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Editor’s Note: this topic is sort of the “Holy Grail” of vibratory pile driving, and has certainly been studied since the publication of this book, mostly in Europe. The Soviet perspective on the problem is interesting. It is also interesting that they discuss the bearing capacity of drilled shafts installed with vibratory assist, as were discussed in the previous chapter. Also discussed in the previous chapter is the business of the liquidity index .
The bearing capacity of piles depends on soil conditions. method of immersion and construction of piles. The first data on the bearing capacity of solid section piles and shell piles driven by vibration were published over 30 years ago (E.M. Perley, A.M. Rukavtsov, 1956.).
Subsequently, these authors jointly, and then E. M. Perley, carried out a program of experimental and field studies of the bearing capacity of steel tubular and reinforced concrete piles of a solid section, shell piles and hollow round piles.
In various soil conditions, the bearing capacity of piles driven by single-acting air-steam hammers with ram mass of 2.5, 3 and 6 tons, a diesel hammer with a ram mass of 2.5 tons, low-frequency vibratory hammers VP-l and VP-3, as well as a high-frequency vibrator VPP-l.
The results of these studies are as follows:
- in soft-plastic clay soils, the bearing capacity of hanging piles with a closed lower end, driven by vibration, can be 30-40% less than the bearing capacity of the same piles driven by impact, and shell piles with an open lower end by 10-20%. Apparently, for shell piles submerged by vibration, the effect of disturbance in the structure of clay soils on their bearing capacity is much less than for piles with a closed lower end. In soft-plastic clays after immersion with vibrators, in some cases it is recommended to finish the piles with hammers to the design capacity;
- in hard-plastic clay soils, the bearing capacity of shell piles driven by vibration is the same as when driven by impact;
- in sandy soils, the bearing capacity of piles and shell piles driven by vibration is 20-30% higher than those driven by impact; under vibration effects, sandy soils are compacted.
A very relevant, but very complex issue is the question of determining the bearing capacity of piles according to the parameters of their vibration penetration. Difficulties arise due to the fact that in the process of vibration immersion, the dynamic resistance to immersion can differ significantly from the static one.
In the mid 1950’s B.P. Tatarnikov developed an empirical formula that relates the bearing capacity of a pile to the power of a vibrator at the end of the pile driving. The empirical coefficients included in this formula turned out to be inconsistent for the same soil when the amplitudes and frequencies of oscillations changed, and therefore the formula gave significant errors (by a factor of 2 or more).
An attempt to apply the usual formula of the dynamic method for driven piles with the substitution of the conditional calculated impact energy of the vibrodriver, determined by its driving force, was made by A. A. Luga (1970). This method also did not give stable results due to the variability of the calculated impact energy of the same vibratory driver when changing the types of piles and soil conditions.
A methodical approach to solving this problem can be based on an analysis of the energy balance of the vibration driving process, as a result of which a relationship is established between the vibration driving parameters – the amplitude and frequency of oscillations with the dynamic and static resistance of the soil to pile driving (Yu. I. Neimark, 1953).
Based on the analysis of the energy balance of the pile oscillation cycle at the final stage of its vibro-immersion at a low speed (up to 20-25 cm/min) A. s. Golovachev (1968) proposed the formula
S = achieved calculated bearing capacity of the pile, kN;
N = the useful power of the vibrator’s engine, consumed for the movement of the vibrosystem, kW;
A = oscillation amplitude, cm;
n = eccentric rotation frequency, min-1;
k – coefficient of soil homogeneity, equal to 0.7;
= a coefficient that takes into account the decrease in soil resistance during vibration driving and the peculiarities of the process dynamics at the final stage of pile driving at low speed.
The values of the coefficient depend on the following factors: the coefficient kb of the reduction of the lateral resistance of the soil in the steady mode of vibration penetration; the ratio of the limiting elastic settlement of the soil under the lower end of the pile and the amplitude of its oscillations, the ratio of the lateral and frontal soil resistance. The higher the last ratio, the closer the coefficient approaches kb. The values of these coefficients are given in a number of works by A. S. Golovachev.
