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1.
The 27 November 1945 earthquake in the Makran Subduction Zone triggered a destructive tsunami that has left important problems unresolved. According to the available reports, high waves persisted along the Makran coast and at Karachi for several hours after the arrival of the first wave. Long-duration sea-level oscillations were also reported from Port Victoria, Seychelles. On the other hand, only one high wave was reported from Mumbai. Tide-gauge records of the tsunami from Karachi and Mumbai confirm these reports. While the data from Mumbai shows a single high wave, Karachi data shows that high waves persisted for more than 7 h, with maximum wave height occurring 2.8 h after the arrival of the first wave. In this paper, we analyze the cause of these persistent high waves using a numerical model. The simulation reproduces the observed features reasonably well, particularly the persistent high waves at Karachi and the single high wave at Mumbai. It further reveals that the persistent high waves along the Makran coast and at Karachi were the result of trapping of the tsunami-wave energy on the continental shelf off the Makran coast and that these coastally-trapped edge waves were trapped in the along-shore direction within a ∼300-km stretch of the continental shelf. Sensitivity experiments establish that this along-shore trapping of the tsunami energy is due to variations in the shelf width. In addition, the model simulation indicates that the reported long duration of sea-level oscillations at Port Victoria were mainly due to trapping of the tsunami energy over the large shallow region surrounding the Seychelles archipelago.  相似文献   

2.
The Kirthar fold belt (KFB) is one of the N-S-trending portions of the thrust belts of western Pakistan. In the study area (some 150 km north of Karachi), the KFB consists of large-scale, open NNW-SSE anticlines that affect the outcropping Kirthar and Nari formations. From an analysis of more than 200 minor strike-slip and reverse faults from 14 different data stations, two main σ1 directions were identified, ENE and ESE. The ESE stress is interpreted as the regional, primary stress field, and the ENE direction as a deflection of the former caused by the presence and activity of NNW-trending macrostructures (folds and strike-slip faults).  相似文献   

3.
Trihalomethanes (THMs) are the most abundant disinfection byproducts (DBPs) of the chlorination disinfection. THMs speciation and their geospatial distribution were examined in 58 locations throughout the water distribution network of Karachi city. THMs (CHCl3, CHCl2Br, CHClBr2 and CHBr3) and physico-chemical parameters (pH, TDS, DO, Residual chlorine, temperature and TOC) were determined. CHCl3 was the major THM found in all water samples of 58 locations, which accounted for 91.69 % of the total THMS followed by CHCl2Br (5.69 %), CHClBr2 (1.78 %) and CHBr3 (0.85 %). Total THMs level exceed the maximum contamination level of WHO and USEPA at some locations. Varying nature of correlation from high to low was found within THMs and among the physico-chemical parameters. GIS linked geospatial analysis revealed the association of THMs level with demographical and geological based variations from east to west of Karachi city. Continuous monitoring program and legislation for the contaminant levels were suggested to avoid adverse public health impact of THMs in drinking water supplies.  相似文献   

4.
Changes in surface air temperature during the last century are widely discussed among researches in the field of climate change. This paper measures the variability of annual surface air temperature of five major cities of Pakistan (Lahore, Peshawar, Quetta, Hyderabad and Karachi) for the period from 1882 to 2003. We perform an exploratory analysis which shows that the annual landmass air temperature series of five relatively more important climate stations of Pakistan obey the normal distribution. A subsequent trend analysis shows that the temperature has been increasing in the twentieth century for the five (major) cities of Pakistan, the increase being 0.3°C to 1.0°C. We computations based on Bayesian analysis for two samples (e.g., for Lahore, we use data for the period from 1882 to 1960 and 1961–2000 for first sample and second sample, respectively) of temperature data of five cites shows that the average of annual mean temperature for the second period is higher than the average of first period. Thus, Bayesian inferencing shows that the general pattern of evolution of temperature over Pakistan is pretty similar to the current global warming configuration.  相似文献   

