| Based on the theories of modern soil technology and soil mechanics, the author has tried to study the engineering characteristics, microstructure of soft clay, and the principle of DCM combination with vacuum dewatering to improving soft foundation. As the background of a major project in Qingdao, the author study its scheme and test using the method to improving soft foundation in this paper. The following conclusions can be drawn after the analysis.1. The maternal rock of soft clay is granite mainly in investigation area, and x-ray diffraction analysis result show that the mineral composition is quartz and felspar mostly, its content about 50～60%; and a few of chlorite and mica, about 30～40%; and farthing of amphibole, calcite, dolomite and gypsum etc. The clay minerials, like chlorite etc is from felspar weathering. The composition of grain size is mainly silt grain, about 60%; clay and sand grains each about 20%.2. The penetrability of the soft foundation is the main factor of vacuum dewatering effect. The penetrability of the soft foundation in investigation area decides the silt grain content in the soft clay; but it is inverse ratio with sand grain content, namely more sand grain, lesser the penetrability, this is different from other references, the reason is sand grain surrounded by clay grain, being isolated in soft clay changing the way of penetrating current. The penetrability is between 2.0×10-6cm/s and 8.0×10-5cm/s, roughly 2.0×10-5cm/s.3. In the soft foundation in investigation area, the relationship formulas between clay grain content (c%) and plasticity index (IP): IP=0.6601c+1.3918; clay grain content (c%) and void ratio (e): e=0.0319c+0.3281;clay grain content (c%) and water content (w%): c=0.5506w-1.1065; clay grain content (c%) and liquidity index (Il): Il=0.0248c+0.6107; water content (w) and void ratio (e): e=0.0255w+0.0186; water content (w%) and plasticity index (IP): IP=0.4184w-1.3476; coefficient of penetrability k (*10-5cm/s) and clay grain content (c%): k=0.6597c+16.271; coefficient of penetrability k (*10-5cm/s) and silt grain content (f %): k=1.1523f-43.895; coefficient of penetrability k (*10-5cm/s) and sand grain content (s %): k=-0.941s+44.754; coefficient of penetrability k (*10-5cm/s) and void ratio (e): k=5.2672e-0.3469.4. It is discovered by observed through SEM: the microstructure of the soft soil foundation is mainly composed of detrinite grains, a few of aggregation filling among the detinite grains; flock on the grains surface is clay mineral formed from felspar weathered. The flock reduces the penetrability of soft soil foundation, pore water cannot be discharged and the over-pore water pressure formed when the soft soil foundation is tamped. Compared tamped soil microstructure with un-tamped soil, the grains in tamped soil microstructure is with more compact and more sequence; the macro-pore have reduced distinctly and micro-pore increased, but the whole pore volume have reduced consumedly, the macro-grain margin have broken distinctly and edges and corners smooth in tamped soil.5. It is found by researched for the first time that: On level, farther from tamping point with littler increment of pore water pressure, pore water pressure is affected little by the tamping energy when the inspect point is far to 10 meters from tamping point; on vertical, the increment of pore water pressure is same roughly on different depth within a certain scope of depth (less than 6m commonly); but to excess a certain scope of depth (7m commonly), the increment of pore water pressure is minished with depth increase. On the same conditions, the over-pore water pressure peak value is higher and dissipate for a longer in soil structure destroyed compare to soil structure un-detroyed. Different tamping energy affect pore water pressure more evidently when tamping times less than 8; due to excess 8 times, pore water pressures increase to a extremum gently. This research result may become to certain tamping times each over on the fact engineering.6. It is in a creative way to save the problem of over-pore water pressure dissipating quickly with vacuum dewater method, namely using power vacuum pump to draw away groundwater, forming some vacuum and negative pressure to enhance effective stress among the clay grains drawing the grains each other, in order to achieve soft foundation consolidation quickly. Showing through test on the effect of same tamping energy: over-pore water pressure dissipating spend for over 8～10d when no setting any dewater system; for 8～10d when setting prefabricated strip drain dewater system; only 1～2d when setting vacuum dewater system in the soft soil foundation. So, it may shorten the time of over-pore water pressure dissipating when setting vacuum dewater system in the soft soil foundation, and the effect of shortening time limit for a project is very obviously.7. DCM combination with vacuum dewatering accords with the principle of soil consolidation. It is coupling static and dynamical to make soft clay to penetrate and consolidate. It is a very complicated and interesting to evaluate the consolidation. Normally combination the pore developing model when the soil don't dewater, with Terzaghi's or Biot's combination theory to evaluate pore water pressure progress course quantificationally. While, the dynamic pore water pressure developing is effected by static pore water pressure and their dissipating are different consolidation process.8. To study DCM to improve soft foundation starts on just now in our country. By reading published thesis, the method mostly combines with prefabricated strip drain or sand drain dewatering. It is good of the sand drain dewatering effect on static pressure consolidation, but the sand drain is easy extrusion distortion when tamping process to reduce dewatering effect. It is first time DCM combination with vacuum dewatering to improving soft foundation, and experimenting success in Qingdao major oil refining project. The inspecting results are totally satisfying to foundation design. It is imaginative and abundance the methods to improving soft foundation for DCM combination with vacuum dewatering success to apply. On economics, DCM combination with vacuum dewatering to improving soft foundation save the cost for 10～20% and 2/3 time limit for a project compared with traditional methods. Because it has the advantages of lower cost and rapidness, DCM combination with vacuum dewatering to improving soft foundation should apply in engineering widely.
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