Fracture grouting is widely used for building foundation reinforcement, however the underpinning mechanisms are still not clear. Using numerical results about a single-hole fracture grouting process as a basis, a model composed of soil and grouting veins has been created to analyze the reinforcement mechanism. The influence weights of the grouting vein skeleton and compaction effect have been studied, thereby obtaining relevant information on the compressive modulus of the considered composite soil. The research results show that the compaction effect plays a leading role in the soil fracture grouting reinforcement. The grouting pressure, the hardened grouting vein modulus, and the shape of the grouting veins all influence the compressive modulus of the composite soil.

Fracture grouting is widely used to strengthen the construction foundation to prevent and control its settlement. For example, before the subway goes under a building, it is usually necessary to reinforce the soil under the building to reduce settlement. Although fracture grouting is widely used in engineering, its reinforcement mechanism is still not very clear yet. After fracture grouting is finished, a grouting composite soil body is formed. The grouting veins and the soil are tightly combined in the composite soil body. The modulus of grouting veins in the composite soil is relatively high, which plays a role in skeleton support. The grout also compacts the surrounding soil during the grouting process. However, it is not clear which plays the leading role.

The mechanism of grouting reinforcement has always been the focus of engineers and technicians. Bai et al. [

Although scholars have carried out some experimental studies, it is difficult for laboratory experiments to fully simulate the state of the soil layer. Pure theoretical research and experiment are difficult to visually reflect the dynamic expansion process and the shape of the grouting veins. The numerical simulation can not only simulate the state of the formation, but also reflect the dynamic expansion process. Eriksson et al. [

Although some studies on the effect of fracture grouting reinforcement were carried out, most of them focused on a single fracture in fractured rock or soil. However, one grouting hole usually produces multiple grouting veins when in clay soil. The grouting effect evaluation rarely focuses on the composite soil formed by the combination of grouting veins and soil. It is impossible to take samples containing multiple grouting veins on-site for confined compression tests. Therefore, the numerical method becomes an effective method. In this paper, we used an FEM/VOF numerical method [

After the fracture grouting in soil finished, the grouting vein and the soil formed a tightly combined grouting composite soil with two solid media, see

During fracture grouting, the infiltration and diffusion of the grout have a compacting effect on the surrounding soil. After the grouting is condensed, it forms a rigid grouting vein to play a skeletal support effect in the composite soil. Compaction and skeleton support are the two main mechanisms of fracture grouting reinforcement. The reinforcement effect is related to the grouting pressure, the shape of the grouting vein, and the mechanical properties of the soil itself. The compressive modulus of grouting composite soil is a quantitative index to evaluate the reinforcement effect. It is mainly affected by compaction and skeleton effects.

The numerical simulation method based on finite element (FEM) and fluid volume function (VOF) hybrid formulation method [

The tensile failure and shear failure of material element are both considered in the numerical model. If the minimum effective stress of an element exceeds its tensile strength, a form of tensile failure is considered to have occurred, see

Shear failure will occur if the shear stress satisfies the Mohr-Coulomb failure criterion, see ^{′} is the effective stress cohesion, φ^{′} is the effective stress friction angle or shearing resistance angle, and

No matter what kind of failure form occurs in the soil element, it is considered that fracturing occurs. The stiffness of the element will decrease. The stiffness of the fracture without grout can be described by ^{−4 }m in this investigation. As the cracks are occupied by the grout, the stiffness of crack can be described by

The permeability of grout and water in a fracture

The flow of fluids through fractures in soil is usually considered as laminar flow. In the grout propagation model, fractures are considered to be equal-width flow fields. In the laminar flow state, the velocity of fluid motion between two flat plates exhibits a quadratic distribution, seen in

The velocity of the grout in the soil crack can be expressed as _{x} is the velocity of the grout in the x-direction,

Then, the unit discharge of the fluid within the fracture can be expressed as:

Therefore, the superficial velocity of the flow can be expressed as:

As shown in

The viscosity and modulus of grout

During the fracture grouting, the modulus and viscosity of the grout increase nonlinearly under the influence of hydration. Experimental study shows that the Young’s modulus and the viscosity of grout varied gradually, see

where _{g}(_{g}(_{g0} is the initial viscosity coefficient of the grout, and

The permeability coefficient of the grout in soil within a certain time can be calculated by _{g0} is the initial permeability coefficient of grout in soil.

Considering the inhomogeneity of soil strength. The material properties of soil elements obey Weibull distribution in numerical calculation.

To validate the numerical method, a simulation model of 6 m × 6 m was established. The soil mass within the simulated area is under the plane strain boundary condition, see _{v} in the model.

At the same time, fracture grouting test was carried out in an underground engineering. Fracture grouting site excavation test and the corresponding numerical simulation results are shown in

The numerical simulation method based on FEM/VOF method can simulate the fracture grouting diffusion process and record the minor principal stress change process during grouting.

According to the Duncan-Chang hyperbolic constitutive model and related parameters, the tangent deformation modulus of the soil element before and after grouting can be calculated by the minor principal stress of the soil layer, see

Material | _{ur} |
_{f} |
_{b} |
|||||
---|---|---|---|---|---|---|---|---|

Soil | 40 | 28 | 50∼500 | 100∼1000 | 0.6 | 0.95 | 20 | 0.5 |

We select the soil element within the range of 1 m × 1 m near the grouting hole as the research object. Then use

The improvement of the compressive modulus of grouting composite soil is one of the methods for evaluating the effect of grouting reinforcement. The shape of the grouting veins in the composite soil is random. The variety of grouting vein shapes increases the difficulty of calculating the compressive modulus of grouting composite soil. Scholars conduct research by simplifying the morphology of the grouting veins. Qian [

The reinforcement radius of a single grouting hole is generally about 1 m. So we selected the fracture grouting composite soil within the 1 m × 1 m rectangle around the grouting hole as the research object. The calculation model was established as shown in ^{−7} m/step applied to the top of the model. By counting the average stress of the top after each loading level, the compressive modulus of the composite soil can be obtained according to the following formula:_{0} is the initial height of the composite soil.

The material parameters of soil and grouting vein are shown in

Material | Modulus (MPa) | Poisson’s ratio | Internal friction angle (°) | Density (g·cm^{−3}) |
---|---|---|---|---|

Soil | Refer to |
0.3 | 28 | 1800 |

Grouting vein | 100, 200, 300, 500, 1000 | 0.2 | 20 | 2500 |

To analyze the relationship between grouting pressure, grouting depth, soil parameters, and the composite soil compressive modulus, a series of research conditions were set up, see

Grouting depth (m) | Grouting pressure (MPa) | Soil modulus parameter K | Grouting vein modulus (MPa) |
---|---|---|---|

3 | 0.33, 0.44, 0.55, 0.66 | 50, 100, 200, 500 | 300 |

4.5 | 0.33, 0.44, 0.55, 0.66 | 50, 100, 200, 500 | |

7.5 | 0.44, 0.55, 0.66, 0.88 | 50, 100, 200, 500 | |

10 | 0.44, 0.55, 0.66, 0.88 | 50, 100, 200, 500 | |

15 | 0.55, 0.66, 0.88, 0.99 | 50, 100, 200, 500 |

Vertical displacement distribution of undisturbed soil before grouting and composite soil after grouting under a vertical load of 150 kPa, see

The vertical displacement curves of the ground surface before and after grouting under different loads are shown in

The upper settlement curve of grouting composite soil when considering different reinforcement factors, see

To study the weight of the compaction and grouting vein skeleton effects on the composite soil modulus, we established three working conditions: only considering the skeleton effect, only considering the compaction effect, and considering the two effects. The weight of the grouting vein skeleton effect is calculated as follows:_{g} is the weight of the grouting vein skeleton, _{g+c} is the composite soil modulus considering both the skeleton and the compaction effect, _{g} is the composite soil modulus considering only the skeleton effect, _{0} is the initial modulus of the soil before grouting.

The weight of the compaction effect _{c} is:

The calculation results of the previous set of numerical simulation according to _{g} = 2.76/6.56 = 42.07%; the weight of the compaction effect is: _{c} = 3.8/6.56 = 57.93%. To find out which reinforcement effect of fracture grouting plays a leading role, Further statistical analysis of reinforcement weights under different grouting pressures and depths are needed to find out which one plays the leading role.

Working condition | Compressive modulus of soil before grouting (MPa) | Compressive modulus of composite soil after grouting (MPa) | Modulus increase (MPa) | Modulus increase ratio |
---|---|---|---|---|

Composite soil modulus | 2.94 | 9.5 | 6.56 | 223.2% |

Only consider the role of the skeleton | 5.7 | 2.76 | 93.9% | |

Only consider compaction | 6.74 | 3.8 | 129.3% |

The weight of the compaction effect and the skeleton effect in the grouting reinforcement under different grouting depths, grouting pressures, and the initial modulus of the soil, see

The fracture grouting composite soil comprises two solid media: the grouting vein skeleton and the soil. The shape of the grouting vein, the modulus of the soil after grouting, and the modulus of the grouting vein skeleton will all affect the compressive modulus of composite soil. According to the simulation working conditions designed in

The relationship between composite soil compressive modulus and grouting pressure, see

With the increase of grouting pressure, the modulus of composite soil tends to stabilize. The soil reinforcement effect will not increase or increase slowly when the grouting pressure increases to a certain extent. It is not that the greater the grouting pressure, the better the reinforcement effect. Excessive grouting pressure will cause deformation and damage, thereby reducing the reinforcement effect. The recommended grouting pressure in the soil can be given in conjunction with

Grouting pressure can influence the spreading range and branches of the grouting vein. The relationship between the grouting pressure and the weight of the grouting vein skeleton, see

After fracture grouting, the slurry hardens to form grouting veins with a certain strength. The grouting veins play the role of skeleton support in the soil. It can also improve the strength and resistance to deformation of the soil. The modulus of the grouting vein skeleton will affect the modulus of the composite soil, see

With the skeleton modulus increases, the weight of the skeleton action increases, see

The initial modulus of the soil before grouting is related to factors such as soil type and buried depth. It is a parameter that reflects the softness and hardness of the soil. The initial modulus of the soil affects the distribution of the grouting veins and the injection rate of the slurry. The study of the expansion law of single-hole splitting grouting shows that with the increase of soil modulus, the volume of the slurry bubble gradually decreases, the width of the slurry vein becomes narrower, and the grouting rate falls. It will also cause the weight of the skeleton action to fall with the increase of the initial modulus of the soil under the same grouting pressure and other conditions, see

It can be seen that as the initial modulus of the soil increases, the weight of the skeleton action shows a downward trend. The skeleton effect is obvious, and the compaction effect is relatively weak when fracture grouting in soft soil. When fracture grouting in hard soil, the skeleton effect is reduced, and the compaction effect is enhanced.

In this paper, a calculation model of fracture grouting composite soil is established based on the grouting veins of single-hole fracture grouting. The composite soil compressive modulus under different grouting depth, grouting pressure, and initial soil modulus were obtained through a series of compression test simulations. The weight analysis of the fracture grouting reinforcement effect was also carried out. The main conclusions are as follows:

The fracture grouting reinforcement effects include the support effect of the grouting vein skeleton and the compacting effect. Through statistical analysis of 64 sets of calculation results, it is found that the compacting effect plays a leading role in the soil fracture grouting reinforcement.

The compressive modulus of fracture grouting composite soil is affected by the grouting pressure, the initial modulus of the soil, and the strength of the grouting vein skeleton.

With the increase of grouting pressure, the compressive modulus of composite soil increases continuously. However, there is an optimal pressure value for fracture grouting reinforcement, not that the bigger the pressure, the better the effect.

With the increase of the grouting vein skeletal modulus, the composite soil compressive modulus and the weight of the skeletal action gradually increase. When the elastic modulus of the grouting vein in soft soil is greater than a certain level, the support effect of the skeleton does not increase significantly.