The mass vibration driving of heavy steel reinforced concrete piles was first done in the beginning of the 1950’s with the aid of vibratory drivers of the VP-1 and VP-3 types (B. P. Tatarnikov). Several tens of thousands of steel reinforced concrete piles with a cross section up to 45 x 45 cm and a length up to 14 m were driven solely in the construction of bridges.
An industrial test on driving heavy steel reinforced concrete piles indicates that the use of the vibration technique is most effective in this case in loose water-saturated soils, while the use of vibratory drivers for this purpose is not feasible in low-moisture and compact soils.
At the present time, the driving of prismatic steel reinforced concrete piles with vibratory drivers is carried out relatively rarely not only due to the limited range of the effective application of vibration during the driving of elements with a high lateral resistance, but also due to additional complications (with the use of nonspecialized vibratory drivers) with separate raising of the vibratory driver and pile and their subsequent rigid connection.
The vibrational driving of prismatic steel reinforced concrete piles is most widely used under the conditions of the construction of electric transmission lines (ETL) and the supports of the contact system of railroads, where specialized units are used for this purpose, and the piles themselves due to the construction conditions are equipped with threaded pins for subsequent fastening of the metallic support to them.
The use of specialized self-propelled units makes it possible, on the one hand, to assure a complete autonomy of these machines and, on the other hand, due to the use of part of the mass of the unit, to achieve an optimal pressure of the end of the pile on the soil.
One of such specialized units is the AVSE-U unit, designed for the vibrational driving of pile foundations for the support of a contact line (M. B. Belkind, A. N. Tarasov, and M. N. Margolin, 1983). The construction of the units was worked out by the Central Scientific Research Institute of Communications (TsNIIS), jointly with the planning and design office (PKB) of Glavstroimekhanizatsiya Mintransstroya. The AVSE-U unit is equipped with a contactless system for controlling the vibratory driver.
The driving of pile foundations with the aid of the AVSE-U unit takes place in the following manner. After it is set up around the point of driving, the crane with the vibrating driver and pile fastened to it is turned perpendicular to the axis of the path, moved to the required distance, after which the slewing guide box, together with the vibratory driver, is moved by hydraulic drive into the vertical position. Then the machine operator, paying out cable, guides the vibratory driver with the pile downward, switches on the electric motor and effects the installation. If necessary, the pile is directed with the aid of an extensible or rotary crane. When the installation is complete, the hydraulic holding clamps are loosened, the vibratory driver is raised, the guide box is moved into the horizontal position, the crane is moved and turned into position along the platform, after which the next pile, which lies on the table, is gripped.
The specialized units also include vibrating impressing units of the VVPS-20/11 (Figure 57) and VVPS-32/19 types, the characteristics of which are given below. The construction of the units was worked out by the Leningrad branch of the Orgenergostroi Institute in collaboration with the All-Union Scientific Research Institute of Hydraulic and Sanitation Engineering Operations (VNIIGS).
|Amplitude value of the compelling force, kN||200||320|
|Maximum value of the crushing effort, kN||110||190|
|Static moment of the mass of the eccentrics, kg-cm||3500||6000|
|Frequency of the compelling force, Hz||12-15||11-13|
|Engine power of the basic machine (tractor), kW||79.4||132.4|
As is evident from Figure 57, the encompassing welded frame, to which all the basic subassemblies of the unit are fastened, is located on the tractor with an elongated travelling section. A generator is placed in the lower stage of the rear portion of the frame; it is driven by the power takeoff shaft of the tractor through a reducer. A two-drum friction winch with an electric motor is installed in the second level of the frame above the generator. The raising of the vibratory driver with pile, placement of the upper section of the crane in the transport position are accomplished with the aid of the winch and, inversely, the loading force is also created.
The vibratory drivers constructed according to the scheme of the VPP-2 (see Figure 45c) do not have any structural peculiarities, with the exception of the sets of the driving cables, located on the spring-mounted part of the driver frame.
