Editor’s Note: the term “cylinder piles” used in the section is a translation of a Russian term that is more exactly translated “shell piles” or “pile shells.” Since that term has an entirely different connotation in English (for one thing, it usually denotes a steel pile, while a concrete one is meant here) we use the term “cylinder pile.” It should be noted that the piles described here have significant differences from the Raymond type that is common in the U.S.
The period of rapid development and propagation of vibration technology in the driving of reinforced concrete cylinder piles was 1952-1963. In the construction of bridges over the rivers Usa and Klyazma, the vibrator VP-1 was successfully used for the first time in 1952-1953 for driving cylinder piles 1.2 and 0.9 m in diameter, in conjunction with the removal of gravelly-sandy soil with an air lift and washing.
Further development of the vibration technology was associated with the construction of the Ukhan bridge over the river Yangtze in China and then with broad application in domestic bridge construction. The foundations of more than 200 bridges over the Volga, Oka, Dnieper, Severnaya Dvina, Don, Neva, Ob and others have been constructed with the vibration technology, and mooring structures were erected in Novo-rossiisk, Tuaps, Sukhumi, Kaliningrad, Leningrad, Nakhodka, Varna and Ventspilsa.
The successful application of deep foundations of cylinder piles made it possible to dispense with the use of caisson foundations almost completely and obtain a substantial technical-economic effect. The bridge construction costs were reduced by 20-35% here (K. S. Silin, 1962), and the concrete consumption was reduced 2-4 fold. In comparison with mooring points of reinforced concrete prismatic piles, the use of cylinder piles 1.2-1.6 meters in diameter, not filled with concrete, makes it possible to save up to 45% of the foundation concrete and reduce the man-hours of labor by 25% (V. V. Kapliev, 1966).
In industrial construction, the use of foundations of cylinder piles 1.2 in in diameter in comparison with foundations of clusters of prismatic piles assures a concrete saving of 25-35%, and of the hardware by 50-60%, a reduction in labor costs of 55-65%, and a shortening of the work time by 50-70% (I. G. Vypov and L. E. Litvintsev, 1981).
In the most general form, the technological scheme for constructing foundations of cylinder piles is described as follows. The cylinder piles consisting of separate sections of factory or on-site manufacture are placed at the driving site with a crane through a preliminarily prepared guide arrangement. Then the piles are driven into the ground with a vibrodriver, employing washing and extraction of the soil or the drilling out of obstacles encountered if necessary. In order to increase the bearing capacity of the piles, a broadening, filled with concrete by the feed method, can be constructed in their base.
Foundations of cylinder piles are constructed on dry land or in water (K. S. Silin, N. M. Glotov, and V. I. Karpinskii, 1966). The work in water is more complicated. The sequence of the operations in the construction of foundations in the water can be presented in the following order:
- The preparation of the reinforced concrete piles;
- The preparation on shore and assembly of the guide housing on flotation means for fixing the piles in the projected position during their driving into the ground;
- The arrangement around the guide housing of a special floating system with raising and regulating compound pulleys for installing the housing in the projected position;
- Removal of the guide housing from the floating system to the construction site of the buttress and consolidation as an anchor;
- Lowering the housing into the water, assembly of its successive sections and placement of the housing in the projected position in the scheme;
- Driving several piles to the’ prescribed mark through the stages of the guide housing and suspension of the housing on them;
- Freeing and removal of the floating system from the constructed support and driving the remaining piles into the ground;
- Removal of the soil from the internal cavities of the piles;
- Drilling holes in the base of the piles, cleaning them of sludge, installation of the fixture housings and the placement of concrete by the vertically movable pipe method with subsequent control drilling;
- Installation of a tongue-and-groove enclosure of the foundation pit on the perimeter of the guide housing;
- Removal of soil from the foundation pit and placement of a concrete mass by the feed method, joining the pillars.
There were special conditions in the construction of bridges in the building of the Baikal-Amur trunk line. In the case of the Amur the flow is rapid and the soil is rocky. Nevertheless, even under such conditions the bridge builders rejected caissons in favor of cylinder piles. In order to accomplish this proposal, it was also necessary to solve the problems of sinking reinforced concrete piles into the rock and their reliable fixation. Drilling machines with ordinary chisel bits did not furnish the proper effect at the Amur. The drilling problem was solved with the aid of an especially improved turbine drill.
