الفهرس 
Literature ReviewOnly 14 pages are availabe for public view
 |  | from | 167 |  |  |
| | | from | 167 | | |
Abstract
Soft soil has large voids ratio leading to low permeability and high compressibility potential. Furthermore, it has low shear parameters leading to low bearing capacity. Accordingly, soil improvement has to be carried out on such soil formation. The most effective soil improvement technique in terms of cost and efficiency is preloading surcharge with a stress higher than the final permanent load accompanied with PVDs to accelerate the consolidation process through creating radial path besides the vertical consolidation path that forms 5%-10% of the total degree of consolidation in this case. The usage of this technique decreases the voids ratio as the soil becomes stiffer than its natural condition. Furthermore, it decreases the soft soil compressibility potential. Moreover, the soil shear parameters increase leading to bearing capacity enhancement. However, this technique undergoes some problems that may affect its efficiency which are smear, well resistance, and kinking.
In this thesis, a case study in the Eastern Nile Delta is used to verify the FEM using the field measurements. The soil properties used in the numerical analysis are concluded from the soil investigation programs that have been carried out in this area. There are several investigation programs performed in this zone due its importance as the Egyptian government has several expansive plans in this area. CPTu is used as the main tool to determine soil characteristics in Eastern Nile Delta. Furthermore, the boreholes executed show that the very soft clay layer extends to around fifty meters depth from the ground surface.
A three-dimensional numerical analysis is performed using the soft soil creep constitutive model that takes creep in consideration as it forms a significant value from the total settlement in case of very soft soil. Moreover, the model simulates the mentioned efficiency problems in order to reach the real actual soil behavior. The output of the numerical analysis shows an agreement with the field results. In the beginning, the field settlement is higher than the FEM as there is a rapid settlement in the field. This difference occurs as there are some deviations in the idealized construction sequence compared with the actual sequence. However, the results are almost identical with 2% - 3% of error after 45 days from the field settlement monitoring. Furthermore, soil behavior, the rate of settlement, is the same after the first 20 days from field results monitoring. An investigation is performed to conclude the effect of using the PVDs on the residual settlement. the utilization of the PVDs decreases the residual settlement by 7 cm under the same construction sequence and soil conditions for the same applied working loads.
The case of the soil treatment using preloading stress accompanied with PVDs can be analyzed using (2-D) or (3-D) FEM. In this thesis, the accuracy of the (2-D) is investigated. There are several approaches to model this problem using (2-D) programs whether using plane strain or axisymmetric. There are three techniques to be able to use the plane strain methodology which are based on either modifying the permeability, the gematrical boundaries or a mix of both of them. The axisymmetric approach almost coincides with the (3-D) results with maximum percentage of error of 6.00%. Furthermore, the rate of settlement in the axisymmetric model is slightly slower than the (3-D) model. Furthermore, plane strain approaches show agreement with the axisymmetric model. The percentage of difference between the equivalent horizontal permeability, equivalent vertical permeability ,and equivalent parallel drain approaches with the axisymmetric model are 9.78&, 6.00% ,and 12.00% respectively
Besides the modeling approaches, the effect of this improvement technique on the settlement behavior is investigated. The construction sequence, soil properties, and the boundaries for the Eastern Nile Delta case are followed. Uniform preloading stresses of magnitude (+76.50 kPa), (+110.50 kPa), (+144.50 kPa), (+178.50 kPa), and (+212.5 kPa) are used in this design chart. Furthermore, working loads of (+10.00 kPa), (+20.00 kPa), (+30.00 kPa), (+50.00 kPa), and (+70.00 kPa) are used with each preloading stress to correlate between the preloading stresses, working loads, and the residual settlement in the Eastern Nile Delta. The outputs show that as the preloading stress increases, the residual settlement decreases. Furthermore, the effect of increasing the preloading stress is low in case that the soil is in the overconsolidated state in both cases. Moreover, there is an optimum preloading stress beyond which any further increase in the surcharge height won’t be effective and the curve between the preloading stress and residual settlement will flatten.
Moreover, the influence of the effective stress bulb is examined through using different footing sized. Three different sizes of footings are investigated which are square footings of length 3m, 5m, and 10m. The stress on the footing area is varied between (+40.00 kPa), (+60.00 kPa), and (+80.00 kPa). Besides the load on the footing itself, a load of (+10.00 kPa) is applied in the area around the footing to cover all the geometrical boundaries. The mentioned three preloading stresses are used for each working load, and for each footing size which are (+76.50 kPa), (+110.50 kPa), and (+144.50 kPa). The geometrical boundary is six times the foundation length in order not to affect the settlement results. The results show that the required preloading stress increases to reach the allowable settlement as the dimension of the footing increases. The rate of the settlement increases as the dimension of the foundation increases from 5 meters to 10 meters more than increasing it from 3 meters to 5 meters as the stress bulb will increase more in the highly compressible layer , the lower 30 meters of very soft clay.
