PVC真空预压发处理软土地基施工方案doc.docx

1. Source of borrows pit indicating the location and quantityWe have license given by Vietnam authority to exploit a quantity of 8, 400,000 m 3 in Nha Be river and the location is in Nhon Trach district, Dong Nai province.2. Physical proprieties of proposed fill materialThe sampling test of sand proposed to use is carried out according to AASHTO M45-89, C144-88 (see Test Result of Analyses of Sand component). Thespecific weight of the sand is 2.634, unit weight 1.5g/cm 3 , organic content 2.87% and modules 2.52.3. Proposed method of statement and monitoring devices3.1 Method statementAccording to the geotechnical report of the site, the void ratio of the softsilty clay (e) is 2.377, and the permeability coefficient (Kv) 5.5*10 -7 cm/s.If we adopt “wick drains and pre - loading ” method to reach a soil bearingcapacity of 50Kpa, 75Kpa, 100Kpa under the site’s conditions, theconsolidation period would be far beyond the proposed work period. For the purpose of reducing the consolidation period of foundation soil an overload should be added, though the loading would be 7.30m high to fulfill the bearing capacity of 100Kpa. Obviously, this technique would bear a tremendous number of loading and unloading, which would also mean a high construction cost. The stability of side slope would also be difficult to monitor.Based on our previous experience of soft soil improvement of China coastal and Hiep Phuoc Power Plant in Nha Be, 15km Southeast of Hochiminh City, the pre-loading method is appropriate only for general area. In process area wesuggest to adopted the following method (details see drawings “ Proposed Process Area Plan ” and “ Proposed Process Area Zone ” in page 6, 7):f = 50 Kpa, vacuum precompression method;f = 75 Kpa, vacuum precompression and filling water (2.0m) on membrane method;f = 100 Kpa, vacuum precompression compound with loading method.The hydraulic filling of sea sand should be adopted under the elevation +2.80, which is specified in the tender documents. But based on our siteinvestigation report, we propose to use sweet water sand instead of sea sand for these following reasons:(1)Fine grade and high mud content of the sea sand;(2)No local facilities available such as hydraulic filling mud absorption.The backfill material (sweet water sand) will be delivered by barges to the nearest dock and then carried by trucks to the site.3.2 Monitoring devicesAccording to the requirement of the tender documents, settlement gauge should be installed at 50mx50m grid and be measured by leveling instruments.In accordance with the requirement of the owner, the static penetration test should be carried out at the 10 specified points and the penetration instruments should be buried through the whole depth of the compressible soil layers.The compaction test of filling materials in site should be carried out with heavy compactor.4. Design philosophy to achieve the requirement of the specification for soil improvement4.1 philosophy of vacuum precompression methodThe vacuum precompression method is used to consolidate foundation soil. This method consists in laying on the surface of foundation soil a sand bed, and in installing a drainage system with wick drains. The sand bed drainline is buried and connected with vacuum system to form an air exhaust and a dewatering system. A set of airtight seal membranes is laid on the surfaceof the sand bed with its edge buried in soil. Thus, a negative pressure area within sand bed and foundation soil can be formed from air exhaust and dewaturing pumps. The vacuum method can reach a soil bearing capacity of 80KPa. Under the vacuum condition, soil is in the state of consolidating and pore water is discharged from soil under the pressure difference. The consolidation is completed when pressure of soil to be consolidated is reaching the same pressure of the drainage system.The philosophy of vacuum recompression method is that under the pressuredifference ( Pa -Pv), soil can be consolidated; Pa indicates atmospheric pressure and Pv indicates air pressure in the sand bed and the verticaldrains. In the calculation of consolidation of the foundation soil, the RENDULIC-TERZAGE Theory and BICOT Theory of Consolidation can be adopted which are the same of those adopted in loading recompression method. Thedrop of the ground water table may also accelerate the consolidation of foundation soil beside the pressure difference.The effective stress in the soil increases successively during the period ofconsolidation of the foundation soil with vacuum recompression method. Thehorizontal displacement of the soil is toward the center of theconsolidation area from the beginning of the period of vacuum recompression,which is different from the loading recompression me。