Method for Improving an Inwards Stability of a Levee

Abstract

A method for improving an inwards stability of an existing levee including a landslide slope (128) and at least one of a landside berm (132), a landside heel, and a landside trench. The method comprises placing columns (160) through the landside berm into one or more soft soil layers (40) with corresponding soil volumetric weights (s), wherein the columns have a column volumetric weight (c) that is at least 10% larger than the soil volumetric weights, and wherein the columns comprise a mineral aggregate and an impermeable filler.

Claims

1. A method for improving an inwards stability of an existing levee including a landside slope and at least one of a landside berm, a landside heel, and a landside trench, wherein the method comprises: placing columns through at least one of the landside berm, the landside heel, and the landside trench into one or more soft soil layers with corresponding soil volumetric weights, wherein the columns have a column volumetric weight that is larger than the soil volumetric weights, and wherein the columns comprise a mineral aggregate and an impermeable filler.

2. The method according to claim 1, wherein the column volumetric weight of the columns is at least 10% larger than the soil volumetric weights of the one or more soft soil layers.

3. The method according to claim 1, wherein the impermeable filler comprises a hydraulic binding agent.

4. The method according to claim 1, comprising: determining a slip surface of most probable sliding fault for the landside slope and the landside berm; partitioning the slip surface into a torque inducing portion related to the crown of the levee and a torque resisting portion related to the landside berm; wherein placing the columns involves arranging the columns in the one or more soft soil layers at the torque resisting portion.

5. The method according to claim 1, comprising: forming the columns with a first column portion having a first column diameter in an upper soil layer, and with a second column portion having a second column diameter in the one or more underlying soil layers, wherein the second column diameter is larger than the first column diameter.

6. The method according to claim 5, wherein the second column diameter is at least 30% larger than the first column diameter.

7. The method according to claim 1, wherein the columns are placed through at least one of the landside berm, the landside heel, and the landside trench, and into the one or more soft soil layers by means of a depth vibrator.

8. The method according to claim 7, comprising: drilling a first void portion and removing soil from the upper soil layer, prior to placing the columns through at least one of the landside berm, the landside heel, and the landside trench into the one or more soft soil layers with the depth vibrator.

9. The method according to claim 1, wherein placing the columns comprises arranging at least two columns at transversally consecutive positions across the landside berm.

10. The method according to claim 1, comprising: placing the columns with a predominantly vertical orientation in the one or more soft soil layers.

11. The method according to claim 1, comprising: placing the columns in a spatially separated distribution in the one or more soft soil layers.

12. The method according to claim 11, comprising: placing the columns with mutual spacing with a value in a range of 0.5 meter to 2.5 meter, preferably with mutual spacing in a range of 1.0 meter to 2.0 meter, and more preferably of about 1.5 meter.

13. The method according to claim 1, wherein the columns comprise further fillers for adapting at least one of the rheological properties, the strength properties, or the hardening properties of the columns.

14. The method according to claim 1, wherein a supporting sand layer is situated underneath the soft soil layers, wherein the columns are arranged so as to extend through the one or more soft soil layers into the supporting sand layer, and wherein the columns comprise reinforcing material for increasing a strength and/or stability of the columns.

15. The method according to claim 14, wherein the reinforcing material comprises fiber materials.

16. The method according to claim 14, wherein the reinforcing material comprises a geo textile for enveloping the columns, and wherein the method comprises: placing the geo textile prior to placing the columns.

17. The method according to claim 14, wherein the reinforcing material is formed into steel rods, and wherein the method comprises: inserting the steel rods into the columns after placing the columns.

18. A method for improving an inwards stability of an existing levee including a landside slope and at least one selected from a group consisting of a landside berm, a landside heel, and a landside trench, wherein the method comprises: placing columns through at least one selected from the group consisting of the landside berm, the landside heel, and the landside trench, into one or more underlying soft soil layers with corresponding soil volumetric weights, wherein the columns have a column volumetric weight that is at least 10% larger than the soil volumetric weights, and wherein the columns comprise a mineral aggregate and an impermeable filler. forming the columns with a first column portion having a first column diameter in an upper soil layer, and with a second column portion having a second column diameter in the one or more underlying soil layers, wherein the second column diameter is at least 30% larger than the first column diameter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0060] FIG. 1 schematically shows a cross-sectional view with typical elements of a levee according to the prior art;

[0061] FIG. 2 presents a cross-sectional view of a portion of a levee and a slip surface according to an embodiment of the invention;

[0062] FIG. 3 shows a column distribution in a landside berm according to an embodiment;

[0063] FIG. 4 shows a column distribution in a landside berm, a landside heel, and a landside trench according to another embodiment, and

[0064] FIG. 5 illustrates a method for applying stabilization columns according to a method embodiment.

[0065] The figures are meant for illustrative purposes only, and do not serve as restriction of the scope or the protection as laid down by the claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0066] The following is a description of certain embodiments of the invention, given by way of example only and with reference to FIGS. 2-4.

[0067] Embodiments according to the invention generally relate to methods for improving a stability of an existing levee 10. The examples presented below will therefore be discussed with reference to the general terminology and reference numbers discussed herein above with reference to FIG. 1.

[0068] FIG. 2 illustrates that the levee 10 and the subsoil elements 16-20 may typically be formed by soil layers 38-46 of different constitution and typical mass densities. An first (uppermost) soil layer 38 may consist essentially of clay, whereas lower subsequent soil layers may essentially consist of peat 40, clay 42, sand clay 44, and a supporting sand layer 46 respectively. For levees in general, the hydrological properties and the response of such soil layers 38-46 to changing water conditions influence the levee's stability. In particular, the peat layer 40 has a high fluid absorbing capacity and a low (unsaturated) mass density compared to the other soil layers. Changing water conditions (e.g. the height of the water body 35 on the water side 12) affect the mechanical properties and fluid dynamics in the soil layers 38-46, and the peat layer 40 in particular. This is highly riskful for foundations in these areas.

[0069] The proposed method comprises applying columns 60 through the existing landside berm 32 and/or heel 33 and/or trench 36 into one or more soft soil layers 40 (and possibly 38 and 42-46, depending on local soil conditions), which are situated directly below the levee 10, and which have corresponding volumetric weight s. The columns 60 have a column volumetric weight c that is significantly larger than the volumetric weights s of the soft soil layer 40 (and possibly 38, 42-46). The columns 60 serve as mass loading structures, intended to increase a total mass of a torque resisting portion 54 of the land side 14 of the levee 10, which corresponds with the landside berm 32. The application of such columns 60 may also reduce the average leakage length for seepage underneath the levee 10, and hence may lower the risk for piping effects.

[0070] In this exemplary embodiment, the soft soil layer(s) is formed by a peat layer 40 with a volumetric weight p having a typical value in a range of 9 kN/m.sup.3 to 14 kN/m.sup.3. In contrast, the columns 60 have a volumetric weight c of at least 18 kN/m.sup.3. The columns 60 are positioned extending at least down to the soft soil layer(s) (e.g. peat layer) 40, to increase the average volumetric weight in this layer. Note that in alternative embodiments, any number of soft soil layers may be present in the levee and any ordering of the layers may be possible. For example, a levee may be formed on a soil layer configuration with multiple soft peat layers and intermediate denser soil layers.

[0071] In this example, the method comprises determining a slip surface 50 of most probable sliding fault for the landside slope 28 and the landside berm 32 of the levee 10.

[0072] This slip surface 50 is subsequently partitioned into a torque inducing portion 52 situated near the crown 24 of the levee 10 on the one hand, and a torque resisting portion 54 near the land side berm 32. These surface portions 52, 54 are separated by a boundary line 56.The columns 60 are then arranged in the soft soil layer(s) 40 (38, 42) at the torque resisting portion 54 of the slip surface 50.

[0073] This slip surface 50 of most probable sliding fault may for example be found by applying Bishop's method on a set of predetermined probable slip circles corresponding with a distribution of adjacent circle centers 58. Sliding fault assessment may also be possible with alternative slip surface calculation methods or finite element methods. In an embodiment wherein Bishop's method is used, the slip surface 50 of most probable sliding fault is mapped to a slip circle 50 with a circle center 58a. The torque resisting portion 54 corresponds in this case with the passive zone of the slip circle 50 of most probable sliding fault. The soft soil layer(s) 40 (38, 42-46) will thus be locally weighted within the torque resisting portion 54 of the slip surface 50, so that the torque component with respect to the circle center 58a of the slip surface 50which counteracts the torque from the soil in the torque inducing portion 52is significantly increased. This torque compensation yields an increased stability for the levee 10.

[0074] Due to the presence of the columns 60 in the torque resisting portion 54 of the slip surface 50, the average shear strength of the soft soil layer(s) 40 (38, 42) may also be locally increased in this region, yielding a further improvement of the stability factor.

[0075] FIG. 3 depicts a cross-section of a column distribution in the landside berm 32, according to an embodiment. The columns 60 have been arranged near the landside heel 33, in such a manner that the local increase of the (average) volumetric weights of the clay layer 38 and the peat layer 40 extends inwards towards the land side 14. As a result, the counter torque generated by the torque resisting portion 54 is significantly increased. Because natural material is not removed or remolded but pushed together, the natural structure of peat is preserved, prohibiting loss of shear strength.

[0076] In this example, the columns 60 consist essentially of a granular mixture of gravel and cement-bentonite, with further fillers for adjusting the rheological properties of the cement-bentonite suspension, and the strength and hardening properties of the resulting granular mixture. The following global component ratio may be adhered to: about 80 weight % gravel and about 20 weight % fillers. Such granular mixture may have a typical volumetric weight with a value of about 20 kN/m.sup.3. The columns 60 are vertically impermeable for reasons of water safety. The phrase vertically impermeable implies in this context that the water/fluid permeability (hydraulic conductivity) of the columns is similar to or even less than a permeability (hydraulic conductivity) of the surrounding upper soil layer 38. As a result, the columns 60 are not formed to act as vertical fluid drainage conduits.

[0077] As is shown in FIG. 3, at least a portion of the columns 60 may extend through the soft soil layers 40, and down to the underlying soil layers 42, 44 as well as the supporting sand layer 46. This column portion 60 provides a dowel effect, that allows transferring of sliding loads occurring along sliding surfaces (e.g. on the slip surface 50) via this column portion 60 to the supporting sand layer 46. Through addition of fibrous material to the granular mixture, the shear strength of the resulting column portion 60 will be increased, which may improve the local (average) shear resistance of the soft soil layer(s) 40 (38, 42-46) in the torque resisting portion 54.

[0078] According to FIG. 3, the columns 60, 60 in this example are formed with column sections 62-66 that have different characteristic cross-sectional dimensions 1-3. Each of the columns 60, 60 is accommodated in a column void (not indicated) that extend through at least some of the soil layers 38-46, and which includes at least and upper void section 72. Such columns 60, 60 may be formed via a method described further below. In this example, the column sections 62-66 have (macroscopically) cylindrical shapes. Here, the diameters 2 of the second column sections 64 located in the peat layer 40 have the largest value, to achieve a significant increase in the average volumetric weight of the (relatively light) peat layer 40. In this example, the first column sections 62 in the upper clay layer 38 have upper diameters 1 of about 0.5 meter, and the second column sections 64 in the peat layer 40 have diameters 2 of about 0.8 meter. A lowest diameter 3 of the third column sections 66 (which belong to the columns 60 that extend down to the supporting sand layer 46) is comparable to the upper diameters 1 i.e. about 0.5 meter.

[0079] In this example, the columns 60, 60 have been positioned in a mutually separated arrangement (viewed along the longitudinal and transversal directions X,Y), such that the columns 60, 60 do not mutually touch or overlap. The column distribution in this example may be characterized by a transversal nearest neighbor distance Y of about 1.0 meter to 1.5 meter, and a longitudinal nearest neighbor distance of about 1.5 meter.

[0080] FIG. 4 shows a cross-sectional view of levee with a distribution of columns in a landside berm 132, a landside heel 133, as well as a landside trench 136. Features in the levee embodiment described above with reference to FIG. 3 are also be present in the levee embodiment shown in FIG. 4, and will not all be discussed here again. For the discussion with reference to FIG. 4, like features are designated with similar reference numerals preceded by 100, to distinguish the embodiments.

[0081] In this embodiment, additional columns 160, 160 with associated wider second column sections 164, 164 have been placed directly through the landside heel 133 and through the bottom of the trench 136. The local (average) volumetric weights of the clay layer 138 and the peat layer 140 is therefore increased in these regions as well, yielding a further increase of the counter torque generated by the torque resisting portion.

[0082] FIG. 5 illustrates a method for forming and placing the columns 60, 60 in the soft soil layer(s) 40 (38, 42-46) according to an embodiment. In this example, a depth vibrator 100 is employed for in-situ construction of the columns 60, 60. An exemplary depth vibrator is described in patent document EP1234916. In the example of FIG. 5, the depth vibrator 100 is attached to a rig 106, and comprises a supply tube 102 with a discharge opening 103 at a lower end thereof, a suspension conduit 104 for supplying a filler suspension to the supply tube 102, and a trough 105 for locally supplying gravel. The supply tube 102 is rotatable with respect to an axis along the height direction Z. The supply tube 102 is rotatable in an eccentric manner about this axis (i.e. the symmetry axis and rotation axis of the supply tube 102 are parallel but laterally shifted), to enable the depth vibrator 100 to force soil components laterally away while the supply tube 102 is lowered into the various soil layers 38-46.

[0083] The depth vibrator 100 and rig 106 require only a single deployment, after which the supply tube 102 may be lowered into the soil layers 38-46 of the existing landside berm 32. The eccentric rotation of the supply tube 102 about its axis of rotation causes local lateral displacement of the soil and allows penetration of the supply tube 102 with its discharge opening at least down to the peat layer 40. To construct the extended columns 60, the depth vibrator needs to be further lowered with its discharge opening 103 down into the further layers 42-46, and down to the supporting sand layer 46 in particular.

[0084] Subsequently, gravel is provided into the trough 105 (e.g. via a power shovel). The gravel and the filler suspension are allowed to mix inside the supply tube 102. Once the depth vibrator 100 has reached its intended depth, the supply tube 102 is slightly lifted, which allows the granular mixture to emanate from the discharge opening 103 into the locally created column void. The depth vibrator 100 is intermittently lowered in a vibrating manner, to locally condense the granular mixture and laterally force the mixture into the respective soil layer 38-46. Via intermittent raising and lowering of the depth vibrator 100, a continuous column 60, 60 is formed that extends through the soil layers 38-46 up to a desired height.

[0085] In one embodiment, the depth vibrator 100 has a maximum tube diameter t of 0.5 meter. When such a depth vibrator 100 is urged though the supporting sand layer 46, the first local column diameter 1 will have a similar value of about 0.5 meters. Due to the weak soil cohesion of the peat layer 40, the second local column diameter 2 in this peat layer 40 will be considerably larger e.g. about 0.8 meter or larger.

[0086] In order to reduce the probability of negative soil displacement effects in the upper clay layer 38, first void sections 72 may initially be drilled and the corresponding clay removed, before the columns 60, 60 are formed in the soil layers 38-46 by means of the depth vibrator 100. Alternative measures may be variations of patterns and diameter around critical objects.

[0087] The descriptions above are intended to be illustrative, not limiting. It will be apparent to the person skilled in the art that alternative and equivalent embodiments of the invention can be conceived and reduced to practice, without departing from the scope of the claims set out below.

LIST OF REFERENCE SYMBOLS

[0088] 10 levee

[0089] 12 water side

[0090] 14 land side

[0091] 16 waterside land

[0092] 18 lowland

[0093] 20 levee base

[0094] 22 levee core

[0095] 24 crown

[0096] 26 waterside slope

[0097] 28 landside slope

[0098] 30 waterside berm

[0099] 31 waterside toe

[0100] 32 landside berm

[0101] 33 landside heel

[0102] 35 water body

[0103] 36 drainage trench

[0104] 38 first soil layer e.g. clay

[0105] 40 second soil layer e.g. peat

[0106] 42 third soil layer e.g. clay

[0107] 44 fourth soil layer

[0108] 46 fifth soil layer e.g. Pleistocene or Holocene sand

[0109] 48 berm sand

[0110] 50 slip surface (e.g. circle)

[0111] 52 torque inducing portion

[0112] 54 torque resistive portion

[0113] 56 boundary line

[0114] 58 set of circle centers

[0115] 60 column (with mineral aggregate and impermeable filler)

[0116] 62 first column section

[0117] 64 second column section

[0118] 66 third column section

[0119] 68 fiber material

[0120] 70 column void

[0121] 72 first void section

[0122] 100 depth vibrator

[0123] 102 supply tube

[0124] 103 discharge opening

[0125] 104 suspension supply conduit

[0126] 105 trough

[0127] 106 rig

[0128] 1 first column diameter

[0129] 2 second column diameter

[0130] 3 second column diameter

[0131] t vibrator diameter

[0132] X longitudinal direction

[0133] Y transversal direction

[0134] Z height direction