CN108376188B - Evaluation and calculation method for loess collapsibility - Google Patents
Evaluation and calculation method for loess collapsibility Download PDFInfo
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Abstract
The invention discloses a yellow soil collapsibility evaluation and calculation method, which comprises the steps of calculating a stress-strain (collapsibility deformation) curve of soaked yellow soil according to a compaction test result beside water injected into a hole, determining the collapsibility initial pressure of the yellow soil according to the curve, and judging the collapsibility of the yellow soil by comparing the stress value after the foundation soil is soaked at a certain depth with the collapsibility initial pressure value, so that the (dead weight) collapsibility bottom boundary of the yellow soil can be determined, the collapsibility bottom boundary can be used as the maximum depth of collapsibility yellow soil foundation treatment, the dead weight collapsibility bottom boundary is an important basis for determining the foundation treatment depth, and the dead weight collapsibility bottom boundary can be used as the neutral point depth of a pile foundation; the modulus of a certain stress section can be calculated according to the stress-strain curve of the loess after being soaked in water, and the collapsibility of the loess can be calculated according to the modulus. The invention has the beneficial effects that: discloses a method for calculating the loess (dead weight) collapsibility, which provides a new way for calculating the loess collapsibility.
Description
Technical Field
The invention relates to the technical field of building engineering, in particular to a method for evaluating and calculating collapsibility of loess, which can be used for determining a collapsibility bottom boundary, a collapsibility initial pressure, pile foundation neutral point depth and loess collapsibility of heavy-thickness self-weight collapsible loess.
Background
Loess is widely distributed in China, loess in many areas not only has collapsibility, but also has self-weight collapsibility, the distribution area and thickness of the self-weight collapsibility loess in northwest, north China and the like are large, and the evaluation and treatment of the large-thickness self-weight collapsibility loess are troublesome problems in engineering. The collapsible amount of loess can be calculated by a soaking load test, a field pit test soaking test and an indoor test. The immersion load test and the field test pit immersion test have high precision, but the investment and the cost are large, the test period is long, and the method is not suitable for being generally adopted in engineering. The method commonly adopted by engineering is carried out according to an indoor test method specified in the existing collapsible loess area building code (GB50025-2004), the method is based on an indoor side-limit consolidation test, the test condition of the method is greater than the stress state of field soil, the change range of the correction coefficient is too large during calculation, the determined collapsible bottom boundary, the position of a pile foundation neutral point, the collapsible amount and the like are greatly different from the field soaking test result, and the test process is relatively complex.
Disclosure of Invention
Aiming at the problems in the prior art, the invention aims to provide a simple and quick evaluation and calculation method for the collapsibility of loess based on an in-situ test.
In order to realize the task, the invention adopts the following technical scheme:
a method for evaluating and calculating the collapsibility of loess comprises the following steps:
step 1, establishing a first drilling hole and a second drilling hole at intervals in a test field, wherein the two drilling holes are drilled to the bottom of a collapsible loess layer; respectively determining a plurality of test points in the two drill holes from top to bottom;
step 2, performing a lateral pressure test at each test point from bottom to top in the first drilling hole respectively;
step 3, performing anti-seepage treatment on the hole bottom of the second drilled hole, then injecting water, and then performing a side pressure test at the first test point; backfilling the second drilled hole below the second test point, performing anti-seepage treatment, injecting water, and performing a second side pressure test; according to the same method, completing a side pressure test until the last test point;
step 4, respectively drawing lateral pressure test curves of each test point in the first drilling hole and the second drilling hole, and then converting the two groups of lateral pressure test curves into vertical stress-strain curves;
step 5, calculating the difference value of the stress-strain curve of the second drill hole and the corresponding point on the stress-strain curve of the first drill hole for the test points at the same positions in the two drill holes, drawing the stress-strain curve according to the difference value, and then determining the initial pressure of the collapse at the positions of the test points according to the curve;
and 6, if the stress of a certain depth in the loess in the saturated state is greater than the initial collapse pressure corresponding to the depth, judging that the loess at the depth is collapsed.
Furthermore, the diameters and the depths of the first drilling hole and the second drilling hole are the same, and the distance between adjacent test points in each drilling hole is the same.
