RU2816722C1 - Multi-blade screw pile - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to multi-blade screw piles and is intended for arrangement of foundations of buildings and structures of various purposes, supports of power transmission lines and communication, pipelines, bridges, advertising structures, residential and public buildings. Multi-blade screw pile, intended for operation in non-rocky soils, consists of a shaft, extension elements connected to the shaft, screw blades of the same size, located along the barrel length, and a conical tip. Screw blades are located within the bearing layer of soil with thickness of H_bl at a distance from each other l_b≥h_f×t_gφ(1+1/t_g ²φ), multiple of screw blade pitch “a”, in amount of: n_b=(N−F_dƒ−F_do)/F_db for piles operating for indentation, where: l_b is the distance between the blades, h_f is the height of the flange of the screw blades, h_f=D−d/2, D is the diameter of the screw blades, φ is the angle of internal friction of the bearing layer of soil, n_b is the number of screw blades, N is the external load, F_dƒ is the bearing capacity of the pile shaft, F_do is the bearing capacity at the tip, F_db is the bearing capacity of the blade, for piles operating for pulling out, F_do=0, note here that shaft bearing capacity is calculated by formula: F_dƒ=ƒ×A_aƒ, where ƒ is the calculated soil resistance along the side surface of the screw pile tf/m² (averaged value for all layers within the immersion depth of the pile), A_aƒ is the surface area of the pile shaft displacement on the ground, is determined by the formula: A_aƒ=π×d[L_pl−n_b×h_f×t_gφ(1+1/t_g ²φ)], where d is pile shaft diameter, L_pl is pile length.

EFFECT: increasing bearing capacity of the pile due to the design of the multi-blade pile taking into account the technological features of the formation of the stressed state of the soil in the zone of the blades location during the piles immersion.

5 cl, 2 dwg

Description

The invention relates to multi-blade screw piles and is intended for constructing foundations of buildings and structures for various purposes: power and communication line supports, pipelines, bridges, advertising structures, residential and public buildings.

A screw pile is known according to the RF patent for utility model No. 28498, published on March 27, 2003), containing a tubular trunk with a conical tip and two blades placed on the tubular trunk, and the diameter of the upper blade is 2 times greater than the diameter of the lower blade, and the distance between the blades is 1.2-1.3 trunk diameter.

This pile is inherently complex in design and highly labor-intensive to manufacture. In addition, the geometric parameters of the pile are not linked to the length of the pile, and the placement of the blades along the height of the pile does not ensure their separate operation, which reduces the load-bearing capacity of the pile.

The closest technical solution to the proposed one is a multi-blade screw pile according to the RF patent for invention No. 2725348, intended for use in non-rocky soils. The pile consists of a shaft, one or more shaft extensions connected to the shaft, and helical blades of the same size located along the entire length of the shaft.

This technical solution has several significant disadvantages. Since the tip is eccentric due to the bevel on one side, the unit for extending the barrel through the coupling is not reliable. The diameter of the propeller blade is insufficient to ensure reliability in pulling out with a load-bearing layer of up to 10 m of pliable soil, and at the same time, the multiplicity of the blade diameter of 0.5 m is too high. The presence of zones of compacted soil (dead zones) is not taken into account, which in calculations leads to an overestimation of the bearing capacity of the pile on the ground.

In addition, the pitch of the blade is underestimated, which can cause the pile to rotate. As Dr. Sc. points out. Zhelezkov V.N. in the book “Screw piles in the energy and other branches of construction” St. Petersburg, Pragma, 2004, “Multi-bladed screw piles with a small pitch of blade screws are more susceptible to turning than others.”

The technical result of the proposed solution is to increase the load-bearing capacity of the pile due to the development of the design of a multi-bladed pile, taking into account the technological features of the formation of the stressed state of the soil in the area where the blades are located when the pile is driven.

To solve the problem, a multi-bladed screw pile design is proposed, represented by a new set of essential design features.

A multi-blade screw pile for working in non-rocky soils, consisting of a barrel, extension elements connected to the barrel, screw blades of the same size, fixed along the length of the barrel, differs from the known ones in that

helical blades are _placed at a distance from each other l _l ≥h _p ×t _g φ(1+1/ t _g ² φ), a multiple of the screw pitch of the helical blade “a”, in quantity: n _l =(N- F _dƒ - F _do )/ F _dл for piles working for indentation, where

N – external load,

F _dƒ – load-bearing capacity of the pile shaft,

F _do – tip load-bearing capacity, F _dл – blade load-bearing capacity,

in this case, the bearing capacity of the trunk is calculated using the formula: F _dƒ =ƒ×A _aƒ , where

ƒ – calculated soil resistance along the lateral surface of the screw pile t/ ^m2 (averaged value for all layers, within the immersion depth of the pile),

And _aƒ is the shear surface area of the pile shaft along the ground, determined by the formula:

A _aƒ =π×d[L _St -n _l ×h _p ×t _g φ(1+1/ t _g ² φ)], where

L _St – pile length,

n_l – number of helical blades,

D – diameter of helical blades,

h _p – height of the flange of the helical blades, h _p =Dd/2,

φ – angle of internal friction of the load-bearing soil layer.

