屏東沖積平原礫石層顆粒形狀與深開挖行為案例探討

本文從屏東沖積平原的歷史談起，藉由屏東市區案例於開挖階段辦理試坑試驗，除瞭解土體單位重與粒徑分布曲線等，尚進行顆粒粒形與顆粒形狀之量測，屏東市區礫石層因母岩以板岩為主，使顆粒粒形以圓盤狀（discs）居多，顆粒形狀多呈現類圓形（subrounded）及類角礫狀（subangular），由這些因素以及抽水試驗結果，探討地層滲透係數較卵礫石地層高之原因，可能係細粒料無泥或黏土，且受制於顆粒形狀使地層採水平堆疊型態。又因為設計階段分析若擋土壁體未貫入不透水層，將因抽水而引致鄰房沉陷，使本案例連續壁挖掘深度達34公尺，為克服砂礫石連續壁挖掘穩定液逸流問題，辦理逸流試驗以決定穩定液參數。砂礫石地層之連續壁施工採多重可圖像化的品質管控，以確保連續壁本身與公母單元接頭之止水性與完整性。而Winkler彈性梁理論似乎較不適用砂礫石地層變形機制，筆者採Terzaghi-Peck視土壓力反推支撐軸力，獲得滿意結果。而鄰近基地200公尺另一開挖工區，在本基址已停止抽水後，開始開挖與抽降水，進一步使本基址之壁體外水位觀測井水位隨之下降，若以各觀測井平面距離與洩降水位，以線性回歸估算影響範圍可能超過1,000公尺，另當地下水位低於地表面6.5公尺以下時，地層沉陷將急速產生，顯示地表面下6.5公尺以內地層應為過壓密地層。

關鍵字

礫石層；顆粒形狀；抽水試驗

並列摘要

This article starts with the history of the Pingtung alluvial plain. Based on the case of Pingtung City, the pit test was carried out during the excavation stage. In addition to understanding the soil unit weight and particle size distribution curve, etc., the particle shape and roundness of the soil are also measured. Because the country rock of the gravel layer in Pingtung City is mainly slate, the particle shape is mostly discs, and the roundness is mostly subrounded and subangular. Due to these factors and the results of the pumping test, the reason why the permeability coefficient of the formation is higher than that of the pebble and gravel formation may be that the fine-grained material has no mud or clay, and the formation is horizontally stacked due to the shape of the particles. And In the design stage, the diaphragm wall did not penetrate into the impermeable layer, which would cause the adjacent building sink due to water pumping, therefore making the excavation depth of the diaphragm wall in this case up to 34 meters to overcome the issue of stabilizing fluid escape in excavation of diaphragm wall. The overflow test was carried out to determine the parameters of stabilizing fluid. The construction of the diaphragm wall in sand and gravel strata adopts multiple image quality control to ensure the water-tightness and integrity of the diaphragm itself and the male and female unit joints. Winkler's elastic beam theory seems to be less applicable to the deformation mechanism of gravel formations. The author adopts Terzaghi-Peck apparent soil pressure to reverse the supporting axial force, and obtains satisfactory results. In another excavation area 200 meters away from the base, excavation and pumping precipitation began after the pumping of the base site had stopped, which further reduced the water level outside the wall of the base site. Based on the horizontal distance of the observation wells and the discharge water level, the impact range may exceed 1,000 meters by linear regression estimation. In addition, when the groundwater level is 6.5 meters below the surface, stratum subsidence will occur rapidly. It shows that the strata within 6.5 meters below the ground surface should be overconsolidated strata.