Terzaghi’s Observational Method – 2020

Our geotechnical construction field is ripe with the lessons of projects gone bad: shoring systems that moved too much causing damage to adjacent properties, collapsed tunnels, unexpected building settlements, soil erosion/scour and failure.  We practice in a world of unseen subsurface conditions characterized by a few discrete soil borings containing intermittent sampling and sparse testing. 

While our borings and CPTs are designed to best “see” what lies at these selected locations, they cannot predict with lies between, and nature has a way of differing from the straight lines we use to connect the dots. Karl Terzaghi, the father of soil mechanics, understood these complications all too well. Terzaghi was strongly influenced by the design and construction of large earth dams built in Europe and the American West, and he saw firsthand how assumptions made for geology and engineering characterization could go awry. To manage this risk, he implemented the Observational Method that we still employ today. 

When it comes to ground improvement works, the Observational Method remains a key part of geotechnical engineering and construction. We use engineering design methods to predict the future of the improved and reinforced ground. Our predictions are as good as our understanding of the site subsurface conditions and knowledge of past performance. It's therefore imperative that we use the observational method to compare predicted behaviors to reality.

The performance of projects supported by Rammed Aggregate Piers depends on pier lengths, aggregate installation volumes, and constructed stiffnesses. Projects supported by Rigid Inclusions are further complicated by structural strengths, grout volumes, and the necessity for uniform application dosage.  We verify our design assumptions by performing modulus tests, which measure stiffness (load versus deflection) and geotechnical capacity. We also deploy the Observational Method during pier installations to identify changed conditions that could adversely affect the performance of our piers. These changed conditions could consist of softer soils, pockets of organics, deeper bearing strata, deeper fill soils, etc., all of which could adversely affect pier performance. That is why having full-time Quality Control implementing the Observational Method is critical to the performance of ground improvement. 

At Geopier, we pride ourselves in working with our geotechnical consulting colleagues to incorporate top-notch Quality Control methods for all installations to deliver the project owner the performance he deserves. We think Karl Terzaghi would agree.

Kord Wissmann, Ph.D., P.E., D.GE
President/Chief Engineer
Geopier Foundations Company
(704) 739-1790
This project included the construction of a supermarket to serve as the anchor amenity of a 2 million-square feet mixed-use development. The soil profile included unsuitable fill material underlain by a thick, soft, sensitive clay layer that extended to approximately 70 feet below the ground surface. Furthermore, the project required up to about 5(+) feet of filling to achieve the proposed ground floor elevation.

Given the challenges with the installation of rigid inclusion ground improvement in thick/sensitive clays, the project paired a state-of-the-art pre-production testing program with the installation of Geopier GeoConcrete® Columns. The pre-production testing program included installing and testing sacrificial rigid inclusions as single elements and in groups. The testing included six full-scale load tests to 200% design load, non-destructive sonic echo tests, and "proof" load tests on production elements. The full-scale load tests consistently resulted in <1/2” of deflection at the design load. Test and production elements were observed by Geopier's licensee, Helical Drilling, Inc's Quality Control team, the Developer’s geotechnical representative, and the Tenant’s geotechnical representative. Concrete installation volumes were checked through careful monitoring of concrete pump strokes and mandrel extrusion pressures.
Geopier Foundation Company
130 Harbour Place Drive, Suite 280
Davidson, NC 28036
T. (800) 371-7470 | (704) 439-1790
F. (704) 439-1799