CN110344387B - Effective reinforcement depth design method for reinforcing saturated sandy soil foundation by dynamic compaction method - Google Patents

Effective reinforcement depth design method for reinforcing saturated sandy soil foundation by dynamic compaction method Download PDF

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CN110344387B
CN110344387B CN201910644013.5A CN201910644013A CN110344387B CN 110344387 B CN110344387 B CN 110344387B CN 201910644013 A CN201910644013 A CN 201910644013A CN 110344387 B CN110344387 B CN 110344387B
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董天文
刘伟
董鑫
赵显涛
谢鹏
白天辉
杨俊岭
崔晓亮
岳玉梅
敖然
赵朕
魏可峰
宋雪松
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Shen Kan Engineering and Technology Corp of MCC
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Abstract

The invention provides an effective reinforcement depth design method for reinforcing a saturated sandy soil foundation by a dynamic compaction method, and relates to the technical field of soft soil foundation reinforcement. The method calculates the maximum depth z of the compaction shell generated by dynamic compaction vibration1Deep influence of the pressed compact shellDegree hyFinally, calculating to obtain the effective consolidation depth Z of the dynamic compaction consolidated saturated sandy soil foundation, which is the maximum depth Z of the compaction shell generated by dynamic compaction vibration1The depth h is influenced by the pressing of the sealing shellyAnd a ramming pit depth h which produces the maximum depth of the compacting shellhAnd (4) summing. The design method for the effective reinforcement depth of the dynamic consolidation saturated sand soil foundation can objectively carry out the engineering design of the dynamic consolidation saturated sand soil foundation, pertinently carry out the trial compaction and detection work of the dynamic consolidation engineering, improve the engineering construction efficiency and reduce the engineering cost.

Description

Effective reinforcement depth design method for reinforcing saturated sandy soil foundation by dynamic compaction method
Technical Field
The invention relates to the technical field of soft soil foundation reinforcement, in particular to an effective reinforcement depth design method for reinforcing a saturated sandy soil foundation by a dynamic compaction method.
Background
At present, researches on tests, theories and the like for reinforcing the soft soil foundation by the dynamic compaction method are carried out at home and abroad, and the dynamic compaction method mainly relates to the dynamic compaction reinforcing work of the soft soil foundation of saturated cohesive soil, unsaturated cohesive soil and unsaturated sandy soil, but still is in the statistical stage of empirical data. In mandatory regulations "building foundation treatment technical specification (JGJ 79-2012)", the effective reinforcement depth "table 6.3.3-1 effective reinforcement depth of dynamic compaction" is summarized according to the field application condition, and other documents do not disclose an effective reinforcement depth design method for reinforcing soft soil foundation by dynamic compaction. With the recent large application of sea sand hydraulic reclamation in island reef and offshore beach construction and the characteristics of short construction time and remarkable economic benefit of the dynamic compaction method, the dynamic compaction method is widely applied to strengthening the saturated sand soil foundation, but a scientific and reliable effective strengthening depth design method is not established, and design parameters are determined only by a trial compaction test, so that the blindness is high, the cost is high, and therefore an effective strengthening depth design method for strengthening the saturated sand soil foundation by the dynamic compaction method is urgently needed to be provided.
Disclosure of Invention
The invention aims to solve the technical problem of providing an effective reinforcement depth design method for reinforcing a saturated sandy soil foundation by a dynamic compaction method, aiming at the defects of the prior art, and meeting the requirement of designing and providing the effective reinforcement depth of the saturated sandy soil foundation.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
a design method for effective reinforcement depth of a forced ramming method for reinforcing a saturated sandy soil foundation comprises the steps of calculating the maximum depth of a compaction shell generated by forced ramming vibration, calculating the influence depth of the compaction shell and calculating the effective reinforcement depth of the forced ramming reinforced saturated sandy soil foundation;
the method for calculating the maximum depth of the compaction shell generated by dynamic compaction vibration specifically comprises the following steps:
assuming that the saturated sandy soil is an isotropic elastic material, the flow of water conforms to Darcy's law, and the compression coefficient is constant; according to the critical pore pressure condition of saturated sandy soil caused by dynamic compaction, considering the dynamic compaction as an axisymmetric problem, taking a section of an axis, researching according to a plane problem, and establishing a theoretical model of the dynamic compaction;
the compression of the deformation of the soil framework caused by the dynamic compaction vibration action has the following relationship,
Figure GDA0002739551500000011
in the formula, the strain of a soil framework is shown, and alpha is the compression coefficient of a saturated sandy soil foundation; e is the void ratio of the sand foundation soil skeleton, sigma'iThe micro-element effective stress of saturated sand soil; dz is the unit length of the infinitesimal body in the z direction; t is the acting time of dynamic compaction in the foundation;
according to Darcy's law, the seepage rate of water passing through the porous medium per unit time is proportional to the cross-sectional area of water and the total head loss, and inversely proportional to the seepage path length, as shown in the following formula,
Q＝kAh1/L＝kAi (2)
in the formula, Q is the flow per unit time of the water passing section; k is the seepage coefficient; a is the area of the water passing section; h is1Total head loss; l is the length of the porous medium seepage path; i is hydraulic gradient;
applying the formula (2) to the flow rate change of the infinitesimal body to obtain a formula (3) which represents the inflow and outflow flow rate change of the upper and lower surfaces of the infinitesimal body in unit area of a certain depth and unit length dz of unit time in the saturated sandy soil foundation,
Figure GDA0002739551500000021
if the compressive deformation of the infinitesimal body soil skeleton of the saturated sandy soil foundation is assumed to be the same as the discharge amount of the fluid, namely:
Figure GDA0002739551500000022
then the process of the first step is carried out,
Figure GDA0002739551500000023
micro element body effective stress sigma 'of saturated sand soil'iThe micro-element effective stress of the saturated sand is obtained according to the concept of effective stress related to the acceleration of the saturated sand soil foundation surface and the pore water pressure, namely
Figure GDA0002739551500000024
Wherein gamma is the volume weight of saturated sandy soil; z is the compaction shell depth of the saturated sandy soil; b is the acceleration amplitude of the dynamic consolidation saturated sand soil foundation ground; omega is the circular frequency of the saturated sand foundation under the condition of dynamic compaction, omega is 2 pi/T, and T is a period; p is a radical offPore water pressure;
substituting the formula (6) into the formula (5) to obtain
Figure GDA0002739551500000025
For a micro-element at a depth of z, the hydraulic gradient of its upper and lower surfaces is expressed as
Figure GDA0002739551500000026
In the formula, gammawIs the volume weight of underground water;
substituting equation (8) into equation (7) and letting
Figure GDA0002739551500000027
CvFor consolidation coefficient, simplifying to obtain a second-order inhomogeneous partial differential equation of the excess pore water pressure, as shown in formula (9),
Figure GDA0002739551500000028
solving the formula (9) by adopting a separation variable method, taking a pore water pressure function of the saturated sandy soil foundation under the dynamic compaction action as a formula (18),
Figure GDA0002739551500000031
in the formula, c1、c2、c3Beta is a coefficient to be solved of a partial differential equation;
if the dynamic compaction action time t in the foundation is t ═ t1When the liquefaction phenomenon of the z depth of the saturated sandy soil foundation can occur, the pore water pressure at the z depth is balanced with the total stress of the upper foundation, namely
Figure GDA0002739551500000032
Wherein g is the acceleration of gravity;
when t is1→∞，
c3＝γzg (20)
When the rammer is just contacted with the saturated sand, namely t is 0, the pore water pressure of the saturated sand is the original value of the pore water pressure of the site, and the function is
pf＝c1 cosβz+c2 sinβz+γzg (21)
At this time, at z ═ 0, the pore water pressure is zero, i.e.
