Journal of Engineering Geology

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Introduction
Grouting is one of the most widely used methods of soils improvement in which pressurized grout penetrates in the voids, of the soil. In the grouting method, in addition to reducing the permeability, shear strength and stiffness of the soil increase significantly. However, application of this method in projects such as dam construction and soil improvement requires the use of a large volume of grouting materials in order to satisfy the design criteria. In more recent years, due to the economic and environmental issues, in the case of cement-based grouts, replacing the whole or a portion of Portland cement with other materials has been experimentally investigated. A special type of kaolinite clay called metakaolin has recently been used in concrete, which has yielded interesting results. However, few studies has been conducted on the use of metakaolin in cement-based grouts. Such that, its effect on the mechanical behavior of the grouted soil are not well understood. Accordingly, in the present study, the mechanical behavior of a type of sandy soil grouted with different combinations of Portland cement and metakaolin was experimentally investigated in laboratory scale.
Material and methods
Different materials used in the present study including sand, cement, metakaolin, bentonite and water were selected based on the standard criteria and with the aim of better interpreting the test  result and their differences due to considered influencing factors. Sand was obtained from Malayer Shushab river bed. According to the Unified Soil Classification System (USCS), it is classified as SP. Ordinary Portland cement was used in this study regarding to its widespread application in the practical works. The metakolin is classified as class N pozzolan according to the ASTM C618. Another constituent material of grout is bentonite which is produced by Iran Barit factory as sodium-calcium bentonite. Its liquid limit and plastic index are 296 and 262 percent, respectively. The water used to prepare the grouts was provided from Hamedan drinking water, which according to ASTM C94 has the required quality for grouting operations in laboratory.
The device for grouting specimens was developed at the Soil Mechanics Laboratory of Bu-Ali Sina University during the present study. It equipped with grouting pressure control system and tool for keeping grout in homogeneous conditions during the grouting operation into specimen. The samples were prepared with 0, 5, 10, 15, 20 and 25 percent substitution of cement with metakaolin. Curing time of grouted samples was considered as14 and 28 days.
In order to investigate the factor affecting stress-strain behavior of the grouted sand, the samples were sheared using advanced triaxial apparatus. After passing considered curing time, the samples were sheared considering three levels of confining pressures of 50, 100 and 200 kPa and by applying axial strain rate of 1 mm per minute. For each test, the maximum deviator stress and its corresponding axial strain were recorded. In addition, for studying post peak behavior of grouted soil, for every one of tests, average ratio of deviator stress to the axial strain as softening modulus, was calculated from the deviator stress-axial strain curves.  The moisture content of the samples was also measured according to ASTM D2216 at the end of tests. In the following, the role of different factors influencing stress-stain behavior of grouted sand including; confining pressure, ratio of water to the mix of cement and metakaolin, percentage of metakaolin, curing time and moisture content were investigated.
Results and discussion
Figure 1 shows the effect of metakaolin as alternative of a portion of cement on maximum confined compressive strength and its corresponding axial strain.
For the samples confined by pressure of 50 kPa the maximum confined compressive strength is almost constant by replacing cement with metakaolin up to 10%. However, the amount of axial strain corresponding to the maximum compressive strength of the specimens increases by 6% (Figure 2). For 25% replacement, the maximum confined compressive strength of the samples decreases by 17% compared to the initial state (pure cement). In contrast, the axial strain value related to peak state of most samples has been increased by 4% in comparison to the initial state.
 
In the case of confining pressure of 100 kPa, by replacing up to 10%, the mean confined compressive strength of the specimens was almost constant. However, the amount of axial strain corresponding to the peak state of the specimens has been increased by a maximum value of 18%. For 20% replacement percentage the compressive strength of the specimens has been decreased by about 15% compared to the initial state. However, in the range of 20 to 25 percent, the reduction process has slowed down, which can be due to various factors such as the effects of sample densification during further increase of metakaolin. According to Figure 1, it can be seen that in the range of 20 to 25% substitution, the amount of strain at failure state increased by an average of 40%, which indicates that the sample is more deformable.
In the case of confining stress of 200 kPa, by replacing 10% of the cement with metakaolin, maximum confined compressive strength and its corresponding axial strain, has been increased by approximately 5 and 14%, respectively. With increasing cement substitution up to 25%, the resistance of the specimens decreased by 8% compared to the result of sample grouted with pure cement. Although, axial strain at peak state has been increased by 28%. From the Figure 1, it is obvious that increasing in confining pressure yeilds a considerable increase in the maximum compressive strength of grouted soil. Besides, post peak behavior of grouted soil is also affected significantly by confining pressure. Such that an increase in confining pressure leads to decrease in softening modulus. On the other word, grouted soil displays a more deformable behavior. It should be noted that these aspects of grouted sand cannot be described by unconfined test. However compressive strength of the grouted soils in the majority of case, has been evaluated based on the unconfined test results. 
Conclusion
The aim of this study was to investigate, in laboratory scale, the mechanical behavior of sand grouted with cement-based grout and considering different percentage of metakaolin as an alternative for a portion of cement. The soil samples were grouted using a specific device developed during present study. After passing curing time the samples were sheared using triaxial apparatus by considering three levels of confining pressures. The main findings of this experimental research are as follows:
- Replacing 10% of cement with metakaolin, increases deformability of grouted soil, without reducing compressive strength. Deformability of grouted soil increases with adding more percentage of metakaolin however, in this case compressive strength decreases.
- By increasing confining pressures, more values of metakaolin can be used instead of cement in the grout.
- Increasing confining pressure, increases compressive strength, increases deformability and deceases softening modulus at post peak behavior.
- Shear strength parameters of grouted sand is affected by adding metakaolin into the grout. Increasing the percentage of metakaolin results in small changes in the internal friction angle of the grouted sand, however, the amount of cohesion decreases.

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Introduction
The ring footings are very important and sensitive due to widespread use in various industries such as oil and gas; so finding some ways for improving the behavior of these types of footings can be very valuable. One of these ways, which is very affordable and also can be help in environmental protection, is the use of granulated rubber that made from disposable materials like scrape tires, as the soil reinforcement. In the present study, the behavior of ring footings with outer constant diameter of 300 mm and variable inner diameters (90, 120 and 150 mm with inner to outer diameter ratio of 0.3, 0.4 and 0.5) placed on unreinforced sand bed and also granulated rubber reinforced bed, has been investigated by field test. The effects of important parameters like inner to outer diameter ratio of ring footing and thickness of rubber-soil mixture on the behavior of ring footing in terms of bearing capacity, settlement and inside vertical stresses of footing bed have been studied and the optimum values mentioned parameters have been determined.
Material and methods
In all tests, a sandy soil was used to fill the test trench which was excavated in the natural bed of the earth with a length and width of 2000 mm and a height of 990 mm. It should be noted that the type of this soil is well-graded sand (SW) according to the Unified Classification System (ASTM D 2487-11). This sand had medium grain size, D₅₀, of 2.35 mm, moisture content of 5.4% and friction angle of 41.7^∘. The granulated rubber particles with dimensions between 2-20 mm, a mean particle size, D₅₀, of 14 mm and a specific gravity, Gs, of 1.15, have been used in all tests for using in rubber-soil mixture layer.
The loading system consists of several parts such as loading frame for providing reaction force, hydraulic jack, load cell, load transfer system (including loading shaft which was located below Load cell and footing cap which was located under the loading shaft) and rigid steel loading plates with different inner to outer diameter ratios (d/D=0.3, 0.4 and 0.5 and constant outer diameter of 300 mm). Some devices like load cell, LVDT, pressure cell, data logger and unit control were applied to collect the data and control the system. Both soil and rubber-soil mixture layers were compacted by vibrating plate compactor to gain their maximum densities. After preparing the tests, the static load was applied on the system at a rate of 1 kPa per second until 1000 kPa or until backfill failure.
Results and discussion
The results of tests on both unreinforced and rubber reinforced beds indicated that the ring footing with inner to outer diameter ratio (d/D) of 0.4 had the maximum bearing capacity in all settlement levels. This behavior can be related to the arching phenomenon within the internal spaces of ring footing with optimum inner to outer diameter ratio. In fact, when the ring footing with optimum inner to outer diameter ratio is subjected to a certain level of loading, the soil inside the ring seems to be compacted due to interface effect of the two sides of the ring. However, by increasing the inner to outer diameter ratio more than its optimum value, the ring behaves like two independent strip footings without any interface effect and therefore the bearing capacity decreases.
The results of tests showed that the vertical inside stresses in different depths of footing bed (both unreinforced and rubber reinforced beds) decrease with increasing d/D ratio. This stress reduction process can be due to the transfer of stress concentration from the points close to the center of the ring to the outer point because of turning from the ring mode with interface effect to the two independent strip footings that mentioned earlier.
The results of rubber reinforced cases illustrated that, regardless of the footing settlement level and also irrespective of d/D ratio, the bearing capacity of ring footing increases with increasing the thickness of rubber-soil mixture layer (h_rs) up to the value equals 0.5 times the outer diameter of ring footing and further increase in this thickness more than mentioned optimum value (h_rs/D=0.5) can decrease the bearing capacity. Even in some cases of reinforced base (h_rs/D=1), the bearing capacity can be reduced to the value less than that of unreinforced cases. It can be due to high compressibility of rubber reinforced layers with higher thicknesses than optimum value.
It should be mentioned that the rubber reinforced layer can reduce the vertical inside stresses compared to unreinforced cases. It can be due to this fact that the rubber reinforced layer acts as a wide slab. Such that it can spread the applied loading over a wider area. Also rubber reinforced layer has high capacity of absorbing energy and therefore can decrease the vertical inside stresses.
Conclusion
In the present study the behavior of ring footing placed on rubber reinforced bed have been investigated by field test. The effect of different parameters such as inner to outer diameter ratio of ring footing and the thickness of rubber-soil mixture layer on the bearing capacity, settlement and vertical inside stresses of the footing bed were studied. The result indicates that:
- In both unreinforced and rubber reinforced bed, the ring footing with inner to outer diameter ratio (d/D) of 0.4 had the maximum bearing capacity, regardless of settlement level.
-The vertical inside stresses in different depths of footing bed decrease with increasing d/D ratio.
-The bearing capacity of ring footing increases with increasing the thickness of rubber-soil mixture layer (h_rs) up to the optimum value equals 0.5 times the outer diameter of ring footing.
-The vertical stresses can be reduced by using rubber reinforced layer../files/site1/files/151/5.pdf
 

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In current paper the effects of surface unreinforced / reinforced sand layers coupled with and without single and group sand columns on the bearing capacity – settlement behavior of soft clays has been investigated. In this regard behavior of soft clay, clay + unreinforced / reinforced sand layer, clay + single / group sand piles and clay + unreinforced / reinforced sand layer + single / group piles samples has been assessed. Geogrid was adopted as the reinforcement, a circular plate 5cm in diameter as the loading surface and C.B.R. apparatus as the loading system. Results show that employing unreinforced / reinforced sand layers at a settlement ratio of 5% improves bearing capacity by 4 t0 7 times the soft clay. Coupling the surface unreinforced / reinforced sand layers with single / group sand piles further increases the bearing capacity by 7 to 9 times that of soft clay.

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