The installation of the foundation of cylinder piles under the bridge supports under the conditions of the Baikal-Amur trunk line construction was accomplished in the following manner. A floating crane with a lifting capacity up to 100 tons lowers the five-section piles into the stages of the guide housing (the piles are 3 m in diameter and 30 m in height). Under the action of the vibrodrivers, the lower end of the piles cuts through the layer of bottom soil and rests in rock. After removing the soil from the pile cavity with a grab bucket, a reaction turbine aggregate is lowered in for drilling spurs in the rock. Then the inner cavity of the spurs and part of the pile are concreted. The operating site is enclosed with a tongue-and-groove wall.
The water is pumped out of the reservoir formed. The three upper sections of the pile are removed. The subsequent operations — installation of a concrete slab (grating) on top of the pile and construction of the buttress — are carried out in the dry.
When the foundations of cylinder piles are laid out on dry land, the technology of the work is substantially simplified. We shall examine the results of applying the complex of vibration equipment in the laying out of the foundation of a building for manufacturing purposes in Leningrad (M. G. Tseitlin, E. G. Godes, and V. M. Klementyev, 1976).
The complex of vibration equipment, which was used on one of the objects, consisted of a vibrator for driving cylinder piles, a vibro-grab bucket for extracting the soil from their cavity, a grooved vibrodriver for setting up the guide conductor and a depth vibrator on a tube or rod for the placement and compaction of the concrete mixture in the cylinder piles.
Cylinder piles 1.2 m in diameter were driven to the 19-meter mark. Their driving was accompanied by periodic extraction of soil from the cavity.
In order to assure the verticalness of the piles and their precise positioning in the plane, the piles were driven through the joint guide conductor for each row of piles. All the elements of the conductor were prepared from the Larsen-V pile, where the vertical pile pillars were driven to a depth of up to 4 m with the VPP-2 vibrodriver. The cutting section of the cylinder pile was lowered through “he guide conductor into the sump and centered. The vibrodriver VTJ-1.6 was fastened on the knife section of the pile prepared for driving; when it was switched on, the section was driven to the upper flange (Figure 62) of the guide conductor, after which the vibrodriver was disconnected from the pile and the soil extracted from its cavity with the vibrating grab bucket. Then the pile was built up and driven to the designated mark.
After extraction of the soil from the driven cylinder pile, the reinforcing cage was lowered into its cavity and then the pile was filled with a grade 200 concrete mixture with a settling of the cone of 4-6 cm. The concrete mixture was poured into the pile cavity with buckets through the receiving bunker and the guide tube and then the mixture was compacted with the aid of a tube fastened to the VPP-2 vibrodriver. The upper layer of concrete was compacted to a depth of 2 m with an IV-59 vibrocompacter. Then, in order to increase the degree of the concrete compaction, high-frequency IV-60 vibrators were used, which were fastened to the tubular rod with a length of 15 m.
An analysis of the results of the studies revealed that the use of the complex of vibration equipment in the construction of foundations of cylinder piles on dry land makes it possible to substantially increase the rhythm of the work due to an increase in productivity in fulfilling all the basic operations associated with driving the piles.
A successful vibrodriving of cylinder piles was accomplished in the first stage of adoption basically with the VP-1 and VP-3 low-frequency vibrators (6-7 Hz).
Adhering to the concept of the independence of the driving ability of the vibrators on the magnitude of the dynamic force, B. P. Tatarnikov developed the low-frequency vibrodriver NVP-56. Powerful vibrators — the two-frequency VP-160 and the one-frequency VP-170 — were developed at the same time. The said vibrators are similar in power, but considerably different with regard to amplitude-frequency mode of the vibrations.
The driving abilities of these machines were compared during the driving of reinforced concrete piles 1.6 m in diameter to a depth of 26 in in clay soils in the construction of a bridge over the Neva (A. D. Prokhorov, 1968).
The VP-170 vibrodriver proved to be the most efficient under the said conditions. In the vibrodrivers tested the same dynamic force was achieved in different ways. In the VP-160 vibrodriver the dynamic force of 1000 and 1250 kN was achieved due to an increased rotational speed of the eccentrics, while in the VP-170 vibrodriver, due to an increase in the moment of the eccentrics.
Due to the extreme reduction in the rotational speed of the disbalances in the NVP-56 vibrodriver, the dynamic force with values of the moment of the eccentrics identical to those of the VP-170 vibrodriver proved inadequate for driving the piles to the specified depth.
In addition, as a result of conducting this cycle of full-scale experimental studies, the following was determined:
- the frequency of the forced vibrations in driving reinforced concrete piles into clays in the presence of soil plugs in the pile cavity should not exceed 6.6-3.3 Hz;
- the most advantageous frequency of the vibrations changes with varying depth of driving;
- the maximum depth and rate of driving the piles into clays are basically dependent on the frontal resistance of the soil;
- with periodic extraction of the soil from the pile cavity the driving ability of the vibrator is sharply increased with the same parameters ;
- the application of washing by means of a pressure of 1.5-2 MPa for breaking down the soil plugs in clays in the pile cavity is not very efficacious; therefore, soil-processing mechanisms are necessary with their forced sinking into the soil.
An analysis of the hodographs and graphs of the vibrodriving of reinforced concrete piles 1.6 m in diameter with the VP-30, VP-3 and VP-80 vibrodrivers into low-moisture sandy soils made it possible to draw rhe following conclusions (A. S. Golovachev and A. D. Prokhorov, 1968):
the driving of the piles to the specified depth is greatly facilitated with periodic advanced extraction of soil from the pile cavity to the mark below the knife;
even with advanced extraction of the soil as the sinking progresses, the amplitude of the vibrations of the vibrosystem and the power required decrease due to an increase in the frictional forces of the pile in the ground.
According to the data of A. D. Prokhorov, the mode in which an amplitude of the vibrations of 8-10 mm is assured at any driving depth must be considered a reasonable amplitude-frequency vibration mode. Driving of the piles to the projected marks, accompanied by the extraction of soil from the cavity, can be assured by vibrators with Po > 1.5 (F—m0g).
An analysis of the driving of the piles into plastic clay soils with stony inclusions with the aid of the VU-1.6, VP-160 and VP-170 vibro-drivers makes it possible to draw the following conclusions:
- a soil plug with stony inclusions is formed in the cavity of the pile during driving;
- The vibrodriving of piles into clay soils with stony inclusions can be effective only with periodic removal of the soil from the cavity and processing of the leading apertures under the pile with a depth of 2.5-5 m and a diameter approximately equal to the inside diameter of the pile;
- The use of external washing (undermining) for facilitating the driving of piles under such conditions is not recommended because undermining results in an intensive washing out and accumulation of gravel inside of the pile.
Advance removal of the soil should be done carefully, due to the danger of its influx inside of the pile. Advance removal of the soil can be carried out to a depth of 4—6 m below the blade of the pile only in stiff-plastic and semihard clays.
In driving piles into compact soils with low-frequency (up to 7-5 Hz) vibrodrivers, the characteristic and most effective mode is accompanied by a rebounding of the vibrodriver from the soil.
During vibration driving into light soils, extraction of the soil from the cavity of the piles is superfluous in most cases. During the vibrations the soil rises along the internal cavity of the pile upward, sometimes 2-3 m above the level of the ground surrounding the pile.
A study of experience in the vibration driving of cylinder piles under various hydrogeological conditions permitted us to note methods and solve a number of current problems, which open up new prospects for further propagation of these progressive edifices in construction. Above all, this is an exploitation of vibrodrivers with regulatable parameters, the use of which makes it possible not only to increase the driving ability of heavy pile vibrators, but also substantially reduce the power of the electrical supply sources. Figure 63 shows the VRP-30/132 vibrodriver in the driving of a reinforced concrete pile 0.6 m in diameter.
An important characteristic of the driving of cylinder piles is associated with the formation of the soil plug in their cavity, which causes a change in the vibration scheme of the cylinder piles and creates the risk of their damage (basically in driving through a layer of water). In addition, the need for extracting soil from the cylinder piles substantially reduces the rhythm of the work in constructing foundations.
The results of the studies conducted by the All-Union Scientific Research Institute of Hydraulic and Sanitation Engineering Operations (VNIIGS) on the said characteristics and the technological measures recommended are considered below.
Tubular piles and cylinder piles 0.6-1.6 m in diameter are usually driven with a low-frequency vibrator with an open lower end, without removal of the soil that penetrates into the cavity of the pile. In such piles, especially with a blade bevelled inward, a compact soil core participating flush with the wall in load transfer is formed when bearing on sands of medium density or stiff-plastic clays. A compacted zone of soil of considerable volume, which is as it were a broadening of the pile base, is created here under the bottom of the pile. When resting on a sandy or stiff-plastic clayey soil, the pile begins to operate as a pile column under conditions close to those that are possible for ordinary solid piles only when resting on rock (A. I. Prudentov, 1971).