5.
Trace metal concentrations were investigated in a recent sediment core collected from the Rehri Creek area of the Karachi coast,Sindh-Pakistan.The core was sliced horizontally at 2.5-cm intervals to determine grain size,sediment composition,pH,organic matter,and acid-leachable trace metals:cadmium,chromium,copper,lead,and zinc.The trace metals were analyzed by ICP.To separate anthropogenic from geogenic input,several approaches were made,including comparison with sediment quality guidelines—ecotoxicological sense of heavy metal contamination and classification by quantitative indexes.Grain-size analysis and sediment composition of core sample show a sandy nature with neutral pH.Elemental sequence(ES)of the trace metals is in the order of Zn(19.2-109.56 ppm)>Si(66.46-101.71 ppm)>Ba(12.05-26.86 ppm)>As(8.18-17.36 ppm)>Ni(4.2-14.69 ppm)>Cr(3.02-9.62 ppm)>Pb(2.79-6.83 ppm)>Cu(2.2-5.29 ppm)>Co(0.9-2.05 ppm).Thus it is likely that the area may face a serious threat of metal pollution with the present deposition rates unless stringent pollution control norms are adopted.The Sediment Geo-accumulation Index shows that there is no Cr,Cu,Ni,Pb,Zn,or Fe pollution;however,the former index and the Pollution Load Index indicate arsenic pollution in the sediments.  相似文献   

6.
《Atmósfera》2014,27(1):35-46
Air pollution has been considered one of the most important environmental challenges because of its direct effect on ecosystems and human health. Temporal changes in the composition of 20 samples of rainwater in the arid region of Karachi, Pakistan during the southwest monsoon of 2009 have been monitored to carry out the indirect assessment of air quality. The study has been fulfilled with the aim of identifying the level of air pollution, and the relative contribution of possible anthropogenic activities. Metal ions were analyzed to predict health risks. Rain samples were found to be alkaline (pH 5.55-7.55) due to the influence of calcium and magnesium rich particles. Continuous rains in consecutive days showed a remarkable decline in air pollution while the dry season boosted up the level of pollution. Strong correlations of total dissolved solids with K+, Na+, Ca2+, Mg2+, and Cl− were observed. A relatively weak correlation of total suspended particles with metals showed the possibility of some other particulate matters being suspended in the air. Principal component analysis and analysis of means were used to predict the anthropogenic source of pollution. This study will be helpful to formulate strategic planning and policies for controlling the level of air pollution in the city.  相似文献   

7.
The dengue infectious disease remnants a human health problem in tropical and subtropical countries. In an Auto Regressive model to assess the role of climatic parameter El Niño Southern Oscillation and land surface mean monthly temperture on dengue outbreaks of the Karachi region over the monthly time interval January 2001 to December 2016, subsequent to stabilization of variance, we are able to apply and predict an Auto Regressive Integrated Moving Average Exogenous-Transfer Function model by using the order selection criteria namely Final Prediction Error and Akaike’s information. The results confirmed that ARIMAX (2,1,2) has fitted model, although an Auto Regressive model predicts a smaller decline in dengue data series than the auto Poisson Regression model. Additionally, we developed an alternative model for the Poisson Autoregressive Exogenous model in order (p) and Negative Binomial Auto Regressive Exogenous model, deliver the best fit as compared to the Poisson Auto Regressive Exogenous model whereas indicated by the deviances. The Pearson test showed a strong positive association between temperature and dengue, while ENSO inverse indication. High dengue outbreaks are detected in the months of September, October, and November. This comparative study exposed a significant relationship among monthly dengue and climatic variation by Auto Regressive Integrated Moving Average Exogenous (ARIMAX), Poisson and Negative-Binomial Autoregressive Exogenous (PARX-NBARX) models, while smallest values of AIC (3.89), Negative Binomial Auto Regressive Exogenous, are preferred more accurate model for the next 12 months forecasting. This study has provided useful information for the development of dengue predictions and future warning systems.  相似文献   

