<p>The indoor leakage water–sand experiment was used to investigate the failure patterns of the sand-pebble formations under different soil stratification and rectangular leakage conditions. The critical length-to-width ratio is related to the size of soil particles and thickness of soil layers. When the length-to-width ratio is less than 30:24, an increase of soil layer thickness no longer affects leakage sand. The total amount of leakage sand is inversely proportional to the opening's length-to-width ratio and directly proportional to the duration of leakage. The leakage sand velocity and the affected area of collapse decreases as the length-to-width ratio increases. Under the critical length-to-width ratio, the quantity, the velocity of leakage sand, the range and depth of collapse reach their minimum values. The formation of a double arch effect above the leakage opening impedes the continuous occurrence of leakage sand. The existence of pebble reduces the total amount of leakage sand. Dividing the area by the collapse plane, the soil within the collapse zone are more susceptible to instability and leakage. The soil in the mobile zone outside the collapse plane experience instability due to the influence of the collapse zone and leak out. The angle θ of shape failure of the collapse plane is related to the natural repose angle α of the soil. The angle θ of shape failure is greater than natural repose angle α₁ of sand layer, and less than that of the angular stone layer α₂. The average difference between the two angles is approximately 13°.</p>