In connection with the limited driving ability of the vibratory drivers, further development was obtained with impact-vibrational driving units, the driving force in which is applied directly to the pile (or other driven element). A representative of such machines is the UWS-60/10 unit.
For the driving of boltless or pinless piles, the impact-vibration hammer of the unit is equipped with a special head that makes it possible to grip and hold the pile that is lying on the ground, with its subsequent raising into the initial position without the aid of any additional load-lifting devices.
The pile is placed in the head of the impact-vibration hammer in the following order (Figure 58). The gripping frame is placed over the top of the pile and a pintle with a fast-acting fastener is inserted. The frame is held with a chain for longitudinal transfer during the lifting of the pile. Due to the fact that the frame is connected with two measuring cables to the head, during the raising of the impact-vibration hammer the pile passes from the horizontal position to a vertical one and gradually enters the cavity of the head. After the lower end is pulled away from the ground, it hangs in the cables in the vertical position and due to the eccentric suspension of the frame the pile is fixed into the front cavity of the head. When the impact-vibration hammer is lowered, the pile is fixed in the internal cavity of the head when it reaches the ground with its lower end.
During the driving of bolted piles, the technology of their lifting involves the following. The pile is placed on a support so that its end with the bolt was located near the point of driving. For lifting, the pile cap of the unit is connected to the pile with two cables. Then with an inclined boom of the unit, the impact-vibration hammer and the pile are raised. With the pile raised, the unit strikes the driving point, after which the vertical position is imparted with the aid of the mechanisms of longitudinal and transverse correction of the boom and pile, and the pile is lowered onto the driving point.
In some cases a 12-meter pile was driven in 5 minutes. Although the underlying soils (loams and dense clays), lying at a depth of 6-8 meters, had a substantial solidity, application of the impact-vibration hammer assured the driving of the pile to the given depth with an accuracy of ± 2 cm. The productivity of the impact-vibrational driving unit, taking into account the time for technical maintenance during the installation of a pile 12 m in length, is about 10 piles per shift.
The impact-vibrational driving apparatuses also include the AVS-1 of VNIIGS construction for the driving of enclosure posts. Its technical characteristics are given below:
|Working element||free springed vibrating hammer with the possibility of transition to the vibratory driver mode|
|Mass of the impact part, kg||450|
|Static moment of mass of the eccentrics of the vibration exciter, kg • cm||300|
|Frequency of the vibrations, Hz||25; 15.5|
|Driving force, kN||20|
|Basic machine||ETU-357 excavator|
The device (Figure 59) was operated in the construction of the guard rails for the Oktyabr railroad (a total of 6000 posts were driven). With the vibration mode, the post was driven in 2.5-30 minutes only to a depth of 0.7-0.8 m, and with the impact mode, to the projected depth of 1.3 m. The use of part of the mass of the pile-driving apparatus for driving the post during its vibrational-impact installation permitted a reduction in the driving time to 1-1.5 minutes. The work was performed according to the following technology:
- The unit was moved to the site of driving the post, raising the impact-vibration hammer upward with the boom to a height of 3.2 m;
- The post was drawn up to the apparatus and raised under the opening of the head with an auxiliary winch of the aggregate;
- The impact-vibration hammer was lowered, the post was clamped in the head with a hydraulic cylinder and positioned over the point of driving with the aid of hydraulic cylinders of longitudinal and transverse correction;
- The post was driven by switching on the impact-vibration hammer;
- The impact-vibration hammer was switched off, it was raised 10-15 cm above the top of the post and the aggregate was moved to the new driving site.
A time study of the basic and auxiliary operations revealed that the post driving cycle had an average duration of 4 minutes and 28 seconds, where the driving process itself occupied about 30% of the cycle; the power required by the electric motor of the impact-vibration hammer did not exceed 2.5-3-0 kW.
