GROUNDWATER BEHAVIOUR IN HIGH- AND LOW-PERMEABILITY SOILS AND ROCKS

The design and analysis of groundwater control systems require an understanding of the factors driving groundwater flow, as outlined in Chapters 3 to 5. We have also seen, when effective stress theory was introduced in Section 6.5, that groundwater pressures influence the available shear strength of soil - this is the basis of soil mechanics analysis. Karl Terzaghi, widely considered the father of modern soil mechanics, said: ‘In engineering practice difficulties with soils are almost exclusively due not to the soils themselves but to the water contained in their voids. On a planet without any water there would be no need for soil mechanics’ Terzaghi (1939).

A detailed knowledge of all aspects of soil mechanics is not necessary for the successful implementation of groundwater control measures and is not the purpose of this book. The interested reader wanting to learn more is directed to accessible textbooks on soil mechanics, such as Powrie (2013), and similar texts on rock mechanics, such as Hencher (2015). However, there is a particular aspect of soil behaviour that is worth discussing here - ‘drained’ versus ‘undrained’ behaviour - because of its profound effect on the temporary stability of excavations in low- and very low-permeability strata.

Drained and Undrained Conditions

It is useful to start with some definitions. Duncan etal. (2014) highlight that the soil mechanics terms ‘drained’ and ‘undrained’ should not be taken literally. In non-technical usage, drained would mean dry or emptied and undrained would mean not dry or not emptied. The soil mechanics usage of these terms actually relates to the transient pore water response to load changes. Duncan et al. (2014) give the following useful definitions:

• • Drained is the condition under which water is able to flow into or out of a mass of soil as rapidly as the soil is loaded or unloaded. Under drained conditions, changes in load do not cause changes in pore water pressure within the soil.
• • Undrained is the condition under which there is no flow of water into or out of a mass of soil in response to load changes. Under undrained conditions, changes in load cause changes in the pore water pressure, because the water is unable to move in or out of the soil as rapidly as the soil is loaded or unloaded. The changes in pore water pressure, relative to original groundwater conditions, are termed ‘excess’ pore water pressures.

When dealing with groundwater control problems, our main interest is in excavations dug below groundwater level, such as that shown in Figure 6.4. Consider point I (at depth Z below original ground level), located immediately below the excavation. Before the excavation is dug (Figure 6.4a), the pore water pressure и will be

where

H_w is the depth to groundwater level y_w is the unit weight of water

The vertical effective stress a'_v is the vertical total stress minus the pore water pressure u:

Figure 6.4 Different groundwater behaviour around excavations in soils of different permeability, (a) Initial conditions, (b) Excavation in high-permeability strata, (c) Excavation in very low-permeability strata, (d) Excavation in strata of intermediate permeability.

which for hydrostatic groundwater conditions can be expressed as where y_s is the unit weight of soil.

Consider now the pore water pressure change that will occur in response to the unloading of the soil due to excavation. The speed of unloading is related to the rate of excavation. For small excavations (or where larger excavations are dug in several cells or sections), rapid deepening is possible, and excavation rates of approximately 2 to 10 m vertically per day are possible. Assuming the unit weight of the soil is 20 kN/m³, this gives an unloading rate of 40 to 200 kPa/day. If excavations of large plan area are dug in a single operation across the whole area, the logistical constraints of digging and removing spoil will often slow rates of excavation to around 0.5 to 2 m/day - this is an unloading rate of approximately 5 to 40 kPa/day. As we will see, the unloading effect will cause a different pore water response in a highly permeable soil compared with a soil of much lower permeability.

Drained Pore Water Pressure Responses during Excavation

Drained responses during excavation typically apply to soils of relatively high permeability that might be termed ‘water-bearing’. To examine this case in soil mechanics terms, consider Figure 6.4b, which shows an excavation in a sandy gravel, where permeability will be in the order of 10-⁴ m/s. In this case, groundwater in the soil can flow rapidly compared with the rate of excavation and unloading - therefore, the soil will act in a drained manner, and water will almost immediately flow into the excavation. The unloading effect will not generate any excess pore water pressure, and the groundwater regime will be controlled by the seepage into the excavation.

