落重撓度儀檢測荷重與堅硬層深度對反算分析之影響

Abstract

落重撓度儀（falling weight deflectometer, FWD）本身經四十年發展雖已成熟，但其試驗進行方法與資料之後續分析仍為一有待加強之重要課題。本研究主要探討剛性鋪面下方堅硬層對FWD試驗與反算分析之影響，並建立靜力反算之動力修正係數，另針對剛性鋪面提出FWD檢測荷重建議。本研究於台灣大學實驗道路面層25 cm與40 cm厚之剛性鋪面進行FWD撓度試驗，並以有限元素法建構實驗道路剛性鋪面之軸對稱動力數值模型，利用此數值模型探討鋪面下方堅硬層對鋪面受FWD衝擊荷重產生之動力效應。若調整鋪面動力有限元素模型之路基厚度等參數，使其模擬結果與FWD之撓度歷時相符，將可用於判定淺層之地下水位與堅硬層所在深度。本研究亦利用靜力反算程式分析有限元素模擬之鋪面動力撓度，建立動力修正係數，以改善靜力線彈性反算程式之分析成效。另外，研究中並利用實驗道路撓度資料，探討落重撓度儀不同檢測荷重對鋪面撓度盤、有效反應深度、動力及靜力反算結果之影響，進而歸納出面層25 cm厚之剛性鋪面結構FWD檢測荷重在70∼220 kN範圍內，而面層40 cm厚者之荷重於130∼250 kN時，其之撓度分析與反算結果較具有一致性，且荷重之衝擊時間應在25 msec以上較佳。

Falling weight deflectometer (FWD) had been well developed in the past forty years, but the procedure of FWD tests and follow-up deflection analyses still need to be improved. The effects of stiff layer beneath the rigid pavement on FWD tests as well as moduli backcalculation are discussed in this research. The dynamic correction factors of traditional static backcalculation and the FWD load recommendations of rigid pavements are also presented. The FWD tests are conducted on rigid pavements with 25 cm and 40 cm thick concrete slabs of experimental pavement sections at National Taiwan University. The axisymmetric dynamic numerical models of the two rigid pavement sections are established through finite element method. The models are adopted to analyze the dynamic effects of rigid pavements due to the stiff layer when subjected to FWD impulse loads. If the simulated and measured deflection time histories are fitted by varying parameters such as the thickness of subgrade, it is possible to determine the water table and the depth to stiff layer. In addition, the dynamic correction factors are derived by statically backcalculating the deflections computed by dynamic finite element models to improve the traditional statically backcalculated results. Moreover, the deflection data of the two pavement sections are used to analyze the normalized deflection basins, depths of effective reaction, dynamic and static backcalculation results affected by different FWD loads. It is concluded that concrete slab with thickness of 25 cm and 40 cm, the applied testing load ranges of 70~220 kN and 130~250 kN are recommended, respectively. And the pulse durations of the FWD impulse loads should be longer than 25 msec.