EXPLORATORY GROUTING PROGRAM TO IMPROVE THE LOVELL POND DAM

摘要

The Lovell Pond Dam is an 80-foot tall, high-hazard, rolled-earth embankment dam, which has undergone nearly 70 years of seepage and at times visible internal erosion (piping) at the abutments. An exploratory grouting program was used to reduce hydraulic gradients upstream of the original concrete core wall, fill possible internal piping pathways, and grout loose soil windows encountered in test borings. Downstream repairs included a filter blanket and toe-drain system to help control and collect seepage as well as slope flattening to increase slope stability. The "exploratory" grouting program allowed modification of the grouting program on a 'real-time' basis to suit the conditions encountered. Initially, a series of exploratory test borings with in-situ permeability testing was performed at approximately 100-foot spacing. Grout holes were then installed in split-spacing, ranging from 20-foot spacing for primary holes to 2.5-foot spacing for limited quaternary holes. Water testing was required at each grouting interval prior to grouting. This allowed the use of different grout mixes (including to a limited extent micro-fine cement grout) and pressures to address the apparent permeability encountered in a given stage. Water testing and grout takes were evaluated on a daily basis to monitor and help optimize the grouting program. The grouting program included nearly 17,000 vertical linear feet of borehole, over 90 in-situ 'k-tests', and the installation of two dozen instruments to measure pore pressures within the dam both pre and post grouting. The grouting subcontractor, Hayward Baker, utilized a Tube-and-Machete system with automated measurement/data collection. This allowed stages to be regrouted as needed and up to six stages could be grouted simultaneously. The use of this system limited some of the flexibility of the exploratory intent of the program and challenged GZA's ability to adjust the program in a timely manner. Grout takes in bedrock were typically near refusal but in localized areas were as high as about 40 bags (roughly 800 gallons) of grout per 10-foot stage. In overburden grout takes were generally higher and ranged from refusal to a high of 60 bags (about 1100 gallons) of grout per 10-foot stage. Grout takes decreased with reduced spacing (i.e. Primary, secondary, tertiary, quaternary holes), as anticipated. Since the seepage collection system was installed simultaneously with grouting, pre-and post-grouting seepage flows could not be compared, however instruments indicate an approximately 10 foot reduction in head downstream of the core wall.