불포화토 삼축압축시험기 Unsaturated Triaxial Testing of Soil

불포화토 삼축압축시험기 Unsaturated Triaxial Testing of Soil

영신컨설턴트 (02) 529 8803

 

Ua : 공기압  Uw: 수압

불포화토 삼축시험은 지하수위 위 지반의 흙수분 상태에 따른 삼축시험이고 현 시험기를 불포화토 시험기로 변경할 수 있다.

시료의 체적변화를 측정하는 방법에 따라 4종류

• method A: GDS pore air pressure/volume controller로 직접 공기체적 측정방법

• method B: HKUST inner cell  공기압으로 셀 압력을 가하고 내부셀과 외부셀이 연결 압력 동일 

• method C: double cell  2개의 내부셀과 외부셀이 격리되여 압력이 다르다.

• method D: 미소변형측정 센서를 시료에 부착하여 시료의 체적변화를 직접측정

 

High-air-entry porous disk

 

 

 

GDSLAB 4D UNSAT software module

 

4차원 시료내부 공기압과 수압, 구속압과 연직압력 제어.

 

 

시료의 공기압과 수압 제어 시험

 

• desaturation ramps 불포화 시험

• soil-water characteristic curve 흙수분 곡선

• drained test saturated conditions 포화토 배수시험

• drained test unsaturated conditions 불포화토 배수

 

 

 

 

 

 

 

 

 

Method A  GDS pore air pressure/volume controller로 시료 공기 체적 직접측정

 

 

GDS 1000cc/2MPa digital pressure/volume controller를 시료상단에 연결하여 공기압과 체적변화 측정

GDS advanced 200cc/2MPa digital pressure/volume controller를 시료하단에 연결하여 물의 압력과 배수량 측정

 

상단의 공기압은 하단의 수압보다 커야 한다.

시료상단은 다공질판을 사용하고 시료하단은 물만 통과되고 공기가 통과되지 않은 세라믹을 사용한다.

공기압과 수압을 조절하여 matric suction을 유발한다.

 

 

 

 

 

The advanced 2MPa/1000cc air pressure/volume controllers.

공기압력 조절기  2MPa with regulation to 1kPa. 체적 1000cc with regulation to 1cu mm (i.e. 0.001cc).

 

Method A technical specification

Advanced 2MPa/1000cc air pressure/volume controllers

• Pressure ranges: 2MPa

• Volumetric capacity (nominal): 1000cc

• Resolution of measurement and control: pressure = <0.1% full range, volume = 0.5cu mm

• Accuracy of measurement: pressure = <0.1% full range, volume = 0.25%

Items required for Method A UNSAT upgrade

• Pedestal with bonded HAEPD

• GDS 2MPa/1000cc air pressure/volume controller

• GDSLAB 4D UNSAT software module

Optional Items required for Method A UNSAT upgrade

• Local strain (Hall Effect or LVDT)

• Atmospheric air pressure transducer

• Access ring for triaxial cell

 

Method B –HKUST inner cell volume measurement

The HKUST (Hong Kong University of Science and Technology) volume change measurement method involves measuring the cell volume displaced by the sample in an inner cell within the main triaxial cell (see Fig. 6).

 

 

 

Method B technical specification

• DPT range: +/- 1kPa (+/- 10cm of water head)

• DPT accuracy: <0.1% FSO

• Theoretical resolution of volume change measurement (16 bit resolution): <2cu mm

• Accuracy of volume change measurement: estimated at 32mm3 or 0.04% volumetric strain for a triaxial specimen 38mm x 76mm

 

Items required for HKUST UNSAT upgrade

• HKUST pedestal with bonded HAEPD

• HKUST pedestal with bonded HAEPD

• High accuracy, low range DPT

• GDSLAB 4D UNSAT software module

• Dual channel pneumatic controller (laboratory air supply or compressor required)

• Cell access ring

 

Method C –double cell volume measurement

 

Fig. 9 Schematic of double cell within a GDS Bishop and Wesley triaxial cell	Fig. 10 Independent application of internal and external cell ressures. Volume reading from inner cell pressure/volume controller equates to ample volume change.
	Fig. 11 Application of internal and external cell pressures using single pressure device. Volume change measured using external volume change gauge.