c1＝-c3＝-γzg (22)
At z-h, the dynamic compaction P wave forms a compaction shell, the pore water pressure at the position is balanced with the hydrostatic pressure of the saturated sand at the lower part, namely the hydrodynamic pressure is zero, and then c is2The solution of (a) is:
Figure GDA0002739551500000033
substituting the formulas (20), (22) and (24) into the formula (18) to obtain
Figure GDA0002739551500000034
For the upper surface of the compressed shell of saturated sand, z ═ h, and the pore water pressure is pfγ hg, then
Figure GDA0002739551500000035
Order to
Figure GDA0002739551500000036
And sin ω t is 1, the formula (26) is substituted, and the pressure packing shell depth z is obtained by solving,
Figure GDA0002739551500000037
in the formula, J is a dynamic response characteristic parameter of a saturated sandy soil foundation; cvThe consolidation coefficient of the saturated sand foundation; t is the acting time of dynamic compaction in the foundation; b is the acceleration amplitude of the dynamic consolidation saturated sand soil foundation ground; g is the acceleration of gravity; z is the pressing shell depth generated by dynamic compaction vibration;
the dynamic response characteristic parameter J and the consolidation coefficient C of the saturated sand foundation are measuredvG, gravity acceleration, t, dynamic compaction action time and peak value B of ground acceleration amplitude of dynamic compaction saturated sandy soil foundation1Substituting max (B) into the formula (27), and calculating the maximum depth z of the compaction shell generated by dynamic compaction vibration1；
The method for calculating the influence depth of the compact shell comprises the following specific steps:
applying the impact load applied by the rammer to the surface of the rammer pit through the pseudo-static load, as shown in a formula (28), so as to obtain the pseudo-static unit load of the impact load acting on the rammer pit;
Figure GDA0002739551500000041
in the formula, p0Pseudo-static unit load, G, acting on the ramming pit for impact loadsHammerFor the weight of the ram, AArea of hammer baseIs the projected area of the ram;
according to the load characteristic that the plane projection of the rammer is circular, the dynamic load at the maximum depth position of the compacting shell is calculated to form dynamic additional stress through the stress solution of the circular load applied to the semi-space elastomer, as shown in a formula (29),
Figure GDA0002739551500000042
in the formula, σz1Is p0Additional stress at the maximum depth of the compact shell is applied; z is a radical of1The maximum depth of the compact shell; r is0Is the projected diameter of the bottom surface of the rammer;
with calculated additional stress sigmaz1The method comprises the steps of taking a standard stress value and zero depth as an initial value, solving the additional stress sigma of the foundation under the action of the compact shell under the stress by adopting the stress solution of the semi-space elastomeryAs shown in the formula (30),
Figure GDA0002739551500000043
in the formula, σyIs σz1Additional stress of the foundation at the depth h below the compaction shell is acted; h is a certain depth of the lower part of the compacting shell; r is0Is the projected diameter of the bottom surface of the rammer;
calculating the dead weight stress at a certain depth with the depth of the pressure seal shell as a starting point, as shown in formula (31),
σh＝γyhy (31)
in the formula, σhThe dead weight stress is gamma of the position where the depth of the compact shell is zero and the depth is hyIs hyWeight of depth, hyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced;
and determining the influence depth of the compact shell by applying a stress ratio method, as shown in a formula (32),
0.1σy＝σh (32) in the formula, σyIs σz1Additional stress, sigma, of the foundation below the compacting shell under the action ofhThe dead weight stress at the depth of h with the depth of the compact shell as the initial value of zero is set;
the effective reinforcement depth of the dynamic consolidation saturated sandy soil foundation is calculated by adopting the following formula:
Z＝z1+hh+hy (33)
wherein Z is effective consolidation depth of dynamic consolidation saturated sand foundation, and Z is1The maximum depth h of a compaction shell of a saturated sandy soil foundation generated by dynamic compaction vibrationhDepth of ramming pit h for producing maximum depth of compacting shellyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: the design method for the effective reinforcement depth of the dynamic consolidation saturated sand soil foundation can objectively carry out the engineering design of the dynamic consolidation saturated sand soil foundation, pertinently carry out the trial compaction and detection work of the dynamic consolidation engineering, improve the engineering construction efficiency and reduce the engineering cost.
Drawings
Fig. 1 is a schematic plan view of monitoring points of a 3000kN · m dynamic compaction test area provided in an embodiment of the present invention;
fig. 2 is a 3000kN · m pore water pressure monitoring time course curve provided by the embodiment of the invention.
