The phenomena can be observed in the tests low seepage pressures can easily form passageways behind the waterproof layer if there is no drainage layer or the drainage layer is blocked in the single - shell test slab ; the waterproof layer can resist higher seepage pressure if there is no drainage layer or the drainage layer is blocked in the double - shell test slab ; the seepage water can be controlled and drained away in the drainage layer by the water proof layer only if there is unblocked drainage layer behind it in the double - shell test slab 由试验可见，在单层试验板中没有排水层或排水层不通畅时，较低的水压就能在防水膜后形成渗流通路；在双层试验板中没有排水层或排水层不通畅时，防水膜能抵抗一定程度的渗漏水压；只有在双层试验板中的排水层畅通时，防水膜才能有效地将渗漏水控制在排水层中排走。
