Methods for Approximate Calculation of the Parameters of Vibratory Drivers and Impact-Vibration Hammers

Calculation of the parameters of longitudinal-action vibrational pile drivers.

The initial data for calculation are: the mass of the element to be driven mo, in kg; the geometric dimensions of the element to be driven;the depth of sinking l, in m; and the soil conditions.

1. The resistance of the soil is determined. The calculated value FCR of the critical separation resistance at a given maximum sinking depth (kN) is determined on the basis of the original data characterizing the soil conditions:

where i is the ordinal number of the soil layer of thickness li passed through during the sinking, k is the total number of layers and Z is the perimeter of the cross section. The values of the specific separation resistance σ are assumed according to the data of Table 5.

2. After determination of the tentative mass value mo, in kg, of the element to be driven and the parts of the vibratory pile driver rigidly connected with it, the approximate value of the static moment of the mass of the eccentrics, in kg • m, is calculated:

where ψ = 0.8 for reinforced concrete piles and ψ = 1 for the other elements sunk.

Table 5 Dependence of the specific separation resistance the soil type.
Type of soil For a pile, kPa For a sheet pile, kN/m
Steel tubes Reinforced concrete piles Tubular piles open on the bottom Light sections Heavy sections
Water-soaked sandy and fluid-plastic clay soils 6 7 5 12 14
The same, soils with interlayers of compact clay or gravelly soils 8 10 7 17 20
Stiff plastic clay soils 15 18 10 20 25
Semihard and hard clay soils 25 30 20 40 50

The recommended vibrational amplitude Ao required for an effective sinking is determined with the data of Table 6.

Table 6. Dependence of the vibration amplitude Ao, in mm, on the types of elements to be driven
Type of elements to be driven Ao, mm
Sandy soils Clayey soils
Vibration frequency, Hz
5-12 13-17 18-25 5-12 13-17 18-25
Steel sheet pile, steel tubes with open end, and other elements with cross-sectional area up to 150 cm2 8-10 4-6 10-12 6-8
Tubular piles (with closed end) with cross-sectional area up to 800 cm2 10-12 6-8 12-15 8-10
Reinforced concrete, square or rectangular cross section with area up to 2,000 cm2 12-15 15-20
Reinforced concrete tubular piles with large diameter, inserted with excavation of soil from tube cavity 6-10 4-6 8-12 6-10

3. The frequency of the vibrations of the vibratory pile driver, Hz, is calculated as follows:

When the set of parameters of the vibratory pile driver is derived with a previously undetermined interval of change in the θ value, it must be determined from the condition

The amplitude of the vibration velocity vo for a successful sinking should be within the interval of 0.5-0.8 m/s ; is a coefficient that takes the resilience of the soil into account: = 0.6-0.8 for low-frequency vibratory pile drivers (5-10 Hz) and = 1 for the others. If the value θ is determined by this method, the static moment of mass of the eccentrics is calculated with the formula, in kg • m:

4. The required minimum mass of the vibratory pile drive and driven element, in kg, is determined as follows:

where Uc is the cross sectional surface, in cm2; po are the recommended pressure values required; the dependence of the pressure po, in MPa, on the type and dimensions of elements driven into water-soaked sandy and loose clayey soils is given below:

steel tubes of small diameter and other elements with a cross sectional surface up to 150 cm2 0.15 – 0.3
tubular steel (with closed end) piles with a cross sectional surface up to 800 cm2 0.4 – 0.5
reinforced concrete piles of square and rectangular section with an area up to 2000 cm2 0.6 – 0.8

5. The value of the ratio of the force of gravity to the amplitude of the compelling force Pθ is verified:

v1 v2
for a steel sheet pile 0.15 0.5
for light piles 0.3 0.6
for heavy piles and tubular ones 0.4 1.0

In performing the calculations with respect to this point, either the mass mo or the amplitude of the compelling force (due to an increase in K or θ) is increased if necessary.

6. Finally, the values K, θ and mo are precisely defined, after which these parameters are verified with the formulas:

In addition, the precisely defined parameters are verified with the formulas of paragraphs 4 and 5.

7. The power of the driving motor is determined by:

where D is the diameter of the journals of the shafts of a vibration exciter, in cm.

The efficiency of transfer from the motor to vibration exciter (equal to 0.9), the coefficient of rolling friction in the bearings of the vibration exciter (equal to 10-3), and the additional consumption of power in the vibration of the soil mass, assumed to be 15% of the power expended to overcome the resistance of the soil, were studied here.

Calculation of the parameters of longitudinal-action impact-vibration hammers.

The original data for the calculation are the same parameters as for the vibratory pile drivers.

1. On the basis of the original data on the element driven, the mass of the impact part of the vibratory hammer is determined, in kg:

m1 =(0.7 — 1.2)m2.

2. The amplitude value of the compelling force, kN, is determined:

Po = dm1 x 10-2.

The lower limit of the parameter d (d = 2-6) is designated for impact-vibration hammers that drive elements with a relatively small cross sectional surface (up to 50 cm2). The parameter d also increases with an increase in cross sectional surface.

3. The preliminary depression force of the working springs is calculated, in kN:

The parameter sin α = 0.3 – 0.5. An attempt must be made to assure the value sin α = 0.4 in designing the hammers.

