2.2
AN INTRODUCTION TO GROUND INSTABILITY

2.21
Introduction to Instability

The term landslide is a convenient name for a wide range of gravity-controlled processes (mass movements) that transport relatively dry material down slope (High-Point Rendel, 1995). Three principal mechanisms are widely recognised: sliding, falling and flowing. In reality it is common for an area of instability to be affected by many different types of landsliding; such an area is known as a landslide complex (eg. the Isle of Wight Undercliff, UK, Study Area G1).

2.22
Causes of Ground Instability

All slopes are under stress due to the force of gravity. Should the forces acting on a slope exceed the resisting strength of the materials that form the slope, the slope will fail and a landslide occurs. A slide therefore involves the displacement of a body of relatively coherent material, the undersides and margins of which are marked by rupture surfaces or zones known as shear surfaces. Thus, blocks of material move 'en masse' over a shear surface, although displacement inevitably leads to internal stresses which result in the break-up of the moving masse. The displaced materials move to a new position so that equilibrium can be re-established between the destabilising forces and the residual strength of the rock or soils along the surface of movement. A landslide, therefore, is a process that changes a slope from a less stable to a more stable state. No subsequent movement will occur until changes take place which, once again, affect the balance of opposing forces (see Figure 2.34).

Casual reponses of pre-existing landslides

In general the resisting strength of material decreases as the clay content rises. Clay slopes, therefore, are particularly prone to landsliding. Slides also often occur on slopes developed in a combination of impermeable fissured clays overlain by massive, well-jointed cap rocks of limestone or sandstone.

The ultimate cause of all landsliding is the downward pull of gravity. The stress imposed by gravity is resisted by the strength of the material. A stable slope is one where the resisting forces are greater than the destabilising stresses and, therefore, can be considered to have a margin of stability. By contrast, a slope at the point of failure has no margin of stability, for the resisting and destabilising forces are approximately equal. The quantitative comparison of these opposing forces gives rise to a ratio known as the "factor of safety" (F):

Factor of Safety = Resisting forces = Shear strength
Destabilising stresses Shear stress

The Factor of Safety of a slope at the point of failure is 1. On slopes of similar materials, progressively higher values represent more and more stable situations with greater margins of stability. In other words, the higher the value the greater the ability of the slope to accommodate change before failure occurs. These changes are usually divided, for the sake of convenience, into internal and external groups. External changes increase the stress placed on slope-forming materials, while internal changes reduce or weaken their resistance to movement. The majority of landslides are, therefore, the product of changing circumstances or alterations to the status quo (High-Point Rendel).

The shear strength of a material depends upon both the nature of the material itself and the presence of water in fissures and pores. A slope is only as strong as its weakest horizon, often a clay. Clays such as the Gault Clay (which contributes to landslip problems in several British Study Areas) are known as brittle materials because once they have been subject to more than the maximum stress they can withstand and have failed, further displacements are possible at lower levels of stress. In other words the shear strength of the clay declines from a peak value to a lower residual value.

Water content has a major influence on reducing shear strength, not because of lubrication, as is often stated, but due to the fact that water in the ground exerts its own pressure which serves to reduce the amount of particle to particle contact. Within saturated horizons the pore-water, therefore, bears part of the load by exerting an upthrust or buoyancy effect known as pore-water pressure. Although soil or rock particles can resist both normal and tangential (shearing) forces, fluids can support compression forces but cannot resist shearing forces. Therefore, friction or resistance to movement depends on the difference between the applied normal stress and the pore-water pressure. This difference, or the part of the normal stress which is effective in generating shear resistance, is known as the effective stress.

Two contrasting sets of conditions are often used to describe landslides:

The importance of this distinction is that once a slide has occurred it can be made to move under conditions that the slope, prior to failure, could have resisted. In other words reactivations can be triggered much more readily than first time failures.

