,

particularly if more permeable laminations or lenses are present Other types

Artificial ground Tunnels and shafts. May not Temporary. A ‘wall’ of frozen freezing using work if groundwater flow ground (a freezewall) is formed, brine or liquid velocities are excessive which can support the side of the nitrogen (2 m/day for brine or excavation as well as excluding

ground-20m/day for water. Liquid nitrogen is expensive liquid nitrogen) but quick; brine is cheaper but

slower. Liquid nitrogen is to be preferred if groundwater velocities are relatively high. Plant costs are relatively high

Compressed air Confined chambers such as Temporary. Increased air pressure tunnels, sealed shafts (up to 3.5 bar) raises pore water and caissons pressure in the soil around the

chamber, reducing the hydraulic gra-dient and limiting groundwater inflow. Potential health hazards to

very low permeability stratum that forms a basal seal for the excavation (Fig. 5.2(a)).

The costs and practicalities of constructing a physical cut-off wall are highly dependent on the depth and nature of any underlying permeable stratum. If a suitable very low permeability stratum does not exist, or is at great depth, then upward seepage may occur beneath the bottom of the cut-off wall, leading to a risk of base instability (see Section 4.3). In such circumstances dewatering Table 5.1 Continued

Method Typical applications Notes

workers. Air losses may be signifi-cant in high permeability soils. High running and set-up costs

Earth pressure Tunnels in most soils Temporary.The TBM excavates for balance tunnel and weak rocks the tunnel, and supports the soil

boring machine and excludes groundwater by

(TBM) maintaining as balancing fluid

pressure in the plenum chamber immediately behind the cutting head.

The fluid is a mixture of soil cuttings, groundwater and

conditioning agents (such as polymer or bentonite muds).TBMs need to be carefully selected to deal with given ground conditions; set-up and running costs may be high

Figure 5.2 Groundwater control by exclusion using physical cut-offs. (a) Cut-off walls penetrate into very low permeability stratum, (b) cut-off walls used in combination with dewatering methods, (c) cut-off walls used with hor-izontal barrier to seal base.

Figure 5.2 Continued.

methods may be used in combination with exclusion methods (Fig. 5.2(b)).

Alternatively, it may be possible to form a horizontal barrier or ‘floor’ to the cut-off structure to prevent vertical seepage (Fig. 5.2(c)). The construction of horizontal barriers is relatively rare, but has been carried out using jet grout-ing, mix-in-place, grouting and artificial ground freezing techniques.

If a complete physical cut-off is achieved, some groundwater will be trapped inside the working area. This will need to be removed to allow work to proceed, either by sump pumping during excavation, or by pump-ing from wells or wellpoints prior to excavation.

One of the attractive characteristics of the exclusion technique is that it allows work to be carried out below groundwater level with minimal effects on groundwater levels outside the site. This means that any potential side effects of dewatering (see Chapter 13) are avoided. In particular, in urban areas exclusion methods are often used in preference to dewatering meth-ods to reduce the risk of settlement damage resulting from lowering of groundwater levels. However, when considering using the exclusion tech-nique to avoid groundwater lowering in areas outside the site, it is essential to remember that almost all walls will leak to some extent. Leakage may particularly occur through any joints (between columns, panels, piles, etc.) resulting from the method of formation.

Leakage of groundwater through cut-offs into the excavation or working area can cause a number of problems:

i During construction the seepages may interfere with site operations, necessitating the use of sump pump or surface water control methods to keep the working area free of water.

ii The leakage into the excavation may be significant enough to locally lower groundwater levels outside the site, creating the risk of settlement or other side effects.

iii If the cut-offs form part of a permanent structure (such as the walls of a deep basement) even very small seepages will be unsightly in the long term and may cause problems with any architectural finishes applied to the walls.

In many cases, the significant seepages that give rise to problems (i) and (ii) can be dealt with by grouting or other treatment. On the other hand, it can be very difficult to prevent or to seal the small seepages of (iii). David Greenwood (1994) has said:

Water penetration is very difficult to oppose. It is comparatively easy to reduce torrents to trickles, but to eliminate trickles is difficult. If it is essen-tial to have a completely dry or leakproof structure, costs rise steeply.

This is an important point to consider if cut-off walls are to be incorporated in the permanent works.