8.
Artesian basins contain the largest mineral water resources of the world. There are several types of mineral therapeutic water: sulfate, chloride, radon-rich, iron-rich waters, etc. Artesian basins occupy very large areas in Russia. However, genesis of water and brines is still not very clear. This is one of the most important hydrogeological problems that is being attempted to solve for many years. Most of the Russian hydrogeologists traditionally consider that these waters are of sedimentary origin. However, higher concentrations of bromine, iodine, iron, radon and other balneologically active components can be of different origin, for example, of infiltration or juvenile water. As an example, two areas will be considered – West-Siberian basin and East-European artesian area.West-Siberian artesian basin has very distinct latitudinal and vertical zonation. Latitudinal zonation is caused by climate changes from north to south. As for the vertical zonation, mineralization and chemical composition change in the vertical cross-section and from the periphery to the center within the same aquifer. The main mineral water resources of West-Siberian artesian basin are concentrated in Mesozoic rocks. Brackish waters and low-saturated brines without specific components are used for medical purposes. The most well-known spa is Karachi, which exploits chloride-hydrocarbonate brackish water. Sodium chloride bromine and iodine-bromine waters are used at other health resorts. It is possible to organize extraction of iodine from brines of Tcherkashinsko-Tobolskoe occurrence in Tumen region.East-European artesian area occupies most of the Russian Platform. The most widespread types of mineral water within the Russian Platform are sodium-chloride and magnesium-sulfate waters and brines. Such well-known spas, like Moscow mineral waters, Krainka, Staraya Russa and many others, belong to this type. Resources of these waters are definitely connected with sedimentogenic processes. The upper hydrodynamic zone contains iron-rich, hydrogen sulfide, and sometimes radon-rich water. Their formation is caused by the interaction between waters of infiltration and sedimentary genesis, or between infiltration waters and host rocks. One of the examples is Polustrovo iron-rich water. There are industrially valuable waters containing bromine and iodine.The resources of therapeutic water of sedimentary basins allow to increase balneological potential of spas in Russia.  相似文献   

9.
A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment.Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements.In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting.The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.  相似文献   

10.
The presence of raised beaches and marine terraces along the Makran coast indicates episodic uplift of the continental margin resulting from large-magnitude earthquakes. The uplift occurs as incremental steps similar in height to the 1–3 m of measured uplift resulting from the November 28, 1945 (M 8.3) earthquake at Pasni and Ormara, Pakistan. The data support an E—W-trending, active subduction zone off the Makran coast.The raised beaches and wave-cut terraces along the Makran coast are extensive with some terraces 1–2 km wide, 10–15 m long and up to 500 m in elevation. The terraces are generally capped with shelly sandstones 0.5–5 m thick. Wave-cut cliffs, notches, and associated boulder breccia and swash troughs are locally preserved. Raised Holocene accretion beaches, lagoonal deposits, and tombolos are found up to 10 m in elevation. The number and elevation of raised wave-cut terraces along the Makran coast increase eastward from one at Jask, the entrance to the Persian Gulf, at a few meters elevation, to nine at Konarak, 250 km to the east. Multiple terraces are found on the prominent headlands as far east as Karachi. The wave-cut terraces are locally tilted and cut by faults with a few meters of displacement.Long-term, average rates of uplift were calculated from present elevation, estimated elevation at time of deposition, and 14C and U–Th dates obtained on shells. Uplift rates in centimeters per year at various locations from west to east are as follows: Jask, 0 (post-Sangamon); Konarak, 0.031–0.2 (Holocene), 0.01 (post-Sangamon); Ormara 0.2 (Holocene).  相似文献   