As slope movements are the result of changes which upset the balance between resistance and destabilisation, the stability of a slope is often described in terms of its ability to withstand potential changes (see Figure 2.35):

factors involved in the decline of stability of a slope

This perspective makes it possible to recognise that the work of destabilising influences can be apportioned between two categories of factors on the basis of their role in promoting slope failure.

These two categories are:

  1. Preparatory factors which work to make the slope increasingly susceptible to failure without actually initiating it (ie cause the slope to move from a stable state to a marginally stable state), eventually resulting in a relatively low Factor of Safety;

  2. Triggering factors which actually initiate movement, ie shift the slope from a marginally stable state to an actively unstable state.

When considering the actual cause of landsliding this relative simplicity gives way to complexity as there is a great diversity of cause or factors. In broad terms, however, they can be sub-divided into internal causes which leads to a reduction in shear strength and external causes which leads to an increase in shear stress (Figure 2.36).

A simple sub-division of the causes of landslides

Factors leading to a decrease in shear resistance (internal):

  1. Materials:

    (i) Strata which decrease in shear strength if water content increases (clays, shale), eg. when the local water table is artificially increased in height by reservoir construction or as a result of stress release following slope formation;

    (ii) Low internal cohesion (eg consolidated clays, silts, porous organic matter);

    (iii) Weaknesses in bedrock: faults, bedding planes, joints, foliation in shifts, cleavage, brecciated zones and pre-existing shears.

  2. Weathering changes:

    (i) Weathering reduces effective cohesion and to a lesser extent the angle of shearing resistance by the absorption of water leading to changes in the fabric of clays (eg loss of bonds between particles or the formation of fissures).

  3. Pore-water pressure increase:

    (i) Higher ground water table as a result of increased precipitation or as a result of human interference.

It is not uncommon for ground movements to be caused by anthropogenic influences. In this way, the overloading of a slope by buildings and embankments, excavations without protection mechanisms on a slope during construction works, the raising of the level of groundwater, dynamiting, the inappropriate use of primary material or unsuitable allocation of land can all increase the ground movement hazard.

Anthropogenic effects can also contribute to long term destabilisation of the slope, in relation to other activities like deforestation, insufficient forest management, over-grazing, intensive exploitation and denuding of the land.

2.23
Types of Ground Instability

Many classifications of ground movements have been proposed based on criteria such as the mechanism of the movements, the composition of the materials, the speed of the processes or the mechanisms of release. For example a European classification was developed for the EPOCH (1991-93) project ('The Temporal Occurrence and Forecasting of Landslides in the European Community; Contract No. 90 0025), which used the following subdivisions: fall, topple, lateral spread, slide (rotational), slide (translational), earthflow, complex and planar.

In essence the processes of ground movement can be grouped as follows:-

  1. The fall process starts with the disaggregation of rocky or loose material on a steep slope along the length of a surface on which only a few detached movements have developed. The material then falls mainly in freefall, rebounding and/or rolling.

  2. The flow process (for example earth flow) results from the continuous movement of a superficial ground area quickly leaving the detachment zone, in a compact way to start with, but not generally keeping this compact character. The distribution of speeds within the moving masse is similar to that of a viscous flow.

  3. The landslide process is the downhill movement of a slope, affecting bedrock and/or loose ground along one or more slide surfaces or following relatively narrow zones of intense deformation by shearing.

These principal types of movements, including those that often produce progressive transition mechanisms, can result in different forms of failure (see Figure 2.37):

Types of ground movement

Other processes of ground movement include:

2.24
Landslide Investigation

Prior to commencing a detailed site investigation it is normal practice to undertake a review of available records, reports and documents relating to the site in question. Historical information, whether from newspaper reports, technical papers or from historical/archaeological sources, can provide important background information in support of an investigation and monitoring programme. With this information in place a range of options exist in terms of the next stage which may comprise geomorphological and geological mapping, aerial surveying techniques (aerial photography or analytical photogrammetry), sub-surface investigations, surveys of land-use and existing structural damage if applicable and a review of local planning and building control policies.