Taking into account the peculiarities of the dynamics of the final stage of vibration driving of piles and shells, as well as experimental data, the values are in the range of 4.5-7.5 for water-saturated sands and 3.5-5 for low-moisture sands. As the sand size decreases, the values increase within the corresponding range.
For clay soils, the refined ranges of actual values of for various soil consistency coefficients are given below:
Equation (81) was used in a number of regulatory documents of the Gosstroy of the USSR, the Ministry of Transport and other ministries, where the values of were taken with various underestimations, providing a guaranteed reserve of the bearing capacity of piles.
The experience of using Equation (81) showed satisfactory accuracy both when using conventional vibratory hammers and vibro-pressing units VVS-19/11 and VVS-32/19. Errors in determining the bearing capacity of a pile according to Equation (81) are usually within +10 to -30%.
The disadvantage of this formula is the use of a single coefficient for piles of various configurations and, as a result, an undifferentiated approach to determining the lateral and frontal soil resistances. As a result, for greater ratios of lateral to frontal soil resistance in the process of sinking, the formula gives the more accurate data, and with significant frontal resistances, it underestimates the achieved bearing capacity of piles by 20-30%.
The bearing capacity of filled piles on the ground is closest to the bearing capacity of these piles in terms of the material from which the pile is made, in the case of using vibration technology for constructing such a foundation design (N.A. Makovskaya, 1974).
This is due to the following processes accompanying vibration:
- compaction of the soil surrounding the well, to a greater or lesser extent, depending on the well drilling technology;
- an increase in the physical and mechanical properties of the concrete mixture in the process of pile formation due to forced (vibration) compaction;
- compaction of the soil surrounding the well, due to the ramming of concrete into the walls of the well during the formation of the pile shaft and the formation of corrugations on its surface;
- ensuring the continuity of the pile shaft in the process of vibratory concrete placement under the influence of a strong dynamic impact.
In VNIIGS, the bearing capacity of piles manufactured using vibration technology was studied both using vibration equipment for general construction purposes (E. M. Perley, N. A. Makovskaya, 1973,) and specialized (M. G. Tseitlin, B. B. Rubin, V. E. Trofimov, N. A. Makovskaya, 1981.)
At a number of construction sites in Ukraine and Belarus, more than 500 filled piles were made with a vibrating grab PV-500, of which about 50 were tested with a static indentation load. All wells for filled piles were developed dry with a vibrating grab. Concreting of the pile shaft was carried out with slow-moving concrete mixtures with layer-by-layer compaction with a deep vibrator. In general, soil conditions can be combined into two groups for objects: sandy in the form of homogeneous layers of silty macro- and microporous sands; clayey, in some cases representing homogeneous loams of hard-plastic consistency, silty or silty sandy loams, underlain by moraine loam with the inclusion of pebbles, gravel and boulders.
Determining the design loads allowed on the pile and comparing them with the results of static tests (N. A. Makovskaya, V. E. Trofimov, 1981) showed that the calculation of the bearing capacity of piles in accordance with SNiP 11-02-03.85 made using a vibrating grab in homogeneous low-moisture silty sands is permissible for vibro-manufactured piles. The design loads on piles made in clayey soils were determined according to the “Guidelines for the design, installation and acceptance of foundations from bored piles” (RSN 263-74). The use of recommendations for the calculation of this standard, which makes it possible to increase the design resistance under the heel of the pile, led to a close convergence of the results of the calculation and the data of static tests of piles.
When referring the bearing capacity of piles along the lateral surface to the bearing capacity of the base, obtained from the calculated data, the following conclusions were made:
- in sandy soils, a large share in the total bearing capacity of the pile falls on the heel of the pile;
- in clayey soils, the lateral surface is most loaded.
This is apparently due to the fact that under the influence of vibration in sandy soils, the bottom of the well is compacted. In clay soils during vibration, their thixotropic properties are manifested. At the same time, it can be assumed that the vibration effect is stronger in the bottom hole than along the side walls of the shaft. As a result, under the heel of the pile, the clay soil is mixed with the concrete mixture and the formation of a zone of low strength in comparison with the soil of natural composition. At the same time, the side surface of a pile molded in clayey soil, under the influence of the consolidation process, turns out to be more strengthened in comparison with the base. From this follows the conclusion that before concreting the bottom of the well must be prepared accordingly. In sandy soils, the bottom should be compacted with special tamping agents; in clay soils, concrete or crushed stone should be tamped into the bottom of the well.