In Figure 6.4b, the simple expedient of sump pumping is used to remove water from the excavation, and water will seep upwards into the base of the excavation. This means that at point I, there will be a pore water pressure greater than zero (i.e. a water pressure greater than atmospheric) associated with the seepage. However, point I will also have experienced considerable unloading due to excavation of the depth d of soil. This will reduce the vertical total stress a), by y_s times d. Effective stress theory (Section 6.5) shows that by decreasing the total stress in a zone of relatively high pore water pressures, effective stresses will reduce to low levels (which could reach zero). Low effective stresses reduce the shear strength that the soil can mobilize, and can lead to the excavation base and side slopes becoming unstable. Such instability is sometimes known as ‘quick’ conditions or ‘running sand’. In waterbearing soils, this type of instability can be avoided by deploying pre-drainage groundwater control methods to lower groundwater levels (Section 9.5.1). This will reduce pore water pressures, preventing effective stress falling to very low levels and hence avoiding instability.

Undrained Pore Water Pressure Responses during Excavation

Drained conditions in highly permeable water-bearing soils are easy to understand. Essentially, groundwater will flow into an excavation at the same time as the excavation is dug. In contrast, undrained conditions that can occur when excavating in very low-permeability soils are more complex and often misunderstood. The unloading effect will generate excess pore water pressure, and at least in the short term, this will control groundwater conditions around the excavation.

Figure 6.4c shows an excavation made in a deposit of silty clay that does not contain any permeable fabric (e.g. laminations of silt or sand), where the permeability will be in the order of 10-⁹ m/s. In this case, groundwater flow will be very slow (due to the very low permeability of the soil), and water flow is unable to keep pace with the rate of unloading during excavation.

In perfectly undrained conditions, the rate of excavation is sufficiently rapid, relative to the soil permeability, that no groundwater flow occurs during excavation. In these conditions, effective stress as expressed in Equation 6.4 will not change, and for every kilopascal that the total stress is reduced by unloading, the pore water pressure will reduce below the pre-excavation values. The pore water pressure changes caused by unloading are termed excess pore water pressures, and in this case will be negative. At point I, the unloading caused by excavation of the depth d of soil will reduce the vertical total stress o'_t. by y_sd, and (in the absence of groundwater flow) the pore water pressure и will reduce by a corresponding amount. If the unloading exceeds the pre-excavation pore water pressure at point I, then the pore water pressure will become negative (i.e. less than atmospheric pressure). Negative pore water pressures are known as ‘soil suctions’.

Because effective stresses are not reduced, the shear strength that the soil can mobilize is unchanged in the short term. This can allow excavations to be made in which the sides of the excavation may be temporarily stable at relatively steep angles, and where the base of the excavation can resist some upward groundwater pressures from deeper strata. No water seepage will occur into the excavation in the short term, because the pore water pressures in the soil bounding the excavation are negative (in the fully undrained condition) due to the effect of unloading. However, from a safety and risk perspective, it is vital to understand that the undrained condition is temporary, and as time passes, the excavation will become less stable, as explained in the following.

The reduced pore water pressures will create hydraulic gradients towards the excavation, which will draw groundwater in from the surrounding soil. This will cause the negative excess pore water pressures to dissipate, and water pressures around the excavation will slowly increase until they come into equilibrium with the surrounding ground. Increased pore water pressures will reduce the effective stresses; instability of the excavation base and/ or side slopes will follow if measures are not taken to artificially keep the pore water pressures low or to stabilize the excavation by other means.

In very low-permeability soils such as the silty clay in Figure 6.4c, excess pore water pressure dissipation may be very slow, occurring over many years or decades, but it is inevitable. As time passes, the soil will move towards a drained condition with reduced soil shear strength. It is now well established by soil mechanics analysis that cases of the failure of clay slopes for road and rail cuttings, occurring many decades after they were first formed, are typically due to the slopes slowly transitioning from undrained conditions to the less stable drained condition (Skempton, 1964).

