4.8　Vibroflotation

4.8.1　The vibroflotation test shall be conducted on site before the construction to determine various construction parameters such as water pressure，vibro-compaction current and reservation time of vibration.

4.8.2　The construction of vibroflotation replacement shall meet the following requirements：

1　The water pressure may be 200kPa-600kPa，water flow rate may be 200L/min-600L/min and penetration rate should be 0.5m/min-2.0m/min；

2　After the steady current reaches dense current value，the vibration should be kept for 30s，and the vibroflotation device should be lifted for 300mm-500mm.The filling height for each lift should not be greater than 500mm；

3　The construction should proceed from cenetr to periphery or by alternate installation sequence；where there are existing buildings（structures）or pipelines nearby the improving area，the construction shall start from the side near buildings outwards gradually；

4　The construction site shall include mud drainage ditches and settling tank for mud disposal.

4.8.3　The vibroflotation compaction construction shall meet the following requirements：

1　The high-power vibroflotation device should be adopted for vibroflotation compaction，and the sinking should be fast.The probe penetration rate should be 8m/min-10m/min，and each lifting height should be within 500mm and vibro-compaction time per m should be 1min；

2　For silty and fine sand soils，the vibroflotation compaction construction may be conducted by two point resonance method.The reservation time of vibration should be 10s-20s；the probe penetration rate and lifting rate should be 1.0m/min-1.5m/min；the water pressure should be 100kPa-200kPa，and each lifting height should be 500mm；

3　The construction should proceed from periphery or both sides to the center.

4.8.4　The quality inspection of vibrofloatation shall meet the following requirements：

1　The allowable tolerance for plan position of vibroflotation hole shall not be greater than 0.2 times the column diameter and the verticality tolerance shall not be greater than 1/100；

2　The quality inspection shall be conducted a certain time after construction；for clayey soils，silty soils and granular soils，the interval time shall be no less than 21 days，14 days and 7 days，respectively；

3　The column itself shall be inspected through dynamic penetration test；the inter-column soil should be inspected by standard penetration，static penetration，dynamic penetration or other in-situ tests，and the test position shall be in the center of equilateral triangle or square；the test quantityshall not be less than 2%of total number of column and shall not be less than 5.