Method for compaction detection and control when compacting a soil with a deep vibrator

Abstract

A method for detecting and controlling compaction when compacting a soil by a depth vibrator which has a rotationally drivable imbalance (3) and at least one sensor (6, 12, 13, 14, 19), comprising the steps of: inserting the depth vibrator (2) into the soil (17) up to a desired final depth (Tm); compaction of the soil (17) during which the forward angle () of the imbalance (3) as well as the oscillation amplitude (A) of the depth vibrator (2) are determined; detection of a soil stiffness profile from soil stiffness values (k) determined over time (t); determination of a first soil stiffness value (k1) and a second soil stiffness value (k2) from the soil stiffness profile (k), for which it applies that a rate of increase (k2) of the second soil stiffness value (k2) exceeds a rate of increase (k1) of the first soil stiffness value (k1) by a defined factor; calculation of a transition soil stiffness value (k12) which is between the first soil stiffness value (k1) and the second soil stiffness value (k2); and storing the transition soil stiffness value (k12) detected in the respective compaction step to the associated depth (T).

Claims

1. A method for compaction of soil by means of a depth vibrator comprising an imbalance rotationally driveable in a vibrator housing and at least one sensor, the method further comprising the steps of: inserting the depth vibrator into the soil to a desired final depth (Tm); compacting the soil by the depth vibrator in a series of compaction steps, wherein during the compaction to a respectively measured depth (T) a forward running angle () of the imbalance and a vibration amplitude (A) of the depth vibrator are determined; during a compaction step, detecting a soil stiffness profile comprising soil stiffness values (k) determined over time (t) on the basis of said forward running angle () and vibration amplitude (A); determining a first soil stiffness value (k1) and a second soil stiffness value (k2) from the soil stiffness profile for which a rate of increase (k2) of the second soil stiffness value (k2) exceeds a rate of increase (k1) of the first soil stiffness value (k1) by a defined factor; calculation of a transition soil stiffness value (k12) that is between said first soil stiffness value (k1) and said second soil stiffness value (k2); storing the transition soil stiffness value (k12) recorded in the respective compaction step to the corresponding depth (T).

2. The method of claim 1 further comprising a step of terminating a compaction step when the rate of increase (k2) of the second soil stiffness value (k2) is greater than 1.5 times the rate of increase (k1) of the first soil stiffness value (k1).

3. The method of claim 2, wherein said step of terminating a compaction step comprises terminating said compaction step when the rate of increase (k2) of the second soil stiffness value (k2) is greater than twice the rate of increase (k1) of the first soil stiffness value (k1).

4. The method of claim 1, wherein after completion of compaction at a compaction depth (T) the depth vibrator is pulled to the next stepwise depth to be compacted.

5. The method of claim 1, wherein said at least one sensor comprises at least one acceleration sensor configured to measure acceleration of the depth vibrator during compaction.

6. The method of claim 5, wherein said at least one sensor comprises at least one position sensor configured to detect a signal representing the position of the imbalance mounted on the depth vibrator.

7. The method of claim 5, wherein said at least one acceleration sensor comprises a plurality of acceleration sensors mounted in different planes on the depth vibrator.

8. The method of claim 1, wherein said step of determining the first soil stiffness value (k1) and second soil stiffness value (k2) comprises a calculation based on modal resonating soil mass (M).

9. The method of claim 8, wherein said step of determining the first soil stiffness value (k1) and second soil stiffness value (k2) comprises calculating based on the imbalance (m.Math.e), oscillation amplitude (A) and mass (M) of the depth vibrator.

10. The method of claim 9, wherein said calculation comprises a formula: $M=\frac{m.Math.e}{A}-M$

11. The method of claim 1, wherein said step of determining the first soil stiffness value (k1) and second soil stiffness value (k2) comprises a calculation based on measured amplitude (A) and a reference amplitude (A) of the vibrator at free oscillation.

12. The method of claim 11, wherein said step of determining the first soil stiffness value (k1) and second soil stiffness value (k2) comprises a calculation according to the formula: $V=\frac{A}{A_{}}=\frac{A.Math.\left(M+M\right)}{m.Math.e}$ where V is the amplification factor, A is the vibration amplitude of the depth vibrator during compaction, A is the vibration amplitude of the depth vibrator while free oscillation or with an excitation frequency towards infinity, M is the mass of the depth vibrator, M is the modal resonating soil mass, m is the imbalance mass, and e the eccentricity of the rotating imbalance m relative to the rotary axis.

13. The method of claim 1, wherein a forward angle () of the imbalance relative to the vibrator motion is set to a value greater than 90 and less than 180.

14. The method of claim 13, wherein the forward angle () of the imbalance relative to the vibrator motion is set to a value greater than 100 and less than 170.

15. The method of claim 1, wherein said step of determining the first soil stiffness value (k1) and second soil stiffness value (k2) comprises a calculation according to the formula: $k=m.Math.e.Math.{}^2.Math.\left(\frac{1}{A_{}}-\frac{1}{A}.Math.\frac{\mathrm{sign}\left(-90\right)}{\sqrt{1+{\tan }^2}}\right)$ where A is the vibration amplitude of the depth vibrator during compaction, A is the vibration amplitude of the depth vibrator while free oscillation or with an excitation frequency towards infinity, m is the imbalance mass, e is the eccentricity of the rotating imbalance m relative to the axis of rotation, is the angular frequency of the rotating imbalance, and is the phase advance of the rotating imbalance m relative to motion of the mass M of the depth vibrator.

16. A method for improving the ground by a depth vibrator, comprising the steps of: establishing a first compaction body by insertion of the depth vibrator into the ground to a desired final depth (Tm); after reaching the final depth (Tm) with the depth vibrator, stepwise vibration and removal of said vibrator out of the ground in vibration intervals of a defined amount (T), thereby compacting the subsoil step by step in depth sections, performing the method of claim 1 during vibration of the vibrator at each compaction depth section.

17. The method of claim 16, wherein a soil stiffness profile derived from transition soil stiffness values (k12) detected over the depth (T) is used for controlling construction of at least one compaction body.

18. The method of claim 17, wherein said soil stiffness profile is used for grid optimization of a grid of a plurality of compaction bodies to be constructed.

19. The method of claim 16, wherein said step of performing the method of claim 1 during vibration of the vibrator at each compaction depth section comprises terminating compaction at said compaction depth section based on a transition soil stiffness value (k12) associated with the respective compaction depth section.

20. A method for controlling compaction of soil with an eccentric-weight depth vibrator comprising the steps of: inserting the depth vibrator into the soil to a desired depth; compacting the soil at said depth by the depth vibrator; during said compaction step, detecting a soil stiffness profile comprising a plurality of soil stiffness values measured over time; determining a soil stiffness value in said profile beyond which further vibration markedly increases subsequent soil stiffness values measured over time, said determined soil stiffness value representing a point at which said soil is as compacted as possible.

21. The method of claim 20, wherein said step of determining a soil stiffness value in said profile beyond further comprises measuring rate of increase of successive soil stiffness values and determining a time at which said rate of increase markedly rises.

Description

DESCRIPTION OF THE DRAWINGS

(1) Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof, in which:

(2) FIG. 1 shows an exemplary depth vibrator suitable for the implementation of the present method for compaction detection and control as well as for the construction of a compaction column;

(3) FIG. 2 shows schematically a method for manufacturing a compaction column with online compaction detection and control according to the invention;

(4) FIG. 3 shows an arrangement with depth vibrators for compacting soil of a field to be compacted with a schematically drawn model for describing the dynamic conditions during vibratory operation; and

(5) FIG. 4 shows a graphical representation of various characteristic values over time during the implementation of the inventive method in a depth stage.

DETAILED DESCRIPTION

(6) The present invention is a method for compaction detection and control when compacting a soil by a depth vibrator, and a method for soil improvement using a depth vibrator that is carried out using data from said method for compaction detection and control.

