Design, construction, economic and performance aspects of anchored earth retaining walls- a case study

Abstract

Thc study addrcsscs design, construction, economic and performancc aspccts of a 4.25m high anchorcd carth retaining wall intended to demarcate property line of thc 16- storied corporate office building of the ACME Laboratories Limited at Kalyanpur, Dhaka. The subsoil of the site typically consists of soft silty clay deposit upto a depth of 7.0m, which poses difficulty for design and construction of an economical foundation system for any type of retaining wall. As an alternative to the anchored earth wall, other types of retaining walls such as conventional R.C cantilever wall, geotextile reinforced walls and metal strip reinforced walls were considered for design. A comparative cost study of these walls along with their relative ease and speed of construction were also examined. The standard schedule of rates (2003) of Public Works Departments (PWD) was used as the basis of cost analysis. It is found that anchored earth wall and geotextile reinforced walls are more economical compared to R.C. cantilever wall and metal strip walls. Although geotextile reinforced walls system is found to be more economical than that of anchored earth wall, the component materials of gcotextile reinforced walls are not always easily available in the local market. On the contrary, component materials of anchored earth wall system are easily available in the local market. Besides, it can be implemented using locally available labour and equipment. Hence this type of wall was selected for implementation at thc projcct site. After selection, the anchored earth wall was constructed in down to top sequence. First, the site was cleaned and leveled and then geotextile mattresses were placed. Footing pad was then placed on the top of geotextile mattresses to keep the facing block vertically aligned. Then layer-by-layer sand, reinforcement and anchor blocks were placed accordingly to complete the AE wall construction upto 4.25 m height. A post construction performance study was made in the form of observing the deflection of facing units of thc retaining system. Both dry and wet seasons were considered in the study as the backfill materials are always prone to washing out during rains and also the water may cause excessive pressure on the retaining system in case of development of poor drainage of backfill. During recording of horizontal displacement data of the wall, some particular points in the outer side column of the main building were chosen as reference. The displacements were measured at a regular interval of time over a period of twelve months. From the measured data, it was found that 25 mm displacement occurred at the top of the wall after 4 months. No displacement occurred afterwards. The study reveals that the anchored earth wall is more economical compared to the other soil retaining systems considered. The system may also be characterized by added advantages of availability of component materials in the local market coupled with its ease and speed of construction.