HIGH-PERFORMANCE LIQUEFACTION-RESISTANCE TREATMENT METHOD FOR GRAVEL PILE OF EXISTING BUILDING FOUNDATION

Abstract

The disclosure discloses a high-performance liquefaction mitigation method forstone columns for protecting the existing buildings during earthquakes. Specifically, a small equipment is used to dig trenches in the soil around the existing building. Then, a spiral driller is used to drill a series of boreholes in the trenches according to the optimized borehole design. Next, two or three layers of optimized gravel material with high permeability are filled into the boreholes to work as the inverted layer. Finally, geotextile is arranged around the trench and the trench is filled with the optimized gravel. Compared with current liquefaction mitigation methods for existing buildings, the disclosure is suitable for liquefaction mitigation in large cities, and has the advantages of low disturbance to the overlaid building, simple construction process, high construction efficiency, low construction cost, long service life and the construction material could be easily obtained.

Claims

1. A high-performance liquefaction mitigation method for a stone column for an existing building, comprising: Firstly, a trench being arranged around a foundation of an existing building; then, a borehole being arranged in the trench, and a bottom end of the borehole extending below a liquefiable soil; and an optimized gravel filler being filled into the borehole and the trench in accordance with a specified construction method to form a stone columns improved composite foundation with good water dissipate ability, so as to protect the existing building; installation details of the borehole and the trench being as follows: first, a first borehole is formed in the trench by using a driller with a thicker drilling pipe based on a predetermined design diameter and depth; then, a first filler is filled into the first borehole layer by layer, accompanied by compaction layer by layer until the borehole is completely filled with the filler; thereafter, a second borehole is formed in the first filler in the first borehole by using the driller with a thinner drilling pipe; then, a second filler is filled into the second borehole; next, a third borehole is formed in the second filler in the second borehole by using the driller with a drilling pipe that its diameter is smaller than that of the second borehole; then, a third filler is filled into the third borehole; the first filler, the second filler and the third filler are all adopted as gravel, average grain diameters of the first filler, the second filler and the third filler are increased in sequence; a three-layer stone column with internal, intermediate and external layers is formed; and the trench is fully filled with the third filler.

2. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 1, wherein the depth of the trench is deeper than that of the foundation of the existing building.

3. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 1, wherein the trench is an elongated ditch arranged around the foundation of the existing building to be protected, but not arranged around the foundation of other existing buildings adjacent to the existing building to be protected.

4. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 1, wherein the borehole is arranged in the trench, a diameter of the borehole is selectively 50 to 80 cm and a spacing of the borehole is not larger than 4.5 times a diameter of a pile in the borehole; and the borehole passes through the liquefiable foundation soil but its length is not larger than 15 m.

5. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 3, wherein the borehole arrangement is a vertical borehole perpendicular to the ground surface; an inclined borehole that is not perpendicular to the ground surface, and it inclined toward the existing building to be protected along the depth; or a combination of the vertical borehole and the inclined borehole.

6. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 5, wherein an included angle between an axial direction of the inclined borehole and a ground surface is larger than 60 degrees.

7. (canceled)

8. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 1, wherein grain distributions of gravel material of the first filler, the second filler and the third filler are determined based on the following formula: 1) a formula for design of the first filler is as follows: $\hspace{1em}\{\begin{array}{c}{C}_{u1}=\frac{D_{60}}{D_{10}}<1.5\\ k_1=2D_{10}^2{e}^2>100k_0\\ \frac{D_{15}}{d_{85}}\le 4-5\\ \frac{D_{15}}{d_{15}}\ge 5\end{array}$ Where: C.sub.u1 represents the non-uniformity coefficient of the first filler in an external layer; k.sub.0 and k.sub.1 represent the permeability coefficients of the ground soil and the first filler in the external layer; respectively; d.sub.10, d.sub.15, d.sub.60 and d.sub.85 represent the particle diameters of the ground soil accounting for 10%, 15%, 60% and 85% of the total weight of the ground soil respectively; D.sub.10 and D.sub.15 represent the particle diameters of the gravel material of the first filler in the external layer-accounting for 10% and 15% of the total weight of the gravel material of the first filler; 2) a formula for design of the second filler is as follows: $\hspace{1em}\{\begin{array}{c}{C}_{u2}=\frac{Z_{60}}{Z_{10}}<1.5\\ k_2=2Z_{10}^2{e}^2>k_1\\ \frac{Z_{15}}{D_{85}}\le 4-5\\ \frac{Z_{15}}{D_{15}}\ge 5\end{array}$ Where: C.sub.u2 represents the non-uniformity coefficient of the gravel material of the second filler; k.sub.2 represents the permeability coefficient of the gravel material of the second filler; Z.sub.10 and Z.sub.15 represent the particle diameters of the gravel material of the second filler in an intermediate layer accounting for 10% and 15% of the total weight of the gravel material of the second filler; 3) a formula for design of the third filler is as follows: $\hspace{1em}\{\begin{array}{c}{C}_{u3}=\frac{Y_{60}}{Y_{10}}<1.5\\ k_3=2Y_{10}^2{e}^2>k_2\\ \frac{Y_{15}}{Z_{85}}\le 4-5\\ \frac{Y_{15}}{Z_{15}}\ge 5\end{array}$ Where: C.sub.u3 represents the non-uniformity of the gravel material; k.sub.3 represents the permeability coefficient of the third layer of gravel material; Y.sub.10 and Y.sub.15 represent the particle diameters of the gravel material of the third filler in an internal layer accounting for 10% and 15% of the total weight of the gravel material of the third filler.

9. The high-performance liquefaction mitigation method for stone column for the existing building according to claim 1, wherein a geotextile is arranged at a bottom and lateral sides of the trench, then a gravel material of the first, the second and the third fillers is arranged thereon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a sectional view of the combination of vertical borehole and inclined borehole.

[0036] FIG. 2 is a cross-sectional view of a typical vertical borehole.

[0037] FIG. 3 is a top view of the design.

[0038] FIG. 4 is a schematic view of arrangement of trenches for different types of construction sites: (a) surrounding trench; (b) trilateral trench; (c) bilateral trench; (d) unilateral trench.

[0039] FIG. 5 is a process flow of a construction method with three times of drilling and filler.

[0040] FIG. 6 is a top view showing drilling in the construction method with three times of drilling and filler.

[0041] In the figures: 1. existing building to be protected; 2. foundation of existing building; 3. trench; 4. inclined borehole; 5. vertical borehole; 6. liquefiable foundation soil; 7. first filler; 8. second filler; 9. third filler.

DESCRIPTION OF THE EMBODIMENTS

[0042] In the following, further description will be made in conjunction with the drawings and embodiments. The following examples are only used to illustrate the disclosure and not to limit the scope of the disclosure. In addition, it should be understood that, after reading the contents of the disclosure, those skilled in the art can make various changes or modifications to the disclosure, and these equivalents also fall within the scope defined by the appended claims of the present application.

[0043] In the specific implementation, as shown in FIG. 1 and FIG. 2, according to the survey data of the existing buildings, firstly the trench 3 is arranged around the foundation 2 of the existing building, and the trench 3 closely surrounds the foundation 2, as shown in FIG. 3. Then, a borehole 4/5 is arranged in the trench 3, and the bottom end of the borehole 4/5 extends below the liquefiable soil 6, and the optimized gravel material is filled into the boreholes 4/5 and the trench 3 following the designed construction process to form the composite foundation with good hydraulic permeability, so as to realize the liquefaction mitigation for the existing building 1.

[0044] The design parameters of width of the trench 3 need to be compatible with the in-situ construction space and construction equipment. Due to the small construction space around the existing building, only small construction machinery and equipment such as small excavators can be selected. Preferably, the trench width is designed to be 50 to 100 cm. In the specific implementation, the depth of the trench 3 exceeds the depth of the foundation 2 of the existing building 1, and preferably, the exceeding depth is 30 to 50 cm.

[0045] The trench 3 is an elongated ditch, and the trench 3 is arranged around the foundation 2 of the existing building 1 to be protected, but not arranged around the foundation of other existing building adjacent to the existing building 1.

[0046] As shown in FIG. 4, the trenches 3 are arranged around the existing building depending on actual circumstances of surrounding buildings. The main principle is to accelerate the dissipation of excess pore water pressure during an earthquake. The trench 3 can be designed into four types, including surrounding trench, trilateral trench, bilateral trench or unilateral trench as shown in FIG. 4.