The geomorphological and geological studies together with sub-surface investigations can assist in formulating a preliminary stability analysis which will lead to an understanding of the nature and extent of landsliding as well as the types of contemporary processes taking place and their magnitude and frequency. By surveying existing structures (roads, buildings, damage to underground services, etc.) the impact of landsliding activity on development can be ascertained and the nature of risks and the vulnerability of structures in the area can be assessed. All this information can lead to the development of a landslide management strategy for the area concerned. An example of this approach was adopted for a study undertaken by Geomorphological Services Ltd between 1988 and 1991 as part of a pilot study of 'Coastal Landslip Potential Assessment' in Ventnor, Isle of Wight, commissioned by the United Kingdom Department of the Environment (see Figure 2.38).

The kind of all-embracing study outlined above can only be achieved when sufficient resources are available and dependant on the investigation requirements of the site in question. It may be that one or more different techniques will be used to carry out the investigation.

A range of instrumentation is available to assist ground investigation, monitoring and risk assessment and a number of these types of instruments are described below.

2.25
Landslide Monitoring Techniques

The purpose of this summary is to explain the range of geotechnical instruments that are commonly used for site investigation, monitoring, management and as an aid to civil engineering construction generally. A number of these instruments are referred to in forthcoming chapters (particularly in the Study Area descriptions, Chapters 4 and 5, and in Volume 2).

Monitoring is an integral part of landslide investigation (both preliminary surveys and detailed studies) because it provides a means of accurately and objectively gauging the stability conditions of unstable or potentially unstable slopes (High-Point Rendel); it can also fulfil an important role in assessing landslide risk. There are a range of monitoring techniques that can be used for a variety of purposes.

The Purpose of Instrumentation

The purposes of geotechnical instrumentation are as follows :

Factors affecting the choice of instruments

Each investigation presents a unique set of critical parameters. The geotechnical engineer must identify those parameters and then select instruments to measure them. What information is required for the initial design? What information is required for evaluating performance during and after construction? When the parameters are identified, the specification for instruments should include the range, resolution and precision of instruments (Slope Indicator Co. 1994).

Ground conditions often determine the choice of specific instrument, but in addition instrument performance must be considered. For example (see below section 2.26 'Commonly used Instrumentation..') a standpipe piezometer is a reliable indicator of pore- water pressure in soil with high permeability, but is much less reliable in soil with low permeability. A large volume of water must flow into the standpipe to indicate even a small change in pore-water pressure. In soils with low permeability, the flow of water into and out of a standpipe is too slow to provide a timely indication of pore-water pressure. A better choice in this case would be a diaphragm type piezometer which offers faster response since it is sensitive to much smaller changes in water volume.

The behaviour of a soil or rock mass typically involves not one but many parameters. In some cases, it may be sufficient to monitor only one parameter, but when the problem is more complex, it is useful to measure a number of parameters and to look for correlations between the measurements. Thus it is common practice to choose instruments that provide complementary measurements. For example, inclinometer data indicating increased rates of movement may be correlated with piezometer data that shows increased pore pressure. The load on a strut, calculated from strain gauge data, should correlate with convergence data provided by inclinometers behind a retaining structure. Another benefit of selecting instruments to monitor complementary parameters is that some data will always be available, even if one instrument fails.

Instrument performance is specific by range, resolution, accuracy and precision. The economical designer will specify minimum performance requirements, as the cost of an instrument increases with resolution, accuracy and precision (Slope Indicators 1994).

Range is defined by the highest and lowest readings the instrument is expected to produce. The designer typically specifies the highest values required. Resolution is the smallest change that can be displayed on a read-out device. Resolution typically decreases as range increases. Sometimes the term "accuracy" is mistakenly substituted for resolution. Resolution is usually many times better than accuracy and is never expressed as a "plus or minus" value.

