4.10　Soil-cement Mixed Pile

4.10.1　At least 2 technique test piles shall be installed and tested before the construction.

4.10.2　The construction by single-axis or double-axis cement-soil mixing method shall meet the following requirements：

1　The construction depth should not be greater than 18m；the mixing pile rack shall be installed and leveled；the verticality tolerance of guide frame shall be less than 1/150；the pile position tolerance shall not be greater than 50mm；the pile diameter and pile length shall not be less than design values.

2　The water-cement ratio of grout for single-axis and double-axis soil-cement mixing pile should be 0.55-0.65；prepared grout shall not segregate；pumping shall be continuous；and automatic pressure flow recorder shall be used；

3　Two guniting and three mixing technique shall be used for double-axis soil-cement mixing piles；additional mixing time is required in coarse sand and gravelly sand；the guniting，mixing and lifting rate of drilling bit should not be greater than 0.5m/min，and the mixing and penetration rare of drilling bit should not be greater than 1.0m/min；the lifting（or penetration）amount for each circle of rotation of drilling bit should be 10mm-15mm；the volume of mixing for single equipment within 24h shall not be greater than 100m3；

4　The flow pump should be used to control the grout flow rate during construction，and the outlet pressure of grouting pump shall be kept at 0.40MPa-0.60MPa and the grout flow rate shall be kept constant；

5　After mixing and sinking of drilling bit to the pre-determined elevation，the drill pipe shall be lifted after guniting and mixing for 30s.

4.10.3　The construction by three-axis cement-soil mixing method shall meet the following requirements：

1　The drilling rod extending technique should be adopted for mixing piles with construction depth greater than 30m，the rack greater than 30m shall be provided with stability measures，and the verticality tolerance of guide frame shall not be greater than 1/250；

2　The water-cement ratio of cement grout for three-axis soil-cement mixing piles should be 1.5-2.0；prepared grout shall not segregate；pumping shall be continuous and automatic pressure flow recorder shall be used；

3　The mixing and penetration rate should be 0.5m/min-1.0m/min；lifting rate should be 1m/min-2m/min，and the penetration or lifting shall be kept with uniform rate；

4　The construction may conducted by piling every other pile，proceeding from one side or predrilling methods；for dense soils where difficult penetration may occur，pre-drilling method shall be used；

5　Low mixing amount shall be used for the disturbed zone above the proposed improving zone；

6　For projects with high environmental protection requirements，three-axis mixing piles shall be used and the construction parameters shall be adjusted according to test pile results and the monitoring data.Where close to the protected objects，the mixing and penetration rate should be 0.5m/min-0.8m/min and the lifting rate should be within 1.0m/min.The guniting pressure should not be greater than 0.8MPa；

7　The flow pump should be used to control the grout flow rate during construction，and the outlet pressure of grouting pump should be kept at 0.4MPa-0.6MPa，and the mixing and lift rate shall match the grout flow rate.

4.10.4　During construction of soil-cement mixed piles，the final grouting surface shall be 300mm-500mm higher than the design pile top elevation.During foundation pit excavation，the top laitance section shall be removed manually.

4.10.5　If the grouting is stopped accidently during construction，the drilling bit shall be sunk to 0.5m below the grouting stop point and guniting，mixing and lifting shall proceed once the grouting supply is restored or the drilling bit shall be raised to 0.5m above the grouting stop point and guniting，mixing and sinking shall proceed once the grouting supply is restored.