於道路透水鋪面下方儲存雨水，為海綿城市建設常用之保水和滋潤土壤作法，但儲水於道路下方，會使基底土壤因泡水而軟化，降低對車行載重之承載力，屢屢造成道路養護單位之困擾。事實上，對道路基底土壤而言，保水滋潤與承載能力一直都是個矛盾的問題。為此，本研究提出改良式道路下方保水構造，在道路下方保水層與基底土壤間導入阻水層，避免基底土壤泡水軟化的問題；同時將雨水入滲土壤的範圍，從原本道路下方之保水層，轉移至道路旁之側溝，利用側溝與保水層結合，來發揮保水滋潤之功能。本研究使用PLAXIS 2D軟體，探討雨水由側溝保水層入滲至土壤之行為，並提出四種型式之入滲側溝的設計，評估因雨水入滲而飽和度增加，對基底土壤承受車行載重能力的影響。再依據分析結果，選出一種能兼顧道路承載能力和保水需求之保水側溝設計型式，期望對建設海綿城市有所幫助。

A sponge city can absorb rain water, reduce surface runoff and flood peak of the river. The space underneath the road pavement is often chosen to be the storage space for rainwater. But after soaked in water, the soil underlying the water storage space will lose its strength and capacity to carry the traffic load. It can cause big trouble for the road maintenance department. This study proposes a modified road base configuration which also integrates with the water preserve side ditch. In other words, the modified road base system allows rainwater infiltrate to soil through side ditch rather than the base material underneath the road. So that the subgrade soil will not lose bearing capacity because no water soaking occurs. The PLAXIS 2D software is used here to study the water infiltration behavior from side ditches with different infiltration boundaries under or around the road. Four infiltration scenarios are studied and the effect of increasing degree of saturation on the traffic load carrying capacity of the sub-base soil is also evaluated. Based on the results of numerical study, a modified road base design for the traffic road with a water preserve side ditch is proposed. The infiltrated water can moisture the soil for the need of vegetation but will have minimal negative effect on the bearing capacity of the road base soil. It is hoped that the proposed road base system can contribute to the development of sponge city.

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