EARTH RETENTION
Earth Retention, also known as Shoring, is used to support something that is otherwise unstable. Whether Condon-Johnson is stabilizing a landslide, underpinning an existing structure or supporting the sides of an excavation, the company has the engineers, experience, and equipment to provide the most cost effective solution to your challenging project. Condon-Johnson’s legacy in earth retention started in 1976 when the company built the first soil nail wall in North America. Our geotechnical professionals continue to be at the forefront of innovation, developing and applying progressive design and construction techniques. Condon-Johnson is recognized as a leader in the industry, applying decades of experience, innovation and technology to successfully complete hundreds of earth retention projects.
Internally-braced shoring relies on a system of walers and rakers/struts used to resist lateral earth, water, and surcharge pressures without the use of tieback anchors. Internally braced systems can be used in conjunction with conventional soldier piles and lagging, sheetpiles, secant piles, or DSM wall systems. Following installation of the perimeter shoring systems, the internal bracing is installed as the excavation progresses downward. Internally braced shoring systems can be extended to any depth and are often used when there are property line and utility conflicts or a high ground water table which preclude the use of tieback anchors. Rakers are structural steel members that diagonally brace vertical walls by transferring the load to the bottom of the excavation. Struts are installed between the two sides of an excavation so that they brace each side of the excavation against each other.
Condon-Johnson is experienced in building hard/soft, hard/firm, and hard/hard secant pile walls over 150 feet deep. With innovative construction methods and modern drilling equipment, we are able to install secant pile walls in the most challenging urban environments with minimal impact to adjacent structures, utilities, and roadways.
Secant pile wall systems are constructed using overlapping piles to create a continuous, water-tight structural wall. The piles are installed in an alternating primary/secondary fashion along the alignment of the wall. Typically the initial pile set is constructed of a “soft” or unreinforced material, and the secondary pile set is constructed using hard concrete with either rebar or steel beam reinforcement.
In order to achieve the required structural overlap and water-tight configuration, secant pile walls must be installed with tight tolerances. Guide walls are typically constructed prior to installing the secants to help provide location control for the secant piles.
Where the bottom of excavation is below the ground water table, secant walls systems can be combined with jet grouting at the base of excavation to form a “bottom plug” so that both the bottom and the sides of the excavation remain watertight.
Compared to sheet pile walls, secant pile walls offer increased stiffness, decreased noise during installation, and are installed with minimal vibrations making them ideal for sensitive areas where settlement and noise are an issue.
Shear Pins are steel beam, pipe or bar reinforced concrete piles typically used to reinforce or stabilize slopes. Shear Pin design should consider both shear force and bending moments. Shear pins can be combined with tiebacks for additional lateral restraint. Condon-Johnson has equipment capable of constructing shear pins in excess of 10 feet in diameter, access permitting.
Sheet piling provides a permanent or temporary shoring or cut-off wall. If installed properly, sheet piles use a mechanical interlock that creates a continuous water-tight seal. Sheet pile materials can be steel, vinyl or composite, depending on the application. The sheets would typically be laced with the crane and held vertical in a large steel template that is designed and fabricated for each job. Historically, the piles were then driven down in incremental stages in order to maintain alignment and minimize interlock binding. These conventional methods are slow and inefficient, and the costs of mobilization and operation often make sheet pile walls an expensive option.
Condon-Johnson’s innovative installation methods and equipment now make sheet piling a more viable option. Condon-Johnson owns and operates ABI mobilram equipment, which was designed specifically for the installation of sheet piling. The ABI runs on a track-mounted carrier with an extendable vertical mast. Vibratory hammers, impact hammers, presses, drills or other attachments are mounted to the mast depending on job requirements. Positive and precise control of the piles during installation is now possible with powerful hydraulics capable of articulation in all directions. With the ABI mobilram, sheet pile driving occurs in a single stroke, without the use of a template and requires a smaller crew than conventional methods. Mobilization is also much more efficient, as the ABI is transported on a single truck and set up in a matter of hours. The result is faster and higher quality sheet piling installation, with reduced construction costs.
