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11.
Ogbonnaya Igwe Wilfred Mode Okechukwu Nnebedum Ikenna Okonkwo Ifeanyi Oha 《Environmental Earth Sciences》2014,71(5):2465-2480
This paper analyses the dominant mechanisms of slope failures and identifies potential obstacles to landslide-hazard reduction at the Iva Valley area, Enugu, Nigeria. The landscape is replete with landslide scars and gullies of varied sizes and the slope deposits comprise unconsolidated, friable sands inter-bedded with thin units of montmorillonitic claystone. Forty-three landslide events were identified in the study area with most being shallow, short run-out movements with slip-surface depth <2 m. The study found the landslides mainly occur in the beginning of rainy season characterized by short duration, high intensity rainfall. An integrated approach comprising field mapping, laboratory tests and numerical analyses reveals that the barren nature of the slopes prior to the outset of rainy season, high rainfall intensity, erosion, overgrazing, soil characteristics and the site’s unique lithologic sequence are the main causes of instability. Shearing tests under several conditions showed that the soils strongly strain-soften until low steady-state strength is achieved. A computer code, based on this strength reduction technique, used input parameters obtained from the field and laboratory studies to simulate a landslide with similar structure, travel distance and distribution area. It is noted that urbanization has gradually increased the vulnerability of the society’s poor to landslide hazards as they now erect unplanned residence (tents and blocks) on the slopes. This work is part of a regional study aimed at finding ways of protecting the vulnerable by generating data that could be used to build future landslide susceptibility map. 相似文献
12.
Ogbonnaya Igwe 《Landslides》2013,10(4):515-521
The ICL/IPL Project achieved results in capacity building, investigation of landslides in West Africa and also evaluated some other slope movements in the region. These include the catastrophic rock–debris avalanche at the Cameroon–Nigeria border and the Iva Valley landslides in Enugu. During the avalanche, an estimated 100 M m3 of rock and debris was moved more than 2 km from the source of the slide at 600 m above sea level to the toe in the valley in a few minutes. The materials range from mud and soil debris to blocks of rocks up to 20 m in diameter. The grain size of moved material tended to increase upslope and closer to the head scarp though it tended to decrease again close to and at the source area. Nature and composition of the basement bedrock with foliation planes dipping in the direction of slope, dominant joint sets oriented perpendicularly to the foliation, the nature of weathered material and high relief were strong factors in the avalanche. Field studies identified 43 landslides at the Iva Valley area of Enugu state, which were shallow, short run-out movements with slip-surface depth less than 2 m. The shallow slides and the avalanche are triggered by water infiltration in slopes with high topographic gradient. The soil saturation leads to a reduction of the shear strength of the soil because of a rise in pore water pressure. These landslides are known to occur during or after intense rainfalls at the beginning or at the tail end of rainy season. 相似文献
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14.
Ogbonnaya Igwe Hiroshi Fukuoka Kyoji Sassa 《Geotechnical and Geological Engineering》2012,30(5):1207-1229
The research investigated the influence of grain size distribution on the shear behavior of sand specimen in loose, medium and dense states. The investigation aimed at understanding the extent or degree at which static shear strength of soil is affected by its density. A standard indicator (coefficient of uniformity) was used in constituting four (4) different sand specimens—narrowly graded (NAG), intermediately graded (ING), well graded (WG) and gap graded (GAG). Different normal stress values were applied during the testing program. The specimens were sheared using a ring shear apparatus until a residual value of shear stress was obtained. Result obtained from the analysis shows two remarkable stress paths. Medium dense to dense specimens show dilative stress path while loose (less dense) specimens show contractive stress path. At loose state, WG specimens show higher peak and residual shear strengths than poorly graded ones. Of importance was the fact that all the NAG specimens underwent complete liquefaction but WG specimens did not undergo complete liquefaction. This was interpreted as strong inter-particle contacts/forces binding the grains which enhanced high shear resistance. The research also showed that increase in relative density leads to higher shear strength, particle size distribution notwithstanding. Also, result of the analysis showed that WG specimens in medium-and dense states have higher peak but lower residual strength than ING and NAG specimens-this phenomenon was termed reverse behavior by the author(s). GAG specimens when sheared have the lowest peak and residual strength at any given value of relative density. 相似文献
15.