 

Method C technical specification

Advanced 2MPa/1000cc air pressure/volume controllers

• Resolution of measurement and control: pressure = <0.1% full range, volume = 0.5cu mm

• Accuracy of measurement: pressure = <0.1% full range, volume = 0.25%

Items required for Method C UNSAT upgrade

• GDS double cell

• Pedestal with bonded HAEPD

• GDS pressure/volume controllers to suit

• GDSLAB 4D UNSAT software module

Optional Items required for Method C UNSAT upgrade

• Local strain (Hall Effect or LVDT)

• Atmospheric air pressure transducer

• Access ring for triaxial cell

 

 

체적변화 측정방법	장점	단점
Method A pore water 압력 체적 controller로 간극수의 체적 변화측정 ΔV water   pore air 압력 체적 controller로 간극의 공기체적변화 측정 ΔV air   시료의 전체 체적변화 ΔV = ΔV air + ΔV water	• 간극수의 체적 측정 (1cu mm)   •간극의 공기체적 측정 (1cu mm)	• 공기체적변화의 측정 • 절대압력에 대한 대기압의 보정 • 공기의 이동에 의한 부압으로 발생한 오류

Method B Wet-wet differential pressure transducer beneath water columns subtending airspace inside an inner cell (HKUST Double Cell). Note: HKUST Double Cell is different to a Double Walled cell

• High accuracy and resolution over full range of volume change measurement due to shape of the inner cell and the use of very accurate differential pressure transducer • Insensitive to the difference in pressure between the inner and outer cells • Does not need two independent pressure control and measurements for cell pressure as in double walled cell (method C) • More stable and less temperature sensitive compared to double walled cell • Good for large test specimens

• Requires careful calibration. • Use high quality de-aired water in cell • Make sure air bubbles are purged out of all connectors and lines

 

Method C GDS cell pressure-volume controller for cell water volume changes ΔVcell   Option to use double walled or metal chamber

• Good accuracy and good resolution (1cu mm) of cell pressure and volume measurement from a GDS pressure/volume controller

• Must use metal (not acrylic) cell chamber, double walled cell or ideal the double cell described here • Use high quality de-aired water in cell and make sure air bubbles are purged out of chamber and all connectors and lines

Method D GDS Hall Effect or LVDT local strain transducers (axial and radial)

• Transducers are suitable for small strains • Provides a good estimate of small volumetric strain • Can be combined with methods A and C above (no space inside inner cell to be used with method B)

• Not suitable for large strains • Assumes right cylinder

 

Requirement for measurement of atmospheric pressure

The on-board pressure transducer in the air pressure controller measures pressure relative to atmospheric pressure (known as ‘auge’pressure). Of course, this is correct for the system measurement of pressure because transducers are using the same reference. However, where the air volume change is concerned, the gas laws relate to the absolute pressure of gas. If we assume that atmospheric pressure can change from 900 milibars to 1100 milibars (this is a large range) this represents about +/- 10kPa around atmospheric pressure. Assuming a volume of 200cc at 15kPa gauge (about 115kPa absolute), the gas laws can be expressed as PV=kRT=constant which gives PV=115*200=23000. If the atmospheric pressure changes by 10kPa the controller will still regulate gauge pressure to 15kPa relative to atmospheric pressure. But this will be now 125(100+10+15) kPa

absolute. Now PV = 23000 = 125*V, which gives 184cc. Therefore, with no apparent change in controlled pressure, there will be a measured volume change of 16cc caused by a change in atmospheric pressure.

From this kind of calculation it may be deemed necessary to take into account the atmospheric pressure. By using an absolute pressure transducer connected to the acquisition system, the measurements of atmospheric pressure can be used to correct the saved results.

시료의 미소변형 측정 장치 (method D)

 

Hall Effect transducers to 1700kPa.   LVDT transducers • Low pressure (up to 3500kPa) version 수압 • High pressure (up to 200MPa) version   for use in non-conducting oil

 

Unsaturated permeability : 불포화토 투수시험

 

Mid-Plane Suction Measurement : 시료중앙의 부압측정