Detailed Description
The following examples are given to further illustrate the embodiments of the present invention. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The effective reinforcement depth design method for reinforcing the saturated sandy soil foundation by the dynamic compaction method comprises the steps of calculating the maximum depth of a compaction shell generated by dynamic compaction vibration, calculating the influence depth of the compaction shell and calculating the effective reinforcement depth of the saturated sandy soil foundation reinforced by the dynamic compaction.
The method for calculating the maximum depth of the compaction shell generated by dynamic compaction vibration specifically comprises the following steps:
assuming that saturated sand is an isotropic elastic material, the flow of water conforms to Darcy's law, and the compressibility is constant. Starting from a critical pore pressure angle of saturated sandy soil caused by dynamic compaction, considering the dynamic compaction as an axisymmetric problem, taking a section passing through an axis, researching according to a plane problem, and establishing a theoretical model of the section;
let us say that the microelement effective stress of saturated sand is sigma'iThe following relationship exists for the compression of the deformation of the soil framework caused by the dynamic compaction action, which is the strain of the soil framework:
Figure GDA0002739551500000051
in the formula, alpha is the compression coefficient of a saturated sandy soil foundation; e is the pore ratio of the sand foundation soil framework; dz is the unit length of the infinitesimal body in the z direction; t is the acting time of dynamic compaction in the foundation;
darcy's law states that the seepage rate of water passing through a porous medium in unit time is in direct proportion to the cross-sectional area of water passing and the total head loss and in inverse proportion to the length of a seepage path;
Q＝kAh1/L＝kAi (2)
in the formula, Q is the flow per unit time of the water passing section; k is the seepage coefficient; a is the area of the water passing section; h is1Total head loss; l is the length of the porous medium seepage path; i is hydraulic gradient;
applying the formula (2) to the flow change of the infinitesimal body to obtain a formula (3) which represents the inflow and outflow flow change of the upper and lower surfaces of the infinitesimal body on a unit area of a certain depth in the saturated sandy soil foundation and in unit length dz of unit time;
Figure GDA0002739551500000061
if the compressive deformation of the infinitesimal soil skeleton of the saturated sandy soil foundation is assumed to be the same as the discharge amount of the fluid, namely
Figure GDA0002739551500000062
Then the process of the first step is carried out,
Figure GDA0002739551500000063
the micro element body effective stress of the saturated sand soil is sigma'iThe micro-element effective stress of the saturated sand can be obtained according to the concept of effective stress related to the acceleration of the saturated sand soil foundation surface and the pore water pressure, namely
Figure GDA0002739551500000064
Wherein gamma is the volume weight of saturated sandy soil; z is the compaction shell depth of the saturated sandy soil; b is the dynamic compaction acceleration amplitude value of the saturated sandy soil foundation ground; omega is the circular frequency of the saturated sand foundation under the condition of dynamic compaction, omega is 2 pi/T, and T is a period; p is a radical offPore water pressure;
substituting the formula (6) into the formula (5) to obtain
Figure GDA0002739551500000065
For a micro-element at a depth z position, the hydraulic gradient of its upper and lower surfaces can be expressed as:
Figure GDA0002739551500000066
in the formula, gammawIs the volume weight of underground water.
Substituting equation (8) into equation (7) and letting
Figure GDA0002739551500000067
CvFor consolidation coefficient, simplified to
Figure GDA0002739551500000068
The formula (9) is a second-order heterogeneous partial differential equation of the excess pore water pressure, and a separation variable method is adopted for solving; let the pore water pressure function be expressed as
pf＝Z(z)T(t)+γzBsinωt+c (10)
Wherein Z (z), T (t) and c are a dynamic compaction influence depth function, a dynamic compaction action time function and a constant term which influence the pore water pressure respectively.
Thereby can push out
Figure GDA0002739551500000071
Figure GDA0002739551500000072
Substituting the formulas (11) and (12) into the formula (9),
Figure GDA0002739551500000073
by analyzing the function of equation (13), the equation exists as the essential condition
Figure GDA0002739551500000074
And
Figure GDA0002739551500000075
is a real constant;
Figure GDA0002739551500000076
Figure GDA0002739551500000077
equations (14) and (15) are ordinary differential equations, and the general solutions of z (z), t (t) are:
Z(z)＝c'1cosβz+c'2sinβz (16)
Figure GDA0002739551500000078
in formula (II), c'1、c′2、c′3Is the undetermined coefficient of the functions Z (z), T (t).
Substituting the formula (16) and the formula (17) into the formula (10), wherein the pore water pressure function of the saturated sandy soil foundation under the dynamic compaction action is the formula (18);
Figure GDA0002739551500000079
in the formula, c1、c2、c3Beta is a undetermined constant.
If the dynamic compaction action time t in the foundation is t ═ t1When the liquefaction phenomenon of the z depth of the saturated sandy soil foundation can occur, the pore water pressure at the z depth is balanced with the total stress of the upper foundation, namely
Figure GDA0002739551500000081
Wherein g is the acceleration of gravity.
When t is1→∞，
c3＝γzg (20)
When the rammer is just contacted with the saturated sand, namely t is 0, the pore water pressure of the saturated sand is the original value of the pore water pressure of the site, and the function is
pf＝c1 cosβz+c2 sinβz+γzg (21)
At this time, at z ═ 0, the pore water pressure is zero, i.e.
c1＝-c3＝-γzg (22)
At the position where z is equal to h, the wave form of the dynamic compaction P forms a compaction shell, the pore water pressure at the position is balanced with the hydrostatic pressure of the saturated sand at the lower part of the position, namely the hydrodynamic pressure is zero;
0＝-γzgcosβh+c2sinβh+γzg (23)
then, c2Is composed of
Figure GDA0002739551500000082
Substituting the formulas (20), (22) and (24) into the formula (18),
Figure GDA0002739551500000083
for the upper surface of the compressed shell of saturated sand, z ═ h, and the pore water pressure is pfγ hg, then
Figure GDA0002739551500000084
Order to
Figure GDA0002739551500000085
And sin ω t is 1, and the formula (26) is substituted to obtain
Figure GDA0002739551500000086
In the formula, J is a dynamic response characteristic parameter of a saturated sandy soil foundation; cvThe consolidation coefficient of the saturated sand foundation; t is the acting time of dynamic compaction in the foundation; b is the dynamic compaction acceleration amplitude value of the saturated sandy soil foundation ground; g is the acceleration of gravity; z is the compaction shell depth generated by ramming vibration;
the dynamic response characteristic parameter J and the consolidation coefficient C of the saturated sand foundation are measuredvG, gravity acceleration, t, dynamic compaction action time and peak value B of ground acceleration amplitude of dynamic compaction saturated sandy soil foundation1Max (b), substituting into equation (27),calculating the maximum depth z of the compaction shell generated by dynamic compaction vibration1；
The method for calculating the influence depth of the pressure sealing shell is specifically as follows.