4. The static moment of mass of the eccentrics is determined, in kg • m:

K = 25.3 Po/θ2.

The frequency of the compelling force of the hammer θ lies within the interval of 6-10 Hz.

5. The optimal stiffness of the working springs of the vibratory hammer is, in N/cm:

c1 = (3.5 – 10) x 10-2 m1 θ2

In designing the hammers it is desirable to obtain the minimum c1, value from the recommended optimal range.

6. The ratio γ1 between the frequency of the natural vibrations of the impact part and the frequency of the compelling force:

7. The dimensionless resistance of the soil f and γ is calculated with the F and R values (9). The existence of an impact-vibrational mode is verified in terms of the parameters, sin α, γ1, and f + γ (see Figure 22).

8. The dimensionless impact velocity is calculated (correlation formula):

9. The velocity of the impact part at the moment of impact is determined, in m/s:

ximp = 6.28 x K θ/m1 y1

With respect to the durability of the hammer, the impact velocity should not exceed 2 m/s. If the value ximp » 2 m/s with the parameters chosen, it is necessary to select other parameters that assure a value ximp ≤ 2 m/s

10. The dimensionless sinking per impact (correlation formula)

11. The possibility of driving under the given soil conditions is checked:

The value ΔL is adopted according to the data (Yu. R. Perkov, V. N. Shaevich, 1974) given below:

water-soaked sands of medium coarseness and compactness 0.16
low-moisture sands of medium coarseness and compactness 0.46
wet sands of medium coarseness and compactness 0.22
loams of stiff-plastic consistency 0.32
macroporous sandy loams of a hard consistency 0.28

If it turns out that xPL < ΔL, it is necessary to reduce the parameter sin α to 0.2-0.3 and repeat the calculation. When xPL again proves to be less than ΔL, it can be assumed that the impact vibrational driving is ineffective under the given soil conditions.

12. The dimensionless rising height of the impact part is calculated:

13. The maximum rise value of the impact part is determined, in cm:

14. The maximum reaction of the vibratory hammer springs is calculated, in kN:

15. The drive engine power required is calculated, in kW:

The transfer efficiency from the vibration exciter to the motor (equal to 0.9) and the friction coefficient in the supports of the shafts (equal to 10-3) are taken into account in this formula.

The selection of the parameters of the the impact-vibrational machines for extracting the elements from the soil is done by the same method, but in this case, in kN,

Calculation of the parameters of longitudinal-rotatory-action vibratory pile drivers.

The initial data for the calculation are the same characteristics as for longitudinal-action vibratory pile drivers.

1. The basic calculation characteristics of the driving system are determined:

where σP,V is the specific resistance on the lateral surface of the shell being driven with longitudinal-rotational vibrations; the σP,V values are given in Table 7, compiled according to the results of experimental studies:

where Ut is the area of the end surface of the shell, in m2 ; R is the calculated resistance of the soil under the end of the shell, in kPa (undertaken according to the data of SNip 11-02 — 03.85).

Table 7 Specific resistance of the soil σ, in kPa on the lateral surface of the driven shell under the effect of longitudinal-rotational vibrations.
soils tubular piles with closed lower end shells driven with removal of the soil
water-soaked and soft -plastic clayey soils 4.0 2.5
the same, with interlayers of compact or gravelly soils 6.0 3.5
clayey stiff -plastic soils 10.0 5.0

2. The required dimensionless velocity values of the vibrations are determined:

where vn is the projected driving velocity in m/min (recommended value: vn = 0.2 -1.0 m/min) ; vn = vo/a2 ; θ is the frequency (recommended value: θ = 6-7 Hz).

3. The amplitudes of the compelling force and the static moment of mass of the eccentrics are determined, in kg – m:

4. The separation conditions of the shell relative to the adjacent soil are verified:

The Amin values are given in Table 8.

Table 8. Minimum separation amplitudes Amin, in mm, under the effect of longitudinal-rotational vibrations
element driven sandy soils clayey soils
6-9 Hz 13-16 Hz 6-9 Hz 13-16 Hz
tubular pile with closed lower end 3.0 1.0 4.0 1.5
shells, driven with removal of the soil 2.4 1.2 3.0 2.0

5. The magnitude of eccentricity of application of the compelling force is determined:

The δ value is selected as a function of the ratio of vibrator mass m1 and the mass of the driven shell m2;

6. The drive power of the vibratory longitudinal-rotational action pile driver is calculated, in kW:

where D is the diameter of the support of the eccentric shaft, in mm; ηver is the coefficient that takes into account the losses in transfer from the engine to the eccentric shafts.

7. It is recommended that the effort in extracting the tubular element from the soil under the action of longitudinal-rotational vibrations be determined with the formula

S = 10 mo + FH3

where FH3 is the mean force required to overcome the lateral resistance of the soil during vibratory extraction: FH3 = ζFe; Fe is the separation resistance along the lateral surface of a tubular element; and ζ is the coefficient of decrease in the lateral resistance as a function of the vibratory mode.

The ζ value is determined by the graphs plotted with Equation (35) and given in Figure 19, as a function of the ratio of the extraction velocity (vH3) to the amplitude of the velocity of the longitudinal component of the vibrations (). The rate of rise in the tube should not exceed 0.5-1.0 m/min in the first extraction stage.

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