臺灣博碩士論文加值系統

本研究利用自行研發之實驗系統與數值模擬配合實際案例，探討浚填砂土地下水位對動力夯實成效之影響。在不增加夯擊能量的情況下，預先降低地下水再進行動力夯實，土壤之有效動應力可望增加，進而增加改良深度，亦可縮短等待超額孔隙水壓消散的時間，以縮短工期。本研究因此研發室內控制排水條件之單點夯擊試驗及自動圓錐貫入儀，探討浚填砂土在不同水位及排水條件下，受夯擊後之改良成效與孔隙水壓消散速率並與現地消散試驗比較。此外，亦設計實驗探討單點夯擊試體之邊界效應。
同時，本研究研發三軸圓錐貫入試驗(Triaxial Cone Penetration Test, TCPT)來模擬現場在降低水位夯擊改良後，水位回升是否影響其改良強度。TCPT更與傳統反覆三軸試驗(Cyclic Triaxial Test, CTT)進行關聯性試驗，發現正規化三軸貫入阻抗尖峰值與抗液化反覆剪應力比之間，有良好之關聯性。接著利用數值方法模擬實際案例進行分析，探討地下水位對動力夯實成效之影響。
實驗結果發現，無水位試體之改良成效均大於含水位試體，又低水位試體之改良成效均大於高水位試體。從高水位降低至低水位時，夯擊後雖然有效動應力僅增加約5 %，但其貫入阻抗卻可增加超過50 %。從探討整體性成效之改良率(Improvement ratio) 與貫入功 (Penetration work) 顯示，無水位或低水位試體之整體成效最佳，且最大改良率大都發生在水位以上。單擊試驗所得到之超額孔隙水壓消散速率約為現地試驗之300~1000倍，應與尺度及排水條件相關。而消散速率隨著開孔率增加而增加，隨著水位降低而減小。高水位時，除了不排水條件外，其它開孔率試體夯擊後之錐頭阻抗分佈與完全排水條件者相近，而低水位時，不論排水條件為何，其錐頭阻抗普遍增高與乾土試體者相近。數值模擬現場結果亦顯示在低地下水位時夯擊之優越成效。地下水位從地下3 m降低至地下6 m 以下再夯擊，其土層有效動應力最大可增加50 % 以上。顯見預先降水動力夯實工法值得進一步研究發展，同時提供大地工程師在面對日益嚴苛的基礎耐震設計規範時，多了一種經濟有效的地盤改良工法替代方案。

Through the self-developed experimental system and testing program as well as the numerical technique on the filed practice, the study presents the effects of water level on Dynamic Compaction (DC) for reclaimed soils. Instead of increasing applied energy of DC, the dynamic compaction under a pre-lowered groundwater table would increases the effective dynamic stresses on the soil mass which leads to a deeper improved depth; and the excess pore pressure is also reduced which could shorten the waiting period and construction period. Hence, Single-point Impact Test (SIT) and Automatic Cone Penetrometer (ACP) were developed to perform the dynamic impacts under different water levels and drained conditions to evaluate the improvement effectiveness and dissipation rate. Besides, the boundary effect in the SIT was also investigated and discussed through a developed test set-up.
Meanwhile, the normalized cone resistance in dry and saturated soils under certain effective confining pressure were performed by the developed Triaxial Cone Penetration Test (TCPT) to investigate if any difference exists between before and after recovery of groundwater level. Besides, a series of correlation tests between TCPT and Cyclic Triaxial Test (CTT) were performed and resulted in a well correlation. The numerical technique was also adopted to simulate the field practice and to figure out the effects of groundwater level on the performance of dynamic compaction.
The test results indicate that after impacts, the effectiveness of dry soils is greater than that of soils with water levels; and the effectiveness of soils with lower water level are better than that with higher water levels. When the water level is lowed from high level to low level, it is found that after impacts, a minor increment of effective dynamic stresses with 5 % in average can increase a great amount of cone resistance over 50 % in average. Through the evaluations on Improvement Ratio and Penetration Work, the dry soils or soils with low water level demonstrated their best performance than others with water levels. In addition, the maximum values of Improvement Ratios are mostly found at dry soils above the water levels. The dissipation rates of excess pore pressures measured in SIT are 300 to 1000 times of that measured in field pilot test which might be attributed to the scale effect and different drained conditions. The dissipation rate increased with the raising of opening ratio and decreased with the lowering of water level. At high water level, the post-impacted cone resistances of soils in partial drained conditions are close to that in full drained condition; while at low water level, all the post-impacted cone resistances of soils are close to that of dry soils in spite of their drained conditions. In numerical simulation, the superiority of dynamic compaction under lower groundwater table is verified. It is indicated that the effective dynamic stress induced by impact can be increased over 50 % as the groundwater table is lowered from -3 m to -6 m. The Pre-dewatering DC, therefore, proves worthwhile for a further development and provides the geotechnical engineer a cost-effective alternative on the soil improvement when facing the progressively severer regulation in seismic design for foundation.