(7) For purposes of description the following reference numerals are assigned to the following features:

REFERENCES

(8) 2 depth vibrator 3 mass bodies 4 vibrator housing 5 rotary drive unit 6 sensor/angle encoder 7 clutch 8 linkage 9 water flushing 10 water flushing 11 pipe 12 sensor/water pressure sensor, water flow meter 13 sensor/acceleration sensor 14 sensor/acceleration sensor 15 tip 16 vibration displacement 17 ground 18 compaction body 19 force sensor 20 compression grid 21 sensor/acceleration sensor 22 spring-damper system with modally resonating ground mass 23 Sensor/displacement transducer A amplitude A amplitude at infinitely high excitation frequency B axis of rotation C1 compaction, grain rearrangement C2 grain tension c damping coefficient e eccentricity of the imbalance E plane k soil stiffness index k soil stiffness increase rate s1, s2, s3 operating phases t time T depth m Mass of the imbalance M modal vibrator mass M modal resonating soil mass P transition point V amplification factor frequency ratio forward angle exciter frequency

(9) Referring now to FIG. 1, a depth vibrator 2 is used for compacting soil by means of an imbalance. An imbalance is defined as a rotating body 3 which mass is not rotationally symmetrically distributed. For example, the mass inertia axis of the mass body 3 is offset from the rotation axis B, so that the imbalance generates vibrations during rotation, with which the soil and possible feed material are compacted. The pressure vibration compaction process is based on the effect that the friction between the soil grains is temporarily eliminated by the vibration of the depth vibrator 2 and existing pore spaces are closed almost up to the densest packing due to gravity. Depending on the condition of the soil and the amount of compaction required, a reduction in volume may occur.

(10) Accordingly, an exemplary depth vibrator 2 suitable for the implementation of the method for compaction detection and compaction control according to the invention generally comprises the rotationally drivable mass body 3, which is rotationally drivable about the axis of rotation B in a vibrator housing 4. The mass body 3 can be driven by a rotary drive unit 5, for example an electric motor, via a drive shaft (not shown). A position signal representing the position of the imbalance 3 can be detected by a corresponding sensor 6.

(11) The depth vibrator 2 can be suspended from a linkage 8 via a flexible coupling 7. Sinking and/or compaction can optionally be facilitated by one or more water flushes 9, 10 via pipes 11 integrated in the linkage 8. The water flow rate and/or water pressure can be measured using appropriate sensors if necessary and can be controlled accordingly.

(12) First acceleration sensors 13 are provided in a first plane E13 of the depth vibrator 2, particularly above the imbalance 3, and the second acceleration sensors 14 are provided in a second plane E14, particularly below the imbalance 3. The acceleration sensors 13, 14 are used to measure the acceleration of the depth vibrator 2 during the vibration process. Bidirectional acceleration measurements in two planes E13, E14 of vibrator 2 can be used to determine the horizontal motion behaviour at any point of the vibrator, e.g. at the tip 15 or in the position of excitation by imbalance 3. In particular, the vibration displacement 16 at the vibrator tip 15 corresponds to twice the vibration displacement amplitude.

(13) Force sensors 19 may also be provided to detect the suspension force of the vibrator 2 or to determine the tip pressure of the vibrator. In addition, at least one sensor 23 can be provided to measure the penetration depth T of the depth vibrator 2.

(14) In operation, with reference to FIG. 2, the depth vibrator 2 is first sunk into the ground 17 to the intended improvement depth Tm, if necessary with the addition of water. Subsequently, the depth vibrator 2 is pulled step by step to lesser depths (Tm1, Tm2 T2, T1) in order to compact the next soil section above. The depth vibrator 2 can be vibrated into deeper ground areas under tamping motions. In total, a compaction body 18 will be constructed step by step. In particular, for the most accurate compaction detection possible, it is intended that the lifting measure (T) which the depth vibrator 2 is drawn in each case in steps to construct superimposed compaction sections is smaller than or equal to the distance dimension formed between the two planes E13, E14 of the acceleration sensors 13, 14. During step-by-step compaction, a stiffness parameter k representing the soil stiffness is determined over time t for each compaction step. A total stiffness profile k(T) of the soil 17 relative to the depth T can be derived from a large number of individual stiffness characteristic values k determined relative to the depth T of the compaction body 18, in particular from characteristic stiffness characteristic values calculated relative to the depth. The stiffness profile k(T) can, for example, be stored using a suitable data memory and used for the compaction of further columns in a compaction field.

(15) A special feature of the present method for compaction detection or compaction control is that the soil stiffness signals k continuously determined over time can be compared with at least one soil stiffness signal k determined at a previous point in time and from this a soil stiffness increase rate k can be derived. This rate of increase k indicates by how much, if any, the stiffness of the soil has increased compared with a previously determined value. By comparing the currently determined soil stiffness increase rate k with one or more previously determined increase rates k, conclusions can be drawn about the degree of compaction of the soil section to be compacted. In particular, it can be seen when the increase in the stiffness of the soil increases by leaps. In accordance with the invention, it is provided that the vibration method is terminated at a respective compaction depth (Tm, Tm1 . . . T1) if the currently determined soil stiffness increase rate k exceeds at least one soil stiffness increase rate k determined at a previous point in time by a predetermined factor.

(16) The choice of factor can be determined according to the specific technical requirements of the compaction column to be constructed and/or the soil conditions. For example, it may be provided that compaction is interrupted at the respective compaction depth (Tm, Tm1, . . . , T1) if the determined soil stiffness increase rate k is greater than 1.5 times, in particular greater than 2.0 times, optionally greater than 5.0 times, one or more soil stiffness increase rates determined at a previous time. Such a significant increase in soil stiffness of at least 1.5 times suggests that the greatest possible compaction has been achieved.

(17) The soil stiffness parameter k is calculated in particular on the basis of at least the forward angle , by which the imbalance mass 3 advances in relation to the direction of motion of the vibrator housing 4 during the vibratory motion, the modal mass M of the depth vibrator 2, a soil mass parameter M representing the soil mass resonating at the depth vibrator 2 and the vibration amplitude A of the depth vibrator 2. The vibrator mass M can be determined by amplitude measurement on the depth vibrator 2 before penetration into soil 17. The determination of the modal total mass from vibrator mass M and modal resonating soil mass M can be determined in the vibrated-in state during operation, in particular in operating phases with a forward angle of approximately 180, which should cover a range of 18010.

(18) Preferably, the soil stiffness signal k representing the soil stiffness of soil 17 is determined taking into account an amplification factor V or a reference amplitude. As a reference amplitude the amplitude of the vibrator 2 at a certain excitation frequency while free oscillation can be used. In particular, the measured vibration displacement amplitude A is converted to the amplitude at a theoretically infinitely high excitation frequency A to calculate the amplification factor V:

(19) $V=\frac{A}{A_{}}=\frac{A.Math.\left(M+M\right)}{m.Math.e}$

(20) The dynamic soil stiffness k can then be determined from the measured values and the exciter circular frequency in particular with the following formula:

(21) $k=m.Math.e.Math.{}^2.Math.\left(\frac{1}{A_{}}-\frac{1}{A}.Math.\frac{\mathrm{sign}\left(-90\right)}{\sqrt{1+{\tan }^2}}\right)$

(22) With the frequency ratio

(23) $={\left(1-\mathrm{sign}\left(-90\right).Math.\left(\frac{A_{}}{A}\right).Math.{\left(1+{\tan }^2\right)}^{-\frac{1}{2}}\right)}^{-\frac{1}{2}}$

(24) the associated Lehr damping factor D can also be determined:

(25) $D=\tan \left(-\frac{\mathrm{sign}\left(-90\right)+1}{2}.Math.180\right).Math.\frac{1-{}^2}{2.Math.}$

(26) In this way it is possible to continuously determine the condition-dependent soil stiffness and the damping effect of soil 17 during the penetration of the depth vibrator 2 into the soil or during the compaction process.

