Editors note: from a practical, equipment standpoint, this is one of the most interesting sections of the book. It describes the first use of vibratory hammers on an actual project, and an interesting description of driving sheet piles and problems that can arise during that installation.
The vibratory method of installation and extracting piles began to be used on the industrial scale in 1949. This method was first used in the construction of the Gorki (Nizhny-Novgorod) hydroelectric power plant, where the pile was driven into water-saturated sands with a vibrating driver of the BT-5 type (D. D. Barkan and V. N. Tupikov).
The high technical and economic indices of vibratory driving attained at the Gorki power plant facilitated acceptance of the new method. Vibratory pile driving enjoyed broad dissemination as a result of the studies at the All-Union Scientific Research Institute of Hydraulic and Sanitation Engineering Operations (VNIIGS) with the use of the VPP-2 vibratory driver developed by this Institute (O. A. Savinov, A. Ya. Luskin, and M. G. Tseitlin,) which was used in the construction of many power plants and other large facilities.
In the construction of the Stalingrad power plant with the VPP-2, more than 24,000 tons of piles of the ShP-1, ShK-1 and Larsen-V types was driven to a depth of up to 12-14 meters, primarily into saturated sands with gravel and pebbles, interspersed with sand-silt or clay soils. Vibratory driving of a pile into the underlying rock was difficult; therefore, the pile was driven with hammers. However, the application of VPP-2 furnished a substantial effect, especially due to the rapid setting of piles in porous dikes.
On the basis of accumulated experience:
- Vibratory pile driving technology was acknowledged to be the most effective, especially in driving into saturated sand and plastic clay soils.
- In driving piles by vibration, the productivity is almost 2 times higher than conventional impact driving.
- Extraction of a pile for its reuse can be reliably accomplished by the vibratory method in those cases when the pile was driven by vibration.
- It was found that a pile pounded in with impact hammers is not always amenable to extraction due to deformation of the interlocks.
The technical-economic advantages of the vibration method are defined not only by the increase in the driving rate and the possibility of extracting the pile, but also a more efficient technology of the auxiliary operations performed by using self-propelled load-lifting devices.
Despite the advantages indicated, the use of the vibratory method in pile driving has been limited to date, as a rule, by its length of no more than 12-15 m, and vibro-extraction, by a depth of no more than 8-10 m (VPP-2, V-401) and up to 15 m (MSh-2M). In this case, the range of efficient utilization of the method was limited by saturated sandy and plastic clayey soils.
A series of research efforts directed toward creating a vibrating device for the driving and extraction of piles with a length up to 20 m was carried out in recent years for the purpose of expanding the range of application of the vibratory method in pile driving and also for a comparison of the efficiency of the means of vibration techniques with pile driving equipment when driving piles into difficult soils.
The results of studies on the processes of vibration and impact-vibration driving and extraction of various elements with a predominant lateral resistance were used in solving these problems.
The fundamental results of these studies are summarized in the following:
- During hammering or ramming into saturated sands and plastic clays, the rate of driving and extraction of the elements in the vibration and impact-vibration modes differs little, but the power consumption is considerably less in the vibration mode;
- During hammering or ramming into low-moisture sands and into stiff-plastic clays, impact-vibration driving or extraction is more effective than mere vibration; with regard to the power required, it can be higher in the vibration or impact-vibration modes, depending on the rate of driving;
- The efficiency of impact-vibration driving and extraction can be substantially increased in the two-impact mode of impact-vibration hammer operation (one impact upward and the second downward in each cycle).
The short-term two-impact action on the pile during its extraction is also useful for the initial oscillation buildup (pulling from the site) in the case of substantial resistance in the deformed interlocks of the pile being extracted.
An analysis of the results obtained and the characteristics of vibration driving and extraction of a pile determined the choice of the type of vibrating device, which could operate effectively as a function of the soil conditions and the mass of the pile in vibration, single-impact and two-impact modes of operation.