The experience of using the vibration complex PVN-2 in Minsk also made it possible to draw conclusions regarding the influence of the method of forming the shaft of a filled pile, manufactured using vibration technology, on its bearing capacity during vibration.
Filled piles were used as foundations for a light tower instead of groups of prismatic piles that resist the wind load. The bearing capacity for pulling out the prismatic piles adopted under the project was 400 kN. The total pull-out load on the support was 9000 kN.
To increase the resistance of filled piles to pull-out forces during the concreting process, the casing pipe was periodically upset during extraction.
The experience of examining excavated piles molded using this technology (E. M. Perley, N. A. Makovskaya, 1972) suggests that this measure contributes to the ramming of concrete into the borehole walls. According to the same studies, the magnitude of the dynamic pressure that occurs in the concrete mixture when removing the casing (without upsetting) is above the static 20-50 MPa. The casing pipe was set in accordance with the geological profile of the site only within the strata of medium density soils with a deformation modulus > 20 MPa.
The results of static tests of two piles showed that when the pile moved (exited) by 8.5-9.5 mm, the pull-out bearing capacity of the pile was 1600 kN. At the same time, the load was far from the limit.
According to the calculation, the bearing capacity of the filled pile on the pull-out was 800 kN without taking into account the ramming of concrete into the borehole walls, i.e., 2 times less. At the experimental site of the Research Institute of Foundations and Underground Structures in Nikopol (A. A. Grigoryan, 1972), a set of comparative static tests of filled piles with a diameter of 500 mm and a length of 16 m was carried out.
The following equipment was used for the production of filled piles: a BS-1 percussion drilling machine with a tamper with a diameter of 0.38 m, a set of SO-2 type drilling equipment with a mechanical expander of the Ukrgidrospetsfundamentstroy trust, and a longitudinal-rotary vibration unit of the PVN-type 1 design of VNIIGS (B. B. Rubin, 1973).
The BS-1 machine installed bored piles with excavation of soil from the well over its entire depth with cleaning and ramming, bottoming, with excavation of soil from the well and without bottom compaction, as well as piles made by punching a well to the full depth without excavation and by hole punching at the last 3 m in depth with excavation of soil from the rest of the well.
Bored piles with a widened heel diameter of 1600 mm were manufactured with a set of drilling equipment SO-2.
Two types of piles were made using the PVN-1 vibration plant: with vibration driving of a hollow casing pipe open from below to the full depth, followed by its vibration extraction and unloading from the ground under the action of vibrations, with vibration driving of a pipe open from below to a depth of 12 m and its unloading under the action of vibrations, then the remaining 4 m were punched with a pipe 426 mm in diameter with a tip 500 mm in diameter without excavation. All piles were concreted using the VPT method with a concrete mixture of grade 300 with a cone draft of 16-18 cm.
The experimental site on which the work was carried out, to a depth of 20 m, was a homogeneous array of m highly porous loess loams with a dry soil density of 1.4 g/cm3 and a natural moisture content of 4%.
The results of testing the bearing capacity of piles made by various methods showed that 1 m3 of a pile made using vibration technology in a punched hole has the highest bearing capacity. Note that the volume of concrete in the manufacture of this pile turned out to be 1.5 times less than the volume of concrete in the manufacture of piles with a wide heel.
Based on the analysis of the results of static tests, it was concluded that it is possible to calculate the bearing capacity of piles made by the PVN-1 installation in punched holes in loess soils using Equation (10) SNiP 11-02-03.85, clause 5.9 with the coefficient of working conditions mf = 1.0 The same, with excavation to the full depth, but with the coefficient of soil working conditions mf=0.9.
Production experience in manufacturing filled piles using vibration technology made it possible to draw the following conclusions:
- the use of specialized vibration equipment makes it possible to build economical and technically feasible foundations from filled piles of high bearing capacity;
- rammed piles made using vibration technology with excavation have a greater bearing capacity than bored piles of equal size;
- The bearing capacity of piles manufactured using vibration technology without excavation corresponds to the bearing capacity of factory-made driven piles with significant savings in metal.
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