عنوان مقاله [English]
One of the subjects about alluvial fans is their sedimentary characteristics. The purpose of this study is to analyze and compare the Chitgar and Kan alluvial fans located in the southern slopes of central Alborz in Tehran province and the correlation between the sediment factors affecting soil resistance and determine the most important factor in soil resistance. The study area in this research is Kan and Chitger alluvial fans, located in the northwestern part of Tehran and in the basin of Kan and Chitgar rivers.
At First, For better study and comparison of the parts, the Alluvial Fan were divided into three upper, middle and distal.Then the physiographic characteristics of each section were calculated. In the field operations, 24 trenches in the study area were excavated and all the layers were sampled and their position was recorded with GPS. After collecting samples, sedimentology studies, including classification and determination of particle size, determination of particle size parameters and clay measurements by pipet method were used. Finally, by using Excel and Gradistate soft wares, all statistical parameters and different charts for particle size by Folk method and morphometric analysis and particle size by pipet method were prepared and these two alluvial fans were compared in terms of sedimentary texture according to their basin. Finaly To compare geotechnical properties of sediments in two Fans, soil resistance factor was used.
Results and discussion
After examination of the sediment, two Alluvial Fans aggregates were observed in which both conifers follow from the exponential pattern of particle size reduction from top to bottom. So that in the upper parts the greatest concentration is observed in the coarse particles and fine particles are in abundance. Of course, in some trenches there are also thick and massive deposits of clay lenses with low thickness, which are due to sudden increase or decrease of energy in this part. There are no coarse particles in the bottom part, but fine particles become noticeable. Although this rule does not apply to the end of the cone, because there are even more than 2000 microns of coarse-grained sediments due to the interference of the Alluvial Fan sediments of Karaj. In both Alluvial Fan, the sedimentation in the flood plain is more mass and the diameter of the sediments is reduced from the vertex to the end of the Alluvial Fans. The relatively good sort, especially in the upper parts and the small amount of clay, represents the sediment deposited by water in the flood plain.
The results of sediment compaction in two conifers from top to bottom resulted in the following results:
- The range of upward/downward ratio changes in both fans shows a downward trend.
- The range of sphincter changes in both Alluvial Fan from top to bottom of the Alluvial Fans shows an incremental trend.
- The magnitude of the factor shape, from the top down to both cones, shows an incremental trend.
- The range of rounding changes in both cones from top to bottom shows an incremental trend.
- The range of three-axis size variations in both cones shows a decreasing trend from top to bottom.
- Investigating the correlation between particle size and Particle form to soil resistance indicates that the average factor of particle diameter and rounding dimension shows the highest correlation with soil resistance, which is the first positive and the latter negative
Based on the results of the laboratory tests and the analysis of the above data, general results of the research are as follows. The Chitgar Alluvial Fan in the middle diameter of the particles from the upper to lower parts shows a gradual decrease, although the cone-shaping from the upper to mid-point shows a decreasing trend, but in the end part due to interference with the intermediate sediments of the cone This is the order of Karaj. The variation of the three dimensions in both cones is normal and with increasing distance from the river's origin, the trend is decreasing. Also, in both fans, the form of sediment changes from top to bottom from the plate and the bar to the sphere and cubic. In general, it can be said that the sediment deposited in both conifers is due to coarse-graining, mass sedimentation (which indicates a sudden decrease in the energy of water), and the insignificant amount of clay from flood-river type. . The results of correlation showed that: sediments with coarse-grained and angular particles are more resistant to soil resistance in terms of construction.