In addition, the extension element is connected to the barrel by welding.

The conical tip has flat cuts on both sides symmetrically to the pile axis at an angle of 90° between the cut planes.

The diameter of the blades is selected from 2d to 4d and is set as a multiple of 0.1m.

The pitch of the blades “a” does not exceed 0.3D.

The proposed improvement of the structural elements of a multi-bladed pile makes it possible to increase the load-bearing capacity of the pile.

The proposed pile is illustrated by drawings:

Fig.1 – pile design.

Fig.2 – location of piles relative to the load-bearing soil layer.

The proposed multi-bladed pile (Fig. 1) contains a trunk 1 with a length of L _St with a conical tip 5 and helical blades 4, located at a distance of l _l from each other. To preserve the soil mass undisturbed, each subsequent blade must repeat the path of the previous one, and for this, the distance between the blades must be a multiple of the pitch of the helical blade “a”.

In addition, during the process of immersing (screwing) the pile into the ground and subsequent loading of the pile with the design load, zones of compacted soil (dead zones, see Fig. 1) are formed along the perimeter of the helical blades b and b ¹ , which must be excluded from calculations of the load-bearing capacity of piles on the ground when determining the load-bearing capacity of the pile shaft.

b ¹ =h _p ×t _g φ, b=h _p ×t _g φ, where

h _p – height of the flange of the helical blade,

φ – angle of internal friction of the load-bearing soil layer.

The dead zone between the blades will be:

b ¹ +b=h _p ×t _g φ(1+1/t _g ² φ)

To eliminate interference between the operation of the blades, the distance between the blades, taking into account dead zones, should be: l _l ≥ h _p ×t _g φ(1+1/ t _g ² φ),

The area of the working surface of the pile, taking into account dead zones, is equal to:

A _aƒ = π×d[L _St -n×h _p ×t _g φ(1+1/ t _g ² φ)],

where L _St is the length of the pile;

d – diameter of the pile shaft;

n – number of helical blades;

The pile is extended using extensions 2 of length L ₂ , which are welded with a continuous seam to the primary shaft 1. At its upper end, extension 2 has a head - crown 6, through which the pile is twisted to the length of the extension. The operations of lengthening and tightening the pile are repeated until the pile reaches the calculated depth (see Fig. 2) and is inserted into the load-bearing layer with a thickness of N _n.sl.

To eliminate axial displacement of the pile and tilt from the vertical, the tip 5 is cut off from 2 diametrically opposite sides symmetrically to the axis of the pile at an angle of 90° between the cut planes.

The sequence of calculating the parameters of a multi-bladed screw pile according to the proposed solution is as follows (option):

- the bearing layer N _n.s. is installed. according to geological survey data, in their absence, based on the results of test drilling; design load on pile No. ¹ at 1 g.p.s. (group of limit states, ts)

- the pile diameter d (m) is assigned;

- blade diameter D(m)=2/4d _St ;

- set the length of the pile L _St ;

- according to SP 24.13330.2011 “Pile foundations”, the load-bearing capacity of the blade F _dl and the load-bearing capacity of the tip F _do are calculated;

- the load-bearing capacity of the pile shaft is calculated F _dƒ =ƒ×А _aƒ , where

ƒ – average value of friction resistance along the lateral surface within the length of the pile, determined according to SP 24.13330.2011

And _aƒ is the shear surface area of the pile shaft along the ground, determined by the formula:

A _aƒ =π×d[L _St - n _l ×h _p × t _g φ(1+1/ t _g ² φ)]

- the number of helical blades is set

n _l =N- F _dƒ - F _do / F _dl for piles working for indentation;

n _l =N- F _df / F _dl for pull-out piles.

- the shear surface area of the pile shaft along the ground is calculated, taking into account dead zones. A _aƒ =π×d[L _St - n _l ×h _p × t _g φ(1+1/ t _g ² φ)]

- the actual load-bearing capacity of the trunk is established, taking into account dead zones F _dƒ =ƒ×A _aƒ , a check is performed

N≤n _l ×F _do + F _df +F _l for indentation

N≤ n _l ×F _do + F _df for pulling

- the pitch of the helical blades is determined

l _l ≥h _p ×t _g φ(1+1/ t _g ² φ)

within 4-5d

An example of calculating the parameters of a multi-bladed screw pile as suggested by the author:

- bearing layer – plastic silty loam with layers of sand and clay up to 10%

φ ¹ =18°; with ¹ =1.0 tf/ ^m2 ; ρ=2.0 t/m ³

- design load on the pile is 1 g.p.s. N ¹ =50 tf

- diameter of the pile shaft d=325 mm;

- blade diameter D=2d=650 mm;

- blade propeller pitch a = 200 mm;

- the length of the pile according to the placement of the load-bearing soil layer is assumed to be L _St = 11.0 m;