Abstract in Chinese I
Abstract III
Acknowledgement V
Contents VII
List of Tables XI
List of Figures XII
List of Photos XVI
Notations XVII
Chapter 1 Introduction 20
1.1 Background and Motivation 20
1.2 Research Objectives and Procedure 24
1.3 Achievement Overview 25
1.4 Outline of Dissertation 26
Chapter 2 Relevant Theory and Literatures Review 28
2.1 Pre-dewatering Dynamic Compaction 28
2.1.1 Groundwater Table and Ground Vibration 28
2.1.2 Waiting Period and Construction Period 29
2.1.3 Effective Energy and Depth of Improvement 31
2.1.4 Water Source for HSE Management 31
2.1.5 Limitation to Pre-dewatering DC method 31
2.2 Dynamic Compaction Review 32
2.2.1 History of Dynamic Compaction 32
2.2.2 Propagation of Dynamic Stresses 33
2.2.3 Construction Procedure for DC 35
2.2.4 Single Impact Energy 35
2.2.5 Effective Depth of Improvement 37
2.2.6 Waiting Period 39
2.2.7 Groundwater Table 39
2.2.8 Stresses in Soils under Dynamic Loading 40
2.3 Cone Resistance in the soils 45
2.3.1 Correction Factor for Effective Confining Pressure 45
2.3.2 Used for Soil Classification and Characterization 47
2.3.3 Used for Evaluate Liquefaction Resistance of Soils 48
2.4 Cyclic Triaxial Test for Liquefaction Resistance 51
2.5 Numerical Simulation 54
Chapter 3 Single-point Impact System 56
3.1 Introduction 56
3.2 Single-point Impact Test (SIT) 57
3.2.1 Capillary Rise of Reclaimed Soils 57
3.2.2 Introduction to SIT 65
3.2.3 Instrumentation 70
3.2.4 Impact Equipment 72
3.3 Automatic Cone Penetrometer (ACP) 72
3.3.1 Introduction 72
3.3.2 Major Components and Capacity 74
3.3.3 Calibrations 74
3.4 Boundary Effects of Soil Column 77
3.5 Limitations of Test Setup 80
3.6 Data Acquisition System 80
3.7 Experiment Program 82
3.7.1 Soil Properties and Preparation of Soil Column 82
3.7.2 Test Procedures 86
3.7.3 Test Conditions 86
3.8 Interpretation for Results of SIT 89
3.8.1 Impact-induced Pore Pressures 89
3.8.2 Effective Dynamic Stresses 99
3.8.3 Cone Resistance and Improvement Ratio 100
3.8.3 Penetration Work 105
Chapter 4 Triaxial Cone Penetration Test 108
4.1 Introduction 108
4.2 Major Components and Concerns 110
4.3 Test Equipment and Data Acquisition 113
4.4 TCPT for Water Level Recovery 114
4.4.1 Simulation Tests 114
4.4.2 Modified Cone Resistances 115
4.5 Correlations with Liquefaction Resistance of Reclaimed Soils 117
4.5.1 Experiment Execution 118
4.5.2 Interpretation for Test Results 122
4.5.3 Correlation between and (CSR)Nc=15 126
4.6 Conclusive Remarks 130
Chapter 5 Numerical Simulations Using Program FLAC 131
5.1 Introduction 131
5.2 FLAC 2D Program 132
5.2.1 General Introduction 132
5.2.2 Large Deformation in Simulation on Dynamic Impacts 133
5.3 Model building 134
5.3.1 Analytical Model and Meshes 134
5.3.2 Boundary Conditions and Monitoring Configuration 134
5.4 Soil Parameters and Field Measurement 136
5.5 Dynamic Contact Pressure and Contact Time 138
5.6 Interpretation for Simulation Results 141
5.6.1 Depth of Crater 141
5.6.2 Impact-induced Pore Pressures 143
5.6.3 Effective Dynamic Stresses 147
5.6.4 Effects of Groundwater Table 154
5.6.5 Conclusive Remarks 155
Chapter 6 Conclusions and Recommendations 160
6.1 Conclusions 160
6.2 Recommendations 163
References 165
Appendix A Technical Specifications of Sensors 174
A-1 Sensors for Single-point Impact Test (SIT) 175
A-2 Sensors for Automatic Cone Penetrometer (ACP) 182
A-3 Sensors for Triaxial Cone Penetration Test (TCPT) 184
Appendix B Technical Specifications for Data Acquisition System 188
Author Information 194

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