Calculating the dynamic load at the maximum depth position of the compacting shell to form dynamic additional stress according to the load characteristic that the plane projection of the rammer is circular;
(1) the impact load applied by the ram can be applied to the surface of the rammer pit by a pseudo-static load, as shown in equation (28);
Figure GDA0002739551500000091
in the formula, p0Pseudo-static unit load, G, acting on the ramming pit for impact loadsHammerFor the weight of the ram, AArea of hammer baseIs the projected area of the ram;
(2) solving the stress solution applied to the semi-space elastomer through the circular load, such as an equation (29), and solving the additional stress at the position of the compact shell;
Figure GDA0002739551500000092
in the formula, σzIs p0Additional stress at the position of the compact shell is acted; z is a radical of1The maximum depth of the compact shell; r is0Is the projected diameter of the bottom surface of the rammer;
(3) by calculated sigmaz1The additional stress is a reference stress value, the depth is zero, a stress solution of a half-space elastomer is adopted, such as a formula (30), and the additional stress sigma of the foundation under the action of the compact shell under the stress is solvedy；
Figure GDA0002739551500000093
In the formula, σyIs σz1Additional stress of the foundation at the depth h below the compaction shell is acted; h is a certain depth of the lower part of the compacting shell; r is0Is the projected diameter of the bottom surface of the rammer;
(4) calculating the dead weight stress of a certain depth with the depth of the compact shell as a starting point, as shown in a formula (31);
σh＝γyhy (31)
in the formula, σhThe dead weight stress at the depth of h with the depth of the compact shell as the initial value of zero is set; gamma rayyIs hyA depth weight; h isyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced;
(5) determining the influence depth of the compact shell by applying a stress ratio method, such as a formula (32);
0.1σy＝σh(32) in the formula, σyIs σz1Additional stress, sigma, of the foundation at depth h under the compacting shellhThe dead weight stress is a certain depth with the compression shell depth being zero.
The effective reinforcement depth of the dynamic consolidation saturated sandy soil foundation is calculated by adopting the following formula:
Z＝z1+hh+hy (33)
wherein Z is effective consolidation depth of dynamic consolidation saturated sand foundation, and Z is1The maximum depth h of a compaction shell of a saturated sandy soil foundation generated by dynamic compaction vibrationhDepth of ramming pit h for producing maximum depth of compacting shellyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced.
In the embodiment, the method is adopted to carry out effective reinforcement depth design on the saturated sand land in a certain coastal region, and the geological conditions and the construction parameters of the dynamic compaction site are as follows:
(1) dynamic compaction site geological conditions
The saturated sandy soil stratum in a certain coastal region respectively comprises the following components from top to bottom:
filling soil: loose, light grey, mainly composed of powder and fine sand, containing shell fragments, wet-very wet and saturated under water.
Filling soil with impurities: loose, mainly comprising slag, local fly ash, broken stone blocks and broken brick blocks, containing shell fragments and local concrete blocks.
The fine sand is ash-grey black, loose-dense, mainly fine sand, and is formed by the mutual layer of fine sand and silt, feldspar-quartz, and has a sub-circular particle structure, contains a large amount of mica and shells, has a cohesive soil interlayer, and is layered and saturated.
Fine sand: ash-grey black, medium dense-dense, mainly fine sand, part of fine sand and silts interbedded, feldspar-quartz, and granules in a sub-circular and uniform particle structure, contain a large amount of mica and shells, have clayey soil interlayers, and have bedding and saturation.
Silt: gray, slightly dense to medium dense, containing organic matter, shell fragments, fishy smell, mulling, powder clay and powder sand thin layers, which are layered and saturated.
Fine sand: ash-grey black, slightly dense-medium dense, mainly fine sand, part of fine sand and silt interbedded, feldspar-quartz, and granules in a sub-circular and uniform particle structure, contain about 25% of cohesive soil, contain a large amount of mica and shells, have cohesive soil interlayers, and have the advantages of layering and saturation.
Powdery clay: grey black, plasticity, containing shell fragments and organic matters, having fishy smell, and local containing thin layers of silt and silt, which are in a mutual layer shape.
Fine sand: ash-grey black, medium dense-dense, mainly fine sand, part of fine sand and silts interbedded, feldspar-quartz, and granules in a sub-circular and uniform particle structure, contain a large amount of mica and shells, have clayey soil interlayers, and have bedding and saturation.
Powdery clay: brown to yellow brown, plastic to hard plastic, iron oxide, mica, iron manganese tuberculosis and gray stripe.
Fine sand: gray yellow-gray brown, dense, feldspar-quartzity, uniform grain structure, shell fragment containing, clay mixed, bedding and saturation.
The underground water burial depth is 1.5-2.0 m during exploration.
According to the geotechnical test, the foundation consolidation coefficient C of the sandy soil layer in the coastal regionvIs 0.00324 to 0.00383m2/s。
(2) Dynamic compaction construction parameters
The deep-processing point ramming hammer has the hammer weight of 200kN, the drop distance of 15m, the bottom area diameter of 2.4m and the hammer bottom projection area of 4.522m2Bottom pressure 44.23kN/m2And the tamping energy is 3000 kN.m.
Aiming at the saturated sandy soil, the effective consolidation depth design method for consolidating the saturated sandy soil foundation by dynamic compaction provided by the embodiment is adopted to calculate the effective consolidation depth of dynamic compaction of the saturated sandy soil foundation by dynamic compaction shown in table 1.
