Landslide hazard susceptibility mapping based on terrain and climatic factors for tropical monsoon regions

 Deep weathering, residual material (colluvium) and random rainfall intensity are mainly responsible for landslides in tropical monsoon regions. These parameters are often not taken into consideration in a landslide susceptibility assessment. Sustainable resources development in this region requires information on the spatial distribution of areas susceptible to landslides. This study highlights various aspects of the landslides that take place on the west coast of India and a methodology developed for landslide susceptibility mapping. Received: 12 August 1998 · Accepted: 15 August 1999     

Evaluating landslide deposits along the Tsurukawa fault zone, Japan, using magnetic susceptibility

 This paper describes the results of in-situ measurements of magnetic susceptibility in landslide deposits along the Tsurukawa fault zone in central Japan. The measured magnetic susceptibility values range from 0.4 to 9.6×10^–3 (in SI) and correspond to the proportions of the two component materials, weathered volcanic ash and faulted rock fragments. The study shows that landslide deposits along the Tsurukawa fault zone are composed of varying proportions of weathered volcanic ash. The results contrast with some general assumptions concerning landslides along fault zones in Japan. Received: 16 April 1999 · Accepted: 10 August 1999     

The engineering geological characteristics of lake sediments in the Yuanshan area, Ilan City, Taiwan

 The paper describes a study of the sedimentology and some engineering parameters related to the geological characteristics of the lake sediments which formed behind a barrier dam in a suburb of Ilan City, north east Taiwan. It is concluded that the variation in engineering parameters is strongly influenced by the proportion of diatoms present. Received: 14 November 1997 · Accepted: 20 January 1998     

Preliminary report on the landslides of 5 May 1998, Campania, southern Italy

 Following intense rainfall on 4/5 May 1998, over 100 mass movements occurred in the Sarno-Quindici area, some 30 km east of Naples, southern Italy. The movements took place in an area where recent pyroclastic materials mantle a Mesozoic limestone bedrock massif which had already suffered karstification over a long period. The debris from these movements extended 3–4 km into the surrounding lowlands and reached the towns of Sarno, Quindici, Bracigliano and Siano, causing severe destruction. One hundred and sixty-one people lost their lives. This preliminary paper discusses a number of scenarios to highlight the possible causes and mechanisms of the movements. Of particular importance are preceding rainfall patterns, the possible perched water conditions, the physical properties of the recent metastable volcanoclastics and underlying palaeosols and the influence of man-made changes in the morphology. Further studies are being undertaken to elucidate the relative importance of the different contributory factors. Received: 18 June 1998 · Accepted: 15 July 1998     

The geomorphological comparison of two debris flows and their triggering mechanisms

 The paper reports on debris flows in the Tungmen and Hoser areas of Taiwan, triggered by Typhoon Ofelia on 23 June 1990 and Typhoon Herb at the end of July 1996 respectively. The intense rainfall resulted in large volumes of debris being eroded from the valleys. The village of Tungmen was almost totally destroyed by in excess of 100,000 m³ of material which was moved by the 1990 debris flow while the 1996 typhoon was responsible for the movement of almost 1.5 million m³ of material, 80% of which extended to the confluence area of the two side gullies and the Hoser River. Attention is drawn to the change in shape of the valley sides from generally V-shaped prior to the debris flow to U-shaped following the storms. A comparison of aerial photographs as well as field data was used to define the main topographical effects of the 1990 storm (Tungmen) and 1996 event (Hoser). A study of the discontinuity pattern within the valleys emphasised the significance of toppling and gravity slumping of material in the sides of the valleys in producing large quantities of debris which, in heavy storm conditions, can flow down the gullies. Laboratory tests were used to simulate the effect of saturation on the materials. These indicated that in their saturated state they would easily become unstable. Received: 1 December 1999 · Accepted: 11 January 2000     

Index properties of weathered rocks: inter-relationships and applicability

 The paper describes the change in physical and strength properties due to weathering of three crystalline rocks – granite, basalt and quartzite. It draws attention to the relationship between unconfined compressive strength and other measurements such as the point load index, the Brazilian tensile strength and the Schmidt hammer rebound number. The strength recorded is negatively related to the porosity which is considered an important indicator of the weathering state. Received: 13 December 1997 · Accepted: 30 April 1998     

Geomorphological impacts of Inanda Dam on the Mgeni estuary, north of Durban, South Africa)

