多年凍土區樁基礎承載力試驗研究

張辰熙，王吉良

(黑龍江省寒地建筑科學研究院，哈爾濱150060)

0 Introdution

There are a lot of distributions of permafrost in the Inner Mongolia region.As Chinese economy continues developing，the number of building construction projects in permafrost regions is increasing，and in the use process of the buildings with the use of heating，the temperature field of the frozen soil under the buildings will continuously change because of the building heat，so the constant changes of temperature field lead to the direct changes of the physical and mechanical properties of the frozen soil under buildings in permafrost regions.Therefore，the thermodynamic equilibrium of the frozen soil will be severely damaged，which results in strength reduction of the frozen soil and a serious decline to the bearing capacity of pile foundation in permafrost regions，finally，directly resulting in the usability of the buildings［1-11］.

Through the static load test on pile foundation designed by gradually melted state，the study on carrying capacity of pile foundation was carried out in permafrost regions.The test results verified that the practicality of the construction of pile foundation in permafrost regions and provided a scientific basis for building construction in areas of permafrost［12-19］.

1 Engineering geological profile

1.1 The stratum from top to bottom of this areais

1)Quaternary holocene plant layer(Q 4pd)

①Humus soil:Black，mainly composed of cohesive soil，the thickness is from 0.50～1.20 m.

2)Quaternary holocene alluvium(Q 4al)

②Silty clay:Yellow，plastic，including sand.Natural water content is 21.1% ～25.2%，the depth of the top layer is 0.50 m～1.20 m，thickness is 0.70 m～5.20 m，permafrost table is 3.80 m～5.40 m，and the total water content is 25.6%.The characteristic value of bearing capacity in unfrozen state is fak= 120kPa.

③Silty sand:Yellow，grey，slightly dense，moist～saturated，and the main contents are quartz，feldspar and viscous soil.The granules whose diameters less than 0.075 mm account for 37.2%.The depth of the top layer is between 1.60 m and 4.40 m and the thickness is between 0.30 m and 4.20 m.Permafrost table is from 3.40 m to 5.50 m and the total water content is from 15.9%to 26.9%.The characteristic value of bearing capacity in unfrozen state is fak=145 kPa。

④Round gravel:yellow，grey，the constituent is igneous clastic，the permafrost is found in this layer，ice crystals is found in the pore occasionally，permafrost table is 4.10～7.40 m，permafrost base is 12.50～19.60 m，total water content is 12.4% ～15.3%.The thickness is not impale，and buried depth of the top of the layer is 4.10～6.50 m.The characteristic value of bearing capacity in unfrozen state is fak=300 kPa.

1.2 Groundwater

Groundwater burial condition:The groundwater type within drilling depth is quaternary pore water，which has two layers of water:one is suprapermafrost water，and the other is infrapermafrost water.With confined aquifer，it is buried in the third layer which is the silt layer and the fourth layer which is the round gravel layer.Initial groundwater depth stays between 3.30 m and 18.60 m，permanent water depth stays between 2.54 m and 3.52 m，and maximum permanent water level is 656.86 m，groundwater is directly replenished by atmospheric precipitation or lateral groundwater，and amplitude of water level variation is about 1.50 m.

1.3 Frozen soil

1)Seasonal frozen soil:the standard frost depth of the seasonal frozen soil is 3.00 meters，and the maximum seasonal frost depth is 3.20 meters.The average frost heaving ratio in frost depth of the seasonal frozen soil is η=5% ～10.5%，which is soil of frost heaving～high frost heaving，with frost heaving level III～IV.

2)Permafrost:permafrost presents an integral structure，and according to the observation results of the ground temperature，the area is in the phase of permafrost degradation which belongs to high temperature unstable permafrost.

①Type of frozen soil:The Permafrost is continuous permafrost by distribution type;According to the burial condition is non cohesive permafrost，according to the characteristics of deformation is the plastic permafrost;according to the temperature is high temperature unstable permafrost.It can be classified into icy or ice-rich permafrost according to the moisture content and the thaw collapse coefficient.

