JP2741413B2 - Ground strength measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION << Field of Industrial Application >> The present invention relates to a method for measuring ground strength using vibration,
In particular, it provides a ground strength index that replaces the conventional N value.

<< Conventional Technology >> In civil engineering and construction work, piles, hollow pipes, and the like are often used as foundation methods, and ground improvement methods are often applied. Diesel hammers using impact force have been mainly used for penetrating piles and hollow tubes, but a vibro hammer using vibration force may also be used. In the ground improvement method, a sand compaction pile method using a vibro hammer is also common.

When adopting the foundation method for various structures, drilling is performed in advance at the site to understand the soil properties and ground condition of the original ground, and to check the N value at the depth of the ground, and based on the average N value, for example, the pile method In the case of, construction conditions such as the type of the pile body and the driving length are determined.

The conventional N value is JIS A1219 (standard penetration test method for soil)
The number of hits required to drop a hammer weighing 63,5 kg 75 cm freely and drive the standard penetration test rod 30 cm is specified in the following.For example, in a wide construction area with relatively uniform ground, What is used is an averaged value based on measurements at a plurality of locations.

<< Problems to be Solved by the Invention >> Since this N value is measured by driving a thin rod having a diameter of about 5 cm, it tends to fluctuate depending on the surveyed location on a ground with large variations. It is.

Therefore, in such a case, the average N value is a rough guide, but is slightly larger or smaller than the actual ground strength at the construction site. In addition, by making the measurement points dense, the average N
Although it is desirable to grasp the value for each small area and minimize the deviation from the original ground strength, it is naturally restricted from the construction period and economical aspects.

For this reason, in the actual driving at the site, for example, it is difficult to drive the piles because the pile strength does not reach the design penetration length because the ground strength at each location differs from the design ground condition according to the preliminary survey depending on the driving location of each pile. In many cases, the pile body does not reach the strong supporting ground after the driving of the design length has been completed.

Such a defect with respect to the average N value is particularly remarkable in a landfill using soil or the like in which large-sized rocks are liable to be mixed. The N value tends to be excessive.

In view of such circumstances, the present invention uses a vibro hammer to penetrate a pile or the like having a relatively large diameter as a ground strength index instead of the conventional standard penetration test (N value) which is a relatively microscopic test method. , The strength of the original ground with higher accuracy,
It is another object of the present invention to provide a relatively macro ground strength measuring method that can be grasped on average.

Also, it is possible to calculate the estimated N value of the ground from the R value obtained by the present invention, and to select the type of foundation and the workability of the foundation method,
Alternatively, it is an object of the present invention to provide a ground strength measuring method that can be used rationally for examining construction management.

<< Means for Solving the Problems >> In order to achieve the above object, the method for measuring the ground strength of the present invention comprises: (Where R is the ground dynamic penetration resistance, Cω is the rate of change of the vibratory hammer circular frequency ω, Cη is the rate of change of the vibrohammer vibration acceleration η, W is the vibrohammer output at the time of intrusion, α is a coefficient relating to device specifications, β Is a coefficient relating to idling characteristics).

Also, the obtained R value is (Here, N _i is the estimated N value at any desired depth, the Soil dynamic penetration resistance in R _i is an arbitrary depth, xi] is the coefficient Dynamic skin friction, zeta coefficient on dynamic tip resistance, [Delta] Z estimation N N depth is estimated from the following equation:

<Operation> The soil strength measurement method of the present invention is an alternative to the N value specified in JIS A1219 (standard soil penetration test method) used as a soil strength index, and a sand compaction pile method using a vibro hammer. It is based on the following vibration intrusion equation derived by analyzing the construction record data of the steel plate cell method.

First, a theoretical study of vibration penetration by a vibro hammer will be performed. Vibration models related to pile driving are classified into three types (b), (c) and (d) as shown in FIG.

When the vibration model in FIG. 3 is generalized by a vibration equation having one mass point and one degree of freedom, the following equation is obtained.

 my + cy + ky = P · sinωt (1) The solution of the above equation is as follows.

Where λ ² = k / m γ = c / 2m m = Q _o / g ψ = tan ^-1 {2ωγ / (ω ² −λ ² )} where m is the mass of the vibration system (tf · sec ² / m) and c are damping coefficients (tf · sec / m), k is spring constant (tf / m), P is starting force (tf), ω is circular frequency (rad / sec), and t is time (sec). ,
y is the vertical displacement (m), λ is the natural frequency of the shock absorber or ground spring (sec ^-1 ), γ is the viscous drag coefficient (sec ^-1 ), _Qo is the total vibration weight (tf), g Is the gravitational acceleration (9.8 m / sec ² ) Next, focusing only on the ground spring from the equation (2), and obtaining the amplitude a and the vibration acceleration η, the following is roughly obtained.

On the other hand, vibro-hammer output W of the pile after casting is represented as the output increment ΔW sum varying been accompanied with the output W _o and embedment increase in idling state.

Furthermore the output W _o is the average product of the vibration velocity and the total vibration weight during idling, and the output increment ΔW of the punching under construction is represented by the product of the average vibration velocity and the ground resistance is as follows.

Here, c _o is the operating efficiency of the vibro-hammer.

Considering the circular frequency ω and the vibration acceleration η during pile driving as the rate of change with respect to the values ω _o and η _o during idling, ω / ω _o = Cω and η / η _o = Cη (5) time during and idling amplitude a, the relationship a _o is expressed by the following equation using the equation (5).

a / a _o = Cη / Cω ² (6) From equation (3), the vibro-hammer output W is obtained from equation (6) as follows.

From the above equation, the output W of the vibro-hammer is expressed by the dynamic penetration resistance R of the ground, the change rate of the circular frequency and the vibration acceleration during the driving C
It can be seen that it is governed by ω and Cη.

From the above inference, when formula (7) is converted to a general formula, Where ω _o is the initial circular frequency of the vibro hammer (rad /
sec), λ _o is the natural frequency of the shock absorber (1 / se
c), the total excitation force of P _o is vibro-hammer (tf), Q _o is the total vibrating weight (tf), c _o is the operating efficiency of the vibro-hammer, g is the gravitational acceleration (9.8m / sec ^2).

The ground dynamic penetration resistance R includes coefficients α and β relating to the specifications of the driving device and the idling characteristics, a change rate Cω, Cη of the vibratory hammer frequency and vibration acceleration obtained from the data at the time of driving, and a vibratory hammer at the time of the penetration. Since it is constituted by the output W, it can be easily calculated.

Next, the relationship between the ground dynamic penetration resistance R and the N value is inferred.

For example, consider a N value as ground conditions piles after casting, assuming ground to increase of GL surface N value (z = 0 m) in a ratio of each N _o, underground depth 1 m, N = mz + N _o (11) Next, assuming that the reduction rate due to vibration is μ _f , μ _p for each of the peripheral friction and the tip resistance, the ground dynamic penetration resistance R at the time of pile driving is expressed by the following equation.

_{R = R f + R p =} μ f · R f '+ μ p · R p' (12) where skin friction and tip resistance R _f, R _p is hitting設時_{(tf) R f ', R} p ′ Is static peripheral friction and static tip resistance (tf), where R _f ′ acts on the outer peripheral surface of the pile and R _p ′ acts on the closed area at the tip of the pile. When used in the calculation formula for bearing capacity of a driven pile from the Architectural Institute of Japan), the ground dynamic penetration resistance R is shown as follows.

From _{R = μ f · 1/10 · πDZ} (mZ + 2N o) + μ p · 7.5 · πD p 2 (mZ + N o) (13) above reasoning, the general formula (13), and the depth Z
= Subsequent sequentially N value of each ΔZm the N value as N _o at the time of the 0
N _1, N _2, and ···· N _i. Similarly, the ground dynamic penetration resistance R
R ₀ , R ₁ , R ₂ ... R _i Here, mu _f is skin friction reduction rate due to vibration, mu _p is the tip resistance reduction rate due to vibration, determined by such soil conditions. The D is the diameter of the straight pipe portion of the casing, D _p represents the diameter of the casing tip.

The ground dynamic penetration resistance R is thus converted into an N value.
Since it is estimated, the correspondence with the N value becomes clear, and it can be directly applied to the ground strength management in the conventional vibration intrusion.

The values of α, β, ζ, and 以上 are determined depending on the specific vibration driving machine, construction specifications, and ground conditions.

<< Example >> Hereinafter, an example of the present invention will be described. In the embodiment, the ground dynamic penetration resistance R is calculated based on the above theoretical formula based on the record when the casing is penetrated by the vibration driving machine,
In addition, the N value was estimated and converted (hereinafter, referred to as an estimated N value).
In the field, boring was performed in advance near the casing penetration, and the N value was determined by a standard penetration test.

 The vibration driving machine used was as follows: (1) The equipment specifications were as follows.

Total vibratory force P _o = 48.8tf total vibration weight Q _o = 11.2tf vibro-hammer straight tube spring constant K _o = 61.2tf / m casing of characteristic frequency λ _o ² = 53.6sec ² shocks Apu Soba of vibro-hammer of vibro-hammer characteristics of idling by the diameter D _p = 0.454m (2) Measurement of the tip portion of a diameter D = 0.406m casing parts are as follows.

Tuning efficiency c _o rpm 568Rpm ∴ circular frequency [ω _{o =} 59.5rad / sec vibration acceleration eta _{o =} 4.4 g vibro-hammer output W _o = 17 kW vibro-hammer K by (7), c _o
= 0.427.

(3) The vibratory hammer output W, vibratory hammer circular frequency ω, and vibratory hammer vibration acceleration η were recorded at the time of vibration penetration.

From the above results, when the tip resistance reduction rate μ _p and the peripheral friction reduction rate μ _f due to vibration are calculated from equations (7), (12) and (13) and the measured ground N value, Met.

From equations (9), (10), (14), and (15), α,
β, ζ, ξ are calculated as follows.

α = 0.277 β = 17.00 ζ = 0.181 0.6 = 0.627 An estimated N value was calculated based on the above results. Incidentally, the estimated value of N performed separately and methods according to the total output of the vibro-hammer, the total output _{(W o + W p + W} f) and output during withdrawal (W _o + W _f) from the tip resistance corresponding output W _p during penetration Output separation was performed.

(I) Estimated N value by full power method From full power of vibro hammer, Was calculated sequentially for each underground depth.

(II) after separation of the tip resistor corresponding output W _p of time estimated N value penetration by the output separation method, From the above two equations, the estimated N value is Calculated from

FIG. 1 is an example in which the calculation results of (I) and (II) are displayed on a graph.

In the same figure, the measured N value by the standard penetration test at the same point is shown for comparison.

The estimated N values obtained by the full output method and the output separation method are both correlated with the actually measured N values. In particular, the estimated N value obtained by the output separation method substantially coincides with the actually measured N value, and from this point, the reliability of the ground strength R value is sufficiently demonstrated.

Note that the estimated N value changes relatively slowly compared to the actually measured N value. The reason for this is that in the case of the estimated N value and the case of the actually measured N, the penetration diameter is about 45 cm for the former and about 5 cm for the latter, so that the estimated N value measures the ground strength in a wider range, This is probably because the average intensity in the vicinity was shown.

Therefore, the present invention is an index of ground strength that can replace the standard penetration test (N value), which is a relatively microscopic test method. In particular, a pile body or the like is vibrated into the ground with a vibro hammer to improve the strength of the original ground. It is a relatively macro ground strength measurement method that can be grasped with high accuracy and on average.
In addition, since the N value of the ground can be immediately estimated by measuring and calculating the vibratory hammer output, vibration acceleration, and circular frequency, it can be reasonably used for the selection of the foundation type 7, the workability of the foundation method, or the study of construction management. It can be used.

The ground strength measuring method of the present invention can be variously modified and developed without departing from the technical idea.

<< Effects of the Invention >> As described above, according to the ground strength measuring method of the present invention, (a) the ground strength is measured by vibrating a pile or the like with a vibratory hammer, so that the The machine can be used as it is.

(B) Since the pile body is several times thicker than the standard penetration test rod, the N value measurement is less affected by local ground changes, and the ground strength is more accurately and more averagely measured. Can be grasped.

(C) Since it is converted and estimated into an N value, the correspondence with the N value becomes clear, and it can be applied to ground strength management and the like in the conventional vibration penetration.

(D) The estimated N value of the ground can be obtained in real time by measuring and calculating three components of vibratory hammer output, vibration frequency and vibration acceleration.

 And the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example in which a measured N value obtained by the ground strength measuring method of the present invention is compared with a standard N, and FIGS.
(D) is a schematic diagram showing a vibration model related to pile driving.

Claims (2)

(57) [Claims] 1. Vibration penetrating a pile body or the like with a vibro hammer, (Where, R is the ground dynamic penetration resistance, Cω is the change rate of the vibratory hammer circular frequency ω, Cη is the change rate of the vibrohammer vibration acceleration η, W is the vibrohammer output at the time of penetration, and α is the coefficient relating to the specifications of the driving device. , Β is a coefficient relating to idling characteristics). 2. From the R value, (Here, N _i is the estimated N value at any desired depth, the coefficient related to R _i is the ground dynamic puncture resistance at any depth, zeta dynamic skin friction, xi] is the coefficient related to the dynamic tip resistance, [Delta] Z estimation N A ground strength measuring method, wherein the N value is estimated from the following formula: