Pore water pressure control systems in fine-grained soils When dewatering methods are applied to high and medium permeability

soils, local lowering of groundwater levels occurs by gravity drainage in response to pumping. The period of pumping necessary to lower the water level is quite short as the pore water is rapidly replaced by air. In fine-grained soils of low and very low permeability, gravity drainage of the pore water is resisted by capillary tension. Such soils drain poorly and slowly by gravity drainage.

Because these soils do not drain easily, any excavation made below the groundwater level will encounter only minor seepages, and is unlikely to flood

10^–8 Dewatering not feasible and may not be necessary

0

Figure 5.6 Range of application of pumped well groundwater control techniques – adapted from Roberts and Preene (1994), and modified after Cashman (1994) (from Preene et al. 2000: reproduced by kind permission of CIRIA).

rapidly. Yet, even the small seepages encountered (perhaps less than 1 l/s even for a large excavation) can have a dramatic destabilizing effect. Side slopes may collapse or slump inwards and the base may become unstable or ‘quick’.

On site, people are often surprised that such small flow rates can be a prob-lem. The theory of effective stress explains the mechanism of instability (see Chapter 4). The seepages imply the presence of high positive pore water pres-sures around and beneath the excavation. This implies low levels of effective stress, and hence low soil strength – instability is the natural result.

The solution to this problem is to abstract groundwater and so lower the pore water pressures around and beneath the excavation. This will main-tain effective stresses at acceptable levels and prevent instability. The aim is not to totally drain the pore water from the soils – in any event this would be very difficult as capillary forces mean that fine-grained soils can remain saturated even at negative pore water pressures (see Fig. 3.7). Because the soil is not being literally ‘dewatered’, pumped well systems in fine-grained soils are more correctly referred to as pore water pressure control systems, rather than dewatering systems. The application of pumped well systems in low permeability soils is discussed further by Preene and Powrie (1994).

Glossop and Skempton (1945) and Terzaghi et al. (1996) indicated that gravity drainage will prevail in soils of permeability greater than about 5
10⁹⁵m/s. Where wells are installed as part of pore water pressure con-trol systems in soils of lesser permeability the well yields will be very low.

This can make the continued operation of conventional wellpoint or deep well systems difficult, as the pumps are prone to overheating at low flow rates. However, if the top of the wells are sealed a partial vacuum can be applied to assist drainage. The increase in yield of a well due to the appli-cation of vacuum is likely to be of the order of 10 per cent, sometimes up to 15 per cent.

A sealed wellpoint system (see Section 9.5) operated by vacuum tank pump (see Section 12.1) may be effective in soils of permeability down to about 1
10⁹⁶m/s. If a vacuum is applied to sealed deep wells (see Section 10.8) the lower limit of effectiveness of deep wells may be extended to about 1
10⁹⁵m/s. Ejectors (see Section 11.1) installed in sealed wells will automatically generate a vacuum in the well when yields are low. Ejector systems can be effective in soils of permeability as low as 1
10⁹⁷m/s.

Again, the permeability ranges quoted above are tentative. In fine-grained soils, structure and fabric has a great influence on the performance of vac-uum well systems. If the soil structure consists of thin alternating layers or laminations of coarser and finer soils, the drainage will be more rapid. This is because the layers of coarse material will more rapidly drain the adjacent layers of finer-grained soil. There have also been cases where pore water pressure control systems have been effective in clays of very low perme-ability – the success of the method was attributed to the presence of a per-meable fissure network in the clay.

5.4.2 Some deep well and ejector projects deeper than 20 m The 20 m depth limit in Fig. 5.6 is not restrictive, but is merely a conve-nience for presentation of the diagram. However, there are insufficient reported case histories to have confidence in extending Fig. 5.6 data below 20 m depth but four known case histories are reported below to substanti-ate the view that, with care it is economically feasible to use wells and ejectors to depths of the order of 40 m or more.

At Dungeness A nuclear power station sited on the south coast of England, sixty deep wells were pumped for more than two years. The soil conditions at the site were:

(a) Gravel with cobbles, ;5.5 mOD to 91.5 mOD, average permeability 3
10⁹³m/s

(b) Original groundwater level from ;2.4 mOD to 0.9 mOD

(c) Gravelly sand, 91.5 mOD to 99.1 mOD, average permeability 8

10⁹⁴m/s

(d) Sand, 99.1 mOD to 933.5 mOD, permeability 1.5
10⁹⁴m/s decreas-ing with depth to 5
10⁹⁵m/s.

The construction excavations were encircled by a continuous girdle of interlocking sheet piles driven to 99.1 mOD level so as to exclude recharge from the overlying high permeability soils. The lowering for the excavations for the Turbine Hall and the Reactors was achieved by pumping twenty-nine wells 20 m deep. The lowering for the Cooling Water Pumphouse and the Syphon Recovery Chamber was achieved by pumping nineteen and twelve wells respectively, each 41 m deep.

Prior to the construction of the East Twin Dry dock in Northern Ireland, site investigation borings had revealed a confined aquifer of Triassic sandstone beneath alluvial and glacial deposits (both mainly clays).

The depth to the upper surface of Triassic sandstone varied between 34 m below ground level and 21 m (which was 1 m below the formation level of the entrance structure). There was a high piezometric level confined in the sandstone – to about 3 m above original ground level. Twenty deep wells were installed to depths between 33.5 m and 43 m below ground level using reverse circulation rotary drilling methods. The wells were operated for over two years.

At the Mufulira mine No. 3 dump, in northern Zambia, about 200 twin pipe ejectors were installed to reduce the moisture content and thereby stabilize slimes lagoon deposits, which were of porridge like consistency and had broken through the roof of the main adit causing disastrous loss of life. The Ministry of Mines, Zambia required that the slimes deposits over the cave-in be stabilized before the mine could be permitted to reopen. After nine months of pumping of the ejector installation the phreatic surface was drawn down about 10 m. Eventually the phreatic surface was drawn down

some 30 m and resulted in an acceptable increase in the shear strength of the slimes deposits over the cave-in. The pumping lift was in the range from 20 m to over 40 m.

At the Benutan dam site in Brunei single pipe ejectors were installed to sta-bilize alluvium comprising heterogeneous loose silty fine sands, very soft clays, thin layers of peat and buried timbers (see Cole et al. 1994). Brunei is in an earthquake zone of moderate seismic activity so it was judged that there was risk of liquefaction of these loose alluvial foundation soils beneath the proposed dam unless they were stabilized or replaced with other more stable materials. As at Mufulira, an ejector system was installed and operated to reduce the pore water pressures of the loose soft soils and thereby increase their stability. The depths to which the ejectors were installed ranged from about 10 m to 38.5 m. The ejector pumping was continuous on the deepest line for about two years. The dam height was 20 m above valley floor level.