Table 1 preliminary design of dynamic consolidation depth of saturated sandy soil foundation
Figure GDA0002739551500000111
In Table 1, B1A/g, which is the ratio of the surface acceleration a of the foundation to the acceleration g of gravity; z is the effective reinforcement depth of the dynamic consolidation saturated sandy soil foundation;
according to the consolidation coefficient C of the foundationv＝0.00324～0.00383m2The relation between the acceleration/s and the site acceleration is 0.15-0.26 g, the dynamic compaction action time is 0.7-1.0 s, the depth of a tamping pit with the maximum depth of a compaction shell generated in the dynamic compaction construction of similar areas is referred to 0.614m, and the effective reinforcing depth of the dynamic compaction is preliminarily designed to be 6.486-7.761 m.
And performing test tamping test and analysis on the dynamic-tamping saturated sandy soil foundation.
The dynamic compaction construction parameters are as follows: 2 crawler cranes, 1 total station and 1 leveling instrument are adopted as construction equipment; the point ramming of deep foundation consolidation in the trial ramming area adopts ramming energy of 3000 kN.m, the full ramming energy of shallow foundation consolidation is 1500kN.m, and the area of the trial ramming area is 35 multiplied by 28 which is 980m2. The construction parameters are shown in Table 2.
TABLE 2 test ramming construction parameter table
Figure GDA0002739551500000121
The dynamic compaction construction process and the technical requirements are as follows:
(1) the construction process comprises the steps of performing point ramming once and full ramming twice, performing full ramming construction on a leveling field after point ramming, and performing rolling construction on the leveling field after full ramming;
(2) the tamping energy of the building site is 3000 kN.m, and the tamping number is 9-10 strokes;
(3) tamping for two times, wherein the tamping energy of the building site is 1500kN.m, and the hammer mark is overlapped with the hammer diameter of 1/4;
(4) because the field soil is soft, the rammer with the diameter of 2.4m and the weight of 150-;
(5) the dynamic compaction adopts a filling construction process, and the filling adopts sand in a yard;
(6) continuously reducing water before dynamic compaction construction, and reducing the water level to below 2.0m below the ground surface;
(7) the two-time ramming intermittent period is determined according to the pore water pressure dissipation condition, and the two-time construction interval time is 3 days;
(8) during dynamic compaction construction, if water gushes in the compaction pit, water is discharged into the drainage ditch by a water pump.
Because the soil of the field is soft and has high water content, the rammer is difficult to lift due to the fact that the depth of a ramming pit is 1.6-1.9m in general when the rammer is hit by 4-5 m. In order to ensure the smooth construction, the point ramming construction is carried out by adopting a method of filling into a ramming pit while constructing, and the average filling amount of each ramming point is 6-8m3Individual rammed points are backfilled by 9-11m3. When point tamping is carried out, the tamping number is 9-10 strokes, and the average tamping settlement of the last two strokes is 10-15 cm; taking 4 trial tamping points, and when the tamping number is 14-15 strokes, the average tamping amount of the last two strokes is also within the range of 10-15 cm.
And (4) counting all point tamping settlement, the average value of the last two-stroke tamping settlement, the backfill sand and soil amount and the tamping number according to the construction record, wherein the result is shown in a table 3.
Statistics of table 33000 kN.m point ramming construction records
Figure GDA0002739551500000122
Figure GDA0002739551500000131
Figure GDA0002739551500000141
From the construction record analysis, the average ramming settlement of the last two times of ramming when the ramming number is 9-10 times and 14-15 times is within the range of 10-15cm, which shows that the 9-10 times of ramming reinforced foundation achieves the effects of eliminating liquefaction of the foundation in the compact shell and having good integrity. From the analysis of the depth of the rammed pit, the average value of the ramming number is 160.979cm from the average value of the depth of the rammed pit of 9.958 strokes, the lower limit value of the depth of the rammed pit is close to 1.6m from 4 to 5 strokes, and the fact that after the compaction shell is basically formed by 4 to 5 strokes indicates that lateral extrusion is generated due to more dynamic compaction because the buried depth of a dense fine sand layer in the lower part of the compaction shell is shallow. Thus, it can be concluded that the depth of the rammed pit for which the maximum compact shell is formed is about 0.621 m. This value is close to the ramming pit depth of 0.614m of the maximum depth of the compaction shell generated by dynamic ramming in the similar area to be referred to.
FIG. 1 shows monitoring points of a dynamic compaction test area with 3000kN m, B4, B5 and B6 are standard penetration test points after compaction and are arranged in the centers of four compaction points with equal intervals; JC4, JC5 and JC6 are the test point positions of pore water pressure, and the burial depths of a test point JC4, a test point JC5 and a test point JC6 are respectively 4.0m, 5.0m and 6.0 m; DQ is N before test ramming area ramming63.5The heavy sounding points are arranged in the centers of the four ramming points at equal intervals; d is N after test ramming area ramming63.5The heavy sounding points are arranged in the centers of the four ramming points at equal intervals; JZ is a shallow flat plate load test point position of a trial tamping area and is arranged in the centers of four tamping points at equal intervals; y is a ring cutter method compactness detection point position of the trial compaction area, and the distance from the plane load test point position is 0.5 m.
Shallow plate load test and analysis are carried out. A counter-force device of a ballast platform is selected for test loading counter-force, and the weight of the ballast platform is 1.25 times of the maximum loading value. And controlling the final loading value to reach 2.0 times of the characteristic value of the designed bearing capacity. The test adopts a slow load maintaining method, namely, the load is added step by step, and the next stage of load is added after the load of each stage is relatively stable. 2m is adopted according to the maximum diameter and the specification requirement of the filler2Square rigid bearing plate. During the test, a 1000kN jack is used for applying a vertical load, an oil pressure meter arranged on the jack is used for measuring the value of the applied vertical load, and 4 dial indicators with 50mm measuring ranges are used for observing the vertical displacement of the bearing plate.
Loading and grading: according to design requirements, each stage of loading is 1/5 of the characteristic value of the designed foundation soil bearing capacity, namely 24 kPa.
And (3) settlement observation: and measuring and reading the settlement amount of the pressure bearing plate once in 10, 20, 30, 45 and 60min after each stage of loading, and measuring and reading once every 30min later.
Relative stability criteria: sedimentation did not exceed 0.1mm per hour and occurred 2 consecutive times, considered to have reached relative stability.
