EFFECT ANALYSIS OF PLASTIC DRAINAGE PLATE AND GRAVEL PILE ABOUT THE TREATMENT OF SOFT FOUNDATION’S SEDIMENTATION
Binbin WEI1，Lu LU1，Qihua ZHAO2，Sheqin PENG2
1CCCC Highway Planning and Design Consultants Co.，Ltd.，Beijing 100088
2State Key Laboratory of GeoHazard Prevention and GeoEnvironment Protection，Chengdu University of Technology，Chengdu 610059
Abstract：In this paper，a soft foundation engineering as the research object，and the treatments of soft foundation which the plastic drainage plate and gravel pile were selected respectively.Then the sedimentation rate and cumulative subsidence of the section were monitored and researched.Take the test data as the model sample value，and the hyperbola method and the S curve method were used respectively.Finally the monitoring curve were fitted and forecasted.Through calculation and analysis，the following conclusion was obtained：the plastic drainage plate in the treatment of soft foundation，drainage consolidation occurs mainly during the roadbed filling，after entering preloading period，sedimentation was basically completed，and the sedimentation of subgrade was high.While the gravel pile in the treatment of soft foundation，embankment sedimentation occurred both in the filling period and preloading period，and the sedimentation is small relative to the former.These two methods have their own advantages，choosing it depended on the engineering geological and construction conditions.From the view point of difficulty degree of cost and quality control，the plastic drainage plate was mastered easily，and the cost was relatively low.
Keywords：the treatment of soft foundation，plastic drainage plate，gravel pile，sedimentation rate，cumulative subsidence
Email：582331065@qq.com
1 Introduction
Soft soil is widely distributed in China’s coastal plains，river delta，lake basins and mountain stream valley around.Build a railway or highway on soft soil foundation，especially the construction of high fill embankment.Due to consolidation and shear deformation，the foundation will produce a lot of sedimentation and differential sedimentation，and the subsidence continues for a long time.The part of reason of soft foundation sedimentation is the foundation consolidation；another part of the reason is lateral deformation of the foundation[1].If treatment is not taken or improper，it will result in the instability of the subgrade or excessive sedimentation，and affect the quality of railway or highway and the progress of the whole project directly.
Subsidence is the main problem to be resolved in soft foundation treatment.Selecting a specific treatment scheme combine the actual conditions.The soft foundation treatment method as following：drainage consolidation method，compaction pile reinforcement method，replacement method，the dynamic compaction method，the pressure to protect the humanitarian law，etc[1].Plastic drain board method[2-6]and gravel pile method[6-10]are respectively the typical representative of the drainage consolidation method and the compaction pile reinforcement method，and they are two of the most commonly used method in soft foundation treatment，they have their use range，advantages and disadvantages.The basic principle of the drainage consolidation method is that compacting the foundation by the embankment load weight，at the same time，using the vertical and horizontal drainage channel of the artificial soil layer shorten the distance of drainage to accelerate the consolidation of the foundation.The basic principle of the compaction pile reinforcement method is that adding pile in soft soil foundation by construction equipment，the pile compact the soft soil and form the composite foundation with natural foundation，then enhancing the overall strength of the soft soil foundation，so as to achieve the purpose of strengthening.
Through the engineering example of a soft foundation，the paper selected two treatment of soft foundation namely the plastic drainage plate and gravel pile respectively，then monitored and researched the sedimentation rate and cumulative subsidence of the section.Take the test data as the model sample value，then fitted and forecasted the monitoring curve according to the hyperbola method[11-14]and the S curve method[15-18]respectively.Analyzed effect and the choosing method of plastic drainage plate and the gravel pile in the treatment of soft foundation.
2 Engineering Situation
2.1 Geology
The study area is located in the central of hilly region in Sichuan Basin，the overall terrain present high north west region and low south east region.Geomorphic type is mainly divided into two types：tectonic denudation terrain and erosion accumulation terrain.Vast hilly terrain is formed by the tectonic denudation，and what scattered in the valley terrace is caused by the erosion uplift after accumulation.In the tectonic denudation terrain，the landscape which is domed wide valley-middle-sized hill.Formation litho logy is the layer between sandstone and mudstone.Topography is wide；the valley is flat and open，the top of the hill is rounded and smooth.There are stair-step shapes on the hill slope so that it presenting the tower or bonnet terrain and beaded distribution in plane.The valley slope is relatively slow which generally 15°-20°.
With the landform corresponding，there are a variety of genetic types in quaternary strata in the area，such as quaternary holocene planting soil（
），fill（
），marsh sediments（
）and diluvium（
），the diluvia layer（
），etc.Underlying bedrock is composed of Mesozoic Jurassic system in Penglai town（
）and Suining group（
）.
2.2 Development regularities of soft ground
According to the tectonic movement in Sichuan Province and big landform conditions combined with the natural geographical environment，the researching area belongs to mountainous regions Ⅲ3-basin tectonic denudation mountain hilly region in uplift subsidence basin in eastern Sichuan[19].As for the origin and properties，deposited soft soil in the mountainous and hilly terrain is different from others in general plain and coastal area，it referred to as “valleys” type soft soil commonly.Due to the control of the topography and exposed position of the parent rock，soft soil are mostly distributed in gully，valley，river terraces，and all kinds of depressions[20].Based on the causes and properties of the soft ground in the study area，the author divided it into three categories：wide valleys，narrow valleys，intermountain depression（Table 1）.
