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Abstract
Ground improvement is a widely used engineering technique to stabilize, reinforce, or improve the physical properties of soil and rocks, thereby reducing the cost and improving the performance of foundations and embankments. As a traditional approach to ground improvement, there are various methods, such as soil compaction, rock mechanical grouting, and stone column, available. In recent decades, a series of new methods and technologies, such as dynamic compaction, dynamic compaction using vibratory equipment, vibratory pile driving, and dynamic soil recomposed, have been developed and applied.
For the purpose of ground improvement, it is important to properly evaluate its effectiveness, especially for the combined ground improvement process with different applications. Ground improvement testing is an essential task in order to monitor and monitor the progress, performance, and reliability of ground improvement works and to determine whether, or not, the adopted improvement measures are taking effect and to identify any deficiencies. To ensure the satisfactory effect of ground improvement operations, an important step is to carry out a detailed field test.
This paper reviews common ground improvement testing methods, including standard penetration test, cone penetration test, dynamic penetration test, seismic test, unconfined compression strength test, field vibratory test, thin-walled tube test, and air grouting injection pressure test. These tests have a different focus when used to determine the effectiveness of ground reinforcement. Meanwhile, data processing based on test results and relationship between seismic/geophysical methods and ground improvement effects are discussed. Finally, a summary of challenges present in ground improvement monitoring and testing is presented.
1. Introduction
Ground improvement is a widely used engineering technique to stabilize, reinforce, or improve the physical properties of soil and rocks for the purpose of the basis for engineering construction of various types of structures. Over the past decades, a series of ground improvement techniques and technologies have been developed, including soil compaction, rock mechanical grouting, stone column, dynamic compaction, dynamic compaction using vibratory equipment, and dynamic soil recomposed, etc. These techniques are effective for decreasing settlement and allow use of shallow or low bearing capacity soils for foundation, or to reduce differential settlements, thus significantly improving the performance and reliability of the foundations or embankments.
Ground improvement offers advantages in comparison with traditional methods of increasing soil strength, such as chemical grouting, and it is attractive due to its relatively low cost and relatively fast application. Furthermore, the improved soils and rocks will have a favourable effect on the performance of the foundation or embankment as well as on the surrounding environment.
However, in order to ensure the satisfactory effect of ground improvement works, it is necessary to properly test the effectiveness of ground improvement, particularly when a combined ground improvement process is applied. Ground improvement testing (GIT) is an essential task to monitor and evaluate the progress, performance, and reliability of the ground improvement works and to determine whether or not the adopted improvement measures are taking effect. The performance evaluation of engineered materials and soils has been studied for several decades, and a number of methods and tests have been developed for that purpose. In this paper, the various available GIT methods and technologies, such as standard penetration tests, cone penetration tests, dynamic penetration tests, seismic tests, unconfined compression strength tests, field vibratory tests, thin-walled tube tests, air-grouting injection pressure tests, etc., are reviewed. Finally, the challenges present in GIT are summarized.
2. Ground Improvement Techniques and Requirements
Various ground improvement techniques and technologies are available for the purpose of foundation or embankment construction. These include soil compaction, rock mechanical grouting, stone column, dynamic compaction, dynamic compaction using vibratory equipment, and dynamic soil recomposed, etc. Each technique is appropriate for different types of soil and rock, and under different conditions. Generally, ground improvement is necessary when the existing soil and rock have low bearing capacity, if settlement needs to be reduced, if liquefaction conditions exist, or if landslides or erosion of slopes are anticipated [1].
Typically, ground improvement works require costly and accurate technical knowledge in order to prevent structure failure or prolonged service life. To be reliable and efficient, ground improvement application must satisfy a number of performance requirements. These may include earthquake resistance, prevention of deformation, dissipation of ground motion energy, limiting the settlement to an acceptable value, reducing differential settlement, ensuring the stability of slopes and retaining walls, reducing or eliminating surface and ground water hazards, and increasing bearing capacity. To verify whether or not the ground improvement works satisfy these requirements and meet the specified design criteria, testing of the ground improvement effects is required [2].
3. Ground Improvement Tests
Testing of ground improvement effects is required, in order to ensure the satisfactory effectiveness of ground improvement. Several testing methods and technologies have been developed, aimed at evaluating soil and rock properties and assessing the resulting improvement effects. A comprehensive set of field tests is necessary in order to assess the performance and reliability of ground improvement works. Such tests generally focus on one of the following aspects: strength and bearing capacity, stiffness, permeability, water content, and compaction [3].
3.1. Standard Penetration Tests
Standard Penetration tests (SPTs) use a mechanical device, such as a split-spoon sampler, to measure the soil resistance to penetration and failure [4]. A split-spoon sampler is forced into the ground with a 140-pound hammer falling 30 inches. The penetration rate is recorded and defined as the number of blows required per unit depth. The standard penetration resistance, or N-value, is normally used to evaluate and compare bearing capacity values.
3.2. Cone Penetration Tests
Cone penetration tests (CPTs) are widely used to measure soil properties, such as strength, stiffness, and permeability. The test involves the insertion of a cone-shaped probe into the ground. The cone is attached to a series of mechanical instruments that measure the normal and frictional forces on the cone. The CPTs are used to determine soil parameters required for the design of foundations and embankments.
3.3. Dynamic Penetration Tests
Dynamic penetration tests (DPTs) are widely used for engineering investigations, particularly for assessing damage caused by earthquakes. The DPTs involve the use of an imposing spike to measure the soil resistance to seismic energy. By recording the transmitted energy and the failure depth, the results may be used to assess the stiffness and strength of the soil, as well as to measure the optimum spacing of piles and the required bearing capacity.
3.4. Seismic Tests
Seismic tests, including field surveys and laboratory tests, are widely used to evaluate the seismic performance of foundations and embankments. Such tests involve the propagation of seismic waves through the soil or rock in order to measure its dynamic characteristics. The test results can later be used to evaluate the ground improvement effects and make decisions regarding the design and strengthening of engineering structures.
3.5. Unconfined Compression Strength Tests
Unconfined compression strength tests are used for assessing the bearing capacity and shear strength of soils. The tests involve loading a sample of material and measuring its resistance to compression or shear. The test results can be used to calculate the bearing capacity factor, which is then used to determine the allowable bearing pressures on the ground improvement.
3.6. Field Vibratory Tests
Field vibratory tests involve the use of vibratory methods to measure the static stress-strain properties of soils or rocks. These tests are used for the investigation of soil or rock characteristics, such as stiffness, strength, and vibration absorption. They can also be used to improve soil compaction and assess the effects of ground improvement works.
3.7. Thin-Walled Tube Tests
Thin-walled tube tests are used to measure the compressibility of soils and rocks. The tests involve the insertion of a metal tube into the ground and measuring the displacement of the tube in response to pressure. The test results can be used to assess the stiffness of the soil or rock and the consolidation settlement.
3.8. Air Grouting Injection Pressure Tests
Air grouting injection pressure tests are used to measure the injection pressure of air grouting. This test involves the insertion of a grout tube into the ground and measuring the pressure of the injected grout. The results of the test can be used to evaluate the improvement effect achieved by air grouting method.
4. Data Processing and Relationship Between Seismic/Geophysical Methods and Ground Improvement Effects
In addition to testing the physical properties of the ground materials, tests can also be used to study the relationship between seismic/geophysical methods and ground improvement effects. By means of seismic/geophysical methods, one can obtain the subsurface information of soil and rock, and analyze the geometry, discontinuity, structure and strength of soil and rock mass [5]. The obtained information can be used to assess the likelihood of failure and provide predictions regarding ground improvement effects.
In data processing, finite element simulations are widely used to alleviate uncertainties and to reduce the cost of field experiments. Comparative studies and simulations can also be used to study the effects of various ground improvement techniques, in order to develop new techniques or optimize existing techniques [6].
5. Challenges in Ground Improvement Testing
Ground improvement testing is a rigorous task, that is complicated due to complex soil and rock properties and the variety of ground improvement techniques
