點振動夯實造成砂土密度及土壓力變化

Abstract

本論文以實驗方法探討點振動夯實造成疏鬆乾砂回填土之地表沉陷、密度變化及土壓力變化。本研究以氣乾渥太華砂作為回填土，回填土高1.5公尺。量測於鬆砂相對密度(Dr = 34%)表面進行單點振動夯實造成之砂土密度及土壓力變化 。本變研究利用國立交通大學模型擋土牆設備來探討經過7秒、20秒、46秒、及123秒的地表振動夯實對地表沉陷、砂土密度及土壓力造成的變化。依據實驗結果，本研究獲得以下幾項結論。 1. 在鬆砂的回填土中，土壤內部的垂直與水平土壓力可以分別用□v = □z與Jaky公式估算。 2. 在無凝聚性土壤表面受夯實的受力情況可模擬成方形鋼樁承受振動式打樁機的貫入模式。在夯實過程中，土層發生地表沉陷，直到樁底的極限承載力qp與振動夯實應力□cyc達到平衡。 3. 隨著夯實時間的增加，相對密度等高線(Dr = 36%)會往深處發展，並且土壤內部之最大相對密度Dr,max，也會隨著夯實時間的增加而增大。 4. 隨著夯實時間的增加，土體殘餘的最大垂直與最大水平應力變化（夯實後測得的應力減去初始未夯實測得之應力）也跟著增加。這是因為隨著夯實時間增加，夯實能量也持續傳遞至土壤內部。 5. 夯實後移走夯實機，土壤內部可測得夯實造成之殘餘應力。最大的垂直應力□□v位置，隨著夯實時間的增加，此位置會從深度250 mm稍微向下移動至350 mm的位置。

關鍵字：夯實、土壤壓力、模型測試、沉陷、砂、相對密度

This paper studies the surface settlement, change of soil density and earth pressures due to vibratory compaction at a point. Dry Ottawa sand was used as the backfill material. The height of backfill was 1.5 m. The variation of soil density and earth pressure due to the vibratory compaction on the surface of the loose sand (Dr = 34 %) was measured. The instrumented model retaining-wall at National Chiao Tung University was used to investigate the variation of the surface settlement, soil density and earth pressures after 7, 20, 46 and 123 seconds of vibratory compaction. Based on the test results, the following conclusions were drawn. 1. For a loose backfill, the vertical and horizontal earth pressures in the soil mass can be properly estimated with the equation □v = □z and Jaky’s equation, respectively. 2. The compaction of a cohesionless soil with a vibratory compactor can be simulated with the penetration of a square steel pile driven with a vibratory pile hammer. In the compaction process, the soils under the compacting plate settled until the ultimate tip resistance qp and the cyclic compacting stress □cyc reached an equilibrium. 3. The depth of the relative density contour (Dr = 36 %) increased with increasing time of compaction. The peak relative density in the soil also increased with increasing time of compaction. 4. It was obvious that the peak □□v (3.60, 3.99, 4.44, 4.96, kN/m2) and □□h (1.93, 2.41, 3.16, 3.32 kN/m2) increased with increasing compaction time. This is because, with increasing compaction time, more compaction energy was transmitted to the soil. 5. After the removal of the compactor, residual stresses in the soil mass were measured due to the vibratory compaction. The point of peak □□v, as the compaction time increased, moved downward slightly from the depth of 250 mm to the depth of 350 mm.

Keywords: compaction, earth pressure, model test, settlement, sand, relative density.