Accuracy is the degree to which readings match an absolute value. Accuracy is expressed as a plus or minus value, such as +1mm, +1% of reading, or +1% of full scale.

Precision or repeatability is often more important than accuracy, since what is usually of interest is a change rather than an absolute value. Every time a reading is repeated, the value returned by an instrument is slightly different. Precision is expressed as a plus or minus value representing how close repeated readings approach a mean reading.

Maintenance costs and the type of power source can also influence choice of instruments. For example in a remote location solar-powered equipment may be preferable to mains supply. Reliability and also the ability to remotely down-load the data collected may also be essential to keep down the costs of monitoring the performance of instrumentation in a remote location.

2.26
Commonly Used Instrumentation and Techniques for Ground Instability Monitoring

A range of techniques are available to those seeking additional information on ground conditions. These include trial pits and trenches which allow samples to be taken of soils and bedrock as well as obtaining a record of the exposure, whilst the commissioning of boreholes or adits are more appropriate for investigation of deep landslide systems. Clearly these techniques, which involve mobilisation of more specialised equipment, are of a relatively higher cost. If boreholes have been provided it is possible to undertake a more detailed assessment of the stratigraphy and locate shear surfaces through "down the hole observation techniques"; in more remote locations, where access is difficult, geophysical techniques may be appropriate. A number of options also exist for testing the shear strength of soils (including vane shear, shear box and cone penetrometer tests).

Data loggers are often used to record and periodically transfer monitoring data to the operator. It can also be used to provide an early warning system if linked to telephone alarms when recorded ground movements exceed pre-set limits.

Monitoring surface movements

Triangulation and Trilateration:

A common approach to monitoring surface movements is the use of ground surveys by triangulation and trilateration. Fixed survey markers are established on the landslide surface and the horizontal and vertical co-ordinates of each are calculated through successive surveys. Movement of these markers is monitored by reference to fixed survey control points located on stable ground outside the boundary of the landslide. This allows the determination of absolute rates of movement as well as relative displacements within the landslide mass. Absolute rates of movement allow the failure process to be assessed with respect to the necessity and design of stabilisation measures. Relative movement rates allow zones of high and low hazard to be defined.

In Great Britain these techniques are assissted by Ordnance Survey benchmarks which provide a convenient means of relating observed ground movements to datum levels. However, sighting to convenient benchmarks may require lengthy traverses and the possibility that earlier benchmarks themselves may have been displaced over time should be considered. This approach was used by Chandler and Hutchinson (1894) who quote benchmark levels surveyed in 1896, 1907, 1939 and 1959-60 in an assessment of ground displacements at Ventnor, Isle of Wight, UK. Geomorphological services Ltd (1991) extended the monitoring period at Ventnor into the 1990's and used the results as part of their ground behaviour assessment for the Undercliff.

GPS:

Global Positioning Satellite (GPS) techniques are increasingly being applied to landslide investigations. The GPS system was established in the late 1970's and now comprises a network of 24 satellites orbiting at 20,000km above the Earth's surface. GPS receivers analyse the phase difference of radio signals transmitted from these satellites. The satellite's predicted position is transmitted from earth-based tracking stations to the satellites, and relayed back to the GPS receiver. By employing two or more receivers accuracy in differential poisition of 5mm + 2ppm can be achieved under ideal conditions. In this way a receiver can be installed onto a fixed station and a differential position obtained using a second roving receiver (High-Point Rendel). For monitoring applications where trends in ground movement rather than precise, sub-millimetre position accuracy is required the technique has several advantages over traditional methods including:

A good view of the sky is required and this may result in limitations in use within wooded areas, or close to structures or cliffs. Also, highly skilled operatives are required, and the need for post-processing of results means that an immediate indication of the station positions is not always possible.

Photogrammetry:

Another technique is the use of photogrammetry for landslide and erosion monitoring which forms a rapid alternative method to conventional survey monitoring and consists of the repeated ground-based photography of unstable slopes (Chandler and Moore 1989; Grainger and Kalaugher 1991). The greatest precision is obtained where landslide boundaries and recognisable slope features can be fixed by reference to control points on photographs taken successively from the same location with a lens of constant focal length.

