불포화토 air entry porous disk 포화 방법

불포화토 air entry porous disk 포화 방법

 

 

영신컨설턴트 (02) 529 8803 2018.8

삼축압축 시험 air entry air entry porous disk

 

   

삼축압축 시료 중앙의 압력 측정기

Mid Plane Pore Pressure and Mid Plane Suction Probes (GDSM4P)

▶ 2종류의 압력측정

 

1. 삼축압축 시료 중앙 압력 측정

 

The Mid Plane Pore Pressure Probe는 시료하단의 간극수압측정보다 관의 체적으로 압력이 정확하다.

 

 

2. 불포화토의 부압 The Mid Plane Suction Probe

 

(끝에 high air entry porous disk가 부착)

불포화토 부압 측정

Measurement of Matric Suction in Unsaturated Soil

One of the two stress state variables for unsaturated soils, is matric suction. The GDS suction probe provides a direct measurement of pore water pressure for the measurement of matric suction. This type of direct measurement is preferred in unsaturated soil tests as changes in pore water pressures are more rapidly reflected. When the tip is fully saturated, the response of the suction probe is generally less than 3 seconds, even for relatively large changes in pore water pressure.

 

The principal of making suction measurements using a suction probe is based on the equilibrium between the pore water pressure in the soil and the pore water pressure in the water compartment of the transducer behind the porous tip. Before equilibrium is attained, water flows from the water compartment into the soil, or vice versa. In an unsaturated soil specimen, negative pore

water pressure causes the flow of water from the water compartment into the soil. on the other hand, in a saturated soil specimen, positive pore water pressure causes the flow of water from the soil into the water compartment.

Importance of stone and sensor range selection for Suction Measurement:

Our GDSM4P suction model is based on the design developed by Take & Bolton (2003). Their research focused on developing a tensiometer particularly suitable for suction ranges below 400kPa.

A combination of a 15 Bar sensor, a 5 Bar High Air Entry Porous Disk with a specific saturation procedure delivered accurate and highly responsive results during their trials.

 

References: Take, W. A. and Bolton, M. D. (2003) Tensiometer saturation and the reliable measurement of soil suction,

Géeotechnique, 53 (2), pp. 159-172.

 

 

TENSIOMETER SATURATION 포화

 

Indeed, Guan & Fredlund (1997) indicated that there was a different maximum

suction associated with each different saturation process and tensiometer.

 

The maximum measurable suction is limited by: 부압 측정 장애요인

 

(a) growth of pre-existing gas bubbles 공기압의 팽창

(b) air entry, or 공기 투과

(c) nucleation of vapour bubbles or release of air trapped in surface crevices.

공기응집

 

When any of these three mechanisms separates the fluid in the water reservoir of the tensiometer from the fluid in the pores of the soil, any further soil suction simply leads to gas expansion. A high degree of saturation of the filter avoids mechanism (a).

포화로 공기압 팽창을 피한다.

 

The use of fine porous filter materials, and the inherent surface tension of water, control item (b). The third mechanism relates to the tensile strength of water, but also involves the presence of nucleation sites such as surface crevices (Harvey et al., 1944).

공기투과는 작은 공극 재료 사용, 공기응집은 물의 표면장력과 관계.

absolute pressure 절대 압력

어떤 용기 내의 가스가 용기의 내벽에 미치는 실제의 압력이며,

완전 진공의 상태를 0으로 기준하여 측정한 압력으로 단위는 [kg/cm2a]이다.

절대 압력＝게이지 압력＋대기압,

절대 압력＝대기압－진공 압력, 게이지압력＝절대 압력－대기압.

The initial saturation procedure adopted is similar to that proposed by Ridley & Burland (1999). Initially, the chamber is horizontal; the base is filled with water and allowed to de-air for one hour.

 

The air-dry tensiometer is then inserted into the chamber, sealed and returned to a high vacuum (Fig. 3(a)).

Once the tensiometer reading indicates that the device is in equilibrium with the evacuated chamber, the saturation chamber is rotated through 90, slowly introducing water to the porous filter while under vacuum (Fig. 3(b)). After the tensiometer has been left in this state for at least 20 min, the vacuum is released and further time is allowed for saturation of the filter under atmospheric pressure.

 

Pre-pressurisation

The second stage of tensiometer saturation involves the application of high positive pressures to force into solution any remnants of the air phase entrained within the tensiometer.

As this process involves the application of pressure before the system is used to generate tensions within the water reservoir, it has often been referred to in the literature as pre-pressurisation.

 

The magnitude of the positive water pressure cycles is limited by the allowable over-range of the measurement diaphragm. The base device has a full-scale

range of 700 kPa, with an over-range limit of 1400 kPa. To ensure that the device was not damaged in the pre-pressurisation process, positive pressure cycles were limited to maximum pressure of 1000 kPa.

A pre-pressurisation apparatus has been fabricated consisting of a chamber for the simultaneous saturation of eight tensiometers, a pressure piston to apply the saturation pressure, and a vacuum chamber to act as a water trap (Fig. 4).

 

Before the tensiometers are inserted into the saturation chamber, water is driven from a pressure piston into the water trap, and the system is de-aired under an absolute pressure below 1 kPa for at least 1 h.

