Chapter 9. Concluding Remarks
9.2 Conclusions
The experimental study indicated that the performance of foundation systems, in unreinforced and reinforced configurations, is largely dependent on subgrade strength,
layer thicknesses, and footing settlement. In this section, conclusions drawn from this study are presented.
Homogeneous Foundations (Clay and Sand)
The model tests performed on the homogeneous clay and sand beds (series A) depicted non-linear variations in pressure-settlement responses. Higher bearing pressures were noticed for stiffer clay beds at all settlement levels. The dense sand bed (Dr = 80%) showed stiffer response (higher bearing pressure) as compared to the clay beds up to cu = 30 kPa, while it was softer w.r.t. 60 kPa (cu) clay bed.
Surface deformation profiles with footing settlement for homogeneous clay beds depicted pronounced heaving at foundation surface; while, initial settlement was seen for the dense sand bed followed by a surface heaving (at x = D). Such behavior was due to undrained behaviour of saturated clay and dilation of dense sand. The deformations were gradually reduced at x = 2D and 3D.
Unreinforced Layered Foundations (Sand over Clay)
In case of layered foundations (series B) with unreinforced sand overlying the clay subgrades of different strength, the softer subgrades (cu ≤ 30 kPa) responded with improved bearing pressures at any level of footing settlements.
For a stiff clay subgrade (cu = 60 kPa), reduced bearing pressures were noticed as compared to corresponding homogeneous clay beds.
Bearing pressures were improved with increase in H and s/D, for softer subgrades (cu ≤ 30 kPa). However, for H > 1.67D, the sand column was failed in shear and squeezed out from footing bottom leading to reduction in overall foundation performance.
The improvement factor, Ifs, decreased with increase in subgrade strengths (cu).
Maximum improvement in bearing pressure of about 5.34-fold was seen for very soft subgrade (cu = 7 kPa), whereas, it was about 2.3 for cu = 30 kPa and about 0.90 for cu = 60 kPa.
Comparison of the experimentally obtained foundation responses with that of the theoretical approach are in considerably good aggrement.
Geogrid Reinforced Foundations
Considerable improvement in bearing pressure was observed with planar geogrid reinforcement placed at the sand-clay interface (series C), compared to similar unreinforced foundation configurations (series B).
Higher bearing pressures were noticed with increase in footing settlement (s/D), sand-layer thickness (H), and subgrade strength (cu). The improvement is attributed to the membrane resistance developed at the interface-geogrid through soil-geosynthetic interaction.
Optimum sand layer thickness (H) was 1.67D for softer subgrades (cu ≤ 30 kPa), while the same was 1.15D in case of stiff clay subgrade of 60 kPa.
The improvement factors, Ifsg, depicted a decreasing trend with increase in cu. A maximum of 5.6-fold improvement (Ifsg) was observed for very soft clay subgrade while, the improvement factor is about 1.3 for stiff clay subgrade (cu = 60 kPa).
However, the contribution of planar geogrid (Ifg) was in the range of 1.0-1.75 depending on cu value.
Theoretical analysis depicted excellent aggrement of the foundation responses, in terms of bearing pressures, with that of the experimental results obtained in the present study.
Geocell Reinforced Foundations
Beam like behavior and confinement action of geocell-sand mattress are inferred to be contributed for significant improvement in bearing pressure for all foundation systems with geocell reinforcement (series D). Higher bearing pressures were noticed with increase in footing settlement (s/D), geocell-height (h), and subgrade strength (cu).
The bearing pressure improvement factors (Ifsgc) were decreased with increase in subgrade strength (cu) and geocell-height (h). For example: a maximum of about 11.6-fold improvement was observed for cu = 7 kPa, while, it is 2.3 fold for 60 kPa subgrade.
The geocells contribution (Ifgc), in improved bearing pressures, is higher for stiffer subgrades which is attributed to the semi-rigid slab like structure that offered greater subgrade support on stiffer subgrades.
Optimum height of geocell-mattress for softer subgrades (cu ≤ 15 kPa) is 1.57D.
In the case of stiffer subgrades (cu > 15 kPa) the optimum height is 1.05D. Beyond these optimum heights buckling of geocell-walls and sand squeezing influenced the performance negatively.
Surface heaving was significantly reduced around the footing (at x = D) and it increased at x = 2D and 3D for geocell-reinforced foundations. It is due to the deep beam action of geocell-sand mattress which undergone sagging below the loading and hogging at away from footing.
Bearing pressures calculated based on the theoretical approaches indicated a very good aggrement with that obtained through physical model tests.
Geocell-Geogrid Reinforced Foundations
Geogrid placed at the base of geocells mattress enhanced the foundation performance, maximum up to 30%, compared to geocell-alone system which was inferred due to the membrane resistance of base geogrid.
The optimum height of geocells with base geogrid is 1.05D, which can attain considerable beneficial effect could be obtained from the base geogrid. It is further inferred that the additional beneficial effect is reduced with increase in subgrade strength.
Design Implications of the Study
The comparative performance analysis presented the superiority of reinforcement types in any layered configuration; i.e. higher bearing pressure can be obtained with higher reinforcement combinations (geocell- geogrid > geocell > geogrid > unreinforced).
The comparative performance analysis and the regression models developed, optimized the use of reinforcements in foundation design. For example, a base geogrid with shorter geocell (h = 1.05D) would be more beneficial compared to thicker geocell (h ≥ 1.57D) alone.