In addition, different bearing pressure ratios are introduced to compare the behavior of foundations and reinforcement contributions. Based on the selected test data, regression models were developed to evaluate the bearing pressure at a given settlement level for a foundation system with any clay base and reinforcement configuration.
Introduction
- Introduction
- Soil Reinforcement
- Importance of Subgrades
- Broad Objective of the Study
- Organization of the Thesis
Most applications of soil reinforcement in improving load-bearing capacity are explored through foundation systems of various configurations. The chapter concludes with a critical review of the literature reviewed and the detailed scope of the current research.
Literature Review
Introduction
Unreinforced Foundations
- Foundations on Homogeneous Soil
- Foundations on Layered Soil
Different 'bearing capacity factors', such as Nc, Nq and Nγ, have been proposed as a function of the friction angle of soil. However, most of the modifications considered the carrying capacity theory and factors proposed by Terzaghi.
Reinforced Foundations
- Studies with Planar Reinforcement
- Studies with Geocell Reinforcement
The study examined the effect of geogrid embedment depth on foundation behavior. The improvement in load capacity was in the range of 2 to 2.5 times that of the unreinforced bed.
Critical Appraisal of Literature Review
Proposed bearing capacity calculation mechanisms (after Zhang et al. 2010) Tafreshi and Dawson (2010) conducted a comparative study on strip footings supported by geocell and geotextile reinforced sand. The optimum placement depth (u) of the geocell mattress was found to be 0.1-0.3D under the foot.
Objective and Scope of the Present Study
A series of model tests will be carried out on unreinforced layer foundations having sand layers of different thicknesses on clay substrates of different hardness. Foundations with different reinforced configurations such as geogrid, geocell and geocell-geogrid, covering clay substrates of different hardness will be carried out in the next phase.
Summary
To design the study, a regression analysis will be carried out to correlate the behavior of the foundations in terms of bearing pressures with respect to various influencing parameters. In the regression models, the bearing pressures of different foundation systems will be expressed as a function of subgrade strength, layer thickness, and foundation settlement level.
Materials and Methodology
- Introduction
- Materials Used
- Clay
- Sand
- Geogrid and Geocell
- Details of Testing Program
- Test Description
- Test Set-up and Instrumentation
- Test Bed Preparation
- Test Procedure
- Summary
Wet sieving (ASTM D6913-04) and hydrometer analysis (ASTM D4221-05) were performed to determine particle size distribution of the clay, which is represented in Fig. Sieve analysis (ASTM D6913- 04) was performed for grain size distribution of the sand shown in Fig.
Unreinforced Foundations
- Introduction
- Homogeneous Foundations (Clay and Sand)
- Unreinforced Layered Foundations (Sand over Clay)
- Summary
It can be seen that the bearing pressure of the sand layer increased to about 175 kPa (at s/D = 18%) and then became almost constant as the foundation settled. The pressure and settlement responses of multi-layer foundations with different sand layer thicknesses are presented in the figure. It is defined as the ratio between the bearing pressure of multilayer foundations (qs) and the corresponding homogeneous clay layer (qc) at similar levels. foundation settlements (s/D) as shown in Eq
The depressions arose due to such penetrations of the sand columns on the subsoil. It was observed that the depth of the depressions was reduced with the increase in subgrade strength and sand layer thickness. However, significant heaving of the sand surface was observed for the stiff clay substrates when the thickness of the sand layer was more than the diameter of the footing, i.e.
Geogrid Reinforced Foundations
Introduction
Test Results
For clay subgrades with cu ≤ 30 kPa, geogrid-reinforced foundations showed significantly higher pressure-settlement responses compared to corresponding homogeneous clay beds. It can be noted that the improvement in bearing pressure is not consistent with respect to layer thickness (H) at all settlement levels. In the case of stiff clay subgrade (cu = 60 kPa), the reinforced beds showed higher performance up to H ≤ 1.15D (compared to the homogeneous bed), for s/D in the range 2-18% as in Fig .
It is the ratio between the bearing pressure of the geonet-reinforced foundation (qsg) and the bearing pressure of the corresponding homogeneous clay base (qc), at the corresponding level of settlement (s/D). It can be noted that the variations of the deformations were not very consistent with regard to different influencing parameters, such as settlement, layer thickness and the subsoil strengths. The detailed discussions of test results, by analyzing the influence of different parameters on the behavior of the foundation, are presented in the following sections.
Discussions on Test Results
- Effect of Footing Settlement (s/D)
- Effect of Layer Thickness (H)
- Effect of Subgrade Strength (c u )
A typical comparison of the pressure-settlement responses of unreinforced and geogrid-reinforced footings is shown in Fig. Significant improvement in bearing pressures can be seen with geogrid reinforced systems compared to the corresponding unreinforced systems (series A and B). The pressure-settlement responses of unreinforced and geogrid-reinforced layered foundations, at H = 0.63D with different subgrades (cu), are shown in Fig.
For all the clay subgrades, geogrid-reinforced foundations showed higher bearing pressure compared to the corresponding unreinforced layered systems. The pressure-settlement responses of homogeneous, unreinforced and geogrid-reinforced layered foundations, for cu = 7 and 60 kPa (at H = 0.63D), are compared in Figs. Significantly higher improvement in bearing pressures can be observed for the geogrid-reinforced foundations compared to corresponding homogeneous and unreinforced layered foundations.
Post Experimental Observations
Summary
Geocell Reinforced Foundations
Introduction
Test Results
However, the increase in bearing pressures is not consistent with the change in geocell heights for h ≥ 1.57D in relatively stiffer subgrades (cu > 7 kPa). A typical surface deformation profile for geocell-reinforced foundations, with a 0.63D thick geocell-reinforced sand layer (H) over a clay base of cu = 7 kPa, is shown in Fig. Significant surface settlement around the center of the base (x = D) and away from the center of the base (x = 2D and 3D) can be observed from the figure.
Variations in bearing pressures are further analyzed in terms of the bearing pressure improvement factor, Ifsgc.
Discussions on Test Results
- Effect of Footing Settlement (s/D)
- Effect of Geocell-Height (h)
- Effect of Subgrade Strength (c u )
It can be noted that compared to the unreinforced foundations, the geocell-reinforced system showed a higher settlement at x = D and more heave at x. This in turn increases the stress concentration on the sand cushion provided between the footing and the geocell mattress. When the sand cushion is pushed down from the footing below, the foot load comes directly onto the geocell mattress.
However, it can be seen that the rate of improvement factor, Ifs, decreased for substrates with sand layer thickness H ≥ 1.67D. The reduction in the improvement rate was the effect of local shear failure and compression of the sand column from the bottom of the footing. It is attributed to the semi-rigid slab-like behavior of the geocellular-sand mattress, which has a hanging deformation around the center of the footing and pinched deformations away from the center of the footing, as previously discussed. Foundation rotation is the result of non-uniform loading-geometry in geocellular walls.
Post Experimental Observations
Summary
Geocell-Geogrid Reinforced Foundations
- Introduction
- Test Results
- Discussions on Test Results
- Effect of Footing Settlement (s/D)
- Effect of Geocell-Height (h)
- Effect of Subgrade Strength (c u )
- Post Experimental Observations
- Summary
It is seen that the bearing pressure of the geocell-geogrid foundations is increased with footing (s/D). Pressure-settlement responses of geocell-geogrid-reinforced foundations (Fig. 7.2 to Fig. 7.5) indicated a significant influence of footing on the foundation performances. Reduced surface deformations can be noted for the geocell-geogrid-reinforced systems, compared to geocell-reinforced foundations.
The effect of geocell height (h) on the pressure and settlement responses of geocell and geogrid reinforced systems can be seen in the figure. pressures for harder foundations (cu) can be observed in foundations reinforced with geocells and geogrids.
Design Implications of the Study
Introduction
Comparative Discussion of Different Foundations
However, in the geocell-geogrid reinforced systems, the negative effects (buckling in the geocell walls and squeezing of the sand cushion) were more pronounced compared to geocell-only foundations. 8.4, significant improvements in bearing pressures are observed for reinforced foundations, compared to unreinforced systems. Higher contributions for geocell reinforcement can be observed compared to geogrid reinforcement, for all substrates and settlement levels.
It can be attributed to the complex interaction between the reinforced soil and the subgrades. The reduced benefits of the base geotextile, in the case of the stiff clay subgrade of cu = 60 kPa, can be attributed to the definition of the quantification of the improvements. Therefore, Ifg is magnified compared to Ifbg, where it (Ifbg) was compared to the geocell reinforced system.
Regression Models for Bearing Pressures
- Regression Analysis
- Regression Models
The SSE and MSE are respectively the sum of squares and the mean square, due to 'error', defined as Eq. The SSR and MSR are the sum of squares and the mean square due to 'regression', as defined in Eq. The SST is defined and related to SSE and SSR as Eq. 8.7) Where, is the predicted value and is the mean of the dependent variables. Bera et al. 2005) which can be evaluated as Eq. 8.9) The F-test is performed to test the overall significance of the regression model.
In the study, the level of significance () is considered to be 0.05 and the null hypothesis for the F-test was that “the independent variables are not related to the dependent variables”. The bearing pressures of different foundation systems such as homogeneous and layered unreinforced and reinforced systems (i.e., qc, qs, qsg, qsgc, and qsgcg) are considered as dependent variables for regression analysis. The contribution of unreinforced sand is directly calculated as the bearing pressures of the homogeneous sand layer as qos.
Limitations of the Study
Illustration of Design Implications of the Study
Although it is the same for geocell and geocell-geogrid reinforced foundations, it is considered 1.15D (with u = 0.1D). For the assumed level of settlement, 5% of s/D, the bearing pressure for various foundation systems, with cu = 10 kPa, is to be determined. Therefore, it can be noted that the target capacity requirement can only be achieved with geocell-geogrid configuration.
Based on the pressure settlement responses, the appropriate foundation configuration can be selected according to the design requirements. It can be observed that the design pressure of 100 kPa (say) can be achieved at a 3.5% (s/D) slump for the geocell-geogrid foundation configuration with H = 1.15D. Similarly, for geogrid and unreinforced foundations, bearing pressures can be reached no faster than 8% and 9% of s/D, respectively, with H = 1.67D.
Summary
Concluding Remarks
- Summary of the Thesis
- Conclusions
- Scope for the Future Research
- D
- D)
Bearing capacity of square and circular footings on a finite layer of granular soil under a rigid base. The effects of foundation width on model tests for bearing capacity of sand with geogrid reinforcement. Effect of immersion on settlement and bearing capacity of surface strip footing on geotextile-reinforced sand bed.
An experimental investigation into the contribution of geotextiles to the load-bearing capacity of foundations on weak clays. Comparison of the load-bearing capacity of a strip foundation on sand with geocell and with flat forms of geotextile reinforcement.