11.
The sea surface temperature (SST) variations play a veryimportant role in the genesis and maintenance of meteorological and oceanographic processessuch as monsoon depressions and subsequent floods, large-scale sea level fluctuationsand genesis of tropical cyclones. Many low lying coastal regions of South Asia are adjacentto river deltas and have large population. The dense population, poor economy and severalother socio-economic factors make these areas most vulnerable to the impact of climate change.Variability of sea surface temperature (SST) is importantas the duration and intensity of SST provide the basis for studies related to climatic changescenario. In this study an attempt has been made to estimate the recent SST trends in the coastalwaters of some cities, which lie on the Arabian Sea and Bay of Bengal. The annual andinterannual variability has also been studied. The SST variations have then been linkedwith the El Nino and La Nina events.The NOAA-NASA Pathfinder Advanced Very High Resolution Radiometer (AVHRR) SST fields from 1985-1998, created in the Jet Propulsion Laboratory(JPL), USA are used in this study. Here the quality of data is an important factor toobtain reliable estimates of Sea Surface Temperature (SST) trends and other related parameters.However, this is not possible with the conventional type data, due to low quality as wellas sparse data in the region. Though the satellite based SST climatologies have shorterobservation lengths, they can provide reliable estimates of recent SST variability overa large oceanic areas with sparse or no data.Increasing trend of SST is observed throughout all theseasons in the northern Arabian Sea extending from Oman to Karachi and Mumbai and furthersouth to Salalah and Colombo. However, in coastal islands stations further south ofIndia such as at Colombo the increment is not significant. Though the increasing trend in SSTduring winter is not significant, nevertheless it shows the increasing influence of coldspells on this Island. An interesting situation has been observed in the Bay of Bengal. On anaverage, increasing trends in the annual SST were observed in Visakhaputnam. But at thestations located in the northeastern part of Bay of Bengal, namely Hiron Point and Cox'sBazar reverse conditions are observed. In the Southern Bay of Bengal variations in SST isnot significant which reflects in the SST analysis of Chennai and Port Blair stations. Locationof these stations at lower latitudes (near by equator) probably is the reason for this insignificantchange. It has been found that the interannual mode of SST variations dominate the linear SSTtrends which is characterized by the El Nino Southern Oscillations (ENSO) scale cycle.  相似文献   

12.
German-Italian military successes and the fame of General Rommel's Afrika Korps in 1941 spelled disaster for the British war effort in Libya-Egypt. American military assistance was requested and freely given; this was several months before the United States entered the war! Pan American World Airways was the chosen instrument for building a series of airfields in Africa capable of receiving the planes ferried across the ocean from Recife, Brazil.Besides its reputation as the world's leading airline, Pan Am already had a major aircraft servicing facility at Recife. Liberia, because of its proximity to South America, became the first major West African bridgehead for this South Atlantic ferry route. A few existing and numerous newly constructed airfields between Liberia and Khartoum served as the emergency landing, refueling, maintenance and housing sites. The Nile was followed downstream from Khartoum; American-supplied warplanes played an important role in the pivotal Egyptian battle of El-Alemain in October, 1942. Once the threat to Egypt had subsided, the Brazil — West Africa air link was expanded to include a route through Central Africa, primarily to tap a supply of uranium from what was then the Belgian Congo (now Zaire). Khartoum served a new air ferry route to British India via Aden and Karachi. Transport aircraft used in the China-Burma-India theatre of operations were supplied over this ever-expanding air link. A spur to Basra (Iraq) and Tehran provided a secret diplomatic connection to the Soviet Union before the epic struggle at Stalingrad began in earnest. General Doolittle's surviving Tokyo raiders returned to the United States over segments of this long air route. Most of the airfields lost their locational significance after the war. A few, however, were to become their countries' international airports.More correctly, the Trans-South Atlantic Transport and Ferry Service. The author appreciates the fact that except for a secondary route through Central Africa, this wartime air connection was to the North of the Equator and technically, in the North Atlantic. Use of terminology then as now is to differentiate this particular route from the important Newfoundland to Prestwick (Scotland) air bridge.  相似文献   

