4.7　Preloading

4.7.1　The preloading test shall be conducted on site before the construction and the construction parameters shall be determined according to test results.

4.7.2　The construction of horizontal drainage sand blanket shall meet the following requirements：

1　The cushion material should be medium or coarse sands，of which the silt content shall be less than 5%；

2　The dry density of cushion material shall be greater than 1.5g/cm3；

3　The preloading zone should be installed with blind drains or drainage pipes connected with sand cushion.

4.7.3　The construction of vertical drains shall meet the following requirements：

1　The materail of sand drain should be medium sand or coarse sand，of which the silt content shall be less than 3%；the actual amount of sand filled in sand drain shall not be less than 95% of the calculated value；

2　The length of sand bag or plastic drainage strip buried in sand cushion shall not be less than 500mm；the horizontal tolerance of drain spacings shall not be greater than drain diameter and the verticality tolerance should be less than 1.5%；after pipe or mandrel withdrawing，the length of sand bag or plastic drainage strip on drains shall not be greater than 500mm，the number of back-drawing of drains shall not be greater than 5%of the total number；

3　Where the plastic drainage strip is extended，the connecting type of plain overlapping of core plate in filter membrane shall be used and the overlap length shall be greater than 200mm.

4.7.4　During construction by fill preloading，the loads shall be applied by stages according to the design requirements.Duirng loading process，such items as the vertical deformation，horizontal displacement and pore water pressure shall be monitored everyday and the loading rate shall be controlled according to the monitoring data.

4.7.5　The construction with vacuum preloading method shall meet the following requirements：

1　The site shall be divided into different improving zonesaccording to the site size，shape and construction ability；each improving zone shall be covered with monoblock of sealing film；

2　The evacuating equipment for vacuum preloading should be jet vacuum pump which shall be able to obtain the vacuum suction above 95kPa during empty pumping and the number of pumps shall be determined according to the improving area，soil layer property，etc.；

3　The connection joints of vacuum line shall be sealed；check valves and gate valves shall be installed in vacuum line；filter pipe shall be intalled within sand blanket and shall be covered with a sand layer of 100mm-200mm in thickness；

4　The flat lapping with double heat sealing seam should be adopted for heat bonding of sealing film and the lap width shall be greater than 15mm；more than two layers of sealing films shall be laid and periphery of the cover film shall be sealed with such methods as trenching，folding and spreading，spreading in horizontal direction and pressing side with clay，covering the trench and the film with water；

5　Where a permeable layer with sufficient water supply or air-permeable layer with obvious outcrop occurs within the treatment area，a closed water cutoff wall shall be used to cut off the permeable layer or air-permeable layer；

6　The construction site shall be provided with continuous power supply；if the actually measured settlement rate for 5 consecutive days is less than or equal to 2mm/d or meets the design requirements，vacuum pumping may be stopped.

4.7.6　During construction by combined vacuum-fill preloading，vacuum pumping shall be conducted first，and the staged-loading shall be conducted after the vacuum pressure meets the design requirements and is stable，and the loading rate shall be controlled according to changes of displacement and pore water pressure.

4.7.7　The construction inspection of fill preloading shall meet the following requirements：

1　The construction quality inspection of vertical drains includes material quality，settlement rate，position，insertion depth and embedment depth in the sand blanket as well as back-drawing length and number of inserted plastic drainage strips etc.；the sand of sand drains must be sampled and then be subjected to particle size analysis and permeability test；

2　The test units of horizontal sand drain blanket shall be determined according to construction zones or one test unit for each 10000m2 of improving area；the test quantity of sands for each test unit shall not be less than 3 groups；

3　The fill height tolerance of staged loads shall not be greater than 5%of the equivalent design height of the corresponding stage，and the final loading height shall not be less than the equivalent height of total design load；

4　The unit weight and density inspection shall be conducted on site upon completion of each staged fillloading，and the test quantity should be 3 points for each group composed of 1000m2；

5　The loading height shall be inspected at one point every 25m2.

4.7.8　The construction inspection of vacuum preloadingshall meet the following requirements：

1　The inspection of vertical drains，horizontal sand drainageblanket and treatment effect shall meet those specified in Article 4.7.7 of this code；

2　The vacuum degree can be classified into vacuum degree in vacuum pipe and vacuum degree under-membrance.The monitoring area of each under-membrance vacuum degree test probe should be 1000m2-2000m2；

3　During vacuumizing period，vacuum degree in vacuum pipe shall be greater than 90kPa and under-membrance vacuum degree should be greater than 80kPa.