[0047] The borehole 4/5 is arranged in the trench 3. The diameter of borehole 4/5 is generally 30 to 80 cm according to the stone column construction regulations. In consideration of the special filling method in the disclosure, the diameter of borehole 4/5 is 50 to 80 cm. The spacing between the boreholes 4/5 is calculated based on the water discharge of the foundation 2, and is not larger than 4.5 times the diameter of the borehole. The borehole 4/5 passes through the liquefiable soil 6, but the depth of the borehole 4/5 is not larger than 15 m.

[0048] As shown in FIG. 1 and FIG. 2, the borehole 4/5 is a vertical borehole 5 with its axial direction being perpendicular to the ground surface, or an inclined borehole 4 that its axial direction is not perpendicular to the ground surface, and it inclines toward the existing building 1 to be protected along the depth, or a combination of the vertical borehole 5 and the inclined borehole 4.

[0049] The included angle between the axial direction of the inclined borehole 4 and the level ground is larger than 60 degrees, and preferably 75 degrees.

[0050] As shown in FIG. 5, the construction and filling of borehole 4/5 and the trench 3 are carried out with multiple times of drilling and filling. The three times of drilling and filling are specifically as described below.

[0051] 1) First, the first borehole is formed in the trench 3 by using a driller with a thicker drilling pipe based on the predetermined design diameter and depth. Then the first filler with optimized designed grain distribution is filled into the first borehole layer by layer, accompanied by compaction layer by layer until the borehole is completely filled with the filler.

[0052] 2) Thereafter, a second borehole is formed in the first filler in the first borehole by using the driller with a thinner drilling pipe, and then the second filler with optimized grain distribution is filled into the second borehole.

[0053] 3) Thereafter, a third borehole is formed in the second filler in the second borehole by using the driller with a drilling pipe that its diameter is smaller than that of the second borehole, and then the third filler with optimized grain distribution is filled into the third borehole.

[0054] In the above process, the first filler, the second filler and the third filler are all adopted as gravel. As shown in FIG. 6, the average grain diameters of the first filler, the second filler and the third filler are increased in sequence, such that the particle diameters of the gravel from internal layer to the external layer are decreased in sequence, and finally a three-layer stone column with internal, intermediate and external layers is formed. The formed stone column is in a shape of concentric cylinder and circular column shape. Moreover, the trench 3 is fully filled with the third filler.

[0055] The first to the third layers are filled layer by layer. Specifically, the fillers adopt graded gravel material, which is optimized designed and not only has high permeability but also acts as an inverted layer to prevent the soil from entering the gravel body along with the excess pore water.

[0056] Furthermore, the grain distribution of the gravel of the first filler, the second filler and the third filler are determined based on the following formula.

[0057] 1) The formula for design of the first filler is as follows.

[00004]$\hspace{1em}\{\begin{array}{c}{C}_{u1}=\frac{D_{60}}{D_{10}}<1.5\\ k_1=2D_{10}^2{e}^2>100k_0\\ \frac{D_{15}}{d_{85}}\le 4-5\\ \frac{D_{15}}{d_{15}}\ge 5\end{array}$

[0058] Where: C.sub.u1 represents the non-uniformity coefficient of the first filler in the external layer; k.sub.0 and k.sub.1 represent the permeability coefficients of the ground soil and the first filler; respectively; d.sub.10, d.sub.15, d.sub.60 and d.sub.85 represent the particle diameters of the ground soil accounting for 10%, 15%, 60% and 85% of the total weight of the ground soil respectively; D.sub.10 and D.sub.15 represent the particle diameters of the gravel material of the first filler accounting for 10% and 15% of the total weight of the gravel material of the first filler.

[0059] 2) The formula for design of the second filler is as follows.

[00005]$\hspace{1em}\{\begin{array}{c}{C}_{u2}=\frac{Z_{60}}{Z_{10}}<1.5\\ k_1=2Z_{10}^2{e}^2>k_1\\ \frac{Z_{15}}{D_{85}}\le 4-5\\ \frac{Z_{15}}{D_{15}}\ge 5\end{array}$

[0060] Where: C.sub.u2 represents the non-uniformity coefficient of the gravel material of the second filler; k.sub.2 represents the permeability coefficient of the gravel material of the second filler; Z.sub.10 and Z.sub.15 represent the particle diameters of the gravel material of the second filler in the intermediate layer accounting for 10% and 15% of the total weight of the gravel material of the second filler.

[0061] 3) The formula for design of the third filler is as follows.

[00006]$\hspace{1em}\{\begin{array}{c}{C}_{u3}=\frac{Y_{60}}{Y_{10}}<1.5\\ k_3=2Y_{10}^2{e}^2>k_2\\ \frac{Y_{15}}{Z_{85}}\le 4-5\\ \frac{Y_{15}}{Z_{15}}\ge 5\end{array}$

[0062] Where: C.sub.u3 represents the non-uniformity of the gravel material; k.sub.3 represents the permeability coefficient of the third layer of gravel material; Y.sub.10 and Y.sub.15 represent the particle diameters of the gravel material of the [first]third filler in the internal layer accounting for 10% and 15% of the total weight of the gravel material of the third filler.

[0063] In this way, the third filler with large particle diameter and the first filler with small particle diameter are formed inside the borehole, which could act as an inverted layer that blocks the external soil particle going into the borehole but only the excess pore water.

[0064] Geotextiles are arranged at the bottom and lateral sides of the trench 3, and then the gravel material of the third filler is put in the trenches. The geotextiles prevent the ground soil particles from blocking the drainage channel of the gravel material in the trenches.

[0065] Further, the graded gravel is filled in the trench in a manner of layer by layer. Preferably, the thickness of each layer is limited within 20 cm, and each layer should be hard-pressed after filling until the filler reaches the same level as the foundation of the protected building.

[0066] Moreover, in specific implementation, the stone column and water drainage calculations are according to the following steps.

[0067] 1) Determine the maximum residual volume strain (ε.sub.vr).sub.max according to the standard penetration base N and the level of seismic shear stress that a site may be subjected to, and use the following formula to multiply the maximum residual body strain by the vertical settlement correction coefficient C.sub.s to obtain the residual settlement ε.sub.vr:

ε.sub.vr=C.sub.s×(ε.sub.vr).sub.max

[0068] In the specific implementation, the value of vertical settlement correction coefficient C.sub.s is 0.84.

[0069] 2) Use the following formula to obtain the volume change V1 of the liquefiable soil 6 (liquefiable layer) under the seismic loading:

V1=L1×L2×T×ε.sub.vr

[0070] Specifically, L1 and L2 represent the length and width of the existing building to be protected, and T represents the thickness of the liquefiable soil 6 right under the existing building to be protected.

[0071] 3) Use the following formula to obtain the water discharge q2 of the stone column per unit time:

V2=n1×V1

q2=V2/t

[0072] Specifically, t represents the time required for dissipating the excess pore pressure generated by the earthquake; n1 represents the parameter determined according to the layout of the trench and ranges from 4 to 9 in specific implementation; V2 represents the total water discharge of the stone columns.

[0073] The parameter n1 is determined according to the arrangement of the trench around the existing building. The trench can be classified into four types, namely, surrounding trench, trilateral trench, bilateral trench and unilateral trench, wherein the total water discharge V2 through the stone columns is 9 times, 6 times, 6 times and 4 times V1 for the four types respectively, and thus the corresponding n1 for the four types of trenches is 9, 6, 6 and 4 respectively.

[0074] 4) It is assumed that the liquefiable sand layer liquefies during earthquake. The vertical hydraulic gradient i of the gravel pile is calculated using the following formula:

[00007]$i=\frac{\gamma }{{\gamma }_w}-1$

[0075] Specifically, H represents the buried depth of the liquefiable soil 6, γ represents the average effective gravity of the overlaid soil layer, and γ.sub.w represents the unit weight of water, which generally equals to 10 kN/m.sup.3.

[0076] 5) All the excess pore water generated during earthquake is discharged from the interface between the stone column and the liquefiable soil 6. The interface area S is the side area of the cylinder. The permeability coefficient k of the gravel pile is calculated according to the following formula:

S=2πrT

k>=q2/S/n2/i

[0077] Specifically, r is the radius of the stone column, n2 is the number of the stone columns, and S is the interface area between the stone column and the liquefiable soil 6.

[0078] 6) The diameter of the borehole is set to 50 to 80 cm. According to the above formula, the borehole diameter parameter is taken into the formula and the maximum integer is taken to obtain the number of boreholes. For example, if the calculation result is 14.2, the number of boreholes should be 15.