13.
Prabhas Pande 《Natural Hazards》2013,65(2):1045-1062
Of the intraplate seismic events, the January 26, 2001 Bhuj earthquake (Mw 7.7) would be remembered as one of the deadliest, in which 13,805 human lives were lost, 0.177 million injured and a total of 1,205,198 houses were fully or partly damaged in 16 districts of Gujarat state with an estimated overall loss of Rs. 284, 23 million. The brunt of the calamity was borne by five districts, namely Kachchh, Ahmadabad, Rajkot, Jamnagar and Surendranagar, where 99?% of the total casualties and damage occurred. In the neighbouring parts of Sindhh Province of Pakistan, 40 human casualties were reported, and some buildings cracked in the Karachi city as well. In the Kachchh district of Gujarat state, the telecommunication links and power supply were totally disrupted, road and rail links partially impaired and water supply snapped at many places. The Bhuj airbase had to be closed for some time due to damage to the infrastructure. The macroseismic survey carried out by the Geological Survey of India in an area as large as 1.2 million?sq?km indicated an epicentral intensity as high as X on the MSK scale in an area of 780?sq?km in the central part of Kachchh rift basin. Apart from damages to civil structures, the January 26 earthquake induced conspicuous terrain deformation in the form of liquefaction features, structural ground deformation and low-order slope failures that were mainly prevalent within the higher intensity isoseists. Liquefaction occurred in an area of about 50,000?sq?km. The extensive plains of Rann of Kachchh, the marshy tracts of the Little Rann and the shallow groundwater table zones of Banni Land provided the most conducive geotechnical environments for the development of seismites. The liquefaction activity was profuse in seismic intensity zones X and IX, widespread in intensity VIII, subdued in intensity VII and stray in intensity VI. The common forms of liquefaction were sand blows/boils, ground fissures, craters, lateral spreading and slumping. Ground deformation of tectonic origin was witnessed in the epicentral tract. Such features, though much less subdued in comparison with the 1819 large earthquake (Mw 7.8) in region, occurred along the Kachchh Mainland fault (KMF) and along a transverse lineament, referred to as Manfara?CKharoi fault. The manifestations were in the form of fractures, displacement of strata, linear subsidence, upheaval, formation of micro-basins/micro-ridges, ripping off of rock surface, and at places violent forms of liquefaction. The localities where coseismic deformations were observed include Bodhormora, Sikra, Vondh, Chobari, Manfara and Kharoi. The 2001 event has demonstrated the role of local geology in influencing the ground motion characteristics and, therefore, the hazard estimation.  相似文献   

14.
Spectral analysis of the digital data of the Bouguer anomaly of North India including Ganga basin suggest a four layer model with approximate depths of 140, 38, 16 and 7 km. They apparently represent lithosphere–asthenosphere boundary (LAB), Moho, lower crust, and maximum depth to the basement in foredeeps, respectively. The Airy’s root model of Moho from the topographic data and modeling of Bouguer anomaly constrained from the available seismic information suggest changes in the lithospheric and crustal thicknesses from ∼126–134 and ∼32–35 km under the Central Ganga basin to ∼132 and ∼38 km towards the south and 163 and ∼40 km towards the north, respectively. It has clearly brought out the lithospheric flexure and related crustal bulge under the Ganga basin due to the Himalaya. Airy’s root model and modeling along a profile (SE–NW) across the Indus basin and the Western Fold Belt (WFB), (Sibi Syntaxis, Pakistan) also suggest similar crustal bulge related to lithospheric flexure due to the WFB with crustal thickness of 33 km in the central part and 38 and 56 km towards the SE and the NW, respectively. It has also shown the high density lower crust and Bela ophiolite along the Chamman fault. The two flexures interact along the Western Syntaxis and Hazara seismic zone where several large/great earthquakes including 2005 Kashmir earthquake was reported.The residual Bouguer anomaly maps of the Indus and the Ganga basins have delineated several basement ridges whose interaction with the Himalaya and the WFB, respectively have caused seismic activity including some large/great earthquakes. Some significant ridges across the Indus basin are (i) Delhi–Lahore–Sargodha, (ii) Jaisalmer–Sibi Syntaxis which is highly seismogenic. and (iii) Kachchh–Karachi arc–Kirthar thrust leading to Sibi Syntaxis. Most of the basement ridges of the Ganga basin are oriented NE–SW that are as follows (i) Jaisalmer–Ganganagar and Jodhpur–Chandigarh ridges across the Ganga basin intersect Himalaya in the Kangra reentrant where the great Kangra earthquake of 1905 was located. (ii) The Aravalli Delhi Mobile Belt (ADMB) and its margin faults extend to the Western Himalayan front via Delhi where it interacts with the Delhi–Lahore ridge and further north with the Himalayan front causing seismic activity. (iii) The Shahjahanpur and Faizabad ridges strike the Himalayan front in Central Nepal that do not show any enhanced seismicity which may be due to their being parts of the Bundelkhand craton as simple basement highs. (iv) The west and the east Patna faults are parts of transcontinental lineaments, such as Narmada–Son lineament. (v) The Munghyr–Saharsa ridge is fault controlled and interacts with the Himalayan front in the Eastern Nepal where Bihar–Nepal earthquakes of 1934 has been reported. Some of these faults/lineaments of the Indian continent find reflection in seismogenic lineaments of Himalaya like Everest, Arun, Kanchenjunga lineaments. A set of NW–SE oriented gravity highs along the Himalayan front and the Ganga and the Indus basins represents the folding of the basement due to compression as anticlines caused by collision of the Indian and the Asian plates. This study has also delineated several depressions like Saharanpur, Patna, and Purnia depressions.  相似文献   