Terminating the loading condition: the loading may be terminated when one of the following conditions occurs:
sedimentation is increased rapidly, and soil is extruded out or obvious bulges appear around the pressure bearing plate;
secondly, under the action of certain load, the sedimentation rate cannot reach a relatively stable standard within 24 hours;
the accumulated settling volume of the pressure bearing plate is larger than 6% of the width or the diameter of the pressure bearing plate;
and fourthly, when the limit load requirement cannot be met, the maximum loading pressure is 2 times greater than the pressure value required by the design.
And drawing a load test P-S curve of each test point according to the field test record. And analyzing the test curve of each test point to obtain the characteristic value of the bearing force after the field dynamic compaction treatment. The test results of the foundation soil shallow slab load after dynamic compaction and reinforcement treatment are shown in table 4. The data obtained by the test can meet the requirements of bearing capacity and deformation modulus provided by the design.
Table 4 table of shallow flat plate load test results
Figure GDA0002739551500000151
And carrying out heavy dynamic penetration test and analysis. During the test, a 63.5kg drop hammer is adopted to freely drop at a drop distance of 76cm so as to enable the dynamic probe to penetrate into the soil, the hammering number of each penetration of 10cm is recorded, the test can be stopped when N63.5 is more than 50 for three times continuously, the heavy dynamic penetration test is carried out after 0.5m is drilled, and the final hole depth is 7.50 m. And the N63.5 heavy dynamic penetration test is carried out between main tamping points, and the uniformity, compactness, strength, deformation parameters and foundation bearing capacity of the foundation soil after dynamic compaction are evaluated according to test indexes and regional experience.
According to the results in tables 5 and 6, the test results of the heavy dynamic sounding detection points of the dynamic compaction treatment site are comprehensively analyzed, and the characteristic value f of the bearing capacity of the foundation soil after compaction is carried outakNot less than 120 kPa; the effective reinforcement depth is 6.50m, and the deformation modulus of a soil layer in the reinforcement depth range is more than 7.5 MPa; from the aspect of reinforcing effect, N with the depth range of 0.0-2.4 m and the ramming energy of 3000 kN.m63.5The number of heavy dynamic penetration is increased from 4.9 strokes to 12.9 strokes, which is increased by 163%, and the number of heavy dynamic penetration is increased from 3.3 strokes to 14.5 strokes, which is increased by 339% in 2.4-6.5 m.
TABLE 5 heavy dynamic penetration test result table for N63.5 before tamping
Figure GDA0002739551500000161
TABLE 6 post-ramming N63.5Heavy dynamic penetration test result table
Figure GDA0002739551500000162
Figure GDA0002739551500000171
Standard penetration test and analysis are carried out. The standard penetration test is carried out in dredged and backfilled soil layers. And when the drill hole is drilled to a position 15cm above the test elevation, the test is carried out after residual soil at the bottom of the hole is removed, and sand gushing or hole collapse is prevented. Adopts a free drop hammer method of automatic unhooking and reduces the frictional resistance between the guide rod and the hammer. Eccentricity and lateral shaking should be avoided during hammering, and the hammering rate should be less than 30 beats/minute. And after the injector is driven into the soil for 15cm, recording the hammering number of each driven 10cm, wherein the accumulated hammering number of the driven 30cm is the standard penetration number N. When the number of hammering had reached 50 strokes and the penetration depth had fallen short of 30cm, the actual penetration depth was recorded and the test was terminated.
Tables 7 and 8 are a standard penetration test liquefaction judgment result table and a standard penetration test result statistical table, respectively. According to the specification, a standard penetration test is carried out on the compacted saturated sandy soil foundation, and the standard penetration value and the site standard penetration statistic value of B4, B5 and B6 drill holes are all larger than the liquefaction critical value of the saturated sandy soil foundation, so that the foundation subjected to dynamic compaction meets the design requirement of eliminating liquefaction.
TABLE 7 liquefaction judgment result table of Standard penetration test
Figure GDA0002739551500000172
Figure GDA0002739551500000181
TABLE 8 statistics of SPT test results
Figure GDA0002739551500000182
And standard penetration test shows that the ramming area is reinforced within the depth range of 6.5m and is not liquefied.
And (5) monitoring and analyzing the dynamic compaction pore water pressure. 3 vibrating wire osmometers are embedded in the dredger fill and backfill soil layer (4.0-6.0 m below the ground elevation +0.5 m) of the tamping test area, and the change condition of the pore water pressure before and after the dynamic compaction construction is monitored.
(1) Single point trial tamping pore pressure monitoring
The single-point trial tamping can be carried out by adopting 3000 kN.m tamping energy. And (3) testing the stable initial reading for 2 times before tamping, testing 1 time after each tamping of the dynamic compaction, observing every two hours after the single-point tamping is finished, and then observing for 1 time every day until the pore pressure is dissipated by 80 percent. And recording the data of each observation in an observation table, and checking, sorting and calculating at any time.
(2) Full ramming pore pressure monitoring
And full tamping is carried out after the pressure of the single-point trial tamping hole is dissipated. And (3) testing the initial reading of the pore pressure meter before tamping, and measuring the pore water pressure value twice a day after tamping until the pore pressure is dissipated by 80 percent, and then stopping observation.
From the analysis of the pore water pressure monitoring time course curve shown in fig. 2, the change trend of the pore water pressure of the JC5 measuring point is generally consistent with that of the JC6 measuring point, the tamping can cause the short-term increase of the pore water pressure, then the pressure is quickly attenuated and approaches to a stable level, the increase and the slowing of the pore water pressure are realized along with the increase of the tamping times, the extreme value of the pore water pressure appears about 5 days after the tamping is finished in the tamping field, and the pore water pressure tends to a stable value about 1 day later; the JC4 measuring point with the burial depth of 4.0m gradually reduces the peak value of the pore water pressure from the 1 st impact, shows an extreme value around the 6 th impact and tends to be stable after 3 days. The test shows that: the pore water pressure change of a deep JC5 measuring point and a JC6 measuring point is more consistent with the tamping compaction process, and the dissipation is faster than the dissipation of the pore pressure of a shallow measuring point; when a shallow layer JC4 measuring point is in the tamping process of a tamping test area, the pore water pressure is in a reduction section, the wave generated by dynamic tamping promotes the flow of the pore water of a saturated sandy soil foundation, but after sandy soil in a compaction shell is compacted, the pressure of pores is slowly dissipated. The pressure difference between the inside and the outside of the pressure shell is quickly balanced by judging that the pressure shell is positioned in the range of the pressure shell around 5.0-6.0 m, and the pore pressure at the position is quickly dissipated because the pressure shell is damaged by shear waves; the 4.0m position is located within the range of the compaction shell, and the pore water pressure balancing process of the saturated sandy soil foundation requires more time.