In order to assure the longest service life, the basic operating mode of the vibrating device should be vibratory. If necessary, the pile should be driven in the impact-vibration mode with impacts downward.
The pile can also be extracted in single-impact (impacts upward) or two-impact modes.
The VSh-1 vibrating device developed by VNIIGS (Figure 56) meets the indicated requirements; it is designed for driving and extracting a Larsen-type pile with a length of 20 m.
The parameters of the vibrating device, calculated by using the results of the studies conducted, assure a substantially greater efficiency in comparison with the VPP-2 (V-401) vibrator used at the present time, with the same adjusted power of the electric drive.
After conducting production tests for several years, the VSh-1 vibrating devices have been successfully used on projects of the trusts Gidrospetsstroi and Ukrgidrospetsfundamentstroi (M. G. Tseitlin, V. V. Verstov, and Ya. K. Baitinger, 1984).
The VSh-1 vibrating device was used by the Volgograd administration of the trust Gidrospetsstroi, with the aid of which a pile with a length of 17 m and of the Larsen-IV and Larsen-V types was extracted from a pile wall that had been driven in the 1950’s into clay soils with steam-air hammers. Attempts prior to this to extract the pile with a V-401 vibrator were not successful.
Extraction of the pile with the VSh-1 vibrating device was done with the aid of a Yubegai floating pile driver with a lifting capacity of 30 tons with piles in three pieces. The middle sheet pile, on which the vibrating device was fastened, was strengthened by the welding on of cover plates, and the interlocks of the side sheet piles were cut to the water level (1.5-2 m) in order to facilitate breakaway of the pile package in the interlocks. The pile was successfully extracted with the passage of the vibrating device to the two-impact mode at an exciter vibration frequency of 13.3 Hz. The vibration mode was also tested at a 20 Hz frequency, yielding, positive results.
In the Ulyanov construction administration of the same trust, the problem of driving a pile 19 m in length into clay soils with seams of pebbles to a depth of 7-8 m was one of the objects. An attempt was initially made to solve this problem with the aid of the V-401 vibrating driver and Diesel hammer with a mass of the impact part of 1.8 tons. The driving ability of the V-401 vibrating driver was inadequate for driving the pile to the specified mark. During driving with a the Diesel hammer the upper part of the pile was deformed and consequently the driving was ineffective. The VSh-1 vibrating device, tuned to the vibration mode at a frequency of 13-3 Hz, made it possible to drive the Larsen-V pile to a depth of 19 m. The results of operating the VSh-1 vibrating device on many objects revealed that its driving and extracting ability is considerably greater with the same power consumption than in the VPP-2 (V-401) vibrator.
In addition, during the mass production of the VPP-2 (V-401) vibrator, its refinement for increasing the service life due to some change in its parameters, reinforcement of the electric motor windings, increasing the efficiency of the suspension during operation with load-lifting means, and also the use of a hydraulic head, excluding the need for cutting an hole in the head of the sheet pile and the possibility of collision, observed when a wedged head is used. Such a modification of the VPP-2 (V-401B) vibrator was worked out by VNIIGS and the trust Gidrospetsfundamentstroi.
An experimental comparison of the effectiveness of the various pile driving means in driving piles into difficult soils was conducted by the Leningrad administration of the trust Gidrospetsfundamentstroi on an area whose geological structure is represented by the following stratifications (m):
|Fill sand||0-2, 2.0|
|Fine sand with grains of gravel||2.2—3.0|
|moraine loam of hard and semihard consistency with gravel, pebbles and boulders (up to 20%)||3 and lower|
A pile with the following profiles was used in conducting the experimental studies: Larsen-IV, Larsen-V, ShK-1 and the flat ShP-1.
The following were used as the driving means:
- Drop hammers with a mass of 3.5 and 5.65 tons and a frequency of the impacts of 15-20 per minute;
- Tubular S-858 diesel hammer with a mass of the impact part of 1.8 tons and a frequency of the impacts of around 50 per minute;
- V-401 vibrating driver with a mass of 2.2 tons, operating at a frequency of 16.6 Hz;
- VP-1 vibrating driver with a mass of 4.5 tons, tuned to operating in the mode of a free impact-vibration hammer with a frequency of 420 impacts per minute.