The ability to maintain precise verticality and alignment is especially important in the installation of vinyl and composite sheet piling. While steel interlocks can be strong enough to hold together when driven slightly out of alignment, plastic and fiberglass interlocks simply break. Because these sensitive materials can’t be forced into place, Condon-Johnson installation methods provide a high level of care and quality.
With extensive experience in operating various types and sizes of ABI equipment, Condon-Johnson has developed state-of-the-art techniques in the construction of specialty foundations. The company’s expertise is evident on many different types of completed projects, including piling (sheet piling, soldier piling, and auger cast piling) and ground improvement (vibro-densification and soil mixing).
Soil mixing is a cost-effective ground improvement method that improves the characteristics of weak soils by injecting and blending cementitious grout with the native materials in-situ. The unconfined compressive strength (UCS) of the resulting in-situ soil-cement is on the order of 75-psi to 300-psi depending on the soil type and design requirements. To create a soil mixed wall, the in-situ soil mixed elements overlap each other and structural steel elements are inserted for reinforcement. Soil mixed walls can be a low-cost method for the construction of a water cut-off, earth retention system. These soil mixed earth retention systems can be combined with internal bracing or tiebacks for deeper shoring applications, and bottom seals for a complete water-tight shoring system.
Condon-Johnson offers two different soil mixing technologies that are most applicable to soil mixed walls: 1. Cutter Soil Mixing; and 2. Multi-Axis Soil Mixing.
Cutter Soil Mixing (CSM) provides a cost-effective solution for constructing an impermeable retaining system by mixing the in-situ soils with a cement/bentonite grout to create a rectangular panel up to 120 feet in depth. The elements are constructed through the cutting and mixing of native soils with an injected cement/bentonite grout via drums mounted on hydraulic motors. Each panel is approximately 9 feet long and can be installed in widths ranging from 30 inches to 60 inches by changing the cutter wheels. The panels are installed in an overlapping manner to form a continuous wall. Condon-Johnson’s CSM walls are constructed using the latest soil mixing technology to ensure a high level of quality control through the use of down-hole verticality monitoring and control. This ensures that our CSM panels overlap to provide a water cutoff.
Multi-Axis Soil Mixing is a technique in which two or more vertical mixing augers are teamed together to create a panel of overlapping columns. Typically, these overlapping soil mixed columns are 3 feet diameter and spaced 2 feet on center. Each panel can be constructed to depths exceeding 70 feet. As with the CSM, the multi-axis panels are installed in an overlapping manner to form a continuous wall.
Condon-Johnson built the first soil nail wall in North America in 1976 and has continued to be at the forefront of innovation in geotechnical construction. Soil nailing can be used for both temporary and permanent earth retention applications as well as for slope stabilization.
Soil nailing is an earth retention technique using grouted tension-resisting steel bars (soil nails) and shotcrete facing to stabilize a section of ground as excavation proceeds from the top down.
The first step of construction is the excavation of the “lift” from the top down to a predetermined height that is typically 4 to 6 feet below existing grade. Next, the soil nail holes are drilled at a slight declination and the threaded steel soil nail bar is inserted and grouted in a 4 to 6 inch diameter hole. The result is a gravity block that resists bearing capacity failure, sliding, and rotation thereby supporting the soil behind it.
In many applications, primarily permanent applications, vertical geosynthetic drainage strips are placed on the excavated soil face to ensure drainage and prevent build-up of hydrostatic pressures on the wall. Steel reinforcing is then installed and shotcrete is applied to the exposed soil face. Excavation then resumes for the next lift. A rod-finish is applied to the shotcrete for temporary walls. Permanent shotcrete walls often have a second layer of shotcrete applied that is finished or sculpted and colored to replicate local rock formations.
Soil nail walls can offer a reduced construction time and cost savings compared to other shoring systems. Condon-Johnson offers a variety of related services, including design/build, construction, permanent shotcrete facing, and shotcrete sculpting for a “natural stone” appearance.