15.
软黏土侧压力系数K0阶段性特征研究   总被引：1，自引：0，他引：1  
赵玉花  沈日庚  李青 《岩土力学》2008,29(5):1264-1268
沿海软黏土的侧压力系数K0具有阶段性特征:当试验荷载P＜Pc'(土样的残余结构强度)时,K0值较小,试样的结构性好,呈现出似刚性的材料特征,侧压力系数小;当P＞Pc'时,K0值增大,土的微结构破坏,呈现出弹塑性体的材料特征,侧压力系数增大.而在工程实践中,受成孔、取土等诸多因素干扰,欠固结-正常固结的软黏性土的微结构的结构性易被破坏,使K0指标的阶段性特征不明显.  相似文献   

16.
隧道施工时地表沉降监测控制标准探讨   总被引：5，自引：0，他引：5  
安永林  彭立敏  张峰  吴波 《岩土力学》2009,30(Z2):446-451
地表沉降是判断浅埋隧道地层稳定的一个重要指标。在分析地表沉降监测重要性和隧道埋深的关系基础上,阐述了地表沉降控制标准确定的原则;针对隧道无邻近结构物段,从地层围岩稳定、经验公式和相关规范的角度探讨地表沉降控制标准,并以武广客运专线浏阳河隧道为例子进行验证。结果表明,城市隧道变形控制标准要比山岭隧道更严格,浅埋隧道要比深埋隧道控制标准更严格;围岩越坚硬、跨度越小、边墙高度越小,则允许的位移越小,反之则越大;允许的变形控制标准主要影响因素是围岩自身条件,其次是隧道的跨度  相似文献   

17.
承德坝上御道口地区是对气候变化非常敏感的生态环境脆弱带,认识本区气候变化对未来增温背景下采取应对措施有重要意义。粒度是气候环境变化敏感代用指标,粒度分析结果可推测沉积物形成条件和环境。文章对御道口地区YDK剖面沉积物进行粒度测试。通过对粒度数据进行组分分离和敏感粒级的提取,利用AMS14C测年结果建立年代框架,探讨了坝上御道口地区8000 a B.P.以来的古气候古环境变化。结果显示气候变化经历了三个阶段:（1）8 000~6 100 a B.P.,粒度指标指示此时段动力较大,粉尘距源区较近,风沙活动较强,携带较多粗颗粒物质造成粒径较粗,估计此时段气候干燥,区域植被较差。（2）6 100~1 700 a B.P.,此时段粒度指标指示动力较小,粉尘距源区较远,粗颗粒物质减少,估计此时段气候湿润,区域植被较好,风沙活动减弱。（3）1 700 a B.P.至今,动力较大,粉尘距源区较近,估计此时段气候变干,区域植被较差,风沙活动强。  相似文献   

