擋土牆的排水孔，扮演著暴雨時，將擋土牆背後壤土中的下雨滲水順利快速排出的重要角色，也同時是協助擋土牆作好水土保持工作的最佳伙伴。惟一般常見擋土牆的排水孔，常因沒有設置適當的排水過濾設施或排水器，而造成排水孔排水不順或阻塞，失去了應有的排水防洪功能。肇使擋土牆不堪荷重而導致崩坍，讓可怕的土石流雪上加霜。本文利用實驗室測試半球型排水器與舊式片狀排水器，在有鋪設級配塊石及無鋪設級配塊石做為背填土濾層情況下獲得之排渾水量及排砂量，分別計算在不同滲流傾角θ，採取梅花型交錯排列的配置下，來分析排水孔應有之最大排渾水量及可能產出之最大排砂量(淘空現象)。試驗結果，舊式片狀排水器於周圍未鋪設級配塊石時，其排渾水量百分比達7.09%，而排出砂量百分比高達61.54%。而於周圍鋪設級配塊石時，其排渾水量百分比達51.75%，而排出砂量百分比達12.88%。排出砂量百分比均因其排渾水量百分比增多而遽增。而半球型排水器於周圍未鋪設級配塊石時，其排渾水量百分比達6.36%，排出砂量百分比僅達22.86%。於周圍鋪設級配塊石時，其排渾水量百分比達45.24%，排出砂量百分比更僅有2.81%。觀察出舊式片狀排水器的排出砂量百分比(12.88%)，高於半球型排水器的排出砂量百分比(2.81%)達4.58倍之多。證明半球型排水器（球槽內有含塑膠濾石）其「排砂量」均相對較少，有著”不淘空”背填砂土之特性。是以提供長期以來一直欠缺標準規範，全憑設計單位(工程師)之經驗而策定的困擾作參考。避免造成擋土牆的鋼筋混凝土設計強度或排水濾層設計，過與不及的現象一再發生。

Quick draining of rainfall in backfill around behind retaining wall by retaining wall drainage holes is critical for the soil and water preservation conservation performance of retaining wall. However, retaining wall drainage hole is commonly clogged by lacking adequate drainage filter or drainage device. Clogged drainage leads to structural overload, even failure of retaining wall. The failure of retaining wall worsens landslide disaster. With laboratory experiment, this paper first examined the turbid water drainage and sediment drainage of hemisphere type drainage device and old sheet type drainage device under conditions with or without graded boulder in backfill around. Then, for various seepage dip angle, the theoretical maximum turbid water drainage and sediment drainage (hollowing) of drainage holes of plum blossom type (staggered) configuration are derived. The experiment results show that without graded boulder in backfill around, old sheet type drainage device has turbid water drainage ratio of 7.09% and sediment drainage ratio of 61.54%; with graded boulder in backfill around, old sheet type drainage device has turbid water drainage ratio of 51.75% and sediment drainage ratio of 12.88%. In both cases, sediment drainage ratio increases rapid seriously because of turbid water drainage ratio increase. Without graded boulder in backfill around, hemisphere type drainage device has turbid water drainage ratio of 6.36% and sediment drainage ratio of 22.86%; with graded boulder in backfill around, hemisphere type drainage device has turbid water drainage ratio of 45.24% and sediment drainage ratio of 2.81%. By experiment results, the 12.88% sediment drainage ratio of old sheet type drainage device is about 4.58 times of the 2.81% sediment drainage ratio of hemisphere type drainage device. Hence, it is verified that the hemisphere type drainage device (plastic filter stones in hemisphere) has less sediment drainage and will not hollow backfill behind retaining wall. Conclusion of this paper provided a reference to department of designing and planning and engineering, those who designed always depend on the designing by empirical, because of bother in lack of proper criterion. To avoid from the reinforce concrete over designed, or lack of filter layer of retaining wall happened repeatedly.

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