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New South Wales Government
Slope and cliff instability hazards refer to the possible structural incompetence of these features and associated potential problems with the foundations of buildings, seawalls and other coastal works. Coastal bluffs and the erosion escarpments of sand dunes can slump, sea cliffs can collapse, and foundations can fail, so imperilling coastal developments and structures.
Slope and cliff instability is a different phenomenon from coastal erosion and recession. This is illustrated in Figure C7.1, which shows the loss of two houses from a foredune area. The first house was lost to erosion. The second house was lost due to the collapse of the dune escarpment. Whilst beach erosion and scour can cause stability problems, the collapse of a foreshore slope or the failure of a foundation depends upon the properties of the associated soil and rock constituents. The disciplines of soil and rock mechanics are essential to a stability analysis of coastal slopes, cliffs and foundations built thereon. Relevant aspects of these disciplines are briefly mentioned in the addendum to this appendix.
Typically, a sandy beach in its accreted state consists of a beach berm, a well grassed incipient dune, a higher frontal dune and hind dunes. Severe storms can cause erosion of the berm and frontal dune, leaving a pronounced erosion escarpment. As it dries out, the escarpment tends to "fail" by slumping back to the angle of repose of the sand (about 1V:1.5H). A zone of "Slope Readjustment" can be defined (see Figure C7.1). Any buildup and seepage of groundwater during the storm event will facilitate slumping of the erosion escarpment.
The term coastal bluff refers to headlands and foreshores of weathered rock and to soils perched on rock platforms. Typically, bluff escarpments are sloping rather than vertical (unlike sea cliffs). Examples of coastal bluffs include the foreshore of Bateau Bay in Wyong Shire, Long Reef Headland in Warringah Shire and the headland at Thirroull near Wollongong. The soil areas of coastal bluffs "fail" by "slipping" along a circular failure surface ("slip circle"); weathered rock areas "fail" by a mixture of collapse and sliding. Figure C7.2 shows a slip circle failure of the coastal bluff at Bateau Bay.
A cliff is a sheer or precipitous rockface that stands nearly vertical. Cliffs are subject to infrequent but sudden collapse. Figure C7.3 shows a recent cliff failure at Whale Beach, Warringah Shire.
Severe beach erosion cuts a pronounced escarpment in the frontal dune, which reduces the stability of the dune area immediately behind it. The stability of coastal bluffs is also reduced by erosion, but this is often a gradual process that only takes effect over many years.
The greater the depth of scour in front of a slope or structure, the greater the resulting instability.
During severe storms, a large body of sand is kept in constant motion over the berm. Immediate post-storm measurements indicate eroded berm levels only 0.5m above MSL. However, post-storm drilling in the berm has indicated erosion limits at depths of 1.0 to 1.5m below MSL, i.e. open beach berms are scoured to a level of at least 1.0m below MSL during severe storms (GS, 1985; 1986).
Scour in front of a reflective seawall is likely to be significantly greater than this amount. A scour level of 2.0m below MSL is often adopted for design purposes.
The rock platform that forms the base of some coastal bluffs is resistant to scour. Typically, this platform is located at about MSL.
Groundwater and Seepage
Seepage reduces the stability of a slope by making it easier for the soil particles to slide over each other during failure. Similarly, pore water pressures associated with a groundwater table reduce the inherent strength and stability of a soil mass.
The heavy rainfall that often accompanies severe storms may cause ponding behind the frontal dune, thereby increasing pore water pressures at the toe of the escarpment and the likelihood of failure. Often heavy rainfall is the triggering mechanism in land slip failures on bluffs. The generally poor drainage characteristics of these soils allow a buildup of pore water pressure.
The groundwater level in the beach berm fluctuates in response to tides and waves. When storms increase the average water level on the beach, groundwater levels in the dunes may rise accordingly. If coastal water levels should fall rapidly, e.g. through reduced wave heights on a falling tide, the water table in the dune may remain elevated for some time. A water table "setup" of 2m has been observed through this effect (PWD, 1988). Elevated groundwater levels reduce the stability of the erosion escarpment.
Potential slope, cliff and foundation stability problems can be overcome by proper investigation and design. The limit of beach erosion/recession adopted for design purposes needs to be determined; field work to determine soil and rock properties and likely seepage and groundwater behaviour is required. The assessment of slope stability and the safe design of foundations lies in the fields of soil and rock mechanics. Salient features of these fields are briefly introduced in the Addendum to this Appendix.
Lord, D.B. and Burgess, A.L., (1987). "The Erodibility of Indurated Sand". 8th Australasian Conference on Coastal and Ocean Engineering, Launceston, Tasmania, Nov. 1987, I.E.Aust.
Lambe, T.W. and Whitman, R.V., (1969). "Soil Mechanics", John Wiley & Sons, New York. ISBN 471-51193-5.
PWD, (1977). "Dee Why Lagoon, Investigation of Dredging Proposal". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales. Report No. PWD 77029. December, 1977.
PWD, (1988). "Wave Setup and the Water Table in a Sandy Beach". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales. Report No. TM 88/1.
Roy, P.S. and Walsh, I.L., (1982) "Quaternary Sediment in Coastal Embayments Sydney Metropolitan Region". Geological Survey of New South Wales, Department of Mineral Resources, Report No. 1982/343, August, 1982.
Skene, D.L. and Roy, P.S., (1985) "The Quaternary geology of the coast and inner shelf at Wooli, Northern NSW". Geological Survey of New South Wales, Department of Mineral Resources, Report No., 1986/219, December, 1985.
Terzaghi, K. and Peck, R.B. (1967). "Soil Mechanics in Engineering Practice", Second Edition. John Wiley & Sons, ISBN 0-471-85281-3.
Cohesive and Non-Cohesive Soils
Sand is a non-cohesive soil that generates its strength or ability to sustain loads by the interlocking of sand grains (internal friction). Clay is a cohesive soil that generates its strength from the cohesion or "stickiness" between clay particles.
The behaviour of frictional and cohesive soils is quite different. A clay face can be cut to a vertical angle and remain stable; a sand face will collapse to the angle of repose of the sand. Most natural soils exhibit a mixture of frictional and cohesive behaviour.
The degree of saturation of a soil and the depth to the water table have a major impact on the strength of the soil. The higher the degree of saturation, the more easily soil and clay particles can slide over each other. Pore water pressures associated with a water table reduce the carrying capacity and strength of a soil.
Dune sands are generally composed of fine to medium sized quartz grains of subangular to rounded shape. Figure B7.1 of Appendix B7 shows the size distribution of a typical dune sand. Often dune sands are loosely packed at the surface, but become denser with depth. There are exceptions to the above rules: dune materials can include silty sands, peats and indurated sands (sands loosely cemented together and known as "beach rock" or "coffee rock").
The behaviour of dune sands is almost wholly frictional with little if any cohesive effects. Particle grading, size, angularity and in-situ density all influence the strength or load carrying capacity of sands.
The behaviour of "coffee rock" is not well understood: some indurated sands are soft when buried, but on exposure to air, turn into a brittle "coffee rock" that fractures easily; other "coffee rock" decomposes when wetted. Recent studies indicate that the long term erosion rates of indurated sands are similar to those of loose dune sands (Lord and Burgess, 1987).
Coastal bluffs are usually composed of weathered igneous rocks. Bluff soils typically comprise clays derived from the weathering of these rocks interspersed with boulders. Bluff soils typically display a predominately cohesive behaviour, but some frictional effects may be present. Because of their clay nature, bluff soils are often poorly drained. This facilitates the build-up of pore water pressure, which in turn lowers the stability of the bluff.
A stability analysis of a frontal dune or bluff requires information concerning the extent and nature of the principal soil strata and the depth to groundwater. This information can be obtained from a drilling program, from a close inspection of the dune face (especially if eroded) and possibly from local authorities (if sewerage, bridge or other construction works have been undertaken in the area).
Field and Laboratory Tests
Field and laboratory testing will often be required to determine the physical properties of the underlying soils. Field tests include Hollow Flight Augering and the Standard Penetration Test (which indicates the in-situ density of sands). Laboratory tests include grading, permeability testing, shear box and triaxial testing (the latter two tests indicate the properties of cohesive soils).
The stability of rock cliff face or a rock bluff depends upon the type, jointing and other properties of the component rocks. The assessment of the stability of rock faces is much less exact than for soil slopes. Cliff failures are difficult to analyse. Analytical methods often incorporate finite element techniques applied to rock mechanics.
Soil slopes generally tend to fail by "slipping" along circular failure surfaces. The presence of a weaker stratum of soil can modify both the failure surface and the failure mechanism (see Figure C7.4).
"Slip Circle Analysis" is a technique for assessing the stability of such slopes and their "Factor of Safety" against failure. Such computations serve as a guide to determining minimum setback distances from the erosion escarpment of the frontal dune or from the crown of a bluff subject to slipping.
A variety of trial failure surfaces needs to be investigated if the slope contains different soil types, weak strata or is affected by groundwater seepage. Computer methods of analysis are of benefit in these circumstances.
Slip Circle Analysis does not provide exact solutions. Major uncertainties can arise in the selection of pore water pressure and soil strength parameters. For these reasons, a minimum Factor of Safety against failure is adopted (Lambe and Whitman, 1969). Where details of soil stratigraphy and soil strength have been determined by field and laboratory measurements, a Factor of Safety of 1.5 is recommended. Where this information is less certain, the Factor of Safety should be increased accordingly.
The Standards Association of Australia has not published any codes of practice for the design and construction of foundations in the active zone of the beach. Moreover, there is no Local Government Building Code in New South Wales for such foundations. Thus, the designer must rely on design codes for non-coastal areas of the State and adapt them for coastline hazards. Particular factors to be considered include the landward limit of erosion/recession, the zone of slope re-adjustment of the erosion escarpment on sandy beaches, and foundation/soil interactions under changing groundwater and seepage conditions.
Shallow foundations include piers, strip footings and slabs. Figure C7.5 shows the failure surfaces for a strip footing. Should such a footing be located within a zone of potential slip circle failure, the bearing capacity of the footing should be adjusted accordingly.
Piled foundations can be used to transmit foundation loads to the soil mass below the zone of potential slip circle failure. In this way, a structure can be safely located within the zone of potential slope instability. However, a slip circle failure can impose high lateral loads on piles that intersect the failure surface. This effect needs to be taken into account in the design of such foundations.
The use of a seawall to protect a foreshore slope does not necessarily ensure stability of the slope. First, the seawall itself may fail under extreme conditions. Vertical, non-porous seawalls are vulnerable to sudden collapse caused by toe scour and by pore water pressure build-up behind the wall. Second, the failure surface of the slope may extend under and beyond the seawall, i.e. the seawall forms part of the mass of soil that fails. To maximise stability, seawalls should be porous with a gently sloping face, especially if development is to be allowed close to the crest.