Soils And Foundations (8th Edition) Free Download
Anastacia Iacono
Geosynthetics-reinforced soil (GRS) foundation can improve the bearing capacity of the foundation and reduce the settlement of the footing, which has been widely applied to the treatment and improvement of soft soil foundation. In recent years, scholars have carried out a large number of studies to reveal the influence of different factors on the load and settlement behaviors of GRS foundations. In this study, the reinforcing mechanisms of reinforced materials are first summarized, and then the literature review of planar geosynthetic-reinforced foundations from the aspects of experimental studies, numerical simulations and theoretical analyses are presented. Finally, the current research trend of reinforced foundation and the prospects in the future study are discussed. Although the researches on the performance of GRS foundations are conducted extensively, there is no unified understanding on the failure mode and reinforcing mechanism of reinforced foundation. It is necessary to propose the accurate and simple calculation method to evaluate the ultimate bearing capacity of reinforced foundations based on the reinforcing mechanism and failure mode. In addition, the dynamic response of GRS foundations under cyclic loading and earthquake needs to be studied intensively. The paper summarizes the past and present developments of the GRS foundations and provide the views for the further researches.
In the engineering projects of soft soil area, shallow footings built on the soft soil foundation are usually subjected to the large settlement and low bearing capacity. To ensure the safety of the infrastructures, it is necessary to adopt ground improvement techniques to deal with above problems. With the development of polymer materials, geosynthetics reinforced soil (GRS) foundation treatment technology has emerged, which has been widely used in highway, railway, building and other engineering fields. The GRS foundation can effectively improve the bearing capacity and reduce the settlement of the footing [1]. Generally, geogrid and geotextile are used as the reinforcements in the reinforced soil foundations. The geogrid available in the market mainly includes uniaxial, biaxial and triaxial geogrids, while the geotextile is divided into nonwoven and woven geotextiles.
This paper first summarizes the reinforcing mechanism of planar reinforcement in reinforced soil foundation based on the existing researches, and then presents the literature review of planar geosynthetic-reinforced soil foundations from experimental study, numerical modeling and theoretical analysis. Finally, the current research trend and the future prospect of the reinforced foundation are discussed. The paper presents the reader with the critical review of the past, present and future of planar geosynthetic reinforced foundations, which would be a very useful reference for further studies.
At present, the reinforcing mechanisms of horizontal reinforcement mainly include the above four effects. They are summarized and proposed based on several assumptions and the results of the macro-scale plate load tests on the GRS foundations, while the interaction mechanism between reinforcement and soil based on the meso-scale tests needs to be studied further.
Up to date, the researches concerning the performance of planar geosynthetic-reinforced foundations and the various influencing factors are mostly based on the laboratory reduced model test. Some scholars have also carried out the field test and large-scale model test to investigate the load-settlement behaviors of reinforced foundations under more actual stress conditions.
Due to the low cost and little time consuming, laboratory reduced model tests have become the main approach to study geosythetic-reinforced soil foundations. The equipment for conducting plate load tests mainly include test box, loading device, rigid footing and sensors (load cell and displacement meter). Figure 2 shows a typical test equipment for performing the model tests on reinforced foundation.
Based on the small-scale model tests, the influence of different factors on the bearing capacity and settlement of reinforced foundation was studied thoroughly. These influencing factors mainly include the layout parameters and properties of reinforcement, the shape and size of footing, the form of load and the properties of foundation soil. Currently, scholars mainly analyzed the effect of different factors on the bearing characteristics of the foundation by comparing the load-settlement curves between the unreinforced and reinforced foundations. In general, the bearing capacity ratio (BCR) and the percentage reduction in settlement (PRS) were used as the dimensionless parameters to evaluate the reinforcement effectiveness of the geogrid or geotextile. The bearing capacity ratio (BCR) is defined as follows:
where qr is the bearing capacity of reinforced foundation as the footing reaches a certain settlement; qu is the bearing capacity of unreinforced foundation when the footing reaches the same settlement.
In addition to the layout of reinforcement layers, the properties of reinforcement layer have also an impact on the performance of reinforced foundations. Some researchers have carried out model tests to study the influence of properties of reinforcements, such as stiffness, strength and aperture size on the bearing capacity of foundation [52, 54]. The bearing capacity of reinforced soil foundation increases with the increase in strength and stiffness of reinforcements. However, when the stiffness is larger than a certain value, it would not result in significant increase of bearing capacity. Based on experimental and numerical studies, Yetimoglu et al. [47] found the critical stiffness of reinforcement is about 1100 kN/m while Latha and Swomwanshi [42] thought the critical value is proximately 333.3 kN/m. The difference between two value is mainly due to the type of reinforcement used in the sand. The aperture size of reinforcement has remarkable effect on the performance of reinforced foundations. The ratio of the apertures size to the medium soil grain size is an important factor in the interaction between soils and geogrid [55]. To achieve the best reinforcement effect, Tavakoli Mehrjardi and Khazaei [54] recommended the optimum aperture size of geogrids should be about 4 times of medium grain size of backfill materials. Except the properties of reinforcements, the type and shape of reinforcement can significantly influence the load-settlement response of reinforced soil foundations. The geogrid performs better than geotextile in improving bearing capacity of foundations [37, 52]. The difference is attributed to the better interlocking between soil particles and geogrid. In addition, the biaxial geogrid is better than uniaxial geogrid to enhance the performance of soil foundation [42, 53].
The load-settlement characteristics of reinforced foundation are not only related to the shape, size and layout of the footing, but also affected by the load form applied to the footing. In the laboratory model tests, the applied load is mainly uniform static load. Besides the uniform static load, some researchers have studied the performance of the reinforced foundation under different forms of load, including central load, eccentric load and cyclic load [54, 58,59,60,61]. Figure 6 presents the different forms of load applied to the footing in the plate loading tests.
Compared to the BCR, the PRS due to the inclusion of reinforcement is not widely considered in the experimental studies. A few of researches have demonstrated that the use of geosynthetic can effectively reduce the settlement of footing. Mandal and Sah [43] conducted the plate load tests on geogrid-reinforced clay bed and found that the maximum PRS by using geogrid is about 45%. Alawaji [48] carried out model tests on geogrid-reinforced sand foundation, and revealed that the maximum PRS can reach up to 95%. Moghaddas Tafreshi and Dawson [51] found that the PRS is approximately 64% by using the geotextile in sand foundation.
Although the deformation and bearing capacity of reinforced foundation under different influencing factors have been widely investigated, most of the model tests aim to study the performance of reinforced foundations under different factors by analyzing load-settlement responses. The researches with respect to reinforcing mechanism and failure mode are very limited. Several researchers have conducted the plate load tests to explore the evolution process of the displacement field and the failure mode of the soil in the reinforced foundation based on the digital image correlation (DIC) technology [32,33,34]. However, no further studies are performed to investigate the deformation and failure process of the reinforcement and the interaction between soil and reinforcement.
The field test is the most direct and reliable way to study the load behavior of reinforced soil foundation. However, due to the complex geological conditions and the high cost to conduct field tests, there are few studies concerning the field loading tests related to the reinforced foundation. Luo et al. [65] reported four different types of geosynthetic reinforced sand cushions in a 324 m long railway section to improve the performance of soft soil foundation, and monitored the embankment settlement, geogrid tension and earth pressure at embankment base under four reinforced sand cushions. They found that geosynthetics reinforced sand cushion can effectively reduce the settlement of soft soil foundation under the embankment load, and spread the embankment load to a wider area. Demir et al. [6] studied the load behavior of geogrid-reinforced sand foundations on the natural clay layer under different sizes of circular footing by means of 16 groups of field tests. The test results show that the soil modulus and bearing capacity of the foundation after reinforcement are significantly improved. Venkateswarlu et al. [20] carried out 38 groups of field resonance tests to study the dynamic response of machine foundation located on GRS foundation. Four test schemes were considered, i.e., unreinforced foundation, single layer geogrid reinforced foundation, double layer geogrid reinforced foundation and geocell reinforced foundation. The results show that the reinforcement especially for geocell can effectively reduce the vibration of the machine foundation.
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