The maximum driving depth of a pile of the Larsen-IV, Larsen-V and ShK-1 type with a suspended mechanical hammer was 5-5-6.0 m, including into moraine loam up to 2.5-3 m. In this case an increase in the hammer mass resulted in a substantial deformation of the upper part of the pile and did not increase the driving depth. In a series of tests with a hammer mass of 5-65 tons and a relatively small drop height (0.3-0.5 m) the deformations of the upper part of the sheet pile were so great at the end of the driving that it was necessary to cut off the deformed part in order to continue the work.
During the driving of a pile of the Larsen-IV, Larsen-V and ShK-1 types with a diesel hammer, the maximum installation of the pile into the ground reached 7.5 m, including 4.5 m into moraine loam with substantial deformation of the top of the pile. The ShP-1 pile was driven with a diesel hammer only to the top of the moraine loams, and then lost its longitudinal stability, and its further installation became impossible. In the driving of piles of all profiles with a V-401 vibrating driver, their complete preservation was assured. However, the pile driving stopped when the roof of the moraine loams was reached.
Impact-vibration pile driving was done with a VP-1 vibrating driver, equipped with a forked guide for the pile and attuned to the mode of a free impact-vibration hammer (without being fastened on the pile.) An effective and stable operation of such a VP-1y impact-vibration hammer was achieved with an increase in its mass to 6-7 tons.
With such an adaptation, the impact-vibration hammer drove successfully without a deformation of the upper part of a Larsen-V pile with the required depth into moraine loam to 2.5-3 m. As a result, the VP-1y impact-vibration hammer-driver was used by the Leningrad administration of the trust Gidrospetsfundamentstroi in the driving of a Larsen-IV pile with a length of 11.5-12 m to a depth of 9 m in the installation of a single-row protection dike.
VP-1y drove three to four sheet piles per shift without their deformation and with a driving depth into moraine loam of 2-2.5 m. The productivity of the suspended mechanical hammer was only half as much under these conditions and the driving was accompanied by deformation of the upper part of the sheet pile.
The results of a cycle of tests in which the driving ability of various types of pile driving means in clay soils with a semihard and hard consistency was compared (V. V. Verstov, M. G. Tseitlin, Ya. K. Baitinger, and G. F. Olshevskii, 1984) indicate the greater efficiency of impact-vibration installation with a comparatively low power of the single impact and a high frequency under the condition of a free impact-vibration hammer and a ratio of the total mass of the impact part to the magnitude of the compelling force that assures a stable operation of the impact-vibration hammer. Under such conditions (in contrast to other pile driving means) no deformation of the driven pile occurs during effective driving.
The studies conducted and analysis of the industrial test on the use of the means of the vibration technology of elevated efficiency in the installation and extraction of a pile indicate the need for driving the pile primarily with vibrators or impact-vibration hammers in order to assure its vibro-extraction and reuse along with a high productivity.
The driving or ramming of a pile with hammers can be expediently used in exceptional cases, in particular difficult soil conditions, in conjunction with supplementary measures that facilitate the driving.
Installation of Sheet Piles
One of the important characteristics of the vibratory technology of pile driving in contrast to the impact method consists in the fact that, in most cases, it is necessary to fasten the sheet pile rigidly with the vibrating driver, and this operation is carried out in the horizontal position at ground level or on special supports with subsequent raising of the vibrating driver with the pile.
The choice of vibrator or impact-vibration hammer for pile driving or extraction is made as a function of the geological conditions, the type of pile, its length, the depth of installation, and also the technological scheme used for effecting the operations. The pile should be driven using self-propelled cranes or pile drivers.
