Laboratory Model Test and Analysis of Excess Pore Water Pressure Dissipation of Pile Driven by Tapered - Perforated Tubular Pipe Pile

Through methods based on the indoor model test, the use of static pressure pile with three different hole arrangement and three different kinds of taper cone - bore log shape composite pipe pile bored into soil by static pressure,and after observing the excess pore water pressure caused by pile driving when testing, and analyzing the process of various pipe pile static pressure, the change rule of the excess pore water pressure varying with taper hole arrangement, size, radial distance, pile depth has been gotten. It is found that stellate shape has the best dissipation rate of excess pore water pressure under the condition of hole arrangement and taper 1/70, which is of certain guiding significance for promoting the engineering application of tapered - perforated tubular piles.


Introduction
Prestressed concrete pipe pile [1] has reliable pile body quality and good pile quality. Since the end of 1990s, prestressed concrete pipe piles have been widely used in deep soft foundation treatment of expressways and railways in China.However, in the soft clay foundation with high water content, the excess pore water pressure will be produced during the construction of static pile, which will have an adverse impact on the surrounding structures.
To effectively reduce adverse influence on surrounding environment when doing the static pressure pile, based on literature [2] Tapered -Perforated Tubular Pipe Pile not only inherits the perforated pipe pile [3] - [7] reduces excess pore water pressure and acceleration time and space in a maximum way , but also inherits the conical pipe pile [8] - [9] increases rate of replacement, improves the advantages of bearing capacity of composite foundation, therefore tapered-perforated tubular pipe pile is adopted to study the rule of super pore water pressure varying with time, depth and radial distance, hole arrangement, and the size of the taper.In this paper, three kinds of tapered-perforated tubular pipe piles with different openings and three kinds of different taper sizes are used to carry out staticpressure sinking pile, and the test results are of certain guiding significance for the analysis and demonstration of taperedperforated tubular pipe pile technology and its application.

Preparation of model box
Make a model box of 1.5m×1.5m×1.5m with angle steel and toughened glass.The specific manufacturing process is as follows: 1.5m×1.5m×1.5m angle steel frame is welded with No.5 angle steel, and the angle steel frame is welded centrally on 2m×2m×0.01m steel plate. The four sides of the model box are spliced by two type 2 tempered glass and one type 3 tempered glass respectively, which is convenient for observation.The joints of tempered glass are not sealed, which ensures that excessive free water in soil can flow out from the joints, as shown in Figure 1.

Preparation of soil samples
The soil samples selected in the test are all from silty clay around Nanchang, which are dried, crushed and screened indoors. According to the test requirements, water is added to the soil and stirred.The treated soil samples are filled into the model box in layers and consolidated by static pressure.Before the static pressure pile driving test, the geotechnical test is carried out to measure the relevant physical and mechanical properties of the soil samples.

Sample preparation of model pile
The purpose of this test is to study the influence of hole arrangement and taper on the generation and dissipation of excess pore water pressure.In order to make the pipe pile more easily, stainless steel material is selected for pipe pile sample preparation, considering that a little soil will enter the pipe pile cavity during pile sinking, resulting in soil plug effect.Therefore, the bottom of the model pile is sealed with steel to prevent excessive soil from entering the inner cavity of the pipe pile.
In this indoor test, there are five types of pipe piles selected, as shown in Figure 2.According to various pile type hole arrangement methods, the pile body is bored at intervals of 200mm along the pile length direction, and three layers of pile holes are respectively arranged at 200mm, 400mm and 600mm pile lengths.

Burying of pore pressure gauge
The location of the buried hole pressure gauge is very important to the data acquisition and research analysis, therefore, nine LY -350 pore water pressure gauge were used in this test, with numbers for U1 -U9 respectively , buried in the soil of different position, horizontal distance respectively pile body 3 D, 6 D, 9 D (D is model of pipe pile diameter is 107 mm), vertical buried three layer respectively 400 mm distance from the soil surface, 600 mm, 800 mm, the concrete layout diagram as shown in figure  3.Fill in the first model to the embedded depth, according to the scheme using the tape measure, measure, determine the pile pile body position, use prepared fine sand for leveling, hole pressure gauge is placed, the wire side toward the DH -3818-2, set with a side of porous stone level up, surface covered with fine sand, wire embedded in the soil is S type, buried pore water pressure gauge and continue to fill on the surface, and when filling, don't produce too much disturbance to its, guarantee the stability of pore pressure meter level, lest affect testing accuracy, to reset test equipment before the test, to ensure that the test data is relatively accurate, which is advantageous to the result analysis.  Fig. 3 Schematic diagram of pore water pressure gauge distribution

Test steps
The main route of the test: determining the position of the pile sinking → determining the scale of the pile body → determining the direction of the hole opening of the pile body → data collection during the piling process (leveling the data before collection) → keeping the soil moisture content stable (covering the soil surface of the model box with impermeable film during the test) → data collection after the piling is completed.The test process was carried out in strict accordance with the Pore Water Pressure Test Procedure [10] (CECS55:93), and the test process was designed for 60h.

Analysis of changes with time
Based on the whole test, the variation law of excess pore water pressure with time during pile sinking of different types of pipe piles is analyzed, as shown in Figure 4, which is the relationship curve of excess pore water pressure in soil around the pile with time caused by five different types of pipe piles during static pressure pile sinking. From each curve it can be found in figure 4 the change tendency is roughly similar, which can be concluded that:in the process of static pressure sinking pile, as the pile sinks, the soil around the pile is subjected to the compressive action during the pile sinking process, and the pore water pressure in the soil cannot be dissipated in time, so a large pore water pressure is generated instantly, that is the excess pore water pressure is accelerating (0 to3 h) time (relation curve of the slope increases gradually, the shaking increase type), at the end of the pile driving process, pile piled pore water pressure within 4-60 (h) gradually dissipate over time (curve slope gradually decreases, and in a slow type)

The relationship with depth
From a local point of view, the variation of pore water pressure in various types of pipe piles with depth is analyzed.According to the observation and analysis in fig. 5, when the radial distance R is constant, the excess pore water pressure increases with the increase of depth H, and the dissipation rate becomes smaller and smaller (the tangent slope of the curve decreases gradually).

The relationship with radial distance
It can be concluded from the observation and analysis in Figure 6 that at the same time, when the depth H remains unchanged, the size of excess pore water decreases with the increase of radial distance, and the dissipation rate becomes smaller and smaller (the tangent slope of the curve decreases gradually).

Fig. 6
Variation law of excess pore water pressure with radial distance

Analysis of excess pore water pressure dissipation rate
Excess pore water pressure dissipation rate refers to the difference between the peak value of excess pore water pressure and the value of excess pore water pressure at a certain time point in dissipation stage after the peak value occurs.That is,

Dissipation rate=
In which Δp represents the difference between the peak value of excess pore water pressure and the peak value of excess pore water pressure at a certain time point in the dissipation stage after the peak value appears;p max represents the peak value of excess pore water pressure;p t represents the value of excess pore water pressure in static pile driving for t hours.
According to the formula, the excess pore water pressure dissipation rate of each group of pipe piles at 20h, 40h and 60h is obtained. By comparing and analyzing the excess pore water pressure dissipation rate of five groups of pipe piles at each time point, the difference of excess pore water pressure dissipation of each pipe pile is obtained. It can be seen from the data in Table 3 that the dissipation rate of excess pore water pressure of all kinds of perforated pipe piles increases continuously in (20-60h), and then the area is stable.Observed that under the condition of the same pile diameter and taper, the dissipation effect of excess pore water pressure in the star-shaped cross-over mode is the best, while under the condition of the same pile diameter and hole arrangement mode, the dissipation effect of excess pore water pressure in time and space of 1/70 pipe piles is the best.

Concluding remarks
1) Five different types of pipe piles will have certain excess pore water pressure around the pile in the process of static pressure sinking.Excess pore water pressure rises sharply within 0-3h and decline slowing in 4-60h.
2)When the radial distance remains constant, the excess pore water pressure will increase with the increase of depth, and its dissipation rate will gradually decrease;When the depth remains constant, the excess pore water pressure decreases with the increase of radial distance, and the dissipation rate becomes slower and slower.
3) By comparing three hole arrangement modes, i.e., one-way piercing, two-way piercing and starshaped piercing, it is found that the dissipation rate from large to small is star-shaped piercing, two-way piercing and one-way piercing.However, in the case of star-shaped piercing, 1/70 taper is the best, followed by 1/90 and 1/80 is the worst.The average dissipation rate of five kinds of conical-perforated column composite pipe piles is more than 60% on average, which indicates that conical-perforated column composite pipe piles can effectively improve the dissipation rate of excess pore water pressure in the process of static pressure pile sinking.