路基土壤多位於地下水位面以上，呈現不飽和狀態，功能為承載道路面層所傳遞而下的荷重。當路基土壤受到季節性降雨及道路排水系統不良等影響時，會衍生出路基土壤的含水量增加，並同時牽動著土壤內部基質吸力下降的情形。而回彈模數會因降雨引致含水量的增加而折減。本研究為了解基質吸力與回彈模數之關係，以軸平移技術控制土壤基質吸力在道路開通服務後的含水量狀態，並利用MTS動力三軸試驗系統，進行一系列路基土壤回彈模數試驗，探討不飽和路基土壤基質吸力與回彈模數特性。試驗結果顯示影響路基土壤基質吸力的因素，除了季節性降雨造成含水量增加的影響外，反覆荷載造成土壤孔隙水壓上升基質吸力下降亦是主要原因。並由試驗結果得知當基質吸力較接近平衡含水量狀態時，回彈模數會隨基質吸力增加而增加。

Subgrade often lies above the groundwater level and appear in unsaturated conditions. Its function is to bear the weight transfered down from the pavement. The subgrade can be affected by the seasonal rainfall or the detrimental road drainage system. As a result, water content of the subgrade may increase and the matric suction of the soils may decrease at the same time .The resilient modulus will be reduced because the increase of the water content. The axis-translation technique is used to control the soil suction and the water content state after road serving in order to investigate the relation between suction and the resilient modulus. This research used the MTS cyclic triaxial test system to carry out the resilient modulus tests on subgrade soils.
The test results show that the main factor influencing the matric suction of subgrade, except the water content increase induced by the seasonal rainfall, is the deviator stress. The deviator stress loaded repeatedly may result in pore water pressure rises and then matric suction decreases. Test results also show that when the matric suction is relatively close to Equilibrium Moisture Content (EMC), the resilient modulus will increase with the increased matric suction.

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