Miao Xinli

(Henan Shengda Construction Co.,LTD,Zhengzhou,Henan 450046)

Abstract:In recent years,with the rapid development of social economy and construction industry,the soil foundation of construction engineering has become one of the most concerned problems in engineering construction.As a difficult and high-tech infrastructure construction project,construction project plays an extremely important role in engineering construction.The dynamic consolidation method in the soil foundation reinforcement treatment in construction engineering has a good effect in practical application,which can effectively improve the bearing capacity of land foundation,reduce the uneven settlement of land foundation,play an important role in saving construction resources,reducing engineering cost and so on,and has received an obvious social effect,therefore,the research on its application has great significance.Based on this,this paper mainly discusses the dynamic consolidation in the soil foundation reinforcement treatment.

Key words:construction engineering;soil foundation reinforcement treatment;dynamic consolidation

The dynamic consolidation in the soil foundation reinforcement treatment is widely used because of its advantages of simple equipment,convenient construction,wide application scope and material saving.Dynamic consolidation is an effective method to increase the bearing capacity of soft soil foundation.This method mainly USES heavy hammer to ram the soil layer from the corresponding height to achieve the effect of rapid consolidation of soil.This method is also called power-solidifying method,its operating principle is to lift the weight hammer (10t to 40t)to a height of 10m to 40m through lifting machinery,and let the weight hammer fall on its own.Under the influence of ramming force and shock wave,the soil layer is compacted.

I.Overview of Dynamic Consolidation

Dynamic consolidation is an effective technology to increase the bearing capacity of soft soil foundation.It is a method of using heavy hammer to ram the soil layer from the corresponding height to achieve the effect of rapid consolidation of soil.This method is also called power-solidifying method,which usually uses lifting equipment to lift giant rammers weighing between 10t and 40t,allows them to fall freely on the ground at a height of 10m to 40m,therefore,a strong vibration and impact force are generated to tamp the soil layer.The stress wave generated by the rammer around the ramming pit makes the soil foundation more compact and strengthens the soil foundation[1].This method is often used in the construction of sandy soil,unsaturated clay soil and miscellaneous soil foundation.Continuous ramming and intermittent compaction are the main construction methods of unsaturated clay foundation.The number and depth of compaction can be determined accurately by field test.In general,the ramming energy is 100t to 200t,and the effective depth of ramming can be controlled from 3m to 6m.If the soil with high moisture content is strengthened by dynamic consolidation,it will be difficult to control the compaction position due to the large amount of settlement,thus increasing the difficulty of consolidation treatment of the soil foundation[2].

II.Design of Dynamic Consolidation

(i)Number of Dynamic Consolidation

According to the use requirements of the building and the nature of the soil foundation,the number of times of compaction is determined,usually 2～3 times of compaction,and the final compaction times are determined according to the maximum amount of compaction in the ramming pit and the minimum amount of uplift around the ramming pit[3].In addition to determining the final compaction according to the relationship curve between the number of tamping attempts and the amount of tamping,it is also necessary to ensure that the ground around the tamping pit does not produce a large uplift and that the tamping pit is too deep for hammer-lifting difficulties.

(ii)Ramming Point Spacing

It should be determined comprehensively according to the structure type,ground water and ground soil condition of the building,generally 1.5～2.0 times of the diameter of the rammer.If the ramming energy is relatively large,the spacing of ramming points can be appropriately increased.

(iii)Interval Time of Dynamic Consolidation

The interval time of dynamic consolidation refers to the interval time between two compacts,which can dissipate the excess pore water pressure in the soil[4].In general,the interval time between two ramming operations should be more than one week (except the soil filling based on stone materials);For the clay soil foundation with poor permeability and high water content,the interval time should be greater than 3～4 weeks,so that the excess pore water pressure dissipation can reach more than 85%.If the construction unit has relevant experience,the interval time will also can be shortened.

(iv)Reinforcing Depth and Best Ramming Energy

Sensitive parameters are the important basis of foundation consolidation treatment scheme.Although dynamic consolidation method can effectively consolidate the depth,the effective consolidation depth should be determined according to local engineering experience or field trial compaction.In theory,the consolidation energy which makes the pore water pressure of soil foundation reach the gravity stress of soil is called the best consolidation energy.If the increment of pore water pressure becomes stable with the increase of the times of consolidation,the consolidation energy can be called the best consolidation energy.In addition,factors such as structure type of soil foundation,required depth of treatment and load size of the proposed building can be considered comprehensively,and the final determination can be made through field trial compaction[5].

