EP0733740A1 - Monkey for dynamic downward compaction of compactable materials - Google Patents

Abstract

The invention relates to a monkey for dynamic downward compaction of all compactable materials deposited or piled up either naturally or not, characterised in that it is composed of several tightenable solid plates (1) situated one above the other in the use position and each separated from one another by a layer of resilient material (2).

Description

• This invention relates to a monkey for dynamic downward compaction of all compactable materials deposited or piled up either naturally or not.
• The principle of the dynamic downward compaction by impact on the surface to improve these materials is a generally known principle, for the first time introduced by Proctor, later further technically developed by L. Menard.
• The process basically consists in dropping important masses, going up to several tons, from a height of 5 to 10 up to 40 m, while applying a certain number of impacts at one place, a certain interval of time between each impact and a well-chosen distribution of the points of compaction spread over a yard in function of local circumstances.
• First of all, the process was invented to compact gravel or stony layers from the surface, for example layers wherein the cone resistances are more important than 10 MN/m².
• When putting this into practice, the process appeared later to be applicable also in certain coherent layers.
• The object of the invention is to develop a new and original concept of monkey with a better impact efficiency thanks to a longer contact time which is achieved between the monkey and, in general, all the materials to be compacted.
• In order to allow this in conformity with the invention, the monkey according to the invention is composed of several tightenable solid plates situated one above the other in the use position and which are each separated from one another by a layer of resilient material.
• In order to adjust the properties of the monkey according to the invention to the most diverse situations, there is proposed to join said solid plates and said layers of resilient material into one unit by means of adjustable tightening screws.
• Other details and advantages of the invention will become apparent from the following description of a monkey for dynamic downward compaction of materials. This description is only given by way of example and does not limit the invention. The reference numerals relate to the figures annexed hereto.
• Figure 1 is a graph wherein for a same time unit the impacts caused by two different monkeys are compared.
• Figure 2 is a top view on a monkey according to the invention.
• Figure 3 is a cross section according to line II-II from Figure 2.
• The monkey for dynamic downward compaction, to which these figures relate, is composed in a preferred but not limitative embodiment of several solid plates 1, in this case three plates, each of which being separated from one another by a layer of resilient material 2. The three solid plates 1 with the layers 2 of resilient material situated therebetween are strongly connected to one another and tightened through adjustable tightening screws 3 in an appropriate way.
• These tightening screws have a functional purpose. Depending on the tension they can either increase, or reduce the self-stiffness of the monkey. Consequently, they have an influence in this way on the nature of the impact and thus on the contact time and the transferred energy.
• In case of a very deformation resistant underground, the monkey is adjusted by means of the tightening screws to "deformable monkey" ; in case of a very soft underground, the monkey is adjusted by means of these tightening screws to "stiff monkey".
• By dropping this type of monkey of a new kind from the desired height (if necessary up to 40 m), a longer contact time is achieved between the monkey and the materials to be compacted so that a better efficiency of the impact can be expected.
• The impedance Z₁* of the monkey is composed of the rigidities and characteristics of the solid plates and of the compressible plates together.
• It could thus be set forth for n₁ solid plates with modulus E_s, specific mass ρ_s, thickness h_s and surface area Ω together with n₂ compressible singly interposed sheets with characteristics E_n, ρ_n, h_n and Ω that $${\mathit{\text{<figure-callout id="1" label="Z" filenames="imgf0002.png" state="{{state}}">Z</figure-callout>}}}_{\text{1}}\text{*= α.Ω}\sqrt{{\mathit{\text{E}}}_{\mathit{\text{s}}}{\text{.ρ}}_{\mathit{\text{s}}}}\text{+ β.Ω}\sqrt{{\mathit{\text{E}}}_{\mathit{\text{n}}}{\text{.ρ}}_{\mathit{\text{n}}}}$$

  

  wherein α and β are "weight" factors which are dependent from h_s, h_n, n₁ and n₂. The tightening screws which hold the monkey together may have prestressed the compressible stack in the rest position. As a result thereof "E_n,o" changes in fact in (1) to a secant modulus $${\mathit{\text{E}}}_{\mathit{\text{n,v}}}\text{=}\mathit{\text{Arctg}}\frac{{\mathit{\text{V}}}_{\mathit{\text{prestress}}}}{{\text{ε}}_{\mathit{\text{prestress}}}}$$