② Distribution range:From a plan view，it should be large continuous permafrost distribution area，in the exploration depth，from the vertical，it should be discontinuous permafrost，permafrost table is between 3.40～7.40 m and permafrost base is between 12.50～19.60 m.

Table 1 Permafrost ground temperature obser vation

③ Engineering features of permafrost:This permafrost is degeneration frozen soil，and is extremely unstable under high temperature.The engineering features are weak thaw collapse～thaw collapse.

2 Test outline

The design depth of pile foundation is 20.5 m，and the height above ground is 500 mm，and pile diameter is 400 mm.The pile was constructed by the technology of the long spiral pump concrete piles.Gradually frozen-soil-thawing state design principle was used in the design of pile foundation in permafrost regions，considering the force of negative friction of the soil around the pile during the test process，so the test should be carried out before the time when the thawed soil refreeze.The temperature monitoring on the soil around pile was conducted simultaneously during the test to test the soil around pile was refreezing or not;at the same time，the internal force test on pile was performed to measure friction resistance in permafrost layer，in order to accurately determine the effective carrying capacity of the pile.

3 Test plan

The tests consisted of three sets of single pile static load test，compression test using method of one pile with four anchor piles;the loading device for single pile static load test compressive was 630 t jack;test apparatus was arranged as shown in Fig.1.

Fig.1 Testing of foundation pile

4 Test process

All the tests for the study included compressive static load test of single pile，temperature monitoring on permafrost around pile，internal force test on pile.

A slow to maintain load test method was used for static load test in permafrost regions，and the test load needed to meet the design requirements of the ultimate bearing capacity，while considering the force of negative friction generated by thawing permafrost around pile.

Test monitoring and testing embedded parts were buried in the process of test piling.

Test equipment got admission to test field on June 21，2013，and in coordination with the construction company，installation and welding work was done.

S1 pile test began on June 24，2013 and ended on June 30，2013 due to the termination of the test pile head damage(Fig.4).

S2 pile test began on July 8，2013，and ended on July 13，2013 due to the termination of the test pile head damage(Fig.5).

S3 pile test began on June 25，2013，and ended on July 3，2013 due to the termination of the test pile head damage(Fig.6).

Static load test was performed also with the temperature monitoring test on permafrost around pile ground and the test on the internal force of pile.

The monitoring depth of permafrost temperature monitoring was 20 m，and a single-hole with 31 temperature measuring points.

Internal forces pile testing，each reinforcement steel of test pile was symmetrically installed with 4 groups of steel bar gauge，of 24 gauge steel.

Fig.2 Temperature monitoring

5 Test results and analysis

From the static load test results，pile permafrost temperature curve follows(Fig.3).

Fig.3 Pile permafrost temperatune curve

Test data and result curves of the compression test of single pile static load.

Fig.4 S1 single pile static load test Q-s、s-lgt curves settling volume:6.42 mm

Fig.5 S2 single pile static load test Q-s、s-lgt curves settling volume:5.26 mm

Fig.6 S3 single pile static load test Q-s、s-lgt curves settling volume:8.27 mm

Through the above three sets of test data，it can be seen that the basic parameters of the selected pile foundation designed by the gradually thawing state design method fully met the normal use requirement in Yakeshi permafrost，the Inner Mongolia region，which can be completely prove by the test results.

6 Conclusion

When the pile foundation was designed in accordance with the gradually thawing state design in permafrost，it shall to be considered that the force of negative friction cause by the thawing of the freezing-thawing soil layer around the pile foundation，and the value of force of the negative friction should be according to the current industry standard"Building Pile technical specifications"JGJ94-2008(Table 1)with access to a negative substitution.If the actual pile thawing permafrost will happen gradually，but the force of negative friction was not considered in the design，and the testing departments should consider adding negative friction carrying capacity can be carried out to detect based on the original design value of bearing capacity of pile foundation in permafrost regions.

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