 

Upon the return to atmospheric pressure, the de-aired water within the water

trap is drawn into the pressure piston, and the system is ready for tensiometer insertion. Pre-pressurisation cycles are then applied consisting of 1 h at 1000 kPa and 1 h below 1 kPa absolute.

probe 포화방법

 

1. initial saturation procedure 진공포화

 

chamber를 수평으로 놓아 하단에 물을 채우고 진공으로 1시간동안 공기를 제거시키고 probe 연결하고 진공압을 가한다. 센서가 안정이 되게 한다.

chamber를 90도 회전시키고 20분 동안 포화 후 대기압으로 만든다.

 

2. Pre-pressurisation 압력포화

 

Boyle’'s law 압력 P = Pi (S-Si)(1-H)/(1-S(1-H)

 

Air within a filter element of initial degree of saturation, Si, at an initial absolute pressure of Pi,

H is Henry’'s constant, which is approximately 0·02 ml of air per ml of water at room temperature

 

P100 : theoretical change in pressure required for full final saturation,

 

P100 = 49 Pi (1-Si) 이론적인 포화 압력 4-9 MPa.

 

The work of Harvey et al. (1944), among others, indicates that any remaining air trapped within crevices of the water reservoir could be forced to dissolve

under the application of high pressures, thus increasing the measurable suction of the system by stabilising these potential cavitation nuclei.

압력으로 공기를 녹임

 

For initial degrees of saturation above 95%, they also demonstrated that

the time required for full saturation drops considerably with an increase in initial saturation. Ridley (1993) also proposed that the time required for saturation under these high pressures can be reduced by cycling from high positive pressures to low negative pressures.

진동압력으로 포화

 

포화장지

포화 예

1. 3 bar air-entry 포화

aximum measurable suction of 3 bar air-entry tensiometer with saturation

effort 3 bar air-entry tensiometer의 포화에 따른 측정

  

0 곡선 : The tensiometer subjected only to the initial saturation procedure could not approach the air-entry value of the filter, but rather slowly descended to a value of -80 kPa. 진공압력으로 포화시 -80 kPa 측정

 

1 곡선 : The tensiometer response after the application of one 1000 kPa pre-pressurisation cycle similarly stalls at a value of -95 kPa. These results indicate that the initial saturation was much less than might have been anticipated. 1000 kPa으로 포화시 -95 kPa 측정

 

A second pre-pressurisation cycle finally increases the degree of saturation to the point where sufficient cavitation nuclei are eliminated to allow the measurement of negative pore water pressures below -100 kPa.

 

Once the air-entry value has been exceeded, air slowly migrates into the filter until an unstable bubble forms within the chamber, and cavitation occurs.

 

This instability is illustrated by a quick ‘snap-through’ to –100 kPa, which

corresponds to the minimum pressure within a bubble. the time required for tension breakdown varies with the following saturation cycles, but breakdown from high suction could be initiated at any time by impact loading.

2. 1 bar air-entry 포화

Maximum measurable suction of 1 bar air-entry tensiometer with saturation

effort 1 bar air-entry tensiometer의 포화

3. 3 bar air-entry tensiometer대기압으로 포화시킨 경우

Maximum measurable suction of 3 bar air-entry tensiometer initially saturated at atmospheric pressure with  saturation effort

3 bar air-entry tensiometer  대기압으로 포화시킨 경우

 

 

To illustrate the importance of initial saturation on the final outcome of the saturation process, a tensiometer fitted with an air-dry nominal 3 bar air-entry filter was intentionally poorly saturated at atmospheric pressure by quickly plunging the device into a cup of water.

Saturation chamber for dry initial saturation of tensiometer 진공포화 장치

 

 

probe 포화도 degree of saturation, S

 

S = P / 0.98 ( Pi +P)

 

Pi : the initial absolute saturation pressure,

 

Tensiometer (1 bar filter) of initial degree of saturation :

(a) stepwise calibration; (b) non-linearity and hysteresis

 

1 bar filter 진공압력 -50KPa 포화

Tensiometer (1 bar filter) of initial degree of saturation :

(a) stepwise calibration; (b) non-linearity and hysteresis

 

1 bar filter 진공압력-75KPa 포화

Tensiometer (1 bar filter) of initial degree of saturation :

(a) stepwise calibration; (b) non-linearity and hysteresis

1 bar filter 진공압력-95KPa 포화

Tensiometer (1 bar filter) initially saturated at an absolute pressure below

1 kPa : (a) stepwise calibration; (b) non-linearity and hysteresis

 

1 bar filter 진공압력 1 kPa 포화

Tensiometer (3 bar filter) initially saturated at an absolute pressure below

1 kPa: (a) stepwise calibration; (b) non-linearity and hysteresis

3 bar filter 진공압력1 kPa 포화

Tensiometer (3 bar filter) initially saturated at an absolute pressure below 1 kPa and subjected to one pre-pressurisation cycle: (a) stepwise calibration; (b) non-linearity and hysteresis

 

3 bar filter 진공압력과 1회 압력반복으로 포화