And finally, carrying out trial compaction test and design verification analysis.
(1) Maximum depth of compaction shell of saturated sandy soil foundation
As can be seen from the monitoring and analysis of the pore water pressure, 4.0m is positioned in the compaction shell, and the pore water pressure balancing process of the saturated sandy soil foundation needs a longer time; the pressure of the holes is rapidly balanced at the position of the pressure sealing shell near 5.0-6.0 m due to the action of shear waves. After ramming N63.5In table 6 of the heavy dynamic penetration test results and table 7 of the standard penetration test liquefaction determination results, the penetration number appeared to decrease after increasing. The number of hammering is judged from liquefaction in Table 7, and the foundation is marked at the depth of 5.1-5.5 m after dynamic compactionThe number of quasi-penetration hits reaches an extreme value, which should be within the maximum depth range of the compact shell. Compared with the initial design value of the reinforcing depth of the dynamic compaction saturated sand soil foundation in the table 1, the design value range is 4.706 m-6.442 m, and the maximum depth of the compaction shell obtained by a dynamic compaction test is within the design value range and meets the design requirement.
(2) Depth of rammed pit producing maximum depth of compacted shell
Analysis from the dynamic compaction record (table 3) predicts that the depth of the rammed pit formed by the maximum compaction shell is about 0.621 m. This value is close to the ramming pit depth of 0.614m of the maximum depth of the compaction shell generated by dynamic ramming in the similar area to be referred to.
(3) Depth of impact of pressing shell
From the analysis of the construction records (table 3), the average ramming weight of the last two times of the 9-10 times of ramming and the 14-15 times of ramming is within the range of 10-15cm, which shows that the 9-10 times of ramming of the reinforced foundation achieves the effects of eliminating liquefaction of the foundation in the compacted shell and good integrity. From the analysis of the depth of the rammed pit, the average value of the ramming number is 160.979cm from the average value of the depth of the rammed pit of 9.958 strokes, the lower limit value of the depth of the rammed pit is close to 1.6m from 4 to 5 strokes, which shows that after the compaction shell is basically formed by 4 to 5 strokes, the compaction shell acts on the lower foundation under the action of the dynamic load of the strokes, the whole foundation in the range of the compaction shell moves downwards, and thus the situation that the ramming settlement is close to the ground occurs.
(4) Meet the design requirement of dynamic consolidation on the effective consolidation depth of the saturated sandy soil foundation
When the area is 35 multiplied by 28 to 980m2The tamping test area selects tamping parameters of 2.4m bottom area diameter, 200kN rammer weight, 15m drop distance and 3000 kN.m tamping energy; after ramming, the steel plate passes a flat plate load test, a standard penetration test and N63.5And carrying out relevant test work on the foundation in the forced ramming saturated sandy soil foundation ramming area by using test means such as dynamic penetration test, cutting ring density test, pore water pressure monitoring and the like. The test result shows that: the design of the dynamic compaction effective reinforcement depth by applying the design method of the invention meets the design requirement of the effective reinforcement depth of the saturated sandy soil foundation of 6.5 m.
Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; such modifications and substitutions do not depart from the spirit of the corresponding technical solutions and scope of the present invention as defined in the appended claims.

Claims (2)

1. A design method for effectively reinforcing depth of a saturated sand foundation by a dynamic compaction method is characterized by comprising the following steps: the method comprises the steps of calculating the maximum depth of a compaction shell generated by dynamic compaction vibration, calculating the influence depth of the compaction shell and calculating the effective reinforcement depth of a dynamic compaction reinforced saturated sandy soil foundation;
the method for calculating the maximum depth of the compaction shell generated by dynamic compaction vibration specifically comprises the following steps:
the depth of a compaction shell generated by dynamic compaction vibration is calculated according to the following formula,
Figure FDA0002739551490000013
in the formula, J is a dynamic response characteristic parameter of a saturated sandy soil foundation; cvThe consolidation coefficient of the saturated sand foundation; t is the acting time of dynamic compaction in the foundation; b is the acceleration amplitude of the dynamic consolidation saturated sand soil foundation ground; g is the acceleration of gravity; z is the pressing shell depth generated by dynamic compaction vibration;
the dynamic response characteristic parameter J and the consolidation coefficient C of the saturated sand foundation are measuredvG, gravity acceleration, t, dynamic compaction action time and peak value B of ground acceleration amplitude of dynamic compaction saturated sandy soil foundation1Substituting max (B) into the formula (27), and calculating the maximum depth z of the compaction shell generated by dynamic compaction vibration1；
The method for calculating the influence depth of the compact shell comprises the following specific steps:
applying the impact load applied by the rammer to the surface of the rammer pit through the pseudo-static load, as shown in a formula (28), so as to obtain the pseudo-static unit load of the impact load acting on the rammer pit;
Figure FDA0002739551490000011
in the formula, p0Pseudo-static unit load, G, acting on the ramming pit for impact loadsHammerFor the weight of the ram, AArea of hammer baseIs the projected area of the ram;
according to the load characteristic that the plane projection of the rammer is circular, the dynamic load at the maximum depth position of the compacting shell is calculated to form dynamic additional stress through the stress solution of the circular load applied to the semi-space elastomer, as shown in a formula (29),
Figure FDA0002739551490000012
in the formula, σz1Is p0Additional stress at the maximum depth of the compact shell is applied; z is a radical of1The maximum depth of the compact shell; r is0Is the projected diameter of the bottom surface of the rammer;
with calculated additional stress sigmaz1The method comprises the steps of taking a standard stress value and zero depth as an initial value, solving the additional stress sigma of the foundation under the action of the compact shell under the stress by adopting the stress solution of the semi-space elastomeryAs shown in the formula (30),
Figure FDA0002739551490000021
in the formula, σyIs σz1Additional stress of the foundation at the depth h below the compaction shell is acted; h is a certain depth of the lower part of the compacting shell; r is0Is the projected diameter of the bottom surface of the rammer;
calculating the dead weight stress at a certain depth with the depth of the pressure seal shell as a starting point, as shown in formula (31),
σh＝γyhy (31)
in the formula, σhThe dead weight at the depth of h is zero as the initial value of the pressing shellForce, gammayIs hyWeight of depth, hyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced;
and determining the influence depth of the compact shell by applying a stress ratio method, as shown in a formula (32),
0.1σy＝σh (32)
in the formula, σyIs σz1Additional stress, sigma, of the foundation below the compacting shell under the action ofhThe dead weight stress at the depth of h with the depth of the compact shell as the initial value of zero is set;
the effective reinforcement depth of the dynamic consolidation saturated sandy soil foundation is calculated by adopting the following formula:
Z＝z1+hh+hy (33)
wherein Z is effective consolidation depth of dynamic consolidation saturated sand foundation, and Z is1The maximum depth h of a compaction shell of a saturated sandy soil foundation generated by dynamic compaction vibrationhDepth of ramming pit h for producing maximum depth of compacting shellyThe depth of the compaction shell acted on the lower foundation by the compaction shell under the action of the tamping dynamic load is influenced.
