Large-displacement piles Advantages and Disadvantages
Large-displacement piles include all solid piles, including precast concrete piles, and steel or concrete tubes closed at the lower end by a driving shoe or a plug, i.e. cast-in-place piles.
Advantages of Displacement Piles
Material of preformed section can be inspected before driving.
Steel piles and driven cast-in-place concrete piles are adaptable to variable driving
Installation is generally unaffected by groundwater condition.
Soil disposal is not necessary.
Driving records may be correlated withinsitu tests or borehole data.
Displacement piles tend to compact granular soils thereby improving bearing capacity and stiffness.
Pile projection above ground level and the water level is useful for marine structures and obviates the need to cast insitu columns above the piles.
Driven cast-in-place piles are associated with low material cost.
Disadvantages of Displacement Piles
Pile section may be damaged during driving.
Founding soil cannot be inspected to confirm the ground conditions as interpreted from the ground investigation data.
Ground displacement may cause movement of, or damage to, adjacent piles, structures, slopes or utility installations.
Noise