為防範地下水所引致開挖或回填邊坡的破壞，工程上經常採用設置擋土牆和埋設水平排水管的方式藉以達成安全穩定的需求。然而實務上，配置水平排水管的擋土牆其滲流壓力分佈屬於三向度變化，包括各項尺寸因子所對應之最大排水量均尚無簡易之評估模式。因此設計上，對於排水管的尺寸或是多管情況下的數量及配置間距難以評估。
研究藉由砂箱試驗的驗證，確認三維滲流數值模型GMS與FEMWATER模組以及將排水材料視同孔隙介質的模擬方式能夠合理的反應集水模式、材料內外的水壓分佈和單位時間排水量。繼而模擬配置單支水平排水管的情況，得以分析比較排水材料滲透係數Kd、土壤滲透係數Ks、排水管管徑Dp、長度Ld、地下水位與管口高程差Dw等個別因子對單位時間排水量Qd及滲流壓力水頭Hp分佈的影響。同時發現極限排水情況下，水平排水管的周圍存在著一特殊的等水壓力曲面，被命名為特徵水壓面 (CIPS)。該曲面敏銳的反應因條件改變而引致的滲流變化，因此可以成為評估水平排水管的排水容量和滲流壓力分佈的指標。
研究也觀察到當排水管超過一定長度時，只有靠近管口的一段管長對排水量的增加和水壓分佈具有影響，稱之為有效管長Le。Le受Dw/Dp及Kd/Ks影響且與滿管流之狀態息息相關，因而推論適當的埋設（向上傾斜）角度可降低極限情況下的管內壓力及增加Le 和Qd。此外，研究也進行濾層和阻塞的模擬，對區段阻塞的影響性更為明確。最後，根據單支水平排水管的模擬分析及一組代表性的成果數據，取得Le、Qd、CIPS之尺寸Dc、Lc及壓力水頭值Hp與主要參數Dw/Dp及Kd/Ks之間的量化關係，並藉此建構一概念性的擋土牆配置水平排水管的簡化評估模式，應有利於排水設計或破壞分析等相關實務或研究工作。

Horizontal drain pipes are frequently used behind a retaining wall or on a slope to prevent damage because of the build-up of groundwater pressure. However, the distribution of pressure heads surround a drain pipe is truly a 3-D situation. There is no easy way to estimate the maximum discharge rate for the design of drain pipes.
An early study has verified a simulation of sandbox drainage test by the 3-D numerical model software named GMS (Groundwater Modeling System) with an option of finite-element module FEMWATER. Through the simulation, the drainage material was treated as porous medial and has a permeable property. Therefore, is capable for simulate horizontal drain pipe under a seepage condition. After that, a sensitivity study to clarify the major influence on the discharge rate Qd and the distribution of groundwater pressure was performed. Permeabilities of drainage material Kd and backfill Ks, dimensions of drain pipe Dp and Ld, the distance between groundwater table and pipe outlet Dw were found to be related to the variation of pressure head distribution. In that study research has revealed a unique equalized water pressure contour existed between the hydrostatic and drainage pressure systems. The unique water pressure contour has the biggest but still closed horn shape was named Characterized Iso-pressure Surface (CIPS). The shape, the dimensions (Dc, Lc) and pressure head Hp of CIPS were found to be good objects to reflect the drainage capacities and the seepage pressure distribution pattern.
The research also observed a limited section of pipe length contribute to the increase of Qd and the decrease of Hp. It was named Effectiveness Length Le. Le is not only influence by Dw/Dp and Kd/Ks but also related to the flow pressure inside the pipe. Therefore, it is assumed a pipe installed with an upward inclination would be a help to decrease the flow pressure inside the pipe and increase Le and Qd. Based on the analysis of simulation results, research create mathematic relationship among Le, Qd, Dc, Lc, Hp, Dw/Dp and Kd/Ks. Eventually has established a conceptual simplified assessment model for horizontal drain pipes behind a retaining wall. It is expected to be beneficial to the drainage design work or the back analysis practice for retaining walls.

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