Further, the step 3 of injecting water after the anti-seepage treatment specifically comprises the following steps:
mixing lime and loess according to the weight ratio of 1:3, laying 15cm at the bottom of the hole, tamping by using a percussion drill, injecting water, and recording the cross section of a second drilling hole as S, wherein the water injection amount is S cubic meters; waiting for at least 8 hours after water injection, and pumping out water in the hole if the water level at the bottom of the hole is higher than 10cm before the test; a side pressure test was then performed at the test points.
A method for determining depth of loess self-weight collapse bottom boundary and pile foundation neutral point comprises the following steps:
calculating the saturated dead weight stress of loess at different depths, wherein the saturated dead weight stress is equal to or closest to the initial collapse pressure, namely the position of a neutral point and is also a dead weight collapse bottom boundary; the total stress (i.e., the sum of the saturated deadweight stress and the additional stress generated from the upper building) is equal to or close to the initial pressure of the collapse, i.e., the collapsed bottom boundary of the loess.
Further, when the position of the neutral point of the pile foundation is determined, under the same coordinate system, an initial pressure curve is drawn according to the collapse initial pressure of different positions, a self-weight stress curve is drawn according to the self-weight stress of loess at different positions, the intersection point of the initial pressure curve and the self-weight stress curve is obtained, or the position, corresponding to the closest point on the self-weight stress curve, on the initial pressure curve is the neutral point position.
A method for calculating the amount of self-weight collapse of loess comprises the following steps:
determining the depth of the self-weight collapsible bottom boundary of the loess, and dividing the depth into different layers at the same interval;
calculating the layered collapse modulus from the stress-strain curve:
in the above formula, EiIs the collapsible modulus, P, of the ith subsoil2iIs the saturated dead weight stress of loess at the upper boundary of the layer, P1iIs the saturated dead weight stress of loess at the lower boundary of the layer, s2i、s1iAre respectively P2i、P1iA corresponding amount of deformation on the stress-strain curve;
the self-weight collapse of each layer was calculated according to the following formula:
in the above formula,. DELTA.siThe self-weight collapsibility of the ith layer soil,is the stress increment of the i-th layer soil, hiIs the thickness of the laminate;
the self-weight collapse amount of the loess is as follows:
in the above formula, s is the self-weight collapsible amount of loess, and n is the number of layered layers; p2iIf the total stress is obtained, the calculated s is the collapsible amount of the loess.
The invention has the following technical characteristics:
the method comprises the steps of converting a stress-strain (collapsible deformation) curve of a lateral pressure test, namely a transverse load test, of collapsible loess after soaking into water into a longitudinal (load test) stress-strain curve, determining the initial collapsible pressure according to the inflection point of the curve, judging whether the loess layer is collapsed or not according to the relation between the initial collapsible pressure and the total stress of a certain point in the foundation soil layer, determining the modulus of the soil according to the longitudinal stress-strain curve, and calculating the collapsible deformation, namely the collapsible amount, according to the modulus and the stress of the certain point in the foundation soil by using a traditional mechanical method, so that the goal of determining the collapsible bottom bound of the loess layer and calculating the collapsible amount of the loess is achieved. The method disclosed by the invention is based on an in-situ test (side pressure test), can simply, conveniently and quickly evaluate the collapsibility of the loess, and provides a new way for determining the neutral point and the collapsible bottom boundary of the pile foundation in the collapsible loess and calculating the self-weight collapsibility or the collapsible amount of the loess.
Drawings
Fig. 1 (a) is a schematic top view of a first borehole and a second borehole, and (b) is a schematic cross-sectional view of the first borehole and the second borehole;
FIG. 2 is a schematic diagram of a P-V curve obtained by a side pressure test;
fig. 3 is a stress-strain curve (second curve) of the collapsible deformation obtained by the conversion;
FIG. 4 is a flow chart of the method of the present invention.
Detailed Description
The invention discloses a method for evaluating the collapsibility of loess by a side compaction test method by injecting water into a hole. The premise of carrying out the side pressure test is that the collapsibility of the whole (field) field is known to a certain extent, and if exploratory well excavation or drilling is carried out, the collapsibility coefficients and the self-weight collapsibility coefficients of different depths are measured.