[0079] In this manner, the permeability coefficient of the gravel material, the diameter of the borehole and the number of boreholes could be determined for later construction.

[0080] During the design process, the most important parameters are the diameter, spacing and depth of the borehole. Currently, the in-situ diameter of stone column is generally ranging from 30 to 80 cm. Considering that the construction method in the disclosure, which requires the multiple drilling and filling, the currently adopted diameter of stone column is increased by 20 cm, preferably 50 to 80 cm. The borehole spacing is calculated based on the subsequently obtained water drainage amount of the liquefiable soil, and is not greater than 4.5 times of the pile diameter. The bottom of the borehole should be deeper than the depth of the liquefiable layer, so that the excess pore pressure accumulated in the liquefiable layer under earthquake can be quickly dissipated through the stone columns, and the depth for stone column is not greater than 15 m.

[0081] The specific embodiment and implementation process of the disclosure are as follows.

[0082] Assuming that the seismic fortification level where the existing building locates is 0.25 g, the standard penetration blow counts of the liquefiable soil layer under the existing building is N=10, and it can be obtained from FIG. 1 that the maximum residual volume strain (ε.sub.vr)=4%.

[0083] In this manner, the residual settlement ε.sub.vr can be obtained, and the value of C.sub.s is 0.84.

ε.sub.vr=C.sub.s×(ε.sub.vr).sub.max=0.84*4%=0.336%

[0084] Assuming that the thickness of the liquefiable foundation is 1 m, and the length and width of the existing building are shown in FIG. 3 and both are set to be 4 m, then it can be obtained that the volume change V1 of the liquefiable layer under seismic loading is:

V1=L1×L2×T×ε.sub.vr=4×4×1×0.336%=0.05376 m.sup.3

[0085] Assuming that the layout of the trench around the existing building is surrounding trench type, and n1=9, then the total water discharge V2 through the stone column can be determined. Assuming that the excess pore pressure generated by the earthquake needs to be dissipated within 30 minutes, and the water discharge through the stone column in unit time is q2, then it can be calculated and obtained that:

V2=n1×V1=0.48384 m.sup.3

q2=V2/t=0.48384/18=0.02688 m.sup.3/s

[0086] Assuming that the liquefiable sand layer liquefies during earthquake, the average effective unit weight of the overlying soil layer is γ=20 kN/m.sup.3, and the unit weight of water is 10 kN/m.sup.3, then according to Darcy's law, the vertical hydraulic gradient i of the gravel pile can be obtained as:

[00008]$i=\frac{\gamma }{{\gamma }_w}-1=\frac{20}{10}-1=1$

[0087] All the excess pore water generated during the earthquake is discharged from the interface between the stone column and the liquefiable sand layer. The interface is the side area of the cylinder, assuming that the radius of the gravel pile is r, then the area S is calculated as follows:

S=2πrT=2*3.14*r*1=6.28r

[0088] Assuming that there are n2 stone columns in total, the permeability coefficient k of the stone column is calculated as follows:

k*r*n2>=4.28e−3

[0089] Generally speaking, the permeability coefficient of the liquefiable sand layer ranges from 1e-5 m/s to 1e-6 m/s, the permeability coefficient herein is set to be 5e-6 m/s, then the permeability coefficient of the gravel material is assumed to be 200 times the ground soil, then k=0.001 m/s, so it can be obtained that:

r*n2>=4.28

[0090] In the specific implementation, the radius of the gravel pile is set to be 0.6 m, then it can be obtained that:

n2>=4.28/0.6=7.13

[0091] 8 stone columns are taken herein as the calculation result.

[0092] That is to say, for the site conditions described in this embodiment, the surrounding trench is arranged, two stone columns are set in the trench on each side, and the spacing is 2 m.

[0093] For gravel materials, the porosity ratio e is generally between 0.4 to 0.6, where 0.5 is taken, the calculation is performed according to the following formula:

k=2D.sub.10.sup.2e.sup.2

[0094] In the calculation result, it is obtained that D10=0.447 for the first filler, and other parameters such as D.sub.60 and C.sub.u for the first filler are calculated and obtained according to the above corresponding formula. And the parameters for the second and third filler could also be determined based on the above formula.