  

  Just before touching the ground the monkey has the velocity "v_o".
• The resistance offered by the n₂ compressible sheets against further compression during the impact is represented by P_g,

  

  wherein R_i is the total prestress force in the screws (in case R_i = 0, P_g ≅ 0 because ${\text{ε}}_{\text{prestress}}{\text{= ε}}_{\text{0}}$

  

  ; only deformation under the weight of the stack).
  The combined monkey has therefore a speed rate of fall at contact $${\mathit{\text{v}}}_{\text{0}}\text{=η}\sqrt{\text{2}\mathit{\text{gh}}}$$

  

  wherein h is the free fall height of the monkey and η the correction factor on this "free" fall, arising from the cable support.
• Upon impact, the velocity of the monkey is slowed down from v₀ to v ; the soil surface velocity changes from 0 to v.
• When the velocity of the lowermost solid plate of the monkey has dropped, after impact, to velocity v, the uppermost solid plate will still have a higher velocity due to the compressibility of the layers of resilient material. The time span wherein this lagging deceleration of the slowing down (to velocity v) of the uppermost plate takes place, determines the length Δt of the aimed contact time of the "plastic collision".
• The longer this contact time, the greater the portion of the available kinetic energy which can be transformed in effective power. For monkeys used hitherto and characterised by the absence of any compressible material, the lagging deceleration of the slowing down to velocity v does not exist. Since a relatively short contact time is achieved in this case, the efficiency of such monkeys is lower.
• The monkey according to the invention permits thus in a clearly more efficient manner to compact downwards, especially when harder surface layers are to be treated. In such cases, three criteria for the falling weight remain very important :
• 1. the centre of gravity of the monkey should be located as low as possible in order to avoid any turning over immediately after impact ;
• 2. a collision which is as plastic as possible should be realized in order to keep the most useful duration Δt of the energy transfer as large as possible ;
• 3. a high "flexibility" of the weight of the monkey in order that this could be adapted for different applications, i.e. different impact energy requirements.
• The ground improvement which can be observed when applying the downward compaction can be monitored by means of the deformation modulus E of the ground, which may be from 2 to 10 times higher after compaction. The settings to be expected can therefore be reduced with a factor 4 to 5 and the stacking density can be increased substantially.
• From the graph according to Figure 1, it can clearly be deduced that the wave force generated by a usual monolithic monkey v' is in the first phase higher than the wave force generated by the monkey v'' according to the invention. Measured in time, the total value of the wave force of the monkey v'', constructed according to the concept of the invention is however substantially higher. This is the result of a collision which is as plastic as possible between the monkey and the materials to be compacted, keeping the useful duration Δt of the energy transfer as large as possible.
• In Figure 1 the wave forces are measured on the Y-axis, whereas the time wherein the effect of the impact takes place can be read on the X-axis.
• In summary, there can thus still be underlined that the monkey according to the invention has to meet the following three criteria :
• 1. The centre of gravity of the monkey should be located as low as possible in order to avoid any turning over immediately after impact ;
• 2. A collision which is as plastic as possible should be realized in order to keep the most useful duration Δt of the energy transfer as large as possible ;
• 3. A high "flexibility" of the weight of the monkey in order that this could be adapted for different applications, i.e. different impact energy requirements.
• The invention is of course not limited to the embodiment described hereinabove by way of example and a lot of modifications can of course be conceived, provided they fall within the scope of the claims annexed hereto.

Claims (3)

1. A monkey for dynamic downward compaction of all compactable materials deposited or piled up either naturally or not, characterised in that it is composed of several tightenable solid plates (1) situated one above the other in the use position and each separated from one another by a layer of resilient material (2).
2. A monkey according to claim 1, characterised in that said resilient material (2) is rubber.
3. A monkey according to claims 1 and 2, characterised in that said solid plates (1) and said layers of resilient material (2) form one unit by adjustable tightening screws (3).

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