 Completed in 1988 at a site some 32 km from the Mgeni river mouth, the Inanda Dam may have influenced bed load transport and deposition. The aims of this study were to ascertain whether the dam had affected fluvial sediment delivery to the lower channel, whether the sediment characteristics and bedform of the estuary had been modified and, within this context, to predict the likely effects of continued sand mining on the Mgeni estuary. Re-examination of estuarine cross sections originally surveyed prior to dam completion, comparison of present estuarine sediment characteristics with those of pre-dam sediments and a bottom survey of the Inanda Dam itself were key elements in the analysis. Mean annual discharge of the Mgeni river has reduced by 4% since dam construction and changes in discharge regime caused by the water release policy resulted in an increase in competent discharge events. Retention of sediment behind the dam wall had led to reduction in available downstream sediment. Sediment volume in the estuary had not changed significantly since 1989, although sediment calibre is now much finer. These changes will probably signal the end of sand mining in the lower river and will ultimately cause the estuary to deepen as residual pre-dam sediment is flushed through and not replaced by new material from upstream. Since Mgeni sediment is a source of material for natural beach nourishment, it is likely that beaches will become narrower. Received: 20 November 1999 · Accepted: 3 April 2000     

Engineering aspects of limestone weathering in Istanbul, Turkey

 Devonian limestones are widespread in the eastern part of Istanbul, Turkey. To assess the influence of weathering on the geological and geomechanical characteristics of these rocks, research was carried out on some profiles from the study area. Field observations show that the weathering has been rapid and the bedrock has been weathered to a depth of up to 8 m. A model for the weathering of limestones is established and modifications proposed to the material and mass weathering schemes for characterisation of the limestone for engineering uses, as both foundation and construction material. The paper also presents the results of field and laboratory investigations, including weathering characteristics and physical and mechanical properties. Interrelationships were determined for all test results. It is concluded that chemical characteristics and the porosity are good indicators of weathering. Received: 10 July 1999 · Accepted: 17 September 1999     

Engineering geological environment of Athens, Greece

 Engineering geological information including geotechnical and geological data from exploratory boreholes and trial pits located in the wider area of Athens were stored in a database system. Based on the data processing and field mapping, the geological formations encountered in the foundation materials of the city were divided into individual geotechnical units according to uniformity in lithological character and relevant age. Engineering geological zonation maps of the studied area for urban planning purposes were compiled at an original scale of 1 : 25,000. Received: 4 October 1999 · Accepted: 3 April 2000     

Engineering geological and geophysical investigation of the reclaimed Lekki Peninsula, Lagos, South West Nigeria

 It was hoped to develop an area of reclaimed land on the Lekki Peninsula of Lagos State. An investigation was therefore carried out in order to establish the subsurface geological/geo-electrical sequence; to identify the sand columns and determine their thickness and prepare a map of the geomorphological features of the original, pre-fill terrain. Vertical electrical resistivity soundings (VES) and penetrometer tests were carried out in a sand filled segment of the Lekki Peninsula. The information obtained showed a good correlation between the geomorphological features as interpreted from the aerial photographs and those identified from maps derived from the VES/penetrometer results. Received: 28 August 1998 · Accepted: 18 January 1999     

Geology and location of major coarse aggregate resources in Lebanon, Eastern Mediterranean

 With the ongoing construction in Lebanon associated with renewal and urban expansion, the demand for coarse aggregates has soared. Traditionally, small quarries have been opened near to the centres of demand. However, in view of the detrimental environmental impact of these workings, a large number have been closed by the Lebanese government. In order to ensure that in the future the effect of quarrying on the environment is minimized, a survey of major coarse aggregate resources in Lebanon was undertaken. This paper reports on the field investigations in twenty quarries, covering some 80% of the Lebanese territory. Received: 29 July 1999 · Accepted: 20 August 1999     

Landslide hazard zonation of the Khorshrostam area, Iran

 Landslide hazard zonation is a method to evaluate the risk where there is the potential for landslides. The factors contributing to the hazard in an area can usually be identified, results of the investigations frequently being presented as a landslide hazard zonation map indicating zones of similar risk of the occurrence of a landslide. Korshrostam is one of the areas most susceptible to landslides in Iran with more than 13% of its surface being affected by landslide activity. The effects include damage or disturbance to villages, farmlands and roads as well as the exacerbation of erosion of the land surface and consequently an increase in the rate of sedimentation in the water flowing into the reservoir of the Manjil dam. The method of landslide zonation used in this study was based on a simple grid unit. A number of factors contributing to the likelihood of landsliding were considered, including lithology, slope, tectonic activity, land use and groundwater. For each grid unit, the incidence of landsliding and an assessment of the likely contributory factors were recorded in terms of a surface percentage index (SPI). A computer program was written using fuzzy sets to calculate the hazard potential index (HPI) for each unit. This was used to prepare the landslide hazard zonation map. Received: 10 June 1999 · Accepted: 16 September 1999     

The environmental impact of gold mine tailings footprints in the Johannesburg region, South Africa