Intermediate Conditions between Drained and Undrained Response

The physical principles of soil behaviour are the same for both drained and undrained responses - the difference is time. In the drained case, groundwater can move so quickly (relative to the rate of excavation) that there is no need to consider transient conditions, and equilibrium pore water pressures apply almost instantaneously. Conversely, in the perfectly undrained case, it is assumed that there is insufficient time for any drainage to occur, and the focus is on the excess pore water pressures generated by the unloading effects of excavation.

Drained and undrained conditions are two extremes. There are many cases where, during the timescales of a construction project, the soil around an excavation will be neither truly drained nor undrained, but its state will be in the continuum between the two extremes. Figure 6.4d shows an excavation made in a soil of relatively low permeability intermediate between the two preceding cases. This condition can occur in laminated silt and clay deposits that may contain sand partings, where the horizontal permeability is in the order of 10-⁷ or 10-⁸ m/s.

Immediately after excavation, the soil will exhibit an undrained response, with negative excess pore water pressures caused by unloading, and the excavation side slopes and base may be temporarily stable. However, in this range of permeability, the excess pore water pressures can potentially dissipate during the period while the excavation is open and the permanent works are being constructed. As the excess pore water pressures dissipate, the soil will lose its ability to resist shear. If an excavation has been designed assuming undrained conditions, and the permeability of the soil is high enough to allow some drainage, significant instability can occur days, weeks or even months after the excavation is first formed. Instability can include softening and slumping of side slopes, excessive pressures on retaining walls and shoring systems, and softening and heave of the base of the excavation.

Relevance of Drained and Undrained Conditions to Groundwater Control Problems

The vast majority of cases where groundwater control is required are in relatively permeable strata - such as sands, gravels and highly fractured rock. These are materials that an engineer would describe as ‘water-bearing’ and a hydrogeologist would classify as an aquifer. In such materials, it can be assumed that excess pore water pressures will not be generated by the unloading from excavations, and drained conditions will always apply.

For the specific case of excavations for civil engineering and mining projects, drained conditions are likely to apply for permeabilities greater than around 10-⁶ m/s. Note that for more rapid loading or unloading (for example the ground motion induced by an earthquake), the lower permeability limit given earlier, where purely drained conditions can be assumed, will be higher, and undrained conditions and resultant excess pore water pressures can occur in relatively permeable sands - this is one of the causal factors in earthquake- induced liquefaction (Madabhushi, 2007).

Undrained conditions are more relevant when we consider the lower limits of permeability at which dewatering is required. Consider again the excavation shown in Figure 6.4c, dug into a clay of permeability of around 10-⁹ m/s. Any experienced excavator operator will know that a trial pit dug into such clay will not need dewatering, and they would report the pit as ‘dry’, by which they mean that no water flowed in and no pumping was needed. It is possible that the operator may assume that the pit is dry because it is above groundwater level, but if we consider undrained behaviour, we can see that this need not be the case. In the United Kingdom, where the copious annual rainfall means that groundwater level is often close to ground level, pits dug into clay will be apparently ‘dry’ despite being below groundwater level. What is happening is that the unloading effect of excavation generates negative pore water pressures in the clay around the pit, so even though the clay is fully saturated with water, there is no hydraulic gradient into the pit, and there can be no water inflow until the excess pore water pressures dissipate significantly.

Undrained behaviour explains why groundwater control by pumping is not required for temporary excavations in very low-permeability soils and rocks, even many metres below groundwater level. Some engineers might describe such soils (erroneously unless the soil grains are bonded) as ‘cohesive’, and a hydrogeologist would classify these types of soil and rock as aquicludes. The key aspect is that the permeability should be sufficiently low that the excess pore water pressures (caused by the unloading effect of excavation) do not dissipate significantly during the construction period; this requirement is typically satisfied when the permeability is lower than 1(H to 10-¹⁰ m/s.