A method for evaluating and calculating the collapsibility of loess comprises the following steps:
step 1, establishing a first drilling hole (1#) and a second drilling hole (2#) at intervals in a test field, and drilling the two drilling holes to the bottom of the collapsible loess layer; respectively determining a plurality of test points in the two drill holes from top to bottom;
in the scheme, the first drilling hole and the second drilling hole are respectively used for performing the side pressure tests in the natural state and the soaking state. The distance between the two holes is proper, the soil property of the two holes is kept consistent as much as possible, but the two holes cannot influence each other in the test process, for example, the distance between the two holes can be 2-3 m, as shown in (a) and (b) in fig. 1. The diameters and the depths of the two drill holes are consistent, and the two drill holes are drilled to the bottom of the collapsible loess layer when drilling, namely to the position where the collapsible coefficient and the self-weight collapsible coefficient are less than 0.015. In the present embodiment, the drilling depth H is 19 m.
Determining a plurality of test points from bottom to top in the two drill holes, for example, determining test points from bottom to top at distances of 1m, 3m, 5m, … and 19m from the bottom of the drill hole for the first drill hole; note that the position 1m is the first test point, the position 3m is the second test point, and so on. The arrangement of the test points in the second drilling hole is the same as that of the first drilling hole, namely the test points in the two drilling holes at the same depth correspond to each other in position.
Step 2, performing a lateral pressure test at each test point from bottom to top in the first drilling hole respectively; in the embodiment, the pressure bypass tests are sequentially carried out on the test points of 1m, 3m, 5m, … and 19m in the first drilling hole by using the pressure bypass devices;
step 3, performing anti-seepage treatment on the hole bottom of the second drilled hole, then injecting water, and then performing a side pressure test at the first test point;
optionally, one method of performing the anti-seepage treatment and the water injection is: mixing lime and loess according to the weight ratio of 1:3, laying 15cm at the bottom of a second drill hole, tamping for 5-7 times by using a percussion drill heavy hammer, and then injecting water, wherein if the cross section of the second drill hole is recorded as S, the water injection amount is S cubic meters, namely the water injection depth is 1 m; if the water level is obviously reduced after water injection, if the water level is reduced by more than half within 1 hour, S cubic meter water needs to be supplemented until the water level is not obviously reduced; and (5) waiting for at least 8 hours after water injection to fully saturate the loess.
After 8 hours, before the test, observing that if the water level at the bottom of the hole is higher than 10cm, pumping out the water in the hole; if the hole is collapsed, the hole is cleaned by a drilling machine, and then a side pressure test is carried out at the test point. The test point here is the first test point, i.e. the position 1m from the bottom of the hole.
After the water injection lateral pressure test is carried out on the second drill hole for the first time, the second drill hole is backfilled to be below the second test point (3 m), and the backfilling amount of the loess can be the difference of the heights of the two test points, for example, 2m of loess is backfilled; at the moment, water is injected after the anti-seepage treatment is carried out on the hole bottom formed after backfilling, the specific anti-seepage treatment and water injection method is the same as that of the first time, and then a second side pressure test is carried out; and (4) according to the same method as the first test and the second test, continuously carrying out backfilling, seepage prevention, water injection and side pressure test until the last test point (19m position) completes the side pressure test.
Step 4, respectively drawing lateral pressure test curves of each test point in the first drilling hole and the second drilling hole, and then converting the two groups of lateral pressure test curves into vertical stress-strain (P-S) curves;
as shown in fig. 2, it is a schematic diagram of a side pressure test curve (P-V curve), where P is the pressure of the test point during the side pressure test, and V is the volume deformation of the side pressure chamber. Each test point corresponds to a P-V curve, and a group of P-V curves are generated by drilling one hole; the P-V curve is then converted to a vertical stress-strain curve (for ease of understanding and presentation, the stress-strain curve is referred to herein as the first curve), where P in the stress-strain curve (P-S) refers to the stress at the corresponding point and S refers to the strain at the corresponding point. The conversion method comes from the following documents:
liu Qiong, Yanguanghua, LiDeji uses the side pressure test result to calculate the p-s curve [ J ] of load test, Guangdong water conservancy and hydropower, 2008(08):82-84.