 Gold mining in South Africa resulted in vast volumes of tailings, which have been deposited in impoundments. Poor management of most of the tailings dams resulted in the escape of seepage, adversely affecting soils and water quality. Some tailings dams have been partially or completely reclaimed leaving contaminated footprints. These zones pose a serious threat to the underlying dolomitic aquifers. In this study, the footprints of seven selected sites situated near Johannesburg have been investigated. It was found that the topsoil is highly acidified and only a minor portion of contaminants is bioavailable. However, phytotoxic contaminants such as Co, Ni and Zn could complicate rehabilitation measures as they limit the soil function. In addition, soil samples contain trace element concentrations, which often exceed background concentrations in soils. As a result, the depletion of buffer minerals and the subsequent acidification could result in the long-term remobilization of large quantities of contaminants into the groundwater. Soil management measures such as liming are required to prevent the contaminant migration from the topsoil into the subsoil and groundwater as well as to provide suitable recultivation conditions to enable future land use. Received: 20 April 1999 · Accepted: 27 September 1999     

Reliability of landslide isopleth maps

  Following very heavy rainfall on 19/20 July 1983, 109 landslides occurred in the east of Findikli, Rize, north-east Turkey. The movements took place in completely weathered andesitic, dacitic and granitic rocks as well as slope debris. They extended over an area of approximately 25 km². The paper discusses the contour (isopleth) map prepared using data from the 1983 landslides and discusses its value for landslide hazard zoning. It is concluded that on the basis of 12 years of field evidence, a contour map derived from a precise inventory map can be a useful tool. Received: 28 November 1998 · Accepted: 15 May 1999     

Instability Phenomena in the Neogene Deposits of Northern Peloponnesus, Greece

 The geomorphic and instability conditions of the Neogene formations in Northern Peloponnesus are examined and related to the lithologies and hydrological data. Instability phenomena caused by erosion are briefly described and a qualitative analysis of the run-off conditions within the Finikas and Meganeitos catchments is presented. Received: 31 July 1997 · Accepted: 12 January 1998     

Pollution in the water supply wells of Khartoum, Sudan

 The supply of water to Khartoum, Sudan, is from wells tapping aquifers in the Nubian Sandstone Formation. The boreholes are located in residential areas where sewage disposal is by means of a siphon septic tank system draining into the water table in the upper alluvial deposits. The aquifers are leaky to unconfined in nature and as the lower groundwater moves from the Nile towards the residential areas, it is susceptible to pollution from the septic tanks. Evidence that such pollution is occurring was found in three production wells where the presence of E. coli bacteria was identified. Received: 9 July 1998 · Accepted: 20 May 1999     

A comparative study of the grading coefficient, a new particle size distribution parameter

 The importance of the particle size distribution (grading) of a gravel material for unsealed roads has long been recognized. Various parameters have been devised to allow interpretation of grading curves in terms of a single parameter for ease of application, control and comparison. It is shown that there are significant correlations between many of the better known parameters, although it is also clear from the scatter of the data that the parameters probably indicate slightly different properties. The investigation has shown that a new parameter, the grading coefficient, is a good indicator of the grading and can be used interchangeably with many other parameters. In general terms the grading coefficient is approximately half the percentage gravel in a material and grading coefficients of 16 and 34 are roughly equivalent to grading moduli of 1.5 and 2.6, respectively. Received: 18 June 1998 · Accepted: 20 July 1998     

Prediction of a fracture system from an excavated ditch in the central Caledonides, Sweden

Prospecting for the dimension stone Offerdalsskiffer in the Finnsäter area, Offerdal Nappe, Swedish Caledonides, is mainly undertaken where no bedrock is exposed in order to assess the likely quality of the rock and the effective recovery ratio. A knowledge of the local fracture system is therefore required. The paper discusses a case study undertaken in the Finnsäter area of Sweden where ditch mapping was carried out to predict the fracture system. A comparsion was made of a fracture map extrapolated from the ditch mapping data and that prepared from a larger cleared bedrock surface in order to assess its potential for quarry viability.     

The influence of mineralogical textural and chemical characteristics on the durability of selected sandstones in Istanbul, Turkey

 The paper describes the analyses and geotechnical testing of sandstones taken from a road cutting to the north east of Istanbul, Turkey. Samples were carefully selected in the field in order to obtain material representative of the complete weathering profile. Although unconfined compressive strength, point load index values and Schmidt hammer rebound numbers were obtained, the slake durability test provided the closest correlation with weathering grade. Received: 4 February 1998 · Accepted: 15 June 1998     

Stabilising the leaning tower of Pisa

 The 56-m high "leaning tower" of Pisa was constructed in three phases between 1173 and 1370 on the alluvial sediments of the River Arno, central west Italy. Although the tower deviated from the vertical at an early stage, the continuing tilt has led to concern about its safety. Having reviewed the construction of the tower and the underlying geology, the paper discusses the history of the tilting and describes the temporary remedial measures undertaken and permanent stabilization proposed. Received: 22 December 1997 · Accepted: 22 January 1998