Geotechnical Services

For the last 15 years O’Terra Geotech Group, Inc. has been committed to provide our clients with a wide range of geotechnical and materials engineering services. Our professional engineering personnel are highly educated, have extensive experience and skills in the field of geotechnical and materials engineering to meet our clients’ need.

O’Terra Geotech Group, Inc. team provides geotechnical engineering analysis, design and recommendations for deep and shallow foundations, all types of retaining walls, geo-synthetic reinforced embankment (GRE), ground improvement, foundation settlements and expansive soils, foundation underpinning, liquefiable soils, slope stability and landslides, groundwater and drainage, and pavement design. We provide our clients with investigation of building foundation failure and cost effective remedial recommendations. We also provide our clients with inspection and testing during construction and provide final construction inspection report required by cities and counties.

Soils and foundation investigation involve soil borings, test pits, CPT sounding, using truck-mounted track-mounted and portable drilling rigs. Standard Penetration Test (SPT) is conducted at 5’ or less intervals. During boring operations, disturbed and undisturbed samples are taken for laboratory testing such as moisture/ density, Plasticity, mechanical analysis, shear strength, and consolidation. Corrosion samples will be taken for corrosion tests if requested. Design and construction recommendations are prepared as a signed document for use by the design team and for submittal to the building departments.

Pavement Design in both asphalt and concrete involve pavement thickness layer design (surfacing, base, sub-base and pervious material). We provide recommendations for new pavement or existing pavement restoration.

Foundation engineering involves the determination of the best suited and most economical foundation types and depth for our clients projects based on the strength of foundation soils and rocks. We provide geotechnical information, design data and recommendations for slab-on –grade, mat foundation and drilled piers foundation to the project structure engineer. We conduct foundation evaluation of the existing structures and provide written recommendations to the project structure engineer for repair such as underpinning.

We conduct investigation for the design of retaining walls and determine the best suited and most economical earth retaining system for our client’s projects. We perform analysis and design for mechanically stabilize earth (MSE) walls, soil nail walls, soldier pile walls, tie-back walls, gabion walls, masonry block walls, concrete crib walls, reinforced concrete walls, secant walls, and segmented block walls.

Safe temporary excavation slopes and temporary shoring systems are the responsibility of the contractors. We will provide adequate geotechnical information in our Geotechnical Report to enable the contractors to design their shoring system and temporary excavation slopes for our clients projects. A structural/civil engineer knowledgeable in this type of construction should be retained by the contractor to design the shoring system. We should review the final shoring plans to check that they are consistent with the recommendations presented in our geotechnical report.

When buildings to be constructed on existing slopes we investigate the condition and stability of the slopes, perform stability analysis using computer program and provide written recommendations. For landslide project, we investigate the site, conduct foundation explorations and provide repair recommendations. The investigations may include borings, instrumentations, and monitoring. The repair solution may be drainage improvement, non-structural solution such as geo-synthetic reinforced earth (GRE) or structural repair involving various type of earth retaining structures.

Expansive soils are soils that experience significant volume change associated with changes in water contents. These volume changes can either in the form of swell or in the form shrinkage and this is why they are sometime known as swell/shrink soils. Key aspects that need identification when dealing with expansive soils include: soil properties, suction/water conditions, water content variations temporal and spatial, e.g. generated by trees, and the geometry/stiffness of foundations and associated structures. Expansive soils can be found in humid environments where expansive problems occur with soils of high Plasticity Index (PI) or in arid/semi-arid soils where soils of even moderate expansiveness can cause significant damage. Damage from to expansive soil often occurs as a direct result of interaction with vegetation and associated water content changes. Soils that experience moderate to severe swell/shrink problems are common in most counties of San Francisco Bay Area.

O’Terra Geotech Group will investigate the nature and extent of expansive soils and provide remediation recommendation. We will investigate expansive behavior both in the field and in the laboratory and the associated empirical and analytical tools to evaluate expansive behavior.

We conduct building foundation inspection and if damaged we will provide recommended solutions to our clients.

Ground improvement, is the modification of existing site foundation soils to provide stable ground condition to carry the building imposed load. Ground improvement techniques are used for new projects to allow utilization of sites with poor subsurface conditions. In short, Ground improvement is executed to increase the bearing capacity, reduce the magnitude of settlements and the time in which it occurs, retard seepage, accelerate the rate at which drainage occurs, increase the stability of slopes, mitigation of liquefaction potential, etc. Based on the soil conditions, a suitable method of ground improvement should be considered keeping in view of the economic feasibility as well as the time frame.

Some of the ground improvement methods available are as follows:

1. For cohesive soils: Vertical Drains, stone columns, geo-piers and in-situ deep mixing.
2. For cohesionless soils: Compaction piles, vibro compaction, stone-columns, geo-piers, dynamic compaction and grouting. The final choice among the methods available will depend on overall economy in the total foundation cost. Normally, ground treated with one of these methods results in less foundation cost compared to the cost of deep (pile) foundation which is to be otherwise used if no treatment is adopted.

If we have determine during foundation investigation that the proposed building structures will be on soft, compressible or loose soil, we develop solutions for improving the ground using various ground improvement techniques if soil removal and replacement would not be cost effective.

When converting or refurbishing and existing building or when excavating adjacent to an existing

structure, underpinning would likely be required to stabilize and strengthen the foundation. Underpinning

Can be useful if following conditions exist:

• Stabilizing foundation undergoing settlement
• Stabilizing foundation that have moved due to land sliding or unstable slopes
• Stabilizing foundation affected by erosion, subsidence or heave and vibration damage
• Strengthening foundation subjected to an increase load such as building a second story
• To accommodate a new adjoining building structure and a new basement story

Underpinning of foundations involve increasing the depth of existing foundations in order to transfer the

building load to a more supportive soil type.

Although, there are number of underpinning techniques available, the most common method is traditional

mass concrete underpinning. This simple technique involves excavation a segment of ground below the

existing building foundation in controlled stages, to a depth where stable soil conditions exist. The

excavation is then filled with concrete and allowed to cure before the next “pin” is excavated. In cases

where existing building is tilted jacking operation and underpinning is required to level the building.

If the existing building is on piers then underpinning involved construction of additional piers.

O’Terra geotech Group will investigate the building site (drilling and sampling may be required) and

provide the project structure engineer with geotechnical data and evaluate the most appropriate

underpinning method for subject properties.

We perform ground water investigation and drainage consultation for new developments, hillsides and for existing buildings experiencing groundwater damage such as wet basements and crawlspaces.

We perform the following construction observation for our clients:

• Construction Observation during building and retaining wall foundation (spread footing and drilled piers) construction (make it clickable)
• Construction observation and testing during removal and fill placement,
• Construction observation and testing for utility trenches and retaining wall during backfilling and observation during subgrade preparation and pavement aggregate base and asphalt placement.