Coastal Water Authority Lake Houston Dam

Our Solution

HVJ’s extensive experience in the area informed our approach. Our first step was to review available subsurface information from previous geotechnical investigations near the project site. We found and carefully reviewed prior relevant HVJ reports, paying particular attention to the logs prepared for borings drilled near the current project site. These logs gave us evidence-based information to select the proper drilling method and grout mixture to minimize the risk of hydraulic fracture within the dam’s foundation soils.

Next, we needed to select a drilling method. Working from historical logs, we developed a soil profile for the site. The previously drilled borings showed low- to medium-plasticity clays with varying sand layers, from the ground surface to the boring termination depths. Using that profile, we recommended using power auger drilling through the clay layers and hollow stem auger drilling through the sand layers. We determined this approach would minimize the risk of hydraulic fracture within the dam’s foundation soils.

We also needed to control the grout pressure, as hydraulic fractures can occur during the injection of grout for borehole backfilling. The governing principle is that the soil’s pressure against hydraulic fracture needs to be greater than the grout pressure. 

Within this framework, HVJ calculated the factor of safety against hydraulic fracturing at each depth in the soil profile.  This can be defined as the factor of safety against hydraulic fracturing, as shown by the equation FS = P_f/P_eff.

Where:
FS= Factor of safety against hydraulic fracturing
P_f= Pressure against hydraulic fracturing, psf
P_eff= Effective fluid or grout pressure, psf

The team computed the pressure for various grout mixtures with different unit weights, assuming there would be no additional applied fluid or pressure. The team adjusted unit weight by changing the grout mix materials, including cement and bentonite content and water volume. Our target was a grout mixture with a unit weight that achieves a minimum factor of safety against hydraulic fracturing of 1.3.  We performed factor of safety calculations using the US Army Corps of Engineers RMC Hydraulic Fracture Toolbox as required by ER 1110-1-1807 for Drilling and Invasive Program Plans. 

Our analysis pointed to clear recommendations for a grout mix with a unit weight of about 70 pcf and with the following composition:
o    80 lbs of Cement
o    10 lbs of Bentonite
o    85 gallons of Water

None of this happened in isolation. Solving this challenge was a team effort. From the early project stages, HVJ collaborated in several project team meetings, especially with Black & Veatch, to exchange ideas and build consensus before beginning the field exploration. 

 The Results

HVJ met its goal of reducing the risk of hydraulic fracturing from drilling operations and grouting pressure during borehole backfilling. By using power auger drilling in the clay layers and hollow stem auger drilling in the sand layers, the field crew avoided using drilling fluids. The lower-unit-weight grout mixture also reduced the risk of fracturing. 

In the end, the boreholes were drilled and backfilled with no signs of hydraulic fracturing. HVJ’s technical expertise and familiarity with subsurface conditions from nearby prior investigations enabled the team to make informed decisions at each step.

Photo Courtesy of the Coastal Water Authority