- calculation of the bearing capacity of the blade

F _dl =γ _s (α ₁ s ₁ +α ₁ γ ₁ h ₁ )A _l ,

A _l =π/4(D ² -d ² )=3.14/4(0.65 ² -0.325 ² )=0.24m ² , h ₁ =9.0 m;

F _dl =0.7(10.1×1.0+4.5×9.0)×0.24=15.3 tf

- calculation of the load-bearing capacity of the pile at the tip

F _dо =γ _с ×γ _сr ×R×A _o , A _o =π/4×d ² )=3.14×0.325 ² /4=0.083 m ²

F _dо =1×0.9×85×0.083=6.35 tf

- calculation of the load-bearing capacity of the pile shaft

F_dƒ=ƒ×A_aƒ, Where

ƒ – average value of frictional resistance along the lateral surface within the length of the pile

ƒ = 2.05 tf/m ²

A_f= πd[L_St.-n×h×t_gφ(1+1/ t_g ²φ) ]

The shear surface area of the pile shaft to the ground, based on the assumed number of screw blades - 3 pcs, will, as a first approximation, be:

A _f =3.14×0.325[11-3×(0.650-0.325)/2×0.325(1+1/0.325 ² )]

A _f =3.14×0.325(11-1.75)=9.44m ² , F _dƒ =2.05×9.44=19.35tf

The total bearing capacity of the pile on the ground will be:

F _d =F _dl + F _dо + F _dj = 3×15.3+6.35+19.35=71.6ts

Conditions: N ¹ ≤F _d /γ _k is satisfied

N¹=50ts<71.6/1.4=51.14ts

- Determine the pitch of the helical blades

l _l ≥ h _p ×t _g φ(1+1/t _g ² φ)

l _l ≥ 0.162×0.325 (1+1/0.325 ² )

l _l ≥ 0.162×0.325×11=0.58m

We take the pitch of the screw blades l _l = 1.6 m, a multiple of the pitch of the screw blade a = 200 mm.

The pile indicated in the calculation example was completed in November 2020. a tested on January 14, 2021, report “PKTI Foundation Test”, code. No. 9988.

The pile test schedule is shown in the attached figure. The load-bearing capacity of a 3-blade pile on soil type SBC 325×8-11000-P×20-650/650/650 was 74.0 tf, which showed the correctness of the initial premises laid down by the author in the calculation methodology, which gives good convergence of the calculated and obtained in the result of static tests of piles results.

Estimated F _d =72.6 tf

The F _{d test obtained as a result of testing.} =74.0 tf

If the blades are placed not a multiple of the pitch of the helical blade, a complete disruption of the soil structure around the pile occurs, with a diameter equal to the diameter of the blade. This leads to a decrease in the load-bearing capacity along the lateral surface

F _dƒ by 9.35 tf, for three blades F _dl by 20.4 tf

In this case, the load-bearing capacity will be significantly lower

F _d = 72.6 – 9.35 – 20.4. = 42.75 tf

Thus, the proposed improvements to individual structural elements of a multi-blade pile make it possible to increase the strength of the connection of the pile extensions, eliminate the axial displacement of the pile, eliminate the interference of the blades, eliminate the rotation of the pile and the destruction of the compacted soil, which ultimately makes it possible to significantly increase the load-bearing capacity of the pile.

Claims (21)

1. Multi-blade screw pile, designed for use in non-rocky soils, consisting of a barrel, extension elements connected to the barrel, screw blades of the same size located along the length of the barrel, and a conical tip, characterized in that the screw blades are located within the load-bearing soil layer power N _n.sl at a distance from each other l _l ≥h _p ×t _g ϕ(1+1/t _g ² ϕ), a multiple of the pitch of the helical blade “a”, in the amount of n _l =(NF _dƒ -F _do ) /F _dл for piles working for indentation, where l _l is the distance between the blades, h _p – height of the flange of the helical blades, h _p =Dd/2, D – diameter of helical blades, ϕ – angle of internal friction of the load-bearing soil layer, n _l – number of helical blades, N – external load, F _dƒ – load-bearing capacity of the pile shaft, F _do – load-bearing capacity at the tip, F _dл – bearing capacity of the blade, for piles working for pulling out, F _do =0, in this case, the bearing capacity of the trunk is calculated using the formula: F _dƒ =ƒ×A _aƒ , where ƒ – calculated soil resistance along the lateral surface of the screw pile tf/ ^m2 (averaged value for all layers within the immersion depth of the pile), And _aƒ is the shear surface area of the pile shaft along the ground, determined by the formula: A _aƒ =π×d[L _St -n _l ×h _p ×t _g ϕ(1+1/t _g ² ϕ)], where d – diameter of the pile shaft, L _St – pile length. 2. The pile according to claim 1, characterized in that the conical tip is cut off on both sides symmetrically relative to the axis of the pile at an angle of 90° between the cut planes. 3. The pile according to claim 1, characterized in that the diameter of the blades is determined from 2d to 4d and is set as a multiple of 0.1 m. 4. The pile according to claim 1, characterized in that the pitch of the blades “a” does not exceed 0.3D. 5. The pile according to claim 1, characterized in that the extension element is connected to the barrel by a weld.