III.Main Points and Notices of Construction Technology Control of Dynamic Consolidation Method

Leveling the site,set the drainage channel or lower the site groundwater in advance according to the distribution characteristics of the site groundwater in the survey report;according to the construction sequence,construction is carried out from the edge to the middle section[6].During the consolidation,in order to guarantee the quality of consolidation,the consolidation parameters must be followed,and the consolidation pit must be filled with new soil;in the construction of dynamic compaction,the rammer should be let fall freely,and the vibration generated by impact will affect the surrounding buildings.The soil quality of the soil foundation determines the intensity of vibration,the consolidation sequence should be moved gradually from the adjacent buildings to the distance,in order to reduce consolidation damage,isolation ditch can be set around;the water content of backfill should be controlled within the range of the optimal water content.If the water content is lower than the optimal water content,drilling irrigation or sprinkling infiltration should be adopted.During heavy compaction in rainy days,measures should be taken to prevent the surrounding rain water from flowing into the site,such as setting flood intercepting ditch,drainage ditch,etc.Make a record of the construction process,and check the falling distance and quality of the rammer,the position of the consolidation point before consolidation,the location of the consolidation pit,the number of consolidation times and the measured elevation data;according to the design arrangement of consolidation point,line setting and ramming should be carried out strictly,and construction should be carried out according to the interval time,number of consolidation times,number of consolidation strikes,falling distance,hammer weight and other parameters determined by test consolidation;manual guidance should be arranged in the construction,and the position of the crane should be adjusted at any time when the crane is fully compacted,so as to avoid the appearance of the sector centered on the crane.

IV.Application of Dynamic Consolidation Method in Soil Foundation Reinforcement Treatment

(i)Project Profile

The consolidation method of the soil foundation of a project is dynamic consolidation method.The construction site is a collapsible loess site with no self-weight.The soil foundation is strengthened and backfilled by the method of dynamic consolidation,adopting second time dynamic consolidation.The first time is point consolidation and the second time is full consolidation,the measuring point is plum shape.After the completion of dynamic consolidation and leveling of the site,use the chain track mechanical vibration rolling to meet the requirement of reinforcing soil foundation.

(ii)Operation Steps

The site leveling is carried out after 70cm loess is backfilled.Partition the test site,mark the first ramming point,and measure the elevation of the site.At the same time,place the crane in a reasonable position as specified,align the rammer and the consolidation point;measure the top elevation of the rammer accurately before dynamic consolidation;when the rammer rises to the corresponding height,the decoupling device will open automatically and the rammer will drop freely.At this time,put down the hook and measure the elevation of the rammer at the top.If the bottom of the pit is inclined or the rammer is inclined,the bottom of the pit should be leveled in time and the above steps should be repeated;to complete a consolidation point ramming according to the design requirements of consolidation times and control standards;move the rammer to the next consolidation point and complete the first ramming of all consolidation points;an excavator is used to fill the ramming pit and then the elevation of the site is measured.

(iii)Hammer after Single Pass Consolidation Collecting and Control Standard

Each consolidation point should be rammed more than eight times per ramming,and the average consolidation amount of the last two consolidation times should be less than 50mm (the tamping energy＜4000KN·M),ensuring that the ground around the consolidation pit cannot have a large uplift,do not appear the hammer-raising difficulties because of the deep consolidation pit.

(iv)Interval

The first consolidation interval is 7 days and the second consolidation interval is 12 days.After the construction is completed according to the operation steps,the elevation of the consolidation site is measured.

(v)Quality Inspection of Dynamic Consolidation

The main methods to test the effect of dynamic compaction include geotechnical test,standard penetration test,static load test,etc.[7].General engineering should adopt two or more inspection methods,the inspection of the project should be added to important engineering projects,and large pressure plate load test can be conducted on site.As the interval time after compaction increases,the strength of soil also increases.The test of the effect of dynamic consolidation should be carried out within 1～4 weeks after the dynamic consolidation.The test should not be conducted immediately after the completion of dynamic consolidation,otherwise the low strength data will be measured.

V.Conclusion

With the development of national economy,higher requirements are put forward for project quality.Dynamic consolidation method has the advantages of simple operation,high efficiency and wide application in construction.It can effectively improve the bearing capacity and stability.This paper analyzes and studies the dynamic consolidation method in the soil foundation reinforcement treatment for reference.At the same time,the construction quality control work should be strengthened,and timely correct various problems and deviations.Through a series of comprehensive and effective measures,the bearing capacity and stability of roadbed are guaranteed and the construction quality is improved.