Abstract
Identification of ground conditions is a very important step before starting to build any geotechnical structure. Geotechnical investigations are performed to determine the soils conditions and to evaluate the cost-effectiveness and design of a proposed engineering construction. Fines contents (FC) in sandy soils also play an important role in the engineering design of geotechnical structures, particularly in areas prone to earthquakes. The Screw Driving Sounding (SDS) is a new in-situ test in which a machine drills a screw point into the ground in several loading steps while the attached rod is continuously rotated. At the same time, a number of parameters, such as torque, load and speed of penetration are logged at every rotation of the rod. Because this machine can continuously measure these parameters, an interpreted overview of the soil profile throughout the depth of penetration can be obtained. In this study, a large number of tests were conducted adjacent to boreholes in New Zealand. An attempt was made to correlate the SDS parameters to the soil type as described in the boring logs. In addition, samples from several SDS sites were obtained and sieve analyses were performed in order to formulate a relationship between the fines content and the SDS parameters. From the results, charts were developed to show how soil can be classified and fines content can be estimated using the SDS data. As a simple, fast and economical test, the SDS method can be a reliable alternative in-situ test for soil characterisation.
1 INTRODUCTION
Adequate information about ground conditions is very important for analyses, design and construction of geotechnical systems. Recently, the use of in-situ soil testing has increased in geotechnical engineering practice mainly due to the development of field testing procedures, better understanding of soil behaviour, and identification of the drawbacks and limitations of some laboratory testing (Eslami & Gholami 2006). Standard penetration test (SPT) and cone penetration test (CPT) are the most common in-situ tests around the world due to their capability in accurately characterising soils. Other field tests which are being used in geotechnical practice, such as dynamic cone penetration test (DCP), Swedish weight sounding (SWS), flat dilatometer (DMT), pressure meter test (PMT), vane shear test (VST) and Piezo-cone (CPTu), are less popular than SPT and CPT. Each of these tests applies specific loading pattern to identify the corresponding soil properties, such as strength and/or stiffness (Mayne, 1988). In order to perform some in-situ tests, such as the SPT, PMT and VST, boreholes are required; however, to conduct CPT, CPTu, SWS and DMT, no boreholes are needed. The SDS machine has been recently designed and developed in Japan to reduce the drawbacks of SWS, as well as to include a method of measuring the friction on the rod. The machine previously used for the SWS test has been modified and improved so that it is suitable for the SDS test. In this method, a rod is drilled into the ground in several loading steps at the same time that the rod is continuously turned. An empirical relationship has been developed between the soil parameters and the SDS data (e.g. Tanaka et al., 2012; Maeda et al., 2015); In this study, based on the results of the SDS tests which were conducted adjacent to boreholes in different soil types around New Zealand, a soil classification graph is presented and it is shown that how the soil type can be identified using the SDS data. Furthermore by performing sieve analysis on the samples obtained from the boreholes, a correlation is developed for obtaining fines content directly from the SDS parameter. The SDS test is fast, small in size and relatively cheap compared to other in-situ testing methods and these advantages make it a good alternative for soil characterisation.
2 Screw Driving Sounding test
2.1 SDS test procedure
A monotonic loading system is used in the SDS test and the number of load steps is set to 7. The rod is continuously turned at a constant rate of 25 rpm while the test is going on. The load steps are 0.25, 0.38, 0.50, 0.63, 0.75, 0.88, 1.0 kN in this order, and the load is increased at every rotation of the rod. The parameters measured in the test are: maximum torque (Tmax), average torque (Tavg), minimum torque on the rod (Tmin), penetration length (L), penetration velocity (V) and number of rotations of rod (N). These data are measured on the completion of each revolution of the rod. In SDS, the rod is automatically moved up by one centimetre after each 25cm penetration and then rotated to measure the rod friction. Due to the effects of rod friction on the measured torque and load during penetration, the amount of measured load and torque required for penetration is greater than that required at the screw point. The rod friction can be divided into a vertical component (Wf) and a horizontal component (Tf) as the rod rotates and penetrates into the ground. The corrected torque (T) and corrected load (W) are defined as follows:
T = Ta -Tf (1)
W = Wa -Wf (2)
Where Wa and Ta are the total applied load and applied torque by the SDS machine, respectively. The procedure of calculating Wf and Tf is explained by Tanaka et al. (2012).