This technique was used to establish long-term ground movement rates in the Ventnor Undercliff on the Isle of Wight (GSL 1991). Analytical photogrammetry was used to compare the positions of 129 points on aerial photographs of the town taken in 1988 with photographs of the same area from 1969 and 1948. Displacement of each point was measured by computer, producing "discrepancy" vectors. Statistical analysis was undertaken to establish whether these discrepancy vectors could be accepted as being statistically significant and representative of a real movement.

Direct measurement:

Instrumentation which can assist in the investigation and measurement of surface movements in unstable slopes range from low-cost routine measurement of pins and peglines to more sophisticated automatic monitoring and early-warning systems.

A simple, low cost approach can be direct measurement across tension cracks, particularly between the top of the rear-scarp and the edge of the slipped mass. This can provide useful information on the rate (and change in rate) of movements. However, it is important to interpret the results carefully as short-term results can be misleading and could reflect inaccuracy in the method of measurement. The most common methodology is the repeated measurement between control blocks or pins grouted into the slope material.

Monitoring shallow subsurface movements

Surface Extensometers:

Surface extensometers are used to determine changes in the distance between reference points anchored in walls or the structures of an excavation. Typical applications include monitoring convergence of tunnel walls, monitoring deformation in underground openings, monitoring displacement of retaining structures, deep excavations, bridge supports and other concrete or steel structures.

The choice of instrumentation to be employed will depend on site conditions and cost. If the separation rates are very small, a high resolution instrument automatically recording on data loggers should be used. If, however, separation rates are relatively high and/or the distance being measured is large, direct manual measurement with a graduated steel tape extended between two reference pegs is more cost-effective. The introduction of remote real-time automatic monitoring systems has several advantages over traditional techniques in providing continuous and accurate measurements of surface movement as well as early warning alarm systems. The use of such systems is necessary and cost effective where the risk of structural damage or loss of life is significant. See Figure 2.39.

A rod extensometer

Settlement Cells:

Pneumatic settlement cells provide a single point measurement of settlement. They can be read from a central location and are particularly useful where access is difficult. They can be used, therefore, to monitor consolidation in the foundation during construction, monitor long-term settlement in foundation or fill, and monitor vertical displacement in areas of instability. See Figure 2.40.

A settlement cell

Beam Sensors and Tiltmeters:

Beam sensors and tiltmeters are used to monitor changes in tilt in a structure or slope (Figure 2.41). The changes in tilt can occur where nearby construction activity affects the ground that supports the structure. Activities such as excavation, tunnelling, or de-watering may cause settlement or lateral deformation. Placement of surcharges and pressure grouting may cause heave. Changes in tilt are caused where a load is applied to a structure or if a load is removed.

A titltmeter

In general these types of sensors can be used to monitor stabilisation measures such as pressure grouting and underpinning, monitor structures for the effects tunnelling and excavation, evaluate the performance of bridges, beams and dams under load, monitor the stability of structures in landslide areas, monitor the deflection and deformation of retaining walls, monitor convergence and other movements in tunnels, provide early warning of threatening deformations allowing time for corrective action to be taken or, if necessary, for safe evacuation of an area, and finally to provide an accurate record of movement in the structure for legal purposes.

Hydrological studies:

Of particular significance in determining slope stability is the question of groundwater levels. In many cases slope failures can be explained by reference to rainfall events and a rise in groundwater levels. Hydrological studies are used both for the evaluation and prediction of slope instability.

An investigation of hydrological conditions usually comprises an analysis of: rainfall input to the slope, evaporation, infiltration and surface runoff. A study of these inputs, transfers and outputs helps to identify thresholds and the possible timing of slope failure in relation to climatic conditions. Rainfall can be monitored most effectively using a package weather station linked to a data logger. Continuous data obtained in this way allows rainfall intensity and storm duration to be determined, enabling analyses of the development of slope failure over time. In locations such as the Ventnor Undercliff, Isle of Wight (Study Area G1), the net input of water or 'effective rainfall' has been calculated by subtracting the evapotranspiration rate from rainfall.