18.
薛海斌  党发宁  尹小涛  雷曼  杨超 《岩土力学》2016,37(8):2238-2246
边坡的失稳是一个从量变到质变的动态渐进破坏过程,此问题也是边坡领域研究的重点与难点之一。在考虑岩土材料软化特性和动力学求解的基础上,建立了边坡渐进破坏仿真的理论框架;利用ABAQUS软件的动力显式求解模块实现了边坡的渐进破坏仿真;根据塑性应变揭露了剪切带的扩展过程,由软化本构确定了滑面材料的分区演化规律,根据等效塑性应变确定了边坡的滑面,通过滑面位置将边坡分为滑体、滑带、滑床,并分别研究了边坡各分区内部特征点运动学变量的发展过程,从而揭示了边坡的渐进破坏过程;基于材料参数沿滑面的时空分布,利用矢量和法得到了边坡不同演化阶段的安全系数。对比该方法与Bishop法确定的滑面位置与安全系数,发现两种方法峰值和残余强度对应的安全系数比较接近,该方法搜索所得滑面位于Bishop法自动搜索的滑面之间,验证了此方法的合理性及可靠性。最后分析了材料软化特征对边坡稳定性的影响,在保持其他参数不变的条件下,增大残余黏聚力,边坡的滑面位置加深,安全系数的初始值减小,安全系数的快速减小阶段有所推迟,并且快速减小阶段经历的时间有所延长,稳定后的安全系数有所增大。保持其他参数不变,增大残余黏聚力对应的等效塑性应变阈值,边坡的滑面位置加深,安全系数的初始值减小,安全系数的快速减小阶段有所推迟,但快速减小阶段经历的时间基本不变,达到稳定的时间有所推迟,同时稳定后的安全系数略微有所增大。  相似文献   

19.
黄河水下三角洲粉质土扰动土层特征及成因探析   总被引：9，自引：1，他引：8  
孙永福  董立峰  宋玉鹏 《岩土力学》2008,29(6):1494-1499
黄河水下三角洲上分布着大量的扰动土层,其性质与周围未扰动土相比有很大的差异。在老黄河口区选择扰动土层的典型分布区,利用工程地质钻探取样、原位静力触探试验和浅地层剖面探测对海底扰动土层的厚度及工程地质特征进行了调查分析。结果表明,与未扰动土相比,扰动土的重度较大、孔隙比较小、含水率较低、强度有一定程度的提高。同时,分别对风暴潮作用下黄河水下三角洲粉质土的瞬时液化极限深度和残余液化极限深度进行了计算。计算结果表明,残余液化极限深度较大。将残余液化极限深度、通过浅地层剖面探测结果所得的扰动土层厚度以及通过土工试验统计结果所得的扰动土层厚度进行了对比,发现三者对应很好。对黄河水下三角洲粉质土中扰动土层的形成机制进行了探讨,认为风暴潮作用过程中产生的土体液化是其主要诱因。  相似文献   

20.
分形结构是土体中普遍存在的一种现象,通过对分形结构连接部位所占据的影响域（Voronoi域）的分析,将颗粒连接承载能力换算为影响域上的应力强度,由此获得了一些有益的结果。土体颗粒系统结构分维值的大小对土体强度有着明显的影响;基本结构单元的颗粒数越多,形成的环的长度越长,占据的空间就越大,从力学上讲就越不稳定,因此,其强度越低;土体中分形结构的级数实际上决定了土体中最大孔隙的尺寸,分形结构的级数越多,其相应的最大孔隙越大,等效应力强度会越低;发现偶应力强度与基本结构单元的面积（二维）成反比,即基本结构体越大,偶应力强度就越小,由此可推论出：在其他条件相同的情况下,颗粒越大,偶应力强度就会越低。  相似文献   

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