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가설 설정
- 3m, the tests were performed with rope turns of2% and 3%, and the results are shown in Fig. 12. The average rod energies are 0.284 KN-m with 2% turns and 0.255 KN-m with 3 % turns.
- 2) Rope condition in rope-pulley system has slight influence on transierred energy. It was found that, on the average, new rope would have about 6% reduction of rod energy, compared with used one.
제안 방법
- A series ofSPT was performed at three sites, measuring the energy transferred to drilling rod usin응 PDA. Through the investigations on rod energy ratio for various test conditions, following conclusions are drawn.
- Donut hammer was used in this site to investigate the average rod energy ratio and the effect of assembly of the rope pulley system. Most tests were performed at a depth of 6~7.
- Since the main purpose of SPT is to evaluate the design parameters, N-value should be converted to desired parameters by empirical correlations and the error in evaluated parameters is likely to be reduced by measuring more reliable N-values. For identical soil conditions, level of energy transmitted to the split spoon sampler is influenced by many factors related lo the detailed testing procedure and equipments, and, therefore, measured N-values would not be identical if di fl erent SPT Rod Energy Ratios for Three Types of SPT Hammers 119 types ofhammer, release mechanism, and anvil were used. The equipments and practices used for SPT at different parts of the world are also different, and the measured N-value can not be directly comparable.
- In this study, a series of standard penetration lests were performed at three construction sites for different combinations of hammer, rope condition, and number of rope turns. Energy transferred to drilling rod due to the impact of SPT hammer is evaluated using PDA(Pile Driving Analyzer) with an instrumented rod.
- The first borehole at this site was drilled and tested with the MAD hammer for the investigation of subsoil to a depth of 9m. Instruments and procedures were changed several times durin으 the test to investigate the effects of influencing factors on rod energy. The tests for each items were carried o니t at the same depths of'diflerent boreholes nearby so as to remove the 이Teel of ground condition.
- 1. The measuring rod used for the tests was designed to have the same size with AW-rod for direct connection to rod stem. Due to high frequency signals generated by the steel to steel impact between the hammer and anvil, piezo-resistive type of accelerometers were used, and a high sampling rate of 20kHz was required when converting the analog signals to digital form.
- Instruments and procedures were changed several times durin으 the test to investigate the effects of influencing factors on rod energy. The tests for each items were carried o니t at the same depths of'diflerent boreholes nearby so as to remove the 이Teel of ground condition.
- Since the fi'iction interfere the free fall of the hammer, the condition of rope is expected to affect the velocity energy. To investigate the effect of rope condition on N-value, rod energy ratio was measured for donm and safety hammers, with a rope used more than a \ ear and a new rope, at P-site. Results of tests are summarized in Table 3.
대상 데이터
- S-Site is in Sang Moon-Dong, northern part of metropolitan Seoul, and the lest was a part of subs 니 rfhee investigation 5 story residential building. The site is mainly consisted of weathered soil and weathered rock.
- 5 —9m with a large size boring macliine and steel casing was installed throughout the depth to support the borehole wall. The SPT were carried out at two groups of boreh이es(Pl, P2), which were located at 200m distance.
- P-siLe is located at PaJu city, northern Kyunggido. The site is a construction Held lor 15 story apartment complex. Boreholes were excavated down to 7.
- Y-siie is located at YoungJong-do where new airport is under construction. The site is mainly consisted of soft to medium clay. Borehole was drilled to a depth of 15.
- The site is mainly consisted of weathered soil and weathered rock. The system for SPT is consisted of 63.5kg donut hammer with an anvil of 8.5cm diameter and 10.5cm height. A 20mm diameter manila rope was employed with the 2% rope turns around the cathead and appeared lo be used more than 6 months.
- 8 shows the distribution of rod energy for each blow by safety hammer with a used rope wrapping cathead 2 % turns. The tests were performed at the depths of 6m and 7.5m. The average rod energy measured at 6m is 0.
성능/효과
- 1) The average rod energy ratio of donut, safety, and MAD hammer are calculated as 42.1%, 66.0%, and 57.3% of the theoretical potential energy, respectively.
- 3) The rod energy were measured with number of rope turns of 1%, 2%, and 3%, and it was found that transferred energy is 6 - 8% larger with 1 % rope turns and 4 ~10% smaller with 3% rope turns.
- 4) The equation to correct N-value, measured by different test conditions, to 60% of theoretical potential energy was proposed.
- It was just after 1980's Lliat researchers (Skempton, 1986; Kovacs and Salomone, 1981; Robertson et al, 1983) began to be concerned about the energy transmitted to SPT rod. It was concluded tlial the transferred energy during typical SPT of early days was about 50 ~60% of the theoretical potential energy of hammer and that many different test systems used around the , world deliver a wide range of impact energy. Skempton(1986) suggested Lo use the N-value corresponding to the 60% of the theoretical energy (N6q) and provided the correction factors lor various SPT testing system and procedure.
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