Yanguanghua, undisturbed soil tangent modulus method of foundation nonlinear settlement calculation [ J ]. geotechnical engineering, 2006(11): 1927-;
step 5, calculating the deformation difference value of the stress-strain curve of the second drill hole and the corresponding point on the stress-strain curve of the first drill hole for the test points at the same positions in the two drill holes, drawing the stress-strain curve according to the difference value, and then determining the initial pressure of the collapse at the positions of the test points according to the curve;
through the steps, each drill hole generates a group of stress-strain curves, namely stress-strain curves at positions of 1m, 3m, 5m, … and 19m, then the difference value of the P-S curves of the test points at the same position in the two drill holes is calculated, and a new stress-strain curve (marked as a second curve) is generated by the difference value, namely after the difference value is made between the corresponding positions of the two first curves of the same test point in the two drill holes, the result is still a curve, namely the second curve.
Specifically, the strain S1 corresponding to the stress on the first curve of the first test point on the first drill hole is subtracted from the strain S2 corresponding to different stresses on the first curve of the first test point on the second drill hole, and the difference is the amount of collapse deformation S0, and then a P-S curve, that is, the second curve, is drawn. A second curve P-S, where P represents stress, but S represents the strain at collapse.
After the second curve is drawn, the initial collapse pressure P is determined from the curveshAs shown in fig. 3, the inflection point of the curve is the initial collapse pressure; p of different curve formsshThe value-taking method is referred to the following documents:
zhenyanwu, the collapsibility of Chinese loess, Beijing: geological Press, 1982.
And 6, if the stress of a certain depth in the loess in the saturated state is greater than the initial collapse pressure corresponding to the depth, judging that the loess at the depth is collapsed.
After the above steps, each test point (i.e. 1m, 3m, 5m, …, 19m in the present embodiment) determines a collapse starting pressure PshAnd then the initial pressure of the collapsible is corresponding to the depth of the soil layer.
In practical application, for example, if it is required to determine whether the loess has a depth of 1m and is collapsible, the stress at a depth of 1m in the loess in a saturated state is calculated by a side pressure test or the data of a exploration point near the side pressure test, and then the stress is compared with the collapse initial pressure at a position of 1m determined by the method, and when the stress is greater than or equal to PshIt can be judged as collapsible when the patient is in use, or else it is judged as non-collapsible.
If the depth to be determined is not consistent with the depth tested by the method, for example, the depth to be tested is 2 meters, and the method only provides P corresponding to the depths of 1 meter and 3 metersshThen, the P corresponding to the positions of 1 meter and 3 meters can be obtainedshAverage value as P at 2mshOr P at different depthsshConnecting to form a curve to define P at different positionssh。
On the basis of the technical scheme, the invention also provides a method for determining the depth of the loess self-weight collapsible bottom boundary and the pile foundation neutral point, which comprises the following steps:
calculating the saturated dead weight stress of loess at different depths, wherein the saturated dead weight stress is equal to or closest to the initial collapse pressure, namely the position of a neutral point and is also a dead weight collapse bottom boundary; the same approach can determine the bottom bound of the collapse when considering the top load: the total stress (i.e., the sum of the saturated deadweight stress and the additional stress created by the upper building) is equal to or closest to the collapse starting pressure, i.e., the collapse bottom boundary.
Preferably, when the position of the neutral point is determined, under the same coordinate system, an initial pressure curve (i.e., a curve formed by sequentially connecting the initial pressures of the different positions of the loess) is drawn according to the initial pressures of the different positions (depths), a saturated deadweight stress curve is drawn according to the saturated deadweight stress of the different positions (depths) of the loess, and an intersection point of the initial pressure curve and the saturated deadweight stress curve is obtained, or a position on the initial pressure curve corresponding to the closest point on the deadweight stress curve is the neutral point position, i.e., the depth.
On the basis of the above technical scheme, the invention further provides a calculation method of loess (dead weight) collapsibility, which can calculate the modulus of a corresponding stress section by using a second curve (P-S), wherein the modulus is not called as the collapsibility modulus, then the dead weight collapsibility or the collapsibility can be calculated according to the collapsibility modulus and the additional stress at a certain depth of soil and according to the mechanical principle, the depth is calculated to a dead weight collapsibility or collapsibility bottom boundary, loess below the bottom boundary has no dead weight collapsibility or collapsibility, and loess above the bottom boundary has dead weight collapsibility or collapsibility. The calculation of the self-weight collapse quantity of the loess is illustrated as follows, and the specific steps include:
determining the depth of the self-weight collapsible bottom boundary of the loess (i.e., the position corresponding to the bottom of the collapsible loess in the previous step 1), and dividing the depth into different layers at the same interval; in the embodiment, the test points are layered at intervals of 2m, for example, 0-2.0 m, 2.0-4.0 m, 4.0-6.0 m, 6.0-8.0 m, 8.0-10.0 m and 10.0-11.0 m … from the bottom of the hole, so that different test points are located in different layers. If the stratum boundary is met, if an ancient soil interlayer exists, the stratum boundary depth is taken as the layered boundary of the small layer.
