里面有许多专业术语都看不懂翻译软件翻译的不准确希望各位大侠帮忙一下诚心感谢文章如下ABSTRACT: Piles are often driven open ended into dense sand with the aim of increasing the ease of penetration of the pile. Generally, the pile tip contains an internal driving shoe in order to allow soil to enter the pile, forming a soil plug. The type of shoe influences the length of the soil plug and the ease of penetration of the pile. The paper describes the results of field tests undertaken on a 2.2 m long, by 51 mm diameter, pile, fitted with five different driving shoes and driven into dense sand. Dynamic and static load tests showed good correlations between the type of driving shoe, plug formation rate, and eventual end bearing capacity. It was found that the dynamic measurements provided a lower bound estimate of' axial capacity. The measured capacities were significantly higher than those estimated from current design codes.l. INTRODUCTIONPiles are driven open ended to increase the ease of penetration, particularly when dense sand layers exist in the soil stratigraphy. This enables the pile to be installed to the full design length and thus the design capacity of the pile to be obtained. This is especially relevant to long piles which arc often designed for friction, with the end bearing component making little contribution to the final capacity. In this mode of penetration a plug of soil forms up the middle of the pile. This project looks to enhance the knowledge base that exists (eg Brucy et al, 1991), on how this soil plug affects the performance of the pile, by analysing a number of' field tests that include both open and closed ended piles. Specifically, the aim of the research was to investigate:a) How different driving shoes affected the penetration of' the soil into the pile. and the required driving energy.b) How the ugging ratio? (incremental ratio of length of soil plug to penetration of pile) affected the shaft and end bearing resistance of the pile.c) The accuracy of dynamic measurements of soil resistance for predicting the tensile and compressive capacity of driven piles.2. OVERVIEWA total of 15 field tests were performed at a site in Shenton Park, encompassing dynamic and static tests on a steel pipe pile driven into dense sand, using five different driving shoes. Further data were gathered from a previous thesis (Kain, 1993) which looked at 6 tests also performed at the same site. All 21 sets of' data were subjected to extensive numerical and comparative analysis.3. APPARATUS AND P ROCEDUREThe 2.2 m long pile was instrumented 200 mm below the pile head in order to take measurements of the transient force and velocity waves during the driving of the pile. The pile outside diameter was 51.1 mm with a wall thickness of 1.6 mm and a variety of shoes were manufactured so as to produce different rates of plugging (see Figure l). In addition to the extreme cases of (a) closed ended (solid shoe), (b)flush pile (no shoe), three different driving shoes were explored with the aim of maximising penetration of the soil plug into the pile. The shoes doubled the wall-thickness of the pile at the tip, and extended up to one radius up the inside of the pile allowing the soil to dilate and move easily up the pile once past the sleeve. Once driven to the final embedment static load tests were performed so that an accurate measure of the pile capacity (both compressive and tensile) could be obtained.3. 1 Design CalculationsThe soil consisted of a dense sand with a cone profile as shown in Figure 2. From this profile an estimate of the pile capacity may be obtained using appropriate averaging formulas for both tip (Fleming et al, 1992) and skin fiiction resistances. Qs =兀dlf =（兀）(0.0511)(1.7)(19.31) = 5.3kN Qb = Abqe = (0.00205 1)(38125) = 7.82kN Qt = Qb +Qs = 5.3+782 = 13.lkNAn alternative approach is to adopt the design code (Fleming et al, 1992) to determine pile capacities. Given that Ts = Kai tan8 and assuming that K = 1.2, 8 = 270 and y = 17 kN/m3 then Ts = 10.4z kPa (where z is the depth in m). Also qb = Nqa: = (40)(17)(1.7) =1156kPa where Nq = 40 for # = 300. The design estimate of the pile capacity is therefore Qs = 2.4 kN and Qb = 2.4 kN and thus Qt = 4.8 kN. These two approaches vary significantly, with the design code end bearing pressure being a factor of 3 less than that actually mobilised by the cone penetrometer. As well, the design skin friction is a very conservative estimate of the friction profile mobilised in the CPT.3.2 Dynamic TestingThe pile was driven into the ground by means of a 4 kg drop weight suspended from an aluminium tripod. At pile penetration