**10. Case studies and lessons learnt**

Liu et al. [66] studied failure of a four-tiered geogrid reinforced slope of a road embankment of height varying from 10 m to 40 m over a length of 430 m to assess mechanism and causes contributing to these failures (**Figures 24** and **25**). A flat natural slope of 28° was converted to a steep geogrid reinforced slope of 0.5 H:1 V (63°) slope for constructing an approach road.

The first slope failure occurred during the construction phase itself soon after the rainy season as rainwater seeped into permeable laterite gravel layer underlain by an impermeable clay layer. The interface of laterite gravel and clay created a detrimental bedding plane whose shear strength was reduced due to infiltration of water. The slide got initiated along the interface when toe was excavated to construct the reinforced zone. The second failure occurred due to very strong earthquake. The overstress initiated near the vicinity of the clay layer extended into the retained natural slope to form a massive slide. The third failure occurred following

**37**

*Geoysynthetic Reinforced Embankment Slopes DOI: http://dx.doi.org/10.5772/intechopen.95106*

**Figure 24.**

**Figure 25.**

*Approach road (after [66]).*

abundant rainfall during a heavy rainstorm that infiltrated into the reinforced slope as no sub-drainage system was provided. The infiltration that was obstructed by the impermeable clay and fine contents in the backfills generated significant water

The interface between laterite gravel and clay is an embedded weak plane, which when saturated softened. In addition, because of its low permeability, it became a barrier to the infiltration. Site investigation failed to find the existence of this clay layer, because the reinforced slope was thought to be subsidiary to campus buildings, and no investigation efforts made specific to the reinforced structures. The succeeding design and construction did not appropriately correspond to this clay layer even when it was observed during construction. The results showed the impact of the clay layer on the slope stability to be very critical. The current practice of considering cohesion needs to be re-evaluated as this apparent cohesion may get reduced to even zero with increasing saturation. The lack of a drainage system was

Yang et al. [67] investigated 26 m high, four-tier geogrid reinforced slope (**Figure 26**), backfilled with low plasticity silty clay that contains more than 60% of fines (marginal backfill). Prior to the completion of construction, tension cracks were discovered along with slope settlement at the top of the slope. The tension cracks and slope settlement were caused by a series of heavy rainfall. The slope

pressure, inducing the slope to fail behind the reinforced zone.

another significant cause for failure.

*Reinforced slope with failure details (after [66]).*

*Geoysynthetic Reinforced Embankment Slopes DOI: http://dx.doi.org/10.5772/intechopen.95106*

**Figure 24.** *Approach road (after [66]).*

*Slope Engineering*

possible solutions with the same level of stability but not necessarily having the same economics. Software facilitates the designer to reach an optimal solution apart from locating critical failure surface by using search techniques and by repeatedly

SSAP release 5.0 (2020) software (https://www.ssap.eu) is a versatile free software and uses advanced limit equilibrium method and FE-LEM combination to get the critical Factor of safety. ReSlope is an interactive, design-oriented, program for geosynthetic-reinforced slopes. For a given problem including geosynthetic strength, reduction factors, and design safety factors, ReSlope produces the optimal layout (i.e., length and spacing) of reinforcement layers. ReSlope was specifically developed for geosynthetics. SVSLOPE, GeoStru, Oasys, ReActive, Secuslope are

Liu et al. [66] studied failure of a four-tiered geogrid reinforced slope of a road embankment of height varying from 10 m to 40 m over a length of 430 m to assess mechanism and causes contributing to these failures (**Figures 24** and **25**). A flat natural slope of 28° was converted to a steep geogrid reinforced slope of 0.5 H:1 V

The first slope failure occurred during the construction phase itself soon after the rainy season as rainwater seeped into permeable laterite gravel layer underlain by an impermeable clay layer. The interface of laterite gravel and clay created a detrimental bedding plane whose shear strength was reduced due to infiltration of water. The slide got initiated along the interface when toe was excavated to construct the reinforced zone. The second failure occurred due to very strong earthquake. The overstress initiated near the vicinity of the clay layer extended into the retained natural slope to form a massive slide. The third failure occurred following

other software available for reinforced slope stability analysis.

Pockoski and Duncan [65] compared several Limit Equilibrium based software available based on features of program, ease of use, range of applicability, accuracy, and efficiency. These programs were rated as well considering accuracy of results, computation time, learning curve, time to enter data and complete an analysis, ease of reinforced slope design, ease of unreinforced data entry, time required to make graphical output Report -Ready and quality of output. Different software included for comparison are: UTEXAS4 & TEXGRAF4, SLOPE/W, SLIDE, XSTABL, WINSTABL, RSS, SNAIL, GoldNail. UTEXAS4 is a precise analysis tool but does not have graphic user interface. TEXGRAF4 is second part of UTEXAS software package and displays information and results of the UTEXAS4 search and generates file for use in CAD software. SLOPE/W having a graphic user interface is user friendly and versatile. Reinforcement inclusion in to the analysis is also graphical. SLOPE/W has Monte Carlo based probabilistic stability analysis option where by soil, porewater pressure and seismic coefficient can be entered with standard deviation. SLIDE is Windows based slope stability program and can search for a critical circular, non- circular or composite slip surfaces. XSTABL is an interactive program which can search critical circular surface. WINSTABL is windows-based program whereby geosynthetic reinforcement, anisotropic soil, seismic loads, etc. can be considered in the analysis. RSS is an interactive program and is capable of exhaustive search performed on reinforced slope. Circular, bilinear, bottom third and top third failure surfaces are considered. SNAIL is window based free software and permits use of seven different types of soils and uses force equilibrium on two and three-part wedge analysis. GoldNail is a very powerful and design program that is

performing the stability calculations for different failure surfaces.

primarily meant for soil nail wall analysis.

**10. Case studies and lessons learnt**

(63°) slope for constructing an approach road.

**36**

**Figure 25.** *Reinforced slope with failure details (after [66]).*

abundant rainfall during a heavy rainstorm that infiltrated into the reinforced slope as no sub-drainage system was provided. The infiltration that was obstructed by the impermeable clay and fine contents in the backfills generated significant water pressure, inducing the slope to fail behind the reinforced zone.

The interface between laterite gravel and clay is an embedded weak plane, which when saturated softened. In addition, because of its low permeability, it became a barrier to the infiltration. Site investigation failed to find the existence of this clay layer, because the reinforced slope was thought to be subsidiary to campus buildings, and no investigation efforts made specific to the reinforced structures. The succeeding design and construction did not appropriately correspond to this clay layer even when it was observed during construction. The results showed the impact of the clay layer on the slope stability to be very critical. The current practice of considering cohesion needs to be re-evaluated as this apparent cohesion may get reduced to even zero with increasing saturation. The lack of a drainage system was another significant cause for failure.

Yang et al. [67] investigated 26 m high, four-tier geogrid reinforced slope (**Figure 26**), backfilled with low plasticity silty clay that contains more than 60% of fines (marginal backfill). Prior to the completion of construction, tension cracks were discovered along with slope settlement at the top of the slope. The tension cracks and slope settlement were caused by a series of heavy rainfall. The slope

#### **Figure 26.**

*Layout and Design of Multitier Geosynthetic Reinforced Slope (after [67]).*

settlement was repaired by placing additional backfill on the top of the slope to compensate for the settlement that had occurred.

During the next rainy season, the slope was subjected to a significant amount of rainfall of 187 mm in May, 350 mm in June, 243 mm in July, and 563 mm in August. During this period, tension cracks and slope settlement got regenerated and gradually developed as the rainfall continued. **Figure 27** displays subsequent development of the tension cracks and slope settlement with time.

Factors that caused the slope failure from the forensic investigation are: (i) The use of marginal soil (over 60% of fines) as the backfill without provision of drainage. (ii) The original design and site investigation overlooked the existence of the weathered and fractured rock layer, which has shear strength less than that of an intact rock. (iii) Tension cracks and slope settlement developed at the top surface of the slope allowed rainwater to pond on the top and to infiltrate into the reinforced zone. (iv)The drainage system may have malfunctioned as joints were poorly and loosely connected and likely got dislocated due to the excessive deformation.

**39**

slope.

**Acknowledgements**

*Geoysynthetic Reinforced Embankment Slopes DOI: http://dx.doi.org/10.5772/intechopen.95106*

stability.

**11. Conclusions**

Lessons learnt from these case histories are: (i) Detailed site investigation should be carried out to asses presence of weak layer, soil weathered rock, etc., (ii) Design cohesive backfill slope for drainage and with provision of draining geosynthetics, (iii) Install drainage systems appropriately and (iv) Design RE slope for global

However, it is pertinent to mention that marginal soil can be used with draining

Steep slope embankment is a necessity for development of rail, road and other infrastructure projects. Safety of embankment slopes is of utmost importance which requires proper site investigation, analysis and design. Limit Equilibrium, Limit Analysis, Slip Line & Finite Element Methods are design methods for RE Slopes. Limit equilibrium method is most commonly used for design including the effect of reinforcement. Jewell's design method [14] for geosynthetic reinforced steep slope soil with granular soil is most commonly used method. Song et al. [28] proposed new approach based on LE principle to evaluate stability of reinforced slope. Slopes with cohesive backfill have been constructed due to limited availability of granular material near project site. Proper design of RE slope using cohesive backfill considering the transmissivity of draining geogrid is important. The method suggested by Giroud et al. [32] for draining geogrid reinforced cohesive back fill slope with 0.5 m/day is practical method as it takes care of pore water pressure dissipation for most common soil parameters. Abd &Utili [33] developed a semi-analytical method for uniform slope with c-ф soil using Limit Analysis (LA). The method provides the amount of reinforcement needed as a function of cohesion, tensile strength, angle of shearing resistance and slope inclination. The reinforcement length optimization from face end leads to economy in reinforcement length of the order of 20–30% without affecting factor of safety [61]. There is an interaction between slope and reinforcement [62, 64]. Inclusion of reinforcement in embankment slope results in to shifting of critical slip circle deep inside slope involving larger slide mass thus increasing factor of safety. Reinforcement also provides stabilizing moment/force. Investigation of failed slopes indicate that detailed geotechnical investigation of site to assess presence of weak layer, provision of drainage by way of draining geosynthetics in case of cohesive backfill, installation of drainage systems to capture rain and subsurface water and global stability of reinforced earth slopes are very critical for stability and performance of reinforced

I express my sincere thanks to Prof M.R. Madhav my mentor and Co-author of this chapter whose continued encouragement and support made it possible to complete the chapter and bring it to the current format. I am also thankful to my wife Abha who always supported me in this endeavor. I thank Ms. B. Geeta Sahithi my office colleague for helping me for improved figures out of her personal time.

geogrids as detailed in Section 7.2 with adequate drainage capacity.

#### **Figure 27.**

*Tension crack and settlement of slope and its failure after rains of 2012(a) tension crack; (b) onset of settlement; (c) excessive settlement over 1 m (after [67]).*

Lessons learnt from these case histories are: (i) Detailed site investigation should be carried out to asses presence of weak layer, soil weathered rock, etc., (ii) Design cohesive backfill slope for drainage and with provision of draining geosynthetics, (iii) Install drainage systems appropriately and (iv) Design RE slope for global stability.

However, it is pertinent to mention that marginal soil can be used with draining geogrids as detailed in Section 7.2 with adequate drainage capacity.