Condon-Johnson’s team of geotechnical professionals has constructed cantilevered, anchored-with-tiebacks, and internally-braced soldier pile walls throughout the West Coast for a wide range of projects. Soldier pile walls, either permanent or temporary, are commonly used to support open excavations adjacent to roads, utilities, and structures and to provide permanent support for new highway cuts or existing landslide conditions.
Soldier pile walls consist of wide flange steel piles that are driven or drilled into the ground before excavation begins. Typical pile spacing is 8 feet along the wall alignment. As the excavation progresses, horizontal timber or in some cases shotcrete lagging is installed between the piles in 5 foot “lifts” to resist the load of the retained soil and to transfer the soil load to the piles. Depending on the depth of the excavation and the sensitivity of surrounding structures and utilities to movement, post-tensioned anchors and/or internal bracing can then be installed at intervals to provide lateral support to the solider beams.
The lagging “lift” and excavation cycle continues until the bottom of excavation is reached. At this point, soldier pile walls used as temporary shoring are considered complete. Soldier pile walls used for permanent support will usually have corrosion protection added to the beam and anchors and may get an additional pre-cast lagging or shotcrete facing for a longer service life. Condon-Johnson offers a variety of services related to soldier pile walls such as design/build services, construction, permanent shotcrete facing, and shotcrete sculpting for a “natural stone” appearance.
Condon-Johnson is recognized as the leading ground anchor contractor on the West Coast, successfully installing anchors for temporary excavation support, permanent retaining walls, landslide control, slope stabilization, marine bulkheads, and dam stabilization.
Tieback anchors (also called ground anchors) are pre-stressed structural elements used to transmit an applied tensile load into competent soils. Tiebacks use high strength steel strands or bar that is grouted into a drilled hole with a free stressing (unbonded) length and a bonded length. The unbonded length is the portion of the steel tendon that is allowed to elongate elastically and bridges the gap between the wall facing and the bond length. The bond length is where the tensile load is distributed to the surrounding soils or rock to resist the applied loading. Tiebacks are stressed and locked off at a design load after the grout has reached sufficient strength.
Permanent tiebacks incorporate a corrosion protection system to increase the expected lift of the anchors. Tiebacks can offer a cost-effective solution over cantilevered shoring or shear pin solutions, and can be installed within limited access.
Horizontal drains are a common method for subsurface drainage of slopes where instability is caused by subsurface water. Horizontal boreholes are drilled at a shallow upward angle (typically 2-3 degrees) up to lengths of 1,000 feet. Perforated pipe is installed in the borehole and the drain is sealed at the surface. The pipe or pipes can be hooked up to a collection system to transport water away from the slope.
Access shafts are an essential component of underground infrastructure. They are constructed to provide access for construction activities such as large diameter piles at the pile/column interface and tunnels.
Methods of access shaft construction include circular secant pile walls and corrugated metal pipe (CMP) lined drilled shafts.
Circular secant pile walls are formed by constructing a series of overlapping “primary” and “secondary” concrete-filled drill holes. The primary piles are constructed first, followed by secondary piles, which are cut into the previously placed primary piles. The overlap forms a continuous concrete ring to support the circular excavation.
Corrugated metal pipe (CMP) lined drilled shafts are constructed by excavating or drilling a shaft with a diameter that is 1 to 2 feet larger than the diameter of the CMP. The CMP is placed into the drilled shaft and the annulus is backfilled with concrete.
Both systems are suitable to applications where the bottom of excavation is below the ground water table. For an impermeable shaft, a “bottom plug” can be added so the excavation remains watertight.
Underpinning is a shoring method used to increase foundation depth to safely transfer the load of an existing structure to a depth sufficiently below an adjacent excavation. Underpinning can be constructed using various techniques including slant piles, micropiles, jet grouting, chemical grouting, compaction grouting and hand-mined pits. Access, excavation depth and soil and groundwater conditions dictate which method is most appropriate and cost effective. Condon-Johnson has the experience and equipment necessary to employ all available options for any unique situation.
Condon-Johnson & Associates – Geotechnical Contractors & Engineers