intervals of 100 mm throughout the installation measurements were taken consisting of pile penetration, plug penetration, and blow count. A drop height of l m was generally used for ease of driving. Eight dynamic tests were performed at each of the embedment levels of 600 mm,1200 mm, and 1700 mm. The dynamic tests were further divided into four having a drop height of 0.5 m and four having a drop height of l m. An example stress wave for pile B06 is shown in Figure 3 at a penetration of 1700 mm with a drop height of 0.5 m. It can be seen that both the force and velocity curves coincide until the soil resistance is encountered (and upward waves are reflected); they then diverge for a full shaft resistance of approximately 5.5 kN. Under dynamic loading the pile fails in an unplugged fashion and this shaft resistance represents the full mobilisation of both internal and external skin friction. The end bearing resistance is fullymobilised as illustrated by the large reflection of the velocity wave from the base of the pile and apermanent set of 2.5 mm. A dynamic capacity maybe calculated from the stress waves by solving the wave equation, which considers the pile as an elastic bar and assumes only axial motion. The total dynamic capacity of 8.4 kN, may be modified by a parameter, jc, to produce an estimate of the static capacity, known as the Case capacity (Rausche et al, 1985). The soil parameter jc is obtained from tables or from previous static load tests performed at the site. For pile B06 the Case estimate of static capacity is 6.0 kN reached by assuming jc = 0.1, which is appropriate for the site.3.3 Static TestingStatic load tests in both compression and tension were performed by steadily jacking the pile into the ground (or out of it) and recording load and displacement data points. A typical load settlement curve is shown in Figure 4 for pile B06. It may be noted from this Figure, that the two strain gauges on either side of the pile lead to different estimates of force. This is due to bending effects which are present for both dynamic and static tests. The actual force is obtained by averaging the two sets of data.There are various methods of determining the ultimate capacity from these plots, especially in compression.A common method takes Qstatic as the load required to displace the pile 1力0 0f its diameter. Whitaker (1976) suggests that this definition may be applied to the results of a constant penetration rate test, and probably represents a lower limit to the penetration required to develop the full end resistance. For the small pile of diameter 0.0511 m the minimum displacement required is 5.1 mm. Subsequently this method produces a lower bound capacity of about 7.3 kN in compression for pile B06. The tensile capacity on the other hand is easier to deduce as only external shaft friction is mobilised under failure. This should be fully mobilised by the displacement of 5.1 mm and thus an estimate of tensile capacity for this particular test is 3.1 kN.3.4 Numerical AnalysisThe dynamic data have been analysed using the Impact program (Randolph, 1992) where soil parameters are varied until a match between measured and numerically derived stress waves is obtained. A typical match is shown for pile B06 in Figure 5. A summary of soil parameters deduced from the stress-wave match shown in Figure 5 is tabulated in Figure 6. It should be noted that these parameters are somewhat operator dependent! It can be seen that the external shaft friction is 3.82 kN whereas the tensile capacity was only 3.1 kN. It has been shown that in non cohesive, free draining soils (such as the sand encountered at Shenton Park) the shaft capacity derived under tensile loading, Q, is significantly lower than under compressive loading Qc (De Nicola and Randolph, 1993). Reasons for this difference in shaft capacities include Poisson扭ratio effect and the change in the total stress field due to the direction of loading. Assuming an interface friction angle 8 = 270 the expression derived by De Nicola and Randolph produces
a ratio Q/Qe of 0.87. The results from Impact suggest the ratio is of the order 0.81 but it should be remembered that the division between shaft and base capacity deduced from the Impact program is operator dependent.
From the Impact output an estimate of pile capacity can be obtained. In all tests undertaken the pile was shown to fail in a fully plugged fashion under static loading; therefore the estimated pile capacity from the dynamic measurements for pile B06 is 7.45 kN which compares well to the measured static value.
