Relationship between P-wave velocity & SPT N values and application to assessment of excavatability of terrain
In order to evaluate excavation in terms of time, cost and risks for site formation works, deep excavation works, etc. where large amount of excavation is necessary, investigation for indicator parameters for quick and reliable correlation purpose is imperative. Furthermore, intrusive investigation is often insufficient in both local and extensive areal coverage, which suggests that parameters acquired from conventional drillhole method are inadequate for appraisal of an excavation construction. Geophysical method, such as seismic refraction survey is a quick and non-intrusive technique with minimal site constraints. Therefore, formulating a degree of understanding of relationship between geophysical parameters, such as P-wave velocity in this study and the excavatability of terrain can be a useful tool for better plan of excavation programme as well as to implement due control and/or mitigation measures upon encountering geology-related problems and constraints during the course of excavation.
This dissertation aims to investigate the relationship between P-wave velocity & SPT N values for both the superficial, and solid geology of different degree of decomposition underlying the part of Tung Chung from Lung Tseng Tau to Cheung Sha. The P-wave velocity data were obtained from the seismic refraction surveys carried out as part of the ground investigation works for the Improvement to Tung Chung Road Project. Whereas, the SPT N values are obtained from drillholes for the same ground investigation works, which are situated in line or slightly offset with the seismic survey lines. This data is presented in this dissertation with formal approval from the HKSAR Highways Department.
The correlation was established by means of comparison of frequency distribution graphs for different types of soil stratum. The results show that colluvium with SPT N values of 0-25, has P-wave velocities ranging from 450-550m/s. For the in-situ weathered soils, completely decomposed tuff (CDT) with SPT N values of 0-50, has P-wave velocities ranging from l,400-l,500m/s. Completely/highly decomposed tuff (C/HDT) with SPT N values of 76-100, has P-wave velocities ranging from 1,400-l,600m/s. Highly decomposed tuff (HDT) with SPT N values of 95-125, has P-wave velocities ranging from 3,000-3,500m/s. These results are used for the establishment of rippability classification system, and subsequently in the rippability assessment for soils. For rock excavatability assessment, the Pettifer & Fookes excavatability graph is employed for the evaluation, with the availability of the laboratory point load test data and the discontinuity spacing data from drillhole logs. Results of the assessment reveal that simple excavators such as backhoe is adequate for excavation in colluvium, whereas for the corestone-bearing saprolitic soils, a variety of soil and rock excavators is deemed necessary. Excavation in tuff bedrock is mainly by Caterpillar D6, D7 & D8 tractors, and sometimes by Caterpillar D9 & Dll tractors or hydraulic breakers while encountering locally stronger rocks.
Since the assessment in this study is both site- and time-specific, it is essential to point out the potentially influencing factors so as to make the rippability/excavatability classifications more pragmatic. Seven factors, including rock type, Quaternary geological processes/history, geomorphological processes, regional topography, thickness of overburden soil, nature of superficial soils and groundwater conditions/soil moisture content, which interact to affect the evaluation of the terrain excavatability, are discussed following the assessment. Apart from these factors, there also exist some
limitations relating to the inadequacies and lower resolution of seismic geophysical method as compared to conventional intrusive drillhole techniques.
It should be emphasized that the soil rippability and rock excavatabihty classification schemes should be revised from time to time as excavation technology become increasingly advanced. Predictions made using the devised classification schemes need to be compared with the performance actually achieved under various geological and site conditions.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:seismic waves excavation china hong kong
Date of Publication:01/01/2004