Infrastructure is the key word in today's progress all over the globe. Civil engineering plays an important role in all infrastructural developments. Any civil engineering activity should start with geotechnical structures. Present Union Government (budget 2014) has focussed on infrastructure. To name a few budget allocations, Deendayal Upadhyaya Gram JyotiYojana for 24×7 uninterrupted power supplies in rural areas with INR 5 Billion, INR 144 Billion for PradhanMantri Gram SadakYojna (PMGSY), raise in the corpus of Rural Infrastructure Development Fund (RIDF) by an additional INR 50 Billion, an amount of INR 1 Billion for the Agri-tech infrastructural funds, to strengthen and modernise the border infrastructure INR 22.5 Billion and for scientific warehousing infrastructure, the allocated budget is INR 50 Billion (The Economic Times, 2014).

Retaining walls are prominent geotechnical structures that play major role in infrastructural development. On hilly terrains, road and building infrastructure, deep excavations for tunnelling, tall structures with several cellar floors, protection to water front structures and many others require the construction of retaining walls (Varghese, 2005).

There have been many instances of failure of retaining walls due to improper design, poor construction or unexpected natural forces such as heavy rains, earthquake etc. Failure of a retaining wall may not necessarily mean total collapse, but rather signs of impending instability and likelihood of a collapse. Total collapses of retaining walls are relatively rare. But the failure of such walls may include overturns, slides or topples. Retaining wall failure may not always cause loss of life, but amounts to huge economic loss and infrastructural failure. Cost of repair can be extremely high. Fortunately, most retaining walls failures can be averted. Hence, it is important to understand the performance of retaining walls under different situations. Many previous works (Abdullahi, 2009, Binici et.al., 2010, Day, 1997, Koseki and Hayano, 2000 and Olso, 1993 and Prasad and Jagadeesh 2014) have reported the failures of retaining walls under different circumstances. The present paper is an attempt to show the practical situations of retaining wall failures and the proposed remedy. With this paper, an attempt is made to provide the database of failures to geotechnical structures which can help the young designers to take care of possible failures.

Ooty is a hill station, attracting many tourists because of beautiful landscape, high altitude and weather. It is the highest altitude in Southern India. The hilly terrain provides scenic beauty to the region, but provides a big challenge to the Civil engineering construction. Naturally special structures including retaining walls will be essential to hold earth on one side.

JSS public school is a residential school on the outskirts of the town of Ooty in a hilly terrain built over last one decade or so. About four years back, a retaining wall failure was reported. Figure 1 shows the panoramic view of retaining wall holding steep slope on one side and providing space for a play- ground in the valley portion. This wall made of R.C.C. about 2.5 m high is holding earth along a fairly steep slope for a height of over 25 m. The wall was built to provide play ground in the premises. Figure 2 shows the steep slope of large height and greenery around along with children enjoying the nature.

Figure 1. A panoramic view of play ground in a valley supported from retaining wall holding steep sloping backfill in JSS Public School, Ooty

Figure 2. Steep slope of over 40^o to horizontal and height of over 25 m

The following were the observations made (Prasad, 2010) at the site.

• Retaining wall is a Reinforced Concrete structure and the quality of construction is acceptable.
• The height of retaining wall is about 2.5 m and it possesses a base thickness which appears to be inadequate.
• The backfill is sloping steeply by over 40o to horizontal and extends to a height of up to 25 m.
• Weep holes are provided at regular intervals in the wall, but are not functioning properly.
• At the top of the sloping ground, drainage is poor. There is sewage disposal in the vicinity. Water and sewage stagnation is observed.
• Close to this region at the top of slope (near the entrance to the school), a small water body exists.
• Many trees in the region along the slope had been up rooted.

Looking at the failure of retaining walls at the base, the following two possibilities are envisaged.

1. Slope failure resulting from poor drainage, stagnation of water on the upstream of slope and removal of trees as detailed in Figure 3 that pushes the retaining wall at the toe of slope.

Figure 3. Possible slope failure leading to the damage to retaining wall at the toe of slope

2. Failure of retaining wall due to wrong estimate of earth pressure (perhaps without taking in to account the sloping backfill). Figure 4 and Figure 5 are plotted to explain the effect of inclination of backfill on the earth pressure coefficient and hence total earth pressure. Figure 4 presents the variation of coefficient of active earth pressure with respect to angle of internal friction of backfill soil for different backfill slopes. For illustration, at a backfill friction angle of 30^o , coefficient of active earth pressure is 0.33 when the backfill is horizontal (ß = 0^o ) and is around 0.9 when ß = 30^o . It means that the earth pressure will be 3 times higher with this slope angle instead of horizontal backfill. Figure 5 shows a graph of variation in earth pressure coefficient factor with backfill slope angle for various angles of internal friction of backfill. The earth pressure coefficient factor is defined as the ratio of active earth pressure coefficient at any slope angle of backfill to active earth pressure coefficient of horizontal backfill. It can be seen that the factor can be 3 or 4 times higher depending on the increase in slope angle of backfill. It should also be noted that the backfill slope cannot be stable if its slope angle is higher than the angle of internal friction of backfill in a purely cohesion-less soil.

Hence, in case of steeply sloping backfill, it can be erroneous to assume the backfill to be horizontal. This assumption of horizontal backfill is made by designers of retaining wall many a times. Perhaps in the present situation, the designer may have wrongly assumed the lateral earth pressure leading to failure of retaining wall. Table 1 also provides the values of coefficient of active earth pressure at different slope angles and friction angles of backfill.

Figure 4. Relation between Coefficient of Earth pressure and angle of internal friction at different slope angles

Figure 5. Relation between Earth pressure coefficient factor and backfill slope angle for backfills with different friction angles

Table 1. Variation of Coefficient of active earth pressure with slope angle and backfill friction angle

It was therefore proposed to remove stagnating water on the upstream of slope and provide proper drainage. Wherever possible, improvement in vegetation along the slope was recommended. Further, the failed retaining wall was advised to be reconstructed with proper design or addition of buttresses at regular intervals. Proper weep holes with free drainage were recommended.

Chamarajanagar is a small town and a district headquarters about 80 km south west of Mysore on Karnataka border. One of its police stations is situated on the outskirts of the town on an earthen embankment. It was built about one decade back on a filled up ground of about 4 m height. It is a two storey structure built with columns and beams. On the rear side of the building (South), it appears that original soil exists which is clayey and expansive in nature. Trial pit # P2 was prepared in this region as shown in Figure 6 and samples were collected from a depth of around 1.8 m and the soil at the bottom is classified as highly plastic clay (CH). On the right side of building (east) where the ground comprises of filled up soil, trial pit # P1 is prepared and the samples were collected from the same depth. The soil in this region is classified as poorly graded sandy soil (SP). A 4 m high retaining wall made of stone masonry separates the police station premises from paly ground on the east side. The play-ground is at a lower elevation. The layout of police station premises along with the locations of trial pits is shown in Figure 6. Table 2 provides the soil properties at the bottom of two trial pits as per indian standards (SP36,1997). Figure 7 shows failure of retaining wall with vertical crack, rotation and poor drainage with backfill settlement.

Table 2. Basic soil properties at Chamarajanagar Police Station site

Figure 6. Layout of Chamarajanagar Police Station with locations of trial Pit and failed retaining wall

Figure 7. Failure of retaining wall with vertical crack, rotation and poor drainage with backfill settlement

The following observations were (Prasad, et.al., 2012) made at the site.

• Retaining wall on the eastern side appears to have moved away from the backfill indicating that the horizontal active pressure could not be properly resisted by it.
• Some weep holes appear to be blocked indicating that the drainage through them is not free.
• The south eastern corner portion of the wall seems to be sufficiently damaged. A slight tilt away from the backfill in the alignment of retaining wall was also observed.
• The tests on soil revealed that the original soil is plastic in nature which, over long years may have experienced settlement and filled up with sandy soil on top.
• There is water stagnation on the downstream of retaining wall at the foundation level which might loosen the foundation soil.
• Excessive rain and improper drainage could have further saturated backfill soil leading to increased density and perhaps reduced strength (friction angle) of backfill and hence increase in earth pressure. Table 3 shows that the total active earth pressure is directly proportional to the density of backfill soil.

Table 3. Variation in total active earth pressure with backfill friction angle and backfill bulk density

1. South eastern corner portion of wall was advised to be replaced with a new one. Strengthening the remaining portion by providing buttresses at intervals of 3 m center to center till the top of the wall from its base at foundation level was recommended.

2. Trees and vegetation both on eastern and southern sides were advised to be completely removed. The removal of the roots which may act as reinforcement to the backfill was considered not necessary.

3. Care was recommended to provide proper drainage on the eastern side. Further, the filled up soil on the backfill side of retaining wall, being silty sand, the present status of compaction was considered inadequate. Hence, further compaction was advised.

4. Foundation soil at the base of retaining walls was recommended to be freed from stagnating water with proper drainage.

5. Weep holes were suggested to be made more functional.

Geotechnical problems can most of the times be solved by simple and rational solution. Most of the problems are associated with unnecessarily stressing the ground and disturbing the nature. Hence, proper understanding of the holistic of the problem is essential and creative and ingenious method of solution can be adopted to save the structure. Most of the times water is the worst enemy to the geotechnical engineer. Perhaps provision of good drainage and maintaining free drainage around the structure is most important. At times, a designer is likely to err in assessing the actual force on the built structure. One has to consider all extraordinary situations and conditions at the site before giving design details. Further, it is important to document the designs. In the present case, design details were not available. Proper design and good construction are the two important factors to be considered in construction industr y for good performance of infrastructure such as retaining wall.