An Overview of Geotechnical engineering methods

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Geotechnical engineering

Geotechnical engineering uses principles of soil mechanics and rock mechanics to investigate subsurface conditions and materials; determine the relevant physical/mechanical and chemical properties of these materials; evaluate stability of natural slopes and man-made soil deposits; assess risks posed by site conditions; design earthworks and structure foundations; and monitor site conditions, earthwork and foundation construction.

Research undertaken for new and improved geotechnical methods and processes can have an enormous impact on productivity.  Now there is the availability of several geotechnical methods that helps to complete complicated work in less time, thereby increasing your productivity.

Given below is a brief of a few geotechnical methods adopted across the globe.

Consolidation grouting

Consolidation grouting is a general term for grouting that is carried out within a rock mass with the intent of filling rock mass discontinuities. The process involves injecting a grout material to fill the discontinuities, which are the pathways through which fluids or gases migrate in most rocks. With grout filling the rock discontinuities, the hydraulic gradient is reduced as the liquids or gases move through the grout, ultimately reducing or stopping migration. This technology is used for Pre-excavation Grouting, Grout Curtain, Foundation Grouting and Water Cut-Off Grouting.

Post grout engineering

Post grouting is a method for increasing the capacity of tiebacks, tie-downs, rock anchors, and micropiles. Post grouting involves injected pressurized grout into the bonded area of the anchor. The injection process significantly increases the capacity of our anchors beyond their theoretical limits. Post grouting increases the capacity in a variety of ways. First, the pressurized grout compacts/consolidate the soil immediately surrounding the anchor. This increases the soils shear strength and its ability to take the loads from the anchor. Secondly, the pressurized grout creates a grout bulb effectively increasing the diameter of the ground anchor. The larger diameter anchor can pass an additional load to a greater volume of soil through an increased area of contact. With post grouting, we are typically able to achieve anchor capacities of two to five times their theoretical values.

Continuous flight auger

Continuous flight auger method involves a continuous flight auger drill which is used to excavate a hole and concrete is injected through a hollow shaft under pressure as the auger is extracted. Reinforcement is then inserted after the auger is removed. This creates a continuous pile without ever leaving an open hole. Continuous flight augering can be used to construct a secant piled wall which can be used as a retaining wall or as shoring during excavation. Once initial piles are set with concrete, other shafts are augured between them, slicing into the original piles, with the new ones receiving rebar. The finished result is a continuous wall of reinforced concrete that aids and protects workers during excavation.

Cross passage

Cross-passages are reinforced concrete structures built between twin tunnels or among a tunnel and the ground surface to serve as a route for access, escape passage, fire safety strategy, and housing for electrical and mechanical equipment, maintenance and ventilation. Generally, it is in the shape of a horse-shoe. Minimum clear headroom and the minimum clear width determine the size of the passage. The structure is normally constructed by excavation done through an opening. The opening is created by dismantling the tunnel segments in the completed section. The temporary opening is primarily supported through a circular support frame until the construction of permanent lintel is completed.

The cross passages are generally constructed using traditional mining techniques to create a link between the two tunnels. These techniques typically include the use of ground treatment measures, excavators, rock breakers, rock bolting and shotcrete lining from within the tunnels.

Diaphragm wall

A diaphragm wall is a type of retaining wall that is used to resist the lateral pressure due to soil or water. They are mainly used in the formation of deep basements, underground stations or to separate two large underground facilities. They may be carried to as deep as 50 metres. They are more economical in such deep construction compared to other traditional methods such as piling. They may also be used as a cutoff wall to avoid seepage of water.

A diaphragm wall is constructed using a narrow trench excavated in ground and supported by an engineered fluid until the mud is replaced by the permanent material. Generally, diaphragm walls are made from reinforced concrete, though un-reinforced walls can also be used. Walls generally range in thickness from 500mm to 1500mm and can be excavated to depths of 50m or more.

Diaphragm walls are often used in congested areas or where the excavation depth is very deep which would otherwise require excavation of much greater soil volumes to provide stable battered slopes. They are well suited for deep basements, underground rail stations, rail car unloaders, tunnel approaches, pumping stations and such like.