Yasuhiko Okada Hirotaka Ochiai Takashi Okamoto Kyoji Sassa Hiroshi Fukuoka Ogbonnaya Igwe 《Landslides》2007,4(2):197-203
A seasonal rain front (Baiu front) accompanied a long-term accumulation of precipitation propagated over the wide areas of
the main island of Japan during 15–24 July 2006. In Okaya City, Nagano Prefecture, several flow-type landslides occurred in
the early morning of 19 July 2006, claiming eight lives. Among these landslides, a most peculiar complex earth slide–earth
flow occurred on a north gentle slope of the upstream portion of the Motosawagawa River. In the source area, volcanoclastic
soils overlying tuffaceous rocks at about 4-m depth slid due to the prolonged precipitation that raised the water table level
in the soil. Along with the travel path, the failed materials fluidized causing the liquefaction of the volcanoclastic soils
underlain by volcanic black ash soils. The resulting flow spread over a wide area up to the final deposition. Constant volume
box-shear tests on undisturbed volcanoclastic soil specimens taken from the source area showed effective normal stress tended
to decrease during shearing. The ring shear tests on saturated disturbed specimens produced the large loss of shear resistance,
which may explain the fluidized motion of the complex landslide. 相似文献
16.
Under the WCoE/IPL project, Landsat Enhanced Thematic Mapper plus (ETM+) and ASTER GDEM data were used to analyze factors associated with frequent debris flow and rock-debris avalanche on the Nigeria-Cameroon mountain range. Detailed processing of the ETM+ imageries of the vulnerable portions on the western part of Adamawa plateau was carried out to identify structural trends relatable to the slope failures. The processing identified the structural characteristics of the study area and clearly distinguished the lineaments associated with the debris flow and rock-debris avalanches. The strongly weathered and fragmented gneissic rocks, shallow groundwater table, as well as the distribution of faults and joint sets were significant factors predisposing the slopes to rapid movements. Recent landslides occurred at the locations with high lineament density and in areas close to major faults and long lineaments. The interaction among the moving mass, slope geometry, local geology, topography, and drainage then resulted in a long runout and a high degree of spreading. Analysis of the ASTER–GDEM delineated landforms, slope morphology, and drainage patterns, which was not possible with conventional mapping techniques. Integration of these results yielded reasonable interpretation of the predisposing factors for the long runout failures and provided logical basis for future landslide susceptibility analysis in the area. Accurate investigation of the predisposing factors and characteristics of landslides in environments that have experienced past failures is therefore important in understanding areas that may be susceptible to landsliding in future. 相似文献
17.
The characteristics and the erodibility potentials of soils from Benin, Ogwashi-Asaba, and Nanka formations in Anambra state, Nigeria were studied. The study involved an integration of field observations, geotechnical analyses, and topographical modelling. About ten gullies were identified in the field. However, six major gullies labeled BN1, BN2, OA1, OA2, NK1, and NK2 were chosen and studied extensively to represent the three geologic formations. The results showed that soil properties, anthropogenic activities, topography, fierce surface runoff, and sparse vegetation are the factors that commonly and systematically influence the characteristics and erodibility potentials of all these soils. However, anthropogenic activities and soil properties play the most, dominant roles in the erosional processes. The specific gravity of the soils are similar, with values ranging from 2.50 to 2.69. Compaction test revealed all the soils were loose and collapsible. Grain size analysis further revealed that they have low content of fines (<25%, except for OA1 with 58.87%). The plasticity index of the fines indicated that they are nonplastic to low plastic soils (values range from 0–12%). The soils are highly to moderately permeable, with values ranging from 4.22×10?6m/sec to 4.34×10?4m/sec. All have low cohesion ranging from 1–7kPa, except for the OA1 with 27kPa. Based on the study, the three geologic formations have similar characteristics and high erodibility potentials; though OA1 has varied characteristics and thus, lower erodibility potential. 相似文献
18.