The bearing ability of such piles with respect to the soil approaches the maximum attainable with regard to the strength of the wall material, amounting to 2000-10,000 kN or more on one pile. The specific carrying capacity of a pile with an earth core reaches 1500-2000 kN per m3 of reinforced concrete.
With appropriately selected parameters of the vibrator, the cylinder pile is driven quite easily to the determined depth and then stops with the formation of the soil plug. In many cases, the stoppage of the cylinder pile occurs at a depth considerably less than the planned mark.
This leads to the need for extracting the soil plug and continuing the vibration driving in order to create a pile with a soil core at the planned mark or to free the pile cavity of the soil in preparation for the subsequent concreting.
The tendency developing in recent years of driving metallic tubular piles, where a soil plug is also formed, should be noted. Thus, an urgent problem is the study of the formation conditions of the soil plug during the vibrational driving of a tubular element.
Experimental data demonstrate (M. G. Tseitlin and V. E. Trofimov, 1973) that the elastic component of the resistance forces can be quite large in a number of cases. In connection with this, it became necessary to study the conditions of formation of the soil plug in the elastic-plastic mechanism of soil resistance (M. G. Tseitlin and A.. A. Kosheleva, 1931).
Figure 64 shows the calculation scheme for driving a tubular element with the formation of a soil plug, taking into account the elastic-plastic nature of the soil resistance.
The soil core does not move relative to the tubular element if there is equilibrium between the forces acting on it:
where R* is the frontal soil resistance acting on the plug. When FBH > FKP, a slippage of the soil core takes place
- sKP is the specific value of the dynamic forces required for separation during vibration driving, assigned to the area of the lateral surface of the driven part of the tube, and 1 is the height of the soil core.
- mn is the mass of the soil plug;
- FBH is the force acting on the side of the tubular element and holding the soil plug; and
- xn is the coordinate corresponding to the maximum deposition achieved during the preceding cycle.
- d = 0.2;
- d = 0.5;
- d = 0.8;
- d – 1.2;
- d = 1.6;
- d = 1-9;
I, II, and III — respectively at f = 0.1; f = 0.4; and f = 0.7.
According to the experimental data, the sKP value is 25-30% higher for the plug than for the external surface of the tube.
The condition of formation of the soil plug:
Slippage occurs when
In determining the maximum height of the soil plug, it is necessary to take the sign of the equality in expression (72) that reflects the equilibrium of the forces acting on the plug in the vibration driving process, substitute the FKP value found with formula (71) into the right-hand side of it, and into the left-hand side — x and R* — the parameters of the process of vibration driving the system, tubular element-soil plug as a single unit , into the ground. For determining d2x/dt2 and R*, it is necessary to solve the differential equation that reflects the process of vibration driving of the tubular element with the soil plug:
In accordance with the calculated scheme assumed, the frontal resistance to driving is
We should note that the problem of driving a pile with the elastic-plastic mechanism of soil resistance was previously solved by various methods (I. I. Blekhman, 1954; M. Ya. Kushul and A. V. Shlyakhtin, 1954; Yu. I. Neimark, 1963; and S. A. Varsanovich, 1968).
We introduce the following dimensionless parameters supplementarily to (9):
The characteristic graphs reflecting the dependence of the maximum value of the force fBH acting on the tube-soil core contact on the parameters of vibration driving of tubular elements and the soil re sistance with the dimensionless generalized characteristic x = 100w2D/g =1.2, sKP = 30 kPa and p = 2 t/m3, are given in Figure 65.
An analysis of the results obtained reveals that the force FBH is basically dependent on the frontal resistance of the soil R*. The lateral resistance and the elasticity of the soil and also the frequency of the vibrations substantially influence the variation in the mode of the vibrations during vibrodriving and in fact the acting forces of soil resistance R*. In determining the height of the soil plug, only the modes at which the driving of a tubular element is possible (R* &Mac179; R) are of interest.
As indicated above, the formation conditions of the soil plug for these modes are determined by the force FBH that acts on the tube-soil core contact, or the force FKP, at which a slippage of the soil core occurs. Figure 65 shows the graphs of variation in fKP as a function of qn.
