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Ahad N. Chowdhury 《Sedimentary Geology》1982,32(4):247-263
Petrographic and geochemical evidence shows convincingly that the diagenetic environment changes from marine phreatic through freshwater phreatic to vadose during lithification of the oolites and the overlying Coral Rag of the Osmington Oolite Formation in the Oxford—Berkshire area. This change is thought to be a consequence of tectonic uplift following deposition of the Coral Rag. Extensive neomorphic replacement of the corals took place in a freshwater phreatic environment created by tectonic uplift. Retention of Sr in the Coral Rag is related to differing diagenesis: pore-fluids remained for protracted periods within the Coral Rag as the downward flow was greatly retarded because of obliteration of porosity in the oolites by marine phreatic cementation before deposition of the Coral Rag. 相似文献
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In this study subsidence due to groundwater withdrawal was investigated. Kerman Province in Iran is struggling with land subsidence
problem due to extensive groundwater withdrawal mainly for farming. The rate and type of groundwater withdrawal has very important
impact on settlement rate. In this research, effective parameters on land subsidence caused by groundwater withdrawal were
determined by laboratory tests. Sampling had done up to depth of 300 m mainly with remolded specimens from Shams-abad, Nouq
plain in Kerman province. Similar to the field preconsolidation pressure was applied on specimens in the laboratory. Rate
of applied stress on prepared specimens was similar to effect of oscillation of groundwater level. In order to model the actual
soil behavior in the laboratory, one-dimensional consolidation device (odometer) was adopted for testing. In these tests,
the effect of loading caused by seasonal oscillation of groundwater table is considered by means of cyclic loading in the
testing which has great effect on rate of settlements. The results of tests show that when the water table level periodically
increases and decreases the amount of settlement decrease, comparing with the case when the groundwater table drop to a constant
level. In order to predict the further effects of groundwater level oscillation and actual field condition on land subsidence,
a finite element model based on Biots’ three-dimensional consolidation theory was developed. After calibration of finite element
model with laboratory tests, this model was used for prediction the effect of groundwater level oscillation on actual field
conditions. 相似文献
3.
Vahid Toufigh Farnam Saeid Ahad Ouria Chandrakant S. Desai Hamid Saadatmanesh 《Geomechanics and Geoengineering》2014,9(3):208-214
Soil is a material which is weak in tension; however, different materials such as geotextiles are used to address this inadequacy. In recent years more than one million square metres of geotextiles were used for reinforcing soil. Nevertheless, there are several significant problems associated with geotextiles, such as creep, low modulus of elasticity and susceptibility to aggressive environments. Carbon fibre reinforced polymer (CFRP) was introduced over two decades ago to the field of structural engineering and it can also be used in geotechnical engineering. CFRP has all the benefits associated with geotextiles and it boasts a higher strength, higher modulus, no creep and reliability in aggressive environments. In this investigation, the interface properties of CFRP-sand and fine sand were investigated using the pull-out test. The pull-out test device was designed and assembled using a triaxial loading device and a direct shear device. In the pull-out test, the normal force applied by the triaxial loading and pull-out force is applied by a direct shear device. CFRP samples were prepared in the lab. Precast and cast-in-place samples were tested. The pull-out force and corresponding displacements of each of the materials were recorded and compared. 相似文献
4.
Behzad Behnabian Masoud Mashhadi Hossainali Ahad Malekzadeh 《Journal of Geodesy》2018,92(11):1329-1350
The cross-validation technique is a popular method to assess and improve the quality of prediction by least squares collocation (LSC). We present a formula for direct estimation of the vector of cross-validation errors (CVEs) in LSC which is much faster than element-wise CVE computation. We show that a quadratic form of CVEs follows Chi-squared distribution. Furthermore, a posteriori noise variance factor is derived by the quadratic form of CVEs. In order to detect blunders in the observations, estimated standardized CVE is proposed as the test statistic which can be applied when noise variances are known or unknown. We use LSC together with the methods proposed in this research for interpolation of crustal subsidence in the northern coast of the Gulf of Mexico. The results show that after detection and removing outliers, the root mean square (RMS) of CVEs and estimated noise standard deviation are reduced about 51 and 59%, respectively. In addition, RMS of LSC prediction error at data points and RMS of estimated noise of observations are decreased by 39 and 67%, respectively. However, RMS of LSC prediction error on a regular grid of interpolation points covering the area is only reduced about 4% which is a consequence of sparse distribution of data points for this case study. The influence of gross errors on LSC prediction results is also investigated by lower cutoff CVEs. It is indicated that after elimination of outliers, RMS of this type of errors is also reduced by 19.5% for a 5 km radius of vicinity. We propose a method using standardized CVEs for classification of dataset into three groups with presumed different noise variances. The noise variance components for each of the groups are estimated using restricted maximum-likelihood method via Fisher scoring technique. Finally, LSC assessment measures were computed for the estimated heterogeneous noise variance model and compared with those of the homogeneous model. The advantage of the proposed method is the reduction in estimated noise levels for those groups with the fewer number of noisy data points. 相似文献
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This paper presents a semi-analytical solution for one dimensional consolidation problem of inelastic clays under cyclic loading considering the effect of the change of the consolidation coefficient of the soil layer. Due to change of the consolidation coefficient, and time-dependant loading, Terzaghi's theory would not be applicable in cyclic conditions. In this research, a method based on the time variable exchange along with the superimposing rule is employed to overcome these shortcomings. Changes in the consolidation coefficient are applied in the solution by modifying the loading and unloading durations introducing a Virtual Time. Based on the superimposing rule a set of continuous static loads in specified times are used instead of the cyclic load in the transformed time space. Each full cycle of loading is replaced by a pair of static loads with different signs. Based on the Terzaghi's theory the pore-water pressure distribution and the degree of consolidation are calculated for each static load and the results are superimposed. A set of laboratory consolidation tests under cyclic load and numerical analysis are performed in order to verify the presented method. The numerical solution and laboratory tests results showed the accuracy of the presented method. 相似文献
6.
