# Research Progress on the Influence of Thermo-Chemical Effects on the Swelling Pressure of Bentonite
## 2 Test Methods for Swelling Pressure  
The swelling pressure can be measured by different methods, but different test methods produce different swelling pressure values [9; 11; 12]. It should be emphasized that it is ideal to think of the swelling pressure as the pressure measured when bentonite is wetted under the condition of limited volume [13]. The constant volume method, constant load method, and paired swell test (PS-test) were proposed to measure the swelling pressure [14]. The constant volume method for measuring the swelling pressure is based on strain control techniques [15; 16] including the zero-swell test and the direct method. The main difference between the two approaches is that the latter measures the swelling pressure directly using a pressure transducer, such as the S-type load cell shown in Figure 1. The zero-swell method prevents the specimen from expanding by applying a vertical stress, which is generally around the year 2000 [11; 17]. It is challenging to guarantee optimal swelling readings and constant volume conditions during the test, and the soil pressure is very sensitive to load increments during the test [9; 18], especially in the zero-swell test [14]. Furthermore, the swelling pressure will be overestimated due to the friction between the specimen and the consolidator ring during the recovery of the specimen to the initial volume [9]. In spite of this, the value measured with the zero-swell method is only one-third of the value measured with the swelling reloading test [9; 17]. The reason is that the soil particles in the swelling reloading test are sufficiently hydrated, especially under low load conditions, whereas soil particle swelling is restricted and hydration is not sufficient in the constant volume method. Large errors with the direct method are due to the device's characteristics, including the stiffness of the sensor [19; 20], and the corresponding swelling pressure error can reach a maximum of 1-2 MPa [9]. The constant load test methods include the swell-consolidation test and the double consolidation test. The swell-consolidation method involves compressing the specimen after free swelling for 24 h until it returns to its initial state of porosity or height, which is typical of the constant load method [14]. The swelling pressure is the pressure necessary for the sample to return to its initial height [11], similarly to the swelling pressure of the free-swelling method. The difference between the swell-consolidation method and the swell-reload method is that the latter applies compression to the specimen in steps, with an interval of 24 h between each step [9], which ensures the full swelling of the specimen and also increases related errors such as friction; the swelling pressure is the vertical stress required to restore the specimen to its original state. The \"extra\" macroscopic void fraction generated during the initial swelling at low pressure results in higher swelling pressure measured by the swell-consolidation method compared to the constant volume method [21]. The double consolidation test was performed on two samples, one of which was wetted, swollen and completely submerged, and its maximum expansion was recorded after 24 h of expansion, while the other was compressed at natural moisture content, and the amount of compression was equal to the swelling of the wetted sample; the vertical stress was defined as the swelling pressure [14]. Apparently, the constant load method is constrained by the free-swelling method [14]. The values measured by the double consolidation test are significantly higher than the values obtained by the free swelling test [22]. The PS-test is a simple, reasonable, and reliable swelling test based on the improvement of the loading of the tested specimen; its validity has also been confirmed by other numerous tests. Its most important advantages are that it does not require a complicated setup or design, uses conventional consolidation equipment, and does not need the provision of constant volume conditions.