Table 1 Development and distribution characteristics of soft ground plane in study area
The buried depth of soft soil in researching area is commonly 2-3 m，the thickness of the soft foundation is 3-10 m，and local thickness is up to 13 m.According to the investigation，from top to bottom the surface shallow soft soil in the study area has the following bedding structure[21].The first layer is the topsoil，such as pool or paddy fields，the underground water buried depth is usually 1-3 m；The second layer is soft soil，divided into three categories：quaternary holocene marsh sediments and low liquid limit clay（thickness of 2-6 m），quaternary Holocene sediments swamp high liquid limit clay（7-9 m thickness），ponds or rice paddies mud（thickness of 7-10 m）；The third layer is the substrate，its litho logy is a more compact mudstone or sand layer（Figure 1）.
Figure 1 Schematic diagram of soft bedding structure in study area
3 Research Method
The paper researched the subsidence monitoring of the section with two treatment of soft foundation namely the plastic drainage plate and gravel pile，then compared the fitted and forecasted monitoring curve according to the hyperbola method and the S curve method respectively.Through the calculation of sedimentation monitoring and prediction，calculating the soft ground sedimentation rate and cumulative displacement after dealing with the plastic drainage plate and gravel pile.Analyzed effect and the choosing method of plastic drainage plate and the gravel pile in the treatment of soft foundation.
4 Monitoring Results and Analysis
4.1 Section treatment by plastic drainage plate
In the section treatment of soft foundation by the plastic drainage plate，the sectionⅠ was selected for research.The original foundation of section Ⅰ was soft foundation，soil was low liquid limit clay，the spacing between plastic drainage plate was 1.6 m，designed fill height was 7.2 m，the roadbed was backfilled 2.0 m already when embedding observation sedimentation plate.The distance of sedimentation plate bottom to the original place was 1.5 m，so the main observation was the original foundation’s sedimentation.The observation duration was 108 days，the cumulative filled height was 5.2 m，the intervallic observation time and subsidence data were shown in the following table（Table 2）.Take the test data as the model sample value，data from a total of 290 days（from 110 to 290 d）were predicted，then the hyperbola method and the method of S curve were used to fitting predicted curve，as shown in Figure 2.
Table 2 Observed subsidence data of sectionⅠ（Plastic drainage plate）
Figure 2 Measured curve and fitting predicted one by each model of sectionⅠ
4.2 Section treatment by gravel pile
In the section treatment of soft foundation by the gravel pile，the section Ⅱ was selected for research.The original foundation of sectionⅡwas soft foundation，soil was low liquid limit clay，the spacing between gravel pile was 1.6 m，designed fill height was 17.9 m，the roadbed was backfilled 5.5 m already when embedding observation sedimentation plate，thus it led to a smaller observed total subsidence in the late.The distance of sedimentation plate bottom to the original place was 1.8 m，so the main observation was the original foundation’s sedimentation.The observation duration was 280 days，the cumulative filled height was 16.25 m，the intervallic observation time and subsidence data were shown in the following table（Table 3）.
Table 3 Observed subsidence data of sectionⅡ（Gravel piles）
Take the test data as the model sample value，data from a total of 180 days（from 285 to 465 d）were predicted，then the hyperbola method and the method of S curve were used to fitting predicted curve，as shown in Figure 3.
Figure 3 Measured curve and fitting predicted one by each model of sectionⅡ
5 Discussion
（1）Through monitoring and analysis on the measurement of soft foundation’s section by the plastic drainage plate，it is showed that，under the limiting filling height，the sedimentation rate was 0.58-0.01 mm/d，within 108 days of monitoring；the cumulative sedimentation reached 183mm.
（2）Through monitoring and analysis on the measurement of soft foundation’s section by the gravel pile，it is showed that，under the limiting filling height，the sedimentation rate was 0.15-0.01 mm/d，within 280 days of monitoring；the cumulative sedimentation reached 115mm.
（3）The plastic drainage plate in the treatment of soft foundation，drainage consolidation occurs mainly during the roadbed filling，after entering preloading period，sedimentation was basically completed，and the sedimentation of subgrade was high.While the gravel pile in the treatment of soft foundation，embankment sedimentation occurred both in the filling period and preloading period，and the sedimentation is small relative to the former.
6 Conclusions
The plastic drainage plate and the gravel pile，these two methods have their own advantages，it should not be understood as a whole，and thus choosing a method depended on the engineering geological and construction conditions.From the view point of difficulty degree of cost and quality control，the plastic drainage plate was mastered easily，and the cost was relatively low.For normally consolidated soft clay foundation，the drainage consolidation method is the most economic and reasonable.
Acknowledgement
In this paper，the referenced relevant information come from theresearch group of the professor Zhao Qihua which the author participatedin during the period of a study for a master’s research in State Key Laboratory of GeoHazard Prevention and GeoEnvironment Protection.
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