As the world focus on sustainability, biodegradable PVDs are environmentally friendly more than the conventional PVDs. Conventional PVDs don’t degrade which leads to pollution. Furthermore, soft soil is sensitive to water which leads to disturbance and contamination. Accordingly, a study to explore the effect of using biodegradable PVDs instead of typical PVDs on residual settlement is carried about. The same construction sequence and soil properties of eastern Nile delta are used. The PVDs are deactivated at the moment of working load application in the models used to simulate the biodegradable PVDs. Working loads of (+10.00 kPa), (+20.00 kPa), (+30.00 kPa), (+50.00 kPa), (+70.00 kPa), and (+100.00 kPa) are used along with preloading stresses (+76.50 kPa), (+110.50 kPa), (+144.50 kPa), (+178.50 kPa), and (+212.50 kPa). The results are identical in case of working loads up to (+70.00 kPa). However, the residual settlement is slightly higher in typical PVDs than in the biodegradable PVDs in case of (+100.00 kPa) as the project lifetime which is 50 years is not enough to finish the consolidation process through vertical consolidation only in case of biodegradable PVDs.
Besides decreasing the primary consolidation and increasing the shear parameters of the soft soil, surcharging decreases the secondary consolidation as well. The value of the secondary consolidation settlement in eastern Nile delta is concluded to be 15 cm which is 78.9 % of the total residual settlement. Furthermore, the effect of the PVDs on the secondary settlement is determined.
A parametric study is carried out to study the effect of various factors on the secondary compression index. a simpler construction sequence is used to limit the number of variables. Furthermore, one soil formation is used to simulate the 45 meters which is the lower layer (2) in the Eastern Nile Delta case study. The effect of four factors on the secondary compression index is examined. These factors are the preloading stress, the length of the PVDs, the spacing between the PVDs, and the degree of consolidation under a preloading stress. The output of this parametric study shows the following:
The main factor affecting the C’α is the value of the preloading stress. The value of the C’α /Cα decreases from 52% to nearly zero as the value of the preloading stress increases from about 50% to 250% of the permanent long-term loads. This is mainly attributed to the increase in the average value of the OCR from 1.376 to 1.951.
The PVDs length has a considerable effect on the C’α value. As the length of the PVDs increases, the secondary compression index decreases. The value of the C’α /Cα decreases from 68% to 45% as the PVDs’ length increases from 22% to 100% of the total compressible layer. However, beyond certain PVDs’ length, the increase of the PVDs’ length has a negligible effect on the secondary compression index. This is because Δ〖σ’〗_v/ 〖σ’〗_o is decreasing in deeper soil layers, the effect of the preloading stress is decreasing, and the presence of the PVDs in this layer is meaningless.
Spacing between PVDs also affects the value of the secondary compression index. The value of the C’α /Cα decreases from 62% to 42% as the spacing between PVDs varied from 6m to 0.75m.
Finally, the degree of consolidation under the preloading stress is affecting the value of the secondary compression index. As the degree of consolidation increases, the value of the secondary compression index decreases. The value of C’α /Cα varies from 56% to 46% as the degree of consolidation changes from 65% to 95%.
Another technique to estimate the value of the secondary consolidation settlement is through using empirical equations. In 2007, Wong’s concluded an equation to estimate the secondary compression index after surcharging. This equation is based on swelling index, voids ratio, the compression index, and the OCR attributed in each case. The determined secondary compression indices for the mentioned preloading stresses in the parametric study are used to indicate the Wong’s equation constant for eastern Nile delta. The constant in the equation which is related to each specific soft clay formation is 9.72 for Eastern Nile Delta. However, this constant is valid only till preloading surcharge of height (+4.00m).
6.2 Future research
Investigate wider range of OCRs with the same factors affecting the secondary compression index especially in the range of OCR between 1 and 1.5.
This study considers the change of the secondary compression from the moment of load application after the preloading removal to the end of the project service time. However, the secondary compression index change with time has to be investigated for the same OCR.
Explore the initiation time of the secondary consolidation whether at the instance of the load application or after the end of the primary consolidation.
Examine the relation between the Cα /Cc and 〖σ’〗_v with time whether increasing, decreasing or constant.
Investigating the effect of changing the discharge capacity of the drain with depth and along the consolidation the process.
Laboratory investigation for the effect of various factors on the secondary consolidation should be carried out.
Study the effect of the preloading stress on the soft soil’s cohesion improvement.