Some of the most challenging groundwater control problems occur in soils of relatively low permeability that are not normally considered water-bearing, but where undrained conditions cannot be relied upon during the period for which the excavation is open. Immediately after excavation, there will be some negative excess pore water pressures, but these will dissipate, and problematic seepage and instability will follow. These conditions typically occur in soils where the permeability is in the range of 10-⁶ to 10-⁸ m/s. This permeability range is tentative. In fine-grained soils, structure and fabric have a great influence on the permeability, especially in the horizontal direction. If the soil structure consists of thin alternating layers or laminations of coarser and finer soils, groundwater flow will be more rapid, and the onset of drained conditions (and the increased risk of instability) will occur sooner after excavation. Such instability is often avoided by the use of pore water pressure control techniques such as ejector wells, vacuum wellpoints and deep wells with vacuum (Section 9.5.4).

POSSIBLE APPROACHES TO GROUNDWATER CONTROL

There are two principal approaches to groundwater control:

• Groundwater control by exclusion (Section 9.4), where low-permeability cut-off walls are used to exclude groundwater from the excavation (Figure 6.5). The cut-off wall may be formed from artificial elements introduced into the ground (e.g. a pile wall) or may involve a zone of ground that has been treated or modified to reduce its permeability (e.g. where grouting is used). The only groundwater pumping requirement will be to drain the water trapped within the soil or rock in the area enclosed by the cutoff, and to deal with leakages through the wall and through the impermeable stratum (pumping for surface water control may also be required; see Sections 6.8 and 9.2).

Figure 6.5 Groundwater control by exclusion. Cut-off walls toe into stratum of very low permeability.

• Groundwater control by pumping (Section 9.5), where groundwater is abstracted from an array of wells or sumps to temporarily lower groundwater levels (Figure 6.6). Groundwater control by pumping is also known as groundwater lowering, construction dewatering or simply dewatering.

Pumping and exclusion methods may be used in combination (Section 9.6).

SURFACE WATER PROBLEMS

This book is primarily concerned with the control of groundwater, but since the aim is to provide workable conditions for construction, the importance of surface water control must not be forgotten. Surface water may arise in many ways: as direct precipitation into the excavation; as precipitation run-off from surrounding areas; as leakage through cut-off walls used to exclude groundwater; as waste water from construction operations such as concreting; or even from washing down of plant and equipment. Since the excavation is obviously going to form a low point in the site, surface water, if given free rein, is likely to collect in the bottom of the excavation.

Whatever the source, if surface water is allowed to pond in the excavation, it will impede efficient excavation and construction. Figure 6.7 shows an example of an excavation before and after surface water was suitably dealt with. Neglecting the management of surface water will undoubtedly cost the project time and money. Methods for control of surface water are described in Section 9.2 and Section 14.3.

CHANGING GROUNDWATER CONDITIONS

It has been discussed in Chapter 3 that groundwater conditions are part of the hydrological cycle and therefore are dynamic and may change with time. It is important when considering the potential need for groundwater control that the possibility of ‘extreme’ conditions should be considered.

Figure 6.6 Groundwater control by pumping.

Figure 6.7 Control of surface water in excavations, (a) Without adequate surface water control, (b) With effective surface water control.

An important case is to assess how the groundwater control requirements would change should the site experience an unusually prolonged and/or intense period of rainfall. This includes the case where the ground investigation data indicate that groundwater levels are below the proposed excavation level. This would imply that the excavation will be ‘dry’ and groundwater control is not needed. It can be useful to review the local hydrogeology (including historic groundwater level data if available) to assess whether, following extended periods of wet weather, the excavation may be below groundwater level (which could be either a general water table or perched water). This issue is discussed in more detail in Section 3.9, and in relation to desk studies and monitoring in Chapters 11 and 22, respectively.