The use of the AVS-1 unit made it possible to free the drilling machine, bulldozer and transport means, previously for transfer of the gravel and cement, and also to reduce the consumption of materials and the manual labor required (V. M. Klementyev, 1976).
In the calculation schemes of impact-vibrational driving the driving forces on the pile are considered constant over the entire driving cycle.
In order to assess the effect of the rigidity of the driving cables and the mass of the pile-driving apparatus on the driving ability of the impact-vibration hammer, it is necessary to consider the calculation scheme depicted in Figure 60.
1.y = 4.17;
2.y = 2.38;
3.y = 1.67;
4.y = 1.28;
5.y = 1.04.
In establishing the basis of the calculation model it is necessary to introduce the following into the previous assumptions in studying impact-vibrational systems:
- The pile-driving apparatus is an absolutely solid body that is supported through an articulation on a nondeformable base;
- The free vibrations of the pile-driving apparatus are damped during the movement of the impact part in breaking away from the pile.
The first stage of the movement of the system examined begins at the time of the impact part of the impact-vibration hammer breaking away from the pile and ends at the moment of applying the impact. By virtue of the assumed mechanism of soil resistance, the pile is immobile during the first stage, i.e., the rigidity of the elements of extraneous driving and the mass of the pile-driving apparatus have no effect on the movement process of the impact part.
The second movement phase begins at the time of impact and joint displacement of the impact-vibration hammer and pile. It ceases with the termination of driving.
The differential equation of movement of the impact part in breaking away from the pile – the flight of the hammer (first movement stage) – has the form described in Section 7, with the same initial and final conditions.
The differential equations of movement for the second stage have the form:
where m3 is the mass of the pile-driving apparatus, QP is the driving force on the pile, c5 is the coefficient of rigidity of the connections between the pile-driving apparatus and the impact-vibration hammer, and l is the distance from the axis of turning of the pile-driving apparatus to the point of application of the driving force to the pile.
In addition to the dimensionless variables and parameters (9) and (41), we introduce the following:
The calculations by the method presented above were performed on a computer. The results of the calculations are presented in the form of the graphs of Figure 61 with the following values of the parameters: sin α = 0.5, ξ1 = 0.5, β = 1.0, and δ = 0.75.
It can be concluded from an analysis of the calculations that for
loose soils (f + γ = 3-5) in the case of elastic connections between the pile-driving apparatus and the pile (ξ52 < 12.5) the operation of the impact-vibration hammer is unstable , while for ordinary impact-vibration hammers with the assumed parameters of the hammer, it is in the zone of stable operation. It should be noted that there is an optimal value ξ5 different from zero, for such soils, at which the maximum driving ability of the impact-vibrational aggregate is attained. In this case, a shift in the optimal ξ5 values takes place toward their decrease with an increase in the forces of soil resistance, and in compact soils (f + γ > 7) the optimal value is ξ5 ≈ 0.
Having compared the results of calculation by the method that takes into account the multiple masses of the impact-vibration hammer-pile-driving apparatus system and the influence of rigidity of the connections between the impact-vibration hammer and the pile-driving apparatus, with the calculations by the simplified method, which does not take into account these specific characteristics of the impact-vibrational pile drivers, it can be concluded that the simplified method expounded in Section 7 yields excessively low values of the driving ability of the impact-vibration hammers for loose soils.
For soils of medium and high density f + γ > 7, the simplified method presents results at ξ5 ≈ 0 that are quite close to the more precise calculation scheme of impact-vibrational driving units (see Figure 60). In soils of medium and high density, an increase in rigidity of the connections between the impact-vibration hammer and the pile-driving apparatus significantly reduces the driving ability of the impact-vibration hammer.
For loose soils there is an optimal value of the mass of the pile-driving apparatus (v ≈ 1.67). At the same time, for compact soils the driving ability of impact-vibrational driving units increases with an increase in the mass of the pile-driving apparatus, which is manifested particularly clearly as the rigidity of the connections between the impact-vibration hammer and the pile-driving apparatus increases.