Ground Anchoring

Ground anchoring is a common name applied to an engineered system that mechanically fixes a structure to the ground, enabling load transfer into a competent stratum.  The tensile forces applied are resisted by the shear strength of the surrounding ground.

Ground Anchors have become an essential component in modern construction techniques and are commonly used in retaining wall tie-backs; resistance to structure overturning; resisting landscape sliding; preloading of ground to minimize structural settlement; pile and plate loading tests; and resistance to structure buoyancy. Drilling rigs are used drilling vertically or at any angle to a retaining wall, enabling our optimised design solution to be addressed in each case, regardless of the restrictions on the access available. Restricted access drilling rigs can be used in tight corners.

Jet grouting

Jet grouting uses high-velocity fluid jets to construct cemented soil of varying geometries in the ground. Jet grouting creates in situ columns of soilcrete (grouted soil), using a grouting monitor attached to the end of a drill stem. The jet grout monitor is advanced to the maximum treatment depth. Then high-velocity jets (cement grout with optional water and air) are initiated from ports in the monitor. The jets erode and mix the in situ soil with grout as the drill stem and monitor is rotated and raised.

Depending on the application and types of soils, one of three variations is used: the single fluid system (slurry grout jet), the double fluid system (slurry grout jet surrounded by an air jet) and the triple fluid system (water jet surrounded by an air jet, with a separate grout port). The jet grouting process constructs soilcrete panels, full columns, or partial columns with designed strength and/or permeability.

Microtunneling

Microtunneling or is a digging method is used to construct small tunnels. These small diameter tunnels make it impossible to have an operator driving the machine itself. Instead, the microtunnel boring machine (MTBM) has to be operated remotely from a control room.

Microtunnel boring machines are very similar to normal tunnel boring machines (TBMs), but on a smaller scale. These machines generally vary from 0.61 to 1.5 meters (2 ft 0 into 4 ft 11 in) but smaller and larger machines have existed. Usually, the operator controls the machine from a control room on the surface. The Microtunneling machine and jacking frame are set up in a shaft at the required depth.

Pipe roofing

The pipe roof support method has been widely applied as one of the important auxiliary methods for shallow tunnel excavation. The pipe roof support can consolidate the ground stress and disperse the ground stress and reduce the excavation release stress, which effectively limits the tunnel crown settlement or prevent ground settlement.

Steel pipes are installed from the opening of the tunnel in the front. Pipes are arranged like an umbrella or canopy around the excavation line with the help of standard tunneling equipment. In a few isolated cases, larger diameter pipes are installed using special rigs and Down-The-Hole (DTH) drilling method. It helps in stabilizing and protecting the ceiling and face of the tunnel by substantially increasing the load-bearing capacity of the ground.

The diameter of the steel pipes is usually measured between 60 mm and 200 mm along with a wall thickness of 4 mm to 8 mm. The height of the pipe is commonly measured between 12-15m. When the end of a pipe roof field is reached, there is around 3-6m of the pipe remaining in the ground ahead of the face. This distance is known as the “overlapping length” of the pipe roof system.

Slope stabilisation

Slope stabilisation method uses permanent design measures used alone or in combination to minimize erosion from disturbed surfaces. The purpose of this technology is to stabilise the soil, to reduce raindrop impact, to reduce the velocity of surface runoff, and to prevent erosion. This applies to cleared, graded, disturbed slopes, or where vegetation alone does not provide adequate erosion protection.

Shoring system

Shoring is the process of temporarily supporting a building, vessel, structure, or trench with shores (props) when in danger of collapse or during repairs or alterations. Shoring comes from shore, a timber or metal prop. Shoring may be vertical, angled, or horizontal. The support may be supplied by shoring the wall with heavy timbers sloping upward at about 65° to 75°. The top of the timber is so arranged that part of the wall load is transferred onto it, while the lower end of the timber is framed onto a base to transfer the load to the ground with minimum deformation. Wedges may be used to bring the shore snugly into contact with the wall. If the wall is several stories high, a vertical series of shores may be required. Shores are also used to support the forms for cast-in-place concrete slabs, beams, and girders in reinforced concrete frames.