Calculating the layered collapse modulus from the stress-strain curve:
in the above formula, EiIs the collapsible modulus, P, of the ith subsoil2iIs the saturated dead weight stress of loess at the upper boundary of the layer, P1iIs the saturated dead weight stress of loess at the lower boundary of the layer, s2i、s1iAre respectively P2i、P1iThe corresponding amount of deformation on the stress-strain curve (here, the second curve);
taking the calculation of the self-weight wet-fall amount of 2.0-4.0 m as an example, P2iIs the dead weight stress of 2m of soil, P1iThe deformation quantity corresponding to the two values, namely s, is found out on a second curve for the self-weight stress of the soil at the position of 4 meters2i、s1i。
The self-weight collapse of each layer was calculated according to the following formula:
in the above formula,. DELTA.siThe self-weight collapsibility of the ith layer soil,the stress increment of the ith layer of soil, namely the difference between the saturated dead weight stress of loess at the bottom of a small layer and the dead weight stress at the top of the small layer, hiThe thickness of the layers is 2m in this example;
the self-weight collapse amount of the loess is as follows:
in the above formula, s is the self-weight collapsible amount of loess, and n is the number of layered layers.
If the hole depth is larger, the test period is longer, and the water immersion hole can adopt a method of simultaneously adopting a plurality of drill holes and carrying out test in sections, such as one drill holeTest at line 1m, 5m, 9m, … …, another drill was tested at 3m, 7m, 11m …; the loess collapsibility can be calculated by the same method, except that P is calculated2iUsing total stress, i.e. P2iIf the total stress is obtained, the calculated s is the collapsible amount of the loess.
Claims (3)
1. The evaluation and calculation method for the collapsibility of the loess is characterized by comprising the following steps of:
step 1, establishing a first drilling hole and a second drilling hole at intervals in a test field, wherein the two drilling holes are drilled to the bottom of a collapsible loess layer; respectively determining a plurality of test points in the two drill holes from top to bottom;
step 2, performing a lateral pressure test at each test point from bottom to top in the first drilling hole respectively;
step 3, performing anti-seepage treatment on the hole bottom of the second drilled hole, then injecting water, and then performing a side pressure test at the first test point; backfilling the second drilled hole below the second test point, performing anti-seepage treatment, injecting water, and performing a second side pressure test; according to the same method, completing a side pressure test until the last test point;
step 4, respectively drawing lateral pressure test curves of each test point in the first drilling hole and the second drilling hole, and then converting the two groups of lateral pressure test curves into vertical stress-strain curves;
step 5, calculating the difference value of the stress-strain curve of the second drill hole and the corresponding point on the stress-strain curve of the first drill hole for the test points at the same positions in the two drill holes, drawing the stress-strain curve according to the difference value, and then determining the initial pressure of the collapse at the positions of the test points according to the curve;
and 6, if the stress of a certain depth in the loess in the saturated state is greater than the initial collapse pressure corresponding to the depth, judging that the loess at the depth is collapsed.
2. The method of claim 1, wherein the first and second boreholes have the same diameter and depth, and wherein the distance between adjacent test points in each borehole is the same.
3. The evaluation and calculation method for the collapsibility of yellow soil according to claim 1, wherein the step 3 of injecting water after the anti-seepage treatment comprises the following specific steps:
mixing lime and loess according to the weight ratio of 1:3, laying 15cm at the bottom of the hole, tamping by using a percussion drill, injecting water, and recording the cross section of a second drilling hole as S, wherein the water injection amount is S cubic meters; waiting for at least 8 hours after water injection, and then observing that if the water level at the bottom of the hole is higher than 10cm, pumping out the water in the hole; a side pressure test was then performed at the test points.
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