The geosynthetic clay liner(GCL)is a kind of waterproofing material used widely in engineering.The waterproof mechanism is understood in terms of bentonite particles becoming water-obstruct colloid layers after they sorb water and swell.The swell pressure stress,however,has not been determined directly till now.In our experiment,swell pressure stress of the GCL under saturated water-sorbing condition was measured directly using a custom-made instrument.The results show that(1)the instrument designed by the authors performs satisfactorily and the test results are reproducible;and(2)the trend line of swell pressure stress variation with time can be divided into three segments.The first segment is characterized by a quick increase of the swell force in the first 0?50 hours.The swell pressure stress increases by 7.00×10?4?1.00×10?3 MPa/h.The second segment shows a slow increase of the swell pressure stress from the 50th to 1730th hour.The swell force increases by 7.54×10?6?2.02×10?5 MPa/h.The third segment is characterized by a little variation in swell pressure stress after 1730 hours.In this segment,the average value of the swell pressure stress measurements is 0.0719 MPa and the maximum value is 0.0729 MPa.It is suggested that the swell pressure stress is mainly raised by water entering pores among montmorillonite particles and interstitial layers in individual montmorillonite crystals,leading to an increase of volume. The geosynthetic clay liner (GCL) is a kind of waterproofing material used widely in engineering. The waterproof mechanism is understood in terms of bentonite particles into water-obstruct colloid layers after they sorb water and swell the swell pressure stress, however, has not been determined directly till now.In our experiment, swell pressure stress of the GCL under saturated water-sorbing condition was measured directly using a custom-made instrument.The results show that (1) the instrument designed by the authors implements satisfactorily and the test results are reproducible; and (2) the trend line of swell pressure stress variation with time can be divided into three segments.The first segment is characterized by a quick increase of the swell force in the first 0? 50 hours. swell pressure stress The increase by 7.00 × 10 -4? 1.00 × 10? 3 MPa / h. The second segment shows a slow increase of the swell pressure stress from the 50th to 1730th hour. The swell force increases by 7.54 × 10? 6? 2.02 × 10 ? 5 MP a / h. The third segment is characterized by a little variation in swell pressure stress after 1730 hours. In the segment, the average value of the swell pressure stress measurements is 0.0719 MPa and the maximum value is 0.0729 MPa. It is suggested that the swell pressure stress is mainly raised by water entering bubbles among montmorillonite particles and interstitial layers in individual montmorillonite crystals, leading to an increase of volume.