Shaft sinking

Shaft sinking is excavating a vertical or near-vertical tunnel from the top down, where there is initially no access to the bottom.

Shallow shafts, typically sunk for civil engineering projects differ greatly in execution method from deep shafts, typically sunk for mining projects. When the top of the excavation is the ground surface, it is referred to as a shaft; when the top of the excavation is underground, it is called a winze or a sub-shaft. Small shafts may be excavated upwards from within an existing mine as long as there is access at the bottom, in which case they are called Raises. A shaft may be either vertical or inclined (between 45 and 90 degrees to the horizontal), although most modern mine shafts are vertical. If access exists at the bottom of the proposed shaft and ground conditions allow then raise boring may be used to excavate the shaft from the bottom up, such shafts are called borehole shafts. Shaft sinking is one of the most difficult of all development methods: restricted space, gravity, groundwater and specialized procedures make the task quite formidable.

Secant pile wall

Secant pile walls are formed by constructing intersecting reinforced concrete piles. The secant piles are reinforced with either steel rebar or with steel beams and are constructed by either drilling under mud or augering. Primary piles are installed first with secondary (male) piles constructed in between primary (female) piles once the latter gain sufficient strength. Pile overlap is typically in the order of 3 inches (8 cm). In a tangent pile wall, there is no pile overlap as the piles are constructed flush to each other. Secant pile wall design when steel beams are used involves the use of weaker than normal concrete. The pile that is lagging the wall between two main beams has to be examined for shear and compression arching.

Rock soil bolting

Rock soil bolting method includes, a rock bolt used as anchor bolt, for stabilizing rock excavations, which may be used in tunnels or rock cuts. It transfers load from the unstable exterior to the confined (and much stronger) interior of the rock mass. Rock bolts work by ‘knitting’ the rock mass together sufficiently before it can move enough to loosen and fail by unraveling (piece by piece). As shown in the photo, rock bolts may be used to support wire mesh, but this is usually a small part of their function. Unlike common anchor bolts, rock bolts can become ‘seized’ throughout their length by small shears in the rock mass, so they are not fully dependent on their pull-out strength.

Soldier piling

A soldier piling is a common retaining wall strategy in which H-shaped steel beams (“piles”) are drilled deep into the earth at regular intervals – usually 2 to 4 yards apart. In between each vertical pile, horizontal support fill the gap, helping to spread the load. The method is also commonly known as the “Berlin Wall” when steel piles and timber lagging is used. Moment resistance in soldier pile and lagging walls is provided solely by the soldier piles. Passive soil resistance is obtained by embedding the soldier piles beneath the excavation grade. The lagging bridges and retains soil across piles and transfers the lateral load to the soldier pile system.

Soldier pile and lagging walls are the most inexpensive systems compared to other retaining walls. They are also very easy and fast to construct.

Substructure bridge pilling

Planning and design of substructure construction is most sensitive to tolerance requirements since it supports the entire bridge. The substructure of a bridge comprises the piers, abutments and foundations. These portions usually consist of masonry in some form, including under that general head stone masonry, brickwork and concrete. The substructure work consists of the following components- Foundations [Use of grouting methods for improving foundation soil], Abutments [Use of semi-integral and integral abutments, with a single row of piles], Retaining walls [Use of three-sided precast wall culverts; use of modular retaining walls], Precast concrete sheeting [It has been used as retaining walls, wing walls, and components of the bridge abutment], Piers [Use of precast concrete pier caps and at river locations; piers need to be avoided as their construction adversely affects the fauna and flora], Bearings [Elastomeric, multi-rotational, or isolation types], Foundation protection [Use modern scour countermeasures, other than riprap] and Improving drainage [At bridge sites during construction].

Conclusion

The above-discussed geotechnical engineering methods are a few popular options available. The selection criteria should be best on your project needs and work required. It is also important to choose the best tools while applying these methods. Geotechnical engineers are responsible for evaluating subsurface and soil conditions and materials, using the principles of soil and rock mechanics. They are commonly appointed as consultants on construction projects. Engineers also examine environmental issues such as flood plains and water tables. By doing so, they can determine whether a particular site is suitable for a proposed project, and can inform the engineering design process about how ground conditions can be made safe and effective for construction.