4.5　Dynamic Consolidation

4.5.1　The trial tamping shall be conducted at representative sites before the construction.For each different engineering geological unit，the number of trial tamping area shall not be less than 1 and the trial tamping area shall not be smaller than 20m×20m.

4.5.2　Where there exist buildings（structures）and underground pipelines are sensitive to vibration or with special requirements nearby，the dynamic consolidation should not be used.

4.5.3　The selection of main equipment，tools and devices for construction by dynamic consolidation shall meet the following requirements：

1　Crane：crawler crane or other dedicated devices with automatic trip mechanism and matched with hammer mass and drop distance shall be selected according to the energy level of dynamic consolidation required in the design；measures to prevent rack from overturning shall be taken for high level dynamic consolidation；

2　Hammer：the bottom surface of hammer should be round，and 4-6 exhaust holes should be arranged uniformly at hammer bottom；the hammer for dynamic replacement should have exhaust duct around，and the static pressure of hammer for dynamic consolidation should be 20kPa-80kPa and that for dynamic replacement should be 100kPa-300kPa；

3　Automatic trip mechanism：it shall possess sufficient strength and durability，construction flexibility and easy accessibility.

4.5.4　The dynamic consolidation construction shall meet the following requirements：

1　The tamping point position and its contour line shall be marked up before tamping，and the ground elevation shall be surveyed before and after tamping，and the tamping settlement per tamping on each point shall be calculated；

2　After each pass of tamping，the tamping craters shall be filled and levelled or bulldozed，and the site elevation shall be surveyed to calculate the site tamping settlement for this tamping pass；

3　After full compacting passes are completed，the loose soil on the surface layer shall be compacted and rolled with low energy tamping following the tamping rule of lapping 1/5-1/3 of hammer diameter，and the site elevation after dynamic consolidation shall be surveyed；

4　The dynamic consolidation shall be conducted by areas，from border area to middle part or from the side of adjacent buildings（structures）to the side away.

4.5.5　The construction of dynamic replacement shall meet the following requirements：

1　The material of dynamic replacement pier should be coarse particle materials with hard texture and stable performance such as well-graded block stone，gravel and slag，and the content of particle with the particle size greater than 300mm should not be greater than 30%by weight；

2　The tamping rule at tamping grids should be interlaced and alternately tamping from inside out；

3　The site elevation shall be surveyed after each tamping pass and uplifting amount of site for each tamping pass shall be calculated，and the part of uplifting amount exceeding design elevation should be timely eliminated.

4.5.6　For soft soil area and area with shallow groundwater depth，the construction by dewatering in conjunction with low energy dynamic consolidation shall meet the following requirements：

1　The dewatering and drainage system shall be installed first before the construction of dynamic consolidation；the vacuum well point dewatering system is recommended.The peripheral closed well points should be installed 3m-4m outside the improved area；

2　The dewatering system in tamping area shall be removed at least 2 days after the groundwater level lowers to design water level and is stable；

3　The low energy dynamic consolidation shall follow the rule of tamping lightly first and then heavily with multiple passes；

4　The waiting period for each pass of dynamic consolidation should be determined based on the criteria that the excess pore water pressure dissipation is not less than 80%；

5　The construction site with shallow groundwater depth should be installed with drainage network，and the maximum spacing of drainage network should not be greater than 15m.

4.5.7　The ponded water in tamping craters or on site shall be drained timely during construction in rainy season.

4.5.8　The following measures shall be taken for construction in winter.

1　The construction of dynamic consolidation shall be conducted only after frozen soil is removed；

2　The construction of local compaction may be conducted when the minimum air temperature is higher than-15℃and the frozen depth is within 800mm，and the energy level and tamping pass number for local compaction shall be increased properly；the full site tamping shall be conducted after thawing，and the energy level for full site tamping shall be increased properly；

3　The ground with finished dynamic consolidation construction shall be protected with covering layer before winter.

4.5.9　For dynamic replacement，the replacement pier bottom depth shall be inspected；for dewatering in conjunction with low energy level dynamic consolidation，the dynamic changes of groundwater level shall be monitored.The allowable tolerance for construction quality of dynamic consolidation shall be in accordance with those specified in Table 4.5.9.

4.5.10　The interval time for quality inspection after construction of dynamic consolidation：should not be less than 7 days，14 days and 28 days for granular soils，silty soilsand cohesive soils，respectively.For dynamic replacement and dewatering in conjunction with low energy level dynamic consolidation，a minimum waiting period of 28 days is recommended.