An integrated physicochemical and hydrogeochemical assessment was carried out at an automobile junk market in Obosi and in residential areas in Anambra State, south-eastern Nigeria to examine the concentration of heavy metals in the groundwater and determine the quality of the water for drinking and other domestic purposes. Forty groundwater samples were collected from boreholes and hand-dug wells (three samples from Obosi and the rest from Onitsha). They were subjected to atomic absorption spectrometry using standard field and laboratory techniques and analysed for physicochemical and hydrogeochemical parameters. Results show that the groundwater in the study area is slightly acidic to neutral, soft to moderately hard when compared with the World Health Organization maximum allowable concentration values and the Nigerian Standards for Drinking Water Quality. The electrical conductivity, dissolved oxygen and biochemical oxygen demand ranged from 58 to 1796 μS/cm, 6.78 to 8.76 and 0.17 to 1.50 mg/L, respectively. Heavy metal concentrations measured (in ppm) in the water included nickel, manganese, copper and zinc and varied from 0 to 1.82, 0 to 0.195, 0 to 0.325 and 0 to 0.09, respectively, while heavy metal concentrations in the soil measured (in ppm) included iron, lead and cadmium and varied from 0 to 3.87, 0 to 1.80 and 0 to 7.38 mg/kg, respectively. Statistical results gave significant correlation (at 0.05) between electrical conductivity and total hardness, biochemical oxygen demand and dissolved oxygen, and several others elements. The study helps in the understanding of the chemistry of groundwater for long-term monitoring and management for the local community. 相似文献
19.
Ogbonnaya Igwe 《Landslides》2018,15(12):2509-2519
This paper described the recent catastrophic landslides in Sierra Leone, DR Congo, and Nigeria and their probable mechanisms. The landslides occurred at the peak of rainy season on steep mountains predisposed to failure by geologic, climatic, hydrologic, and tectonic factors. The landslides killed over 500 people and displaced more than 2000 people. The DR Congo landslide occurred a few days after the Sierra Leone landslide of August 14, 2017. The Sierra Leone mudslide occurred in the Regent area of Freetown on the steep slopes of Mount Sugar Loaf. Sugar Loaf forms a part of extensive and forested highlands south of the city. The cities of Freetown, Lokoja in Nigeria, Conakry in Guinea, and other cities in Africa have grown in recent times to almost encircle vulnerable highlands that are prone to failure. The internal friction angles of the slope materials were low at all saturation and normal stress levels used in the study. The result correlated with the stability analysis which showed that the factor of safety along the potential sliding surfaces was at critical value at failure. The number of casualties following a slip of the weak slope materials increased because of the housing pattern and the lack of effective drainages in urban cites in Africa. These factors are important considerations for future prevention and mitigating measures aimed at saving Africa from catastrophic slope failures. 相似文献
20.
Outcrop-based sequence stratigraphic analysis and palynological biofacies were used to define depositional sequences and their bounding surfaces, and build a sequence stratigraphic model for the Upper Cretaceous succession of the Afikpo Sub-basin. Four unconformity-bounded third-order depositional sequences were identified. Sequence 1 comprises the Nkporo Formation and is subdivided into lowstand system tract (LST) representing an incised valley fill and transgressive systems tract (TST) consisting of estuarine and marine shales and mudstones. The base of the sequence is an angular unconformity correlated to the 77.5 Ma sequence boundary (SB) and the maximum flooding surface (MFS) is dated at 76 Ma. Sequence 2 is diachronous and straddles the lithostratigraphic boundary of the Nkporo and Mamu formations. The upper SB is dated at 71 Ma while associated MFS is dated at 73.5 Ma. Sequence 3 consists of the upper Mamu Formation and the Ajali Formation. The upper SB of sequence 3 is at 68 Ma while the MFS is dated at 69.8 Ma. Sequence 4 is the topmost depositional sequence belonging to the Nsukka Formation. The upper SB is placed at 66.5 Ma. The MFS within this sequence is dated at 67.8 Ma. The sequences encompass from tidally influenced bay head delta and central estuarine environments to coastal and shallow marine shelf environments. Stratigraphic architecture and facies types show that sequence development was controlled to a great extent by eustatic sea level variations though differential subsidence rates encouraged differential rates of sediment supply and rates of sea level change along different segments of the shoreline. 相似文献