The maximum height of the soil plug that forms for a given tube diameter is determined by its parameter qn, which is found by the points of intersection of the fKP graph with the fBH curve with the corresponding parameters of vibration driving.
The value g at which the driving of a tubular element with a soil plug is possible, is a function of the force of gravity of the system being driven, the frequency of the vibrations and the elasticity zone of the soil during vibration driving.
The case z = 0 corresponds to a purely plastic model of soil resistance, examined previously.
We should note that when the elastic-plastic soil resistance model is used, the calculated height of the soil plug is somewhat less than in the purely plastic model.
The graphs given, plotted for various z, make it possible with the specified parameters of vibration driving to determine the height of the soil plug formed. It is also possible with the aid of these dependences to select the parameters of the vibration driving of a tube, at which the height of the soil plug will be maximal.
The studies conducted revealed the superiority of the regimes characterizable by large values of dimensionless frontal resistance of the soil: the maximum height of the soil plug is achieved at the minimal dynamic forces that assure the driving of a tubular element at a given frontal resistance of the soil.
The particular difficulties in driving cylinder piles arise during the extraction of clay soils from their cavities, when the use of hydro-mechanization means or drills is impossible or of little effect. The use of jaw grippers for this purpose usually does not assure the required productivity of the work. Penetrating under the effect of its own weight, the gripper or grab bucket (even of heavy construction) does not fill the bucket in compact soils. The resistance to closing of the jaws and the conditions of cutting the soil in the closing of the grab bucket limit the height of the jaws and, consequently, the working capacity of the grab bucket at the given transverse dimensions. Besides the said shortcomings of a cable grab bucket, it should be noted that its geometric form does not match the cylindrical cavity of the tubular element. Apart from this, the small capacity of the grab bucket is also limited by the size of its open jaws, which must be less than the inside diameter of the pile. The problem of extracting compact clay soils from the cavity of the driven cylinder piles is solved by using of vibrating grab buckets with a longitudinal-rotational action.
Vibrating grab buckets have been developed at the present time, which assure extraction of the soil from cylinder piles (drive pipes) with an internal diameter of 400-1200 mm. A vibrating grab bucket with a longitudinal-rotational action (Figure 66) is a mobile driving apparatus operating on a cable with a load-lifting mechanism that assures its raising, lowering and transport to the site of unloading.
a – PV-530 vibrating grab bucket; b – PV-820 vibrating grab bucket; 1 – special vibration exciter; 2 – drive electric motor; 3 – shock absorbing suspension; 4- – soil sampler; 5 – interchangeable attachment.
For reducing the dynamic load on the crane construction, the vibrating grab bucket is equipped with a shock absorber. Vibrating grab buckets are easily hung on the suspension hook of any crane mechanism with an adequate lifting ability without having to remodel it, which is an important operational achievement because it permits the use of the same load-lifting crane for various operations on the construction platform.
The operation of the vibrating grab bucket of longitudinal-rotational action is carried out in the following modes:
- Driving the vibrating grab bucket into the soil — with longitudinal vibrations, which permit an effective collection of the soil;
- Separation and vibratory extraction of the vibrating grab bucket with the soil core filling the soil sampler — with rotational vibrations that assure the slippage of soil sampler relative to the adjacent soil and reduce the extraction effort in comparison with the static effort by 2-3 times;
- Discharging of the vibrating grab bucket — with longitudinal vibrations.
The peculiarity of the vibrating grab bucket operation consists in the fact that its soil sampler cuts through the soil with thin walls, and then the core filling the internal cavity is extracted to the surface and discharged. This operating principle of the vibrating grab bucket makes it possible (in contrast to rotational drilling machines) to sink shafts of large diameter without a substantial increase in the power of the mechanism, its mass and dimensions.
Vibrating grab buckets with a longitudinal-rotational action assure the extraction of sandy and clayey (up to semihard consistency) soils from the cavity of the cylinder piles. The hermetically sealed construction of the vibrating grab buckets permits their use under water or with a clay suspension.
Experience has accumulated in recent years on the use of vibrating grab buckets for extracting soil from the cylinder piles on dry land (from depths up to 30 in) and under water (from depths up to 25 m). The construction of foundations of cylinder piles 1.2 m in diameter on a construction project within the boundaries of a city can serve as an example of driving cylinder piles under industrial construction conditions. The construction area was comprised of clay soils, basically of stiff-plastic to semihard consistency with the inclusion of pebbles and gravel. The cylinder piles consisted of two sections 8 m in length; they were driven with a VU-1.6 vibrator. Driving cylinder piles under the said soil conditions was possible only with the use of drills or vibrating grab buckets, the average productivity of which was 5 m3/h in the extraction of soil from the piles, i.e., 10 times higher than the productivity of drilling machines. The production factor of the operations with soil extraction by a vibrating grab bucket is shown in Figure 67.
The technology of driving cylinder piles, providing for periodic removal of the soil plug, made it possible to drive the piles without a substantial exceeding of the power consumed and with a minimum level of vibrations of the surrounding soil.
In the initial stage of driving (down to a depth of 6-7 m) the soil core was not removed from the pile and had practically no effect on the driving rate, which was 0.8-0.9 m/min. In this case, the power consumed by the vibrodriver was 200-210 kW. In the interval of 9-16 m the driving was carried out with extraction of the soil from the pile cavity at an. average driving rate of the pile of 0.6-0.7 m/min.
Advance removal of the soil was effected to a depth up to 6 m below the pile blade. Without extraction of the soil from the cavity, the cylinder pile was practically not driven at all in this interval. The power consumed in the vibration driving was reduced to 120 kW with removal of the soil plug.
It was possible through the use of vibrating grab buckets to increase the productivity with regard to the extraction of cohesive soils by 3-4 times (in comparison with ordinary grab buckets) and reduce the time lost in soil removal from 85 to 50% of the total duration of the working cycle in driving the piles, and eliminate the operations with respect to reloading the soil extracted from the cylinder piles due to the possibility of discharging it from the vibrating grab bucket directly into a motor transport means.
According to the LSU data of the Gidrospetsfundamentstroi organisation, the application of the vibration technology complex for this purpose permitted the execution of the layout of foundations of cylinder piles with a total carrying capacity of 250 MN after 5 months, which is twice as fast as the existing standard periods.
Under the conditions of hydrotechnical construction, experience in conducting these operations on projects of the Gidrospetsfundament-stroi and Dalmorgidrostroi organisations, in which sandy and clayey soils with various stony inclusions were extracted from cylinder piles from 1000 to 1920 mm in diameter with vibrating grab buckets from under water from depths up to 30 m can be indicated as typical examples of driving cylinder piles employing vibrating grab buckets with a longitudinal-rotational action.
The use of vibrating grab buckets of longitudinal-rotational action in these operations permitted a 2-3 fold increase in productivity as compared with cable jaw grippers, and it was the only means in some cases that assured removal of the soil plug and the possibility of driving cylinder piles.
The characteristics of the vibrational driving of cylinder piles under deep-water construction conditions were first investigated in connection with elucidating the causes of the damage of cylinder piles in the construction of a moorage in the Tallinn ocean fishing port (M. G. Tseitlin, B, P. Shik, I. L. Krymskii, G. A. Pridchin, Yu. V. Shachkin, and A. M. Ivanov, 1966). The moorage had a stockade construction on reinforced concrete cylinder piles 1.2 m in diameter, 14-16 m in length and with a wall thickness of 10 cm (300 grade concrete). The piles were driven with a VP-5 vibrator into moraine loams covered with a thin layer of silt. The water depth was 6-9.5 m. It was established by examination that most of the piles have longitudinal cracks and chipping of the concrete that develop during vibration driving. The nature of these defects is as follows: longitudinal cracks develop in the lower sections of the piles, the length and width of which are dependent on the duration and intensity of the vibration driving; chipping and holes are found in the concrete in some piles. The cracks are formed at the level of the bottom and propagate upward, extending to the junction of the lower section. The opening width of the cracks is from several millimeters to 15-20 cm.
It should be noted that this type of damage became typical at this point in time (1963-1965), but it was usually explained by extreme longitudinal stresses in the walls of the cylinder piles, which developed during their vibrational driving.
In explaining the causes of the damage of the piles, a complex of large-scale experimental studies was conducted on the process of driving reinforced concrete cylinder piles through layers of water with measurement of the vibration and stress parameters in the walls of the piles that appear during the vibration driving. Tensometric sensors were implanted in the piles during their preparation for this purpose.
An analysis of the results obtained in the experimental studies demonstrated that the normal stretching and compression stresses in the annular sections of the tubular piles that arise during vibrational driving are not sufficiently large to be the cause of the formation of longitudinal cracks. It was established that periodic stretching stresses that reach values at which crack formation is possible in the concrete arise in the radial cross sections of the driven tubular piles.
The appearance of stretching stresses in the longitudinal sections of the pile during its upward movement can be explained by the dynamic action of the water column present inside of the pile.
In order to verify these hypotheses, a special cycle of tests was conducted, the program of which included the driving of tubular piles by both the usual technology and with the preliminary pumping out of water from the internal cavity of the tubular pile in order to eliminate the dynamic pressure.
These tests confirmed the hypothesis that the cause of damage to the tubular piles under the hydrogeological conditions of the construction of the Tallinn marine fishing port is the dynamic action of the water.
The experimental studies and large-scale observations conducted afterwards (G. A. Pridchin, L. I. Afanasyeva, et al., 1970) also revealed that the cause of damage to the reinforced concrete cylinder piles driven through a layer of water is an increase in the hydro-dynamic pressure in its cavity during the vibration driving.
It was established in examining the oscillograms of the driving process that stretching stresses arise in the longitudinal sections of the pile during its upward movement. This phenomenon can be explained if it is assumed that the soil plug moves together with the pile and it interacts with the water column present inside of the pile (M. G. Tseitlin, 1966).
During the downward movement of the pile with an acceleration roughly equal to the acceleration of the force of gravity, (this type of movement was observed during the vibratory driving of piles in the Tallinn marine fishing port inertial forces counterbalance the force of gravity and the water exerts practically no pressure on the pile walls.
During the impact of the pile on the soil and its subsequent movement upward, there is an impact of the water on the soil plug. By virtue of the fact that perturbations in the water are propagated at a finite velocity v1, at first there is a change in the velocity of the layer of water adjacent to the soil plug Dl1 = v1 Dt, which prior to impact on the soil plug had the velocity of the water column vo, and after impact it acquired the velocity v* (the velocity of the boundary between the soil plug and the water). At the same time, the soil layer adjacent to the boundary Dl2 = v2 Dt, which has the velocity v01 (velocity of pile movement), acquires the velocity v*(v2 is the propagation velocity of the perturbation in the soil).
By designating the force of interaction between the soil and the water by P and employing the theorem on the variation in the amount of movement of the layers Dl1, and Dl2, we obtain:
where r1, and r2 are the densities of the water and soil respectively and Un is the cross sectional area of the soil plug.
We find from formulas (75) and (76) that
By substituting (77) into (75) and assuming that P = DpUn (Dp is the pressure increment in the water layer D1), we obtain
During the following time interval Dt, the water and soil layers adjacent to the interacting layers change their velocities. Thus, the gradually increased pressure that arises at the boundary between the water and the soil propagates in the form of an increased pressure along the water column and the soil plug. With a subsequent movement of the pile downward, the pressure in the water column is equalized and the cycle of phenomena described is repeated.
Equation (78) does not take into account the process of wave reflection from the free surface cf the water and from the soil, and gives an assessment of the maximum possible increase in pressure with a change in velocity from vo to v01. In the meantime, it is obvious that at shallow water depths the pressure values calculated with formula (78) can be overstated.
Observance of the following inequality is a reliable criterion of the applicability of formula (78)
ty < 2 l/v1
where ty is the time cf the impact interaction between the water column and the soil plug and 1 is the height of the water column.
The physical significance cf this inequality consists in the fact that the impact is completed before the expansion wave arrives.
As the preliminary calculations revealed, a large number of cases are encountered in practice where ty > 2 l/v1 . These cases obviously take place when the water column is not large and the impact time is not very short. It all takes place as if all the layers of the column inside the pile changed their velocity simultaneously. This assumption makes it possible to consider the movement of the entire water column as an incompressible body in determining the excess pressure (the force of interaction between the soil plug and the water).
The accumulation of experimental data on the impact time (under different soil conditions) and on the nature of perturbation propagation in the water column inside of the pile makes it possible to select for calculation either the wave model given above or the model where the movement of the water column is considered as the movement of an incompressible body.
In accordance with the experimental data obtained, the impact time of a tubular pile on the soil is roughly:
|for compact soils||ty (0.05— 0.08) T|
|for soils of medium density||ty (0.1— 0.4) T|
|for loose soils||ty (0.3— 0.5) T|
The propagation rate of the impulse in the water inside of the elastic reinforced concrete tube is determined with the formula of N. E. Zhukovskii
where EB, Ed and Ea are the moduli of elasticity of water, concrete and reinforcement; d is the wall thickness of the tubular pile; and a is the coefficient of reinforcement of the annular reinforcing material.
The propagation rate of perturbation in the soil v2 can be selected from the table (D. D. Barkan, 1948), which contains the results of measuring the propagation rate of longitudinal elastic waves in some types of soils.
|Tight gravelly-sandy soil||480|
The results of experimental studies confirmed the hypothesis that with a water column height of l < v1ty/2 it is possible by assuming further simplifications in the calculation scheme to consider the movement of the water column inside of the pile as the movement of an incompressible body. In this case, the pressure of the water inside of the cylinder pile at the level of the soil plug that develops during vibrodriving can be determined with the formula
pmax = p1l1a (79)
where l1 is the height of the water column and a is the maximum acceleration.
Correspondingly, in the layer of water separated from the soil plug by the value 1*,
p1 = p (l1 — 1*) a. (80)
The maximum acceleration value a for the various soil conditions and vibration modes is determined by the results of studies on the process of driving tubular elements with the formation of a soil plug. Comparison of calculation by the above formulas with the experimental data offers a satisfactory concordance of the calculated and experimental values (G. A. Pridchin, L. I. Afanasyeva, M. F. Reut, and I. D. Bobrik, 1970).
In order to reduce the hydrodynamic loads, a specially prepared shock absorber, consisting of several automobile chambers mounted in a container with ballast, was proposed and used. The vibration driving of tubular piles, using a shock absorber, was carried out in the following manner. The shock absorber was suspended to the head of the vibrodriver on cables, the length of which somewhat exceeded the length of the tubular pile driven. In fastening the vibrator on the pile the shock absorber was lowered to the ground. After driving the pile, the vibrator together with the shock absorber fastened to it are placed on another pile. The experimental work conducted demonstrated the high efficiency of such a shock absorber.
An apparatus proposed by VNIIGS for the vibration driving of piles under conditions of deep-water construction, obviating the possibility of withdrawing the shock absorber from operation during its suction into the soil and assuring maintenance of the shock absorber in the zone of maximum water pressure during the entire period of vibration driving was used in another organisation, Baltmorgidrostroi.
The apparatus (Figure 68) consists of a shock absorber suspended on a cable to a float. The length of the cable is selected so that the shock absorber hangs on the float close to the bottom. The entire apparatus is suspended to the head of the vibrodriver on cables and is introduced inside the pile when the vibrodriver is set up .
Because the height of the water column inside the pile varies practically not at all during the vibration driving of the pile, the shock absorber always remains in the zone of the greatest water pressure.
The studies conducted on the causes of the development of cracks during the driving of cylinder piles also permitted us to conclude that the elasticity of the medium filling the cylinder pile is an important factor, on which the magnitude of dynamic pressure in the pile cavity is dependent.
Stand tests confirmed that a reduction in the modulus of elasticity of the medium filling the pile with the aid of aeration makes it possible to reduce the amount of dynamic pressure to safe values.
An air-injection arrangement was proposed and incorporated, the essential operation of which consists in saturating the water and soil in the cavity of the cylinder pile with air .
The water and soil in the cylinder pile cavity are saturated with air from a compressor through a steel tube, which is sunk into the soil core together with the washing tube. The use of this apparatus in the construction of piers assured a 100% integrity of the cylinder piles. VNIIGS worked out the new protective arrangement that made it possible to improve the technology of carrying out the operations .
Air is fed into the cavity of the driven cylinder pile through a polyethylene conduit, laid in the wall of the cylinder pile during its manufacture, and exits through an annular collector, also located in the wall of the cylinder pile at a distance of 2.5 diameters from the pile toe. The proposed protective apparatus was investigated in the projects of the organisation Baltmorgidrostroi (E. N. Perlei, G. A. Pridchin, and I. Sh. Gonikman, 1981).
The studies conducted revealed that the stresses in the concrete of the walls of the cylinder piles, caused by internal dynamic pressure, in driving without the supplying of air into the cavity of the cylinder piles exceed by 1.6 times the calculated resistance to expansion of grade 400 concrete. When a protective apparatus is used, the stretching stresses in the concrete are considerably less than the calculated stresses.
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