Monitoring deeper subsurface movements

A wide range of automatic monitoring and early warning instrumentation allows accurate measurement of subsurface movements in unstable slopes.

An appraisal of groundwater movements and trends is vital to the completion of a stability analysis and for instability investigations generally. The distribution of groundwater is controlled by the soil or rock permeability. Stability analyses necessitate the identification of pore-water pressure variations allowing determination of the depth of the shear surface. It is then possible to install instrumentation such as piezometers as pore-water pressures on a shear surface are most crucial to slope stability. The calculation of groundwater levels is of course more easily achievable in homogenous slope materials than in slopes of variable composition.

Piezometers:

Piezometers measure pore water-pressure and ground water levels, see Figure 2.42. They are used in geotechnical, environmental and hydrological applications. They assist geotechnical engineers to:

A cheaper alternative to the inclinometer system described below, but one which can also give useful results, is the installation of a continuous flexible plastic tube (often known as a slip indicator) such as piezometer tubing down a borehole to a level well below the expected location of the shear surface. A non-corrosive metal mandrel is lowered to the bottom of the borehole on a rot-proof cord of known length. If slip movements have been sufficiently large to flex the tube since installation, the mandrel will jam just below the point of flexure as it is raised in the hole. A second mandrel is then lowered down the tube from the surface to the upper part of the flexures and hence with the previous measurement the depth of the shear surface can be approximately located.

Type of Pleziometer

Inclinometers:

Inclinometers are used to monitor lateral earth movements in landslide areas and embankments. They are used to monitor the deflection of retaining walls and piles under load. Horizontal inclinometers, which are discussed separately, are used to monitor settlement in foundations and embankments. Principally inclinometers are used for site investigation, verification of design assumptions, determination of the need for corrective measures, monitoring long-term performance and safety monitoring. See Figure 2.43.

Measurement of sub-surface movement can be determined using a variety of different methods, including inclinometer systems and slip indicators. Inclinometers have been developed to accurately measure the movement of pre-existing landslides and to detect signs of pre-failure creep in intact (unfailed) coastal slopes. The basic principle of operation is that a torpedo probe is lowered down the entire length of a near vertical access tube installed in a borehole. The inclination of the access tube from the vertical in two planes at right angles to one another is measured at pre-determined depths or continuously. Providing that the locations of one end of the access tube is known or fixed to a datum it is possible to obtain a complete profile of the tube by taking a succession of readings. Depending upon site conditions, the datum used can be either the bottom of the access tube, keyed into stable ground, or the top, whose position is fixed by ground survey methods. By comparing these profiles the rate and magnitude of horizontal displacement of the tube may be determined over a period of time. If the inclinometer access tube crosses an active shear surface it will tend to develop a convexo-concave slope in section, with the point of inflexion of the curve approximately at the level of the shear surface. Eventually the magnitude of such displacements will become too large for the inclinometer instrument to pass through the access tube.

Horizontal Inclinometers:

Horizontal inclinometers provide settlement profiles of embankments, foundations and other structures. They provide a useful settlement profile of the embankment or for a foundation, and give an indirect indication of how much the base of a structure may have deflected. In most cases in-place inclinometer sensors are connected to a data acquisition system that continuously monitors movements and can trigger an alarm when it detects change or rate of change that exceeds the present value.

Borehole Extensometers:

Borehole extensometers are used to monitor settlement, heave, convergence and lateral deformation in soil and rock. Typical applications include monitoring settlement or heave in excavations, foundations and embankments, monitoring settlement or heave above tunnels and other underground openings, monitoring convergence in tunnel walls and monitoring lateral displacement in slopes.