2. The design method for the effective reinforcement depth of the dynamic compaction reinforced saturated sandy soil foundation according to claim 1, is characterized in that: the specific method for calculating the depth of the compaction shell generated by dynamic compaction vibration comprises the following steps:
assuming that the saturated sandy soil is an isotropic elastic material, the flow of water conforms to Darcy's law, and the compression coefficient is constant; starting from a critical pore pressure angle of saturated sandy soil caused by dynamic compaction, considering the dynamic compaction as an axisymmetric problem, taking a section passing through an axis, researching according to a plane problem, and establishing a theoretical model of the section;
the compression of the deformation of the soil framework caused by the dynamic compaction fluctuation action has the following relationship,
Figure FDA0002739551490000022
in the formula, the strain of a soil framework is shown, and alpha is the compression coefficient of a saturated sandy soil foundation; e is the void ratio of the sand foundation soil skeleton, sigma'iThe micro-element effective stress of saturated sand soil; dz is the unit length of the infinitesimal body in the z direction; t is the acting time of dynamic compaction in the foundation;
according to Darcy's law, the seepage rate of water passing through the porous medium per unit time is proportional to the cross-sectional area of water and the total head loss, and inversely proportional to the seepage path length, as shown in the following formula,
Q＝kAh1/L＝kAi (2)
in the formula, Q is the flow per unit time of the water passing section; k is the seepage coefficient; a is the area of the water passing section; h is1Total head loss; l is the length of the porous medium seepage path; i is hydraulic gradient;
applying the formula (2) to the flow rate change of the infinitesimal body to obtain a formula (3) which represents the inflow and outflow flow rate change of the upper and lower surfaces of the infinitesimal body in unit area of a certain depth and unit length dz of unit time in the saturated sandy soil foundation,
Figure FDA0002739551490000031
if the compressive deformation of the infinitesimal body soil skeleton of the saturated sandy soil foundation is assumed to be the same as the discharge amount of the fluid, namely:
Figure FDA0002739551490000032
then the process of the first step is carried out,
Figure FDA0002739551490000033
micro element body effective stress sigma 'of saturated sand soil'iThe micro-element effective stress of the saturated sand is obtained according to the concept of effective stress related to the acceleration of the saturated sand soil foundation surface and the pore water pressure, namely
Figure FDA0002739551490000034
Wherein gamma is the volume weight of saturated sandy soil; z is the compaction shell depth of the saturated sandy soil; b is the acceleration amplitude of the dynamic consolidation saturated sand soil foundation ground; omega is the circular frequency of the saturated sand foundation under the condition of dynamic compaction, omega is 2 pi/T, and T is a period; p is a radical offPore water pressure;
substituting the formula (6) into the formula (5) to obtain
Figure FDA0002739551490000035
For a micro-element at a depth of z, the hydraulic gradient of its upper and lower surfaces is expressed as
Figure FDA0002739551490000036
Substituting equation (8) into equation (7) and letting
Figure FDA0002739551490000037
CvFor consolidation coefficient, simplifying to obtain a second-order inhomogeneous partial differential equation of the excess pore water pressure, as shown in formula (9),
Figure FDA0002739551490000038
solving the formula (9) by adopting a separation variable method, taking a pore water pressure function of the saturated sandy soil foundation under the dynamic compaction action as a formula (18),
Figure FDA0002739551490000041
in the formula, c1、c2、c3Beta is a coefficient to be solved of a partial differential equation;
if the dynamic compaction action time t in the foundation is t ═ t1When the liquefaction phenomenon of the z depth of the saturated sandy soil foundation can occur, the pore water pressure at the z depth is balanced with the total stress of the upper foundation, namely
Figure FDA0002739551490000042
Wherein g is the acceleration of gravity;
when t is1→∞，
c3＝γzg (20)
When the rammer is just contacted with the saturated sand, namely t is 0, the pore water pressure of the saturated sand is the original value of the pore water pressure of the site, and the function is
pf＝c1cosβz+c2sinβz+γzg (21)
At this time, at z ═ 0, the pore water pressure is zero, i.e.
c1＝-c3＝-γzg (22)
At z-h, the dynamic compaction P wave forms a compaction shell, the pore water pressure at the position is balanced with the hydrostatic pressure of the saturated sand at the lower part, namely the hydrodynamic pressure is zero, and then c is2The solution of (a) is:
Figure FDA0002739551490000043
substituting the formulas (20), (22) and (24) into the formula (18) to obtain
Figure FDA0002739551490000044
For the upper surface of the compressed shell of saturated sand, z ═ h, and the pore water pressure is pfγ hg, then
Figure FDA0002739551490000045
Order to
Figure FDA0002739551490000046
Sin ω t is 1, and equation (26) is substituted, and the compression shell depth z shown in equation (27) is obtained by solving.
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