The cranes or pile drivers should satisfy the following requirements:
- The height of the crane boom should permit the lifting of the sheet pile arrangement with the vibrating driver fastened to it into the interlock of the sheet pile previously driven;
- The outreach of the crane boom should be sufficient to bring the vibrating driver to the sheet pile and permit placement of the raised sheet pile with vibrating driver on the site of installation without moving the crane;
- The load-lifting capacity of the crane or pile driver should be sufficient to lift the sheet pile to be driven with the vibrating driver fastened to it.
The preparatory work, performed prior to beginning the pile driving, includes levelling of the land, the layout of the plan of the pile structure, establishment of the guides (templates) for pile driving (if necessary,) and delivery of the piles to the site and their preparation for driving.
The driving of a pile with vibrating drivers is divided into the following basic operations:
- Servicing and fastening of the sheet pile in the head of the vibrating driver;
- Raising and placement of the sheet pile at the driving site;
- Driving the sheet pile;
- Detachment of the vibrating driver from the driven sheet pile.
During the vibratory driving of the sheet pile, its connection with the vibrating driver should have an assured immobility; this is accomplished with the aid of a hydraulic head or wedge clamp.
During servicing, the sheet pile is placed near the site of the operations on a support (cross beam) with a height of 1—1.5 m so that the upper end projects 1—1.2 m beyond the cross beam.
During the driving of the sheet pile, it is necessary to keep track of the state of the cable and the hook of the crane, to which the vibrating driver is suspended.
The rate of lowering of the crane hook should be such that the crane does not inhibit the installation of the sheet pile; in addition, there should not be excessive free cable because with a great length of the sheet pile it is possible that it could buckle under the weight of the vibrating driver fastened on it. The cable is fully slackened in the final stage of the driving.
The following deviations from the planned position are possible during the driving of the pile:
- Deviation of the pile from the vertical in the plane of the alignment;
- Deviation of the pile perpendicular to the alignment;
- Installation of the sheet pile below the planned mark due to its withdrawal with the adjacent driven sheet pile;
- Not driving the sheet pile to the planned elevation.
Elimination of fanning in the case of a slight deviation is achieved by drawing out the pile during installation in a direction opposite the deviation, and with a substantial deviation and the impossibility of its correction by drawing out, by the installation of wedge-like sheet piles. Deviation of the pile from the line of direction is eliminated by drawing out the sheet pile in the opposite direction.
If the drawing out of the sheet pile does not straighten its position, the sheet pile is pulled out and again driven in, using the necessary measures for maintenance of its projected position. Passage of the sheet pile below the projected mark is corrected by its building up or plating.
Incomplete installation of the pile to the projected mark is eliminated by one-two-fold raising of the sheet pile by 0.5-0.8 m and its subsequent reinstallation. Extraction of the pile with the application of vibration is usually effected with cranes.
In a tentative selection of the load-lifting capacity of the crane for the vibro-extraction of a pile that has been in the ground for a short time (less than one month), it is necessary that the force at its hook exceed the weight of the system (the vibrating driver and pile) driven into the ground by at least 2 times.
When the pile has been in the ground for a prolonged period, the force at the crane hook should exceed the weight of the system driven by vibration by 3-4 times. To reduce the force transfered to the crane boom, it is permissible to use vertical props (outriggers,) joined to the crane boom in an articulated manner and based on the ground with the aid of a plate or slab. Prior to vibro-extraction, the vibrator should be rigidly fastened on the sheet pile with the aid of a wedge or hydraulic head. The rigid connection should be assured during the entire process of vibro-extraction.
If the pile has been in the ground for a long time, a preliminary vibration (prior to lifting) is indispensable, the time of which is determined by testing on each object. The crane cable should not be taut in this case. In the following stage it is necessary with the force of the crane to tension the springs until the coils almost touch and continue the vibration up to the beginning of the vibro-extraction under the action of the force of the opening springs.
Further vibro-extraction is effected with the minimum lifting rate of the crane hook, not allowing the complete compression of the shock absorber springs here. In the final stage the extraction of the pile is carried out without vibration.