Adinarayana R. Punuru Ahad N. Chowdhury Niraj P. Kulshreshtha K. L. Gauri 《Environmental Geology》1990,15(3):225-232
The study of the porosimetric data obtained from intrusion, extrusion, and reintrusion of mercury in limestone samples from the region of the sphinx reveals the characteristics of the pore system and allows analysis of durability factors.The pore system in all these rocks consists of ink-bottle pores. As the initial intrusion and extrusion have been completed, some mercury always remains in the sample. This trapped mercury represents the volume of large voids of the ink-bottle pore system. The distribution of the volume of the narrow throats of this system is revealed by the reintrusion curves.The curves obtained by plotting extrusion and reintrusion volumes against corresponding pressures enclose a loop. These curves relate to pore throats only. As entrapment of mercury does not occur in these pores, their distribution frequency and sorting seem to cause this hysteresis.The pressure/volume data also have been interpreted in terms of work needed to inject and extrude mercury from the pores. This thermodynamic analysis of the data has provided an additional, although less well-defined, means to characterize the porous stones.We have used in this study the model-dependent pore-size distributions and model-independent thermodynamic properties to develop durability factors. The factors based upon the combination of pore sizes in the range of <0.5, 0.5-5, and >5 µm precisely fit the observed durability of limestones at the sphinx, some of which have been exposed for more than 5,000 yr to the ambient atmosphere. 相似文献
7.
The weathering characteristics of the sphinx limestones are evaluated in the context of their pore system. The latter consists of ink-bottle pores. This pore system varies from one stratum to another. In the lower micritic rocks the large cavities of the ink-bottle pores are interconnected through the fine capillary network of the narrow throats. In the upper skeletal-rich rocks the interconnections are provided by large as well as small throats.The pore systems in these limestones reflect the depositional regime as well as the diagenetic alterations. The development of microspar from the micritic mud resulted in the formation of microporosity, and the extended circulation of freshwater enlarged some micropores preferentially, forming large cavities.The relationship of micro- and macropores has strongly influenced the weathering of the sphinx rocks. 相似文献
8.
V.?K.?Sissakian A.?D.?Abdul Ahad N.?Al-AnsariEmail author S.?Knutsson 《Geotectonics》2018,52(2):240-250
Upper reaches of the Arabian Gulf consist of different types of fine sediments including the vast Mesopotamia Plain sediments, tidal flat sediments and estuarine sabkha sediments. The height of the plain starts from zero meter and increases northwards to three meters with extremely gentle gradient. The vast plain to the north of the Arabian Gulf is drained by Shat Al-Arab (Shat means river in Iraqi slang language) and Khor Al-Zubair (Khor means estuary). The former drains the extreme eastern part of the plain; whereas, the latter drains the western part. Shat Al-Arab is the resultant of confluence of the Tigris and Euphrates rivers near Al-Qurna town; about 160 km north of the Arabian Gulf mouth at Al-Fao town; whereas, the length of Khor Al-Zubair is about 50 km; as measured from Um Qasir Harbor. The drainage system around Khor Al-Zubair is extremely fine dendritic; whereas around Shat Al-Arab is almost parallel running from both sides of the river towards the river; almost perpendicularly. The fine dendritic drainage around Khor Al-Zubair shows clear recent erosional activity, beside water divides, abandoned irrigation channels and dislocated irrigational channels and estuarine distributaries; all are good indication for a Neotectonic activity in the region. These may indicate the presence of subsurface anticlines, which may represent oil fields; since tens of subsurface anticlines occur in near surroundings, which are oil fields. 相似文献
9.
Jason M.E. Ahad Johannes A.C. Barth Raja S. Ganeshram Robert G.M. Spencer Günther Uher 《Estuarine, Coastal and Shelf Science》2008,78(4):685-693
In order to evaluate the respiration–photosynthesis dynamics in two contrasting North Sea estuaries, pH, temperature, alkalinity, chlorophyll-a (chl-a), and isotopic ratios of dissolved inorganic carbon (δ13CDIC) and dissolved oxygen (δ18ODO) were measured in the Tyne (July 2003) and Tweed (July 2003 and December 2003) estuaries. Using a concentration-dependent isotope mixing line, δ13CDIC values in the Tweed (July 2003) demonstrated mostly conservative behaviour across the estuary, reflecting mixing between riverine and marine sources, although some samples were slightly more 13C-enriched than predicted δ13CDIC values. Low pCO2 (less than 2 times atmospheric pressure) and 18O-depleted δ18ODO signatures below equilibrium with the atmosphere provided further evidence for net autotrophy in the Tweed estuary in summer 2003. Conversely, in the Tyne during the summer and in the Tweed during the winter higher pCO2 (up to 6.5 and 14.4 times atmospheric partial pressure in the Tweed and Tyne, respectively), slightly 13C-depleted δ13CDIC and 18O-enriched δ18ODO values indicated heterotrophy as the dominant process. The relatively large releases of CO2 observed during these two estuarine surveys can be attributed to significant oxidation of terrigenous organic matter (OM). This study therefore demonstrates the usefulness of combined δ18ODO and δ13CDIC isotopes in examining the relationship between respiration–photosynthesis dynamics and the fate of terrestrially derived OM during estuarine mixing. 相似文献
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