(27) It goes without saying that the calculation of a characteristic value representing the soil stiffness is not limited to the described possibility, but that in principle other calculation methods can also be used to determine the soil stiffness characteristic value.

(28) The mechanical model of a harmoniously excited spring/damper system 22, as shown in FIG. 3, can also be used as the basis for the determination of the soil stiffness for the vibration method using a depth vibrator 2. M is the modal resonating soil mass, k is the spring stiffness or soil stiffness and c is a damping coefficient. Further, a compaction grid 20 on the soil surface and a sensor 21 for measuring surface waves are visible. Measurements of surface waves on the soil around a compaction point can provide information about the dynamic vibrator-soil interaction system 22.

(29) FIG. 4 shows various parameters of compaction by means of a depth vibrator 2 over time for an exemplary compaction process in a compaction section. The upper graph (A) shows the oscillation amplitude A, the second graph (B) the forward angle , the third graph (C) the soil stiffness k and the fourth graph (D) the derivation k of the soil stiffness, in each case over the time t or in different operating phases s1, s2, s3. Therein, s1 denotes the operating phase of lifting (pulling) the depth vibrator 2 from a deeper depth Tm to the current depth (Tm1). In the following step, which is marked s2, the vibration in takes place into the soil section to be compacted. The soil 17 is compacted by grain rearrangement, whereby a permanent increase in stiffness is achieved by reducing the pore volume in soil 17. It can be seen that the soil stiffness parameter k rises steadily over time t during vibration and compaction in the operating phase s2. This means that the slope of the soil stiffness parameter k over time is almost constant or increases slightly.

(30) With increasing compression the vibration amplitude A of the depth vibrator 2 increases slightly until a maximum amplitude Amax is reached at resonance. From this point, the operating phase s3 post resonance is available. The soil stiffness k is further increased with an essentially constant gradient as long as the soil is further compacted by grain rearrangement. If maximum compaction is reached, the soil stiffness k(t) increases in a kink, which is marked here as point P. The soil stiffness k(t) increases in a kink. The associated soil stiffness transition value k12 is between a first soil stiffness value k1 with a smaller gradient and a larger soil stiffness value k2 with a larger gradient. According to the kink in point P of curve k(t), the derivation or gradient rate k of the soil stiffness rises sharply here. This bend point P marks the transition from compaction C1 by grain rearrangement to grain tension C2, which causes a temporary increase in stiffness due to the currently increased vertical loading of soil 17 by vibrator 2. This temporary increase in the stiffness of soil 17 leads to a significantly higher slope of the calculated soil stiffness parameter k from this transition point P. Subsequently, the soil stiffness parameter k increases further with an almost constant gradient until overpressing is achieved in the operating phase s4. In the subsequent step s1, the vibrator 2 is lifted again so that the stiffness parameter k drops again.

(31) It can also be seen that the forward angle decreases slowly during compaction C. The forward angle decreases slowly during compaction. At the beginning of compaction, the forward angle is just below 180 and drops to about 120 until the transition point P is reached. As soon as the transition soil stiffness value k12 is reached, the forward angle decreases more steeply, except for about 90, which are applied in the overpressed state. When s1 is lifted again, the phase angle returns to the initial value.

(32) According to the present method it is intended that the reaching of the transition soil stiffness value k12 or the abrupt increase of the slope of the soil stiffness value k is used as termination criterion for terminating the vibrating process at the respective compaction depth. This results in a particularly efficient compaction process, since on the one hand too little compaction and on the other hand too long a dwell in a compaction stage without additional compaction success is avoided.

(33) It should now be apparent that the foregoing description provides an improved method for compaction detection and control when compacting a soil using a depth vibrator, as well as an improved method for soil improvement using a depth vibrator and the method for compaction detection and control described herein.

(34) Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept.