A COMPREHENSIVE FIELD INVESTIGATION FOR REHABILITATION OF AN EMBANKMENT POND

Planning and implementation of a comprehensive field investigation for the rehabilitation of an embankment pond,
damaged by bed erosion and liner rupture, are discussed. The pond can store 1.5×106 m3 water and is formed by a
peripheral 16-m-high embankment. It regulates the water issuing from the tailrace tunnel of a hydropower station.
Insufficient energy dissipation of the flow from the tunnel caused serious bed erosion downstream of a large concrete
apron leading to the rupture of a deep 2-mm-HDPE (high density polyethylene) liner of the pond floor. The pond
floor is made of four layers placed on the compacted soil with an overall 1.2 m thickness. The waterproof liner in
the mid-depth of the bed layers covers the whole pond floor and extends to the top of the embankment. Being a
major part of the national power grid to help at peak hours, the station could not be shut-down during the study for more than a couple of days. This time limitation posed a serious challenge in drying the pond for a vital inspection of the damages. The study team planned and implemented a comprehensive intense site investigation during and after the pond was drained. The purpose of the site study was fourfold: (i) Provide for an accurate description of the damage. (ii) Collect data for later scale modeling and design studies. (iii) Find clues as to the cause and timing of the damages. (iv) Make use of the water drawdown to monitor concurrently the drainage flow to see if the pond was leaking. The field investigation was undertaken in two stages: (1) during drawdown and (2) after drying the pond. The first involved three actions: (1a) Concurrent discharge measurement at two river sections to detect contribution to the river flow by the seepage, if any, from the pond (1b) Monitoring the drainage from the perforated pipe network buried under the soil layers (1c) Instantaneous measurement of outflow from the pond and water level fall in the pond to validate the continuity equation. Major investigations of the second stage included: (2a) Visual inspection of the inner face embankment for signs of cracks or settlement (2b) A complete survey of the pond floor topography
(2c) Photographical record of the damaged area (2d) Sediment samples from various places in the pond (2e) Visual
inspection of all the appurtenant structures (2f) Electrical resistivity survey on selected places looking for cavities or seepage. Pieces of the large body of the data were then interrelated to one another as well as to the historical data such as those of the water level, discharge, precipitation, and piezometric levels. The investigation revealed that the damage must have happened soon after the operation of the pond eight (8) years prior to the investigation, the scour holes were progressing downstream, the embankment was safe, no significant water escape from the pond was detected, and the intruded sediment from outside was responsible for formation of several depositional islands. The rationales to attain major conclusions are presented. The data and the resulting insights from the field investigation were employed later in the physical modeling study and engineering design of the remedial modifications.