Flume test demonstration of landslide in stable gentle soil slope triggered by small mass of pressurized pore gas

Abstract

This paper presents a triggering factor causing landslide in stable gentle soil slope. The triggering factor is a small mass of pressurized pore gas in the soil slope and illustrated with a flume soil slope model. The pore gas has a small mass of oxygen and is generated via the injection of a small volume of H₂O₂ solution into the cement powder core of the soil slope. The core is covered by a layer of saturated soft clay that forms a trap for the new pore gas to build its pore gas pressure. The pore gas pressure is the driving force to cause the landslide in the stable gentle soil slope. The mass of the pore gas is smaller than 0.1% of the mass of either the gentle soil slope or the landslide body. The videos of the landslide are captured and used for the analysis of movement and displacement of the sliding soil mass with time. The pressure of the pore gas is estimated from the acceleration model of the sliding soil mass. A calibration test is further carried out for the generation of the oxygen gas mass and pressure via the decomposition of the H₂O₂ solution in cement powder. The results quantitatively demonstrate that the small mass of the oxygen gas in the voids of the cement powder core can have enough pressure to trigger the landslide in the stable gentle soil slope model. Such pressurized pore gas triggering factor is generally not noticeable since its mass can be very small and disappear rapidly without a trace. This triggering factor could be responsible for the huge and disastrous landslide that suddenly and dramatically occurred in a huge gentle fill soil slope on December 20, 2015, at Hengtaiyu Industrial Park, Shenzhen, China.

Appendix. Force analysis for the initial overall landslide

According to the model in Fig. 18, the gas pushing force $P_{x} (t)$ can be estimated using the following equation:

$$P_{x} (t) = BY_{EF} p(t)$$

(A-1)

where Y_EF (= 18 mm) is the height between the point E and the point F at t = 13 s and B (= 200 mm) is the flume width.

The resistance acting on the sliding soil mass can be calcualted by following equation:

$$R_{x} (t) = R_{{{\text{base}}}} (t) + R_{{{\text{side}}}} (t)$$

(A-2)

where $R_{{{\text{base}}}} (t)$ is the soil shear resistance along the sliding base of the soil and $R_{{{\text{side}}}} (t)$ is the shear resistance along the interface between the flume side wall and the slope soil.

The shear resistance along the sliding base can be calculated as follows according to the Mohr–Coulomb criteria:

$$R_{{{\text{base}}}} (t) = c_{{{\text{cement}}}} S_{1} + \left( {M_{1} g\cos \theta - p(t)S_{1} } \right)\tan \phi_{{{\text{cement}}}} + c_{u} S_{2}$$

(A-3)

where c_cement (= 0) is the cohesion of cement powder, M₁ (= 12,845 g) is the soil mass above the interface between the sliding base and cement core, S₁ (= L₁B = 92,000 mm²) is the area of the interface area between the sliding base and cement core, c_u is the undrained cohesion of the clay, $S_{2} ( = L_{2} B = 190 \times 200{\text{ mm}}^{{2}} )$ is the contact area of clay and sliding base, and L₁ (= 460 mm) and L₂ (= 190 mm) are respectively the interface lengths between the M₁ and M₂ soil masses and the sliding base.

When the cement powder is fully saturated by the gas with the pressure p(t) and the normal stress at the interface can be reduced to zero or negative (i.e., $p\left( t \right) \ge \frac{{M_{1} g\cos \theta }}{{S_{1} }} = 1.321{\text{ kPa}}$), the shear resistance R₁ = 0; hence, $R_{{{\text{base}}}} (t)$ can be estimated as follows:

$$R_{{{\text{base}}}} (t) = c_{u} S_{2}$$

(A-4)

Similarly, since the side frictional resistance between the sand/cement layer and the vertical side wall of the flume is small, $R_{{{\text{side}}}} (t)$ can be estimated as follows:

$$R_{{{\text{side}}}} (t) = 2c_{u} S_{{{\text{side}}}}^{{{\text{clay}}}}$$

(A-5)

where $S_{{{\text{side}}}}^{{{\text{clay}}}} ( = 18,300{\text{ mm}}^{{2}} )$ is the contact area between the clay seam and the side wall of the flume.

The above dynamic analysis shows that the generated pore gas can have two triggering effects. One is the active sliding force in Eq. (A-2). The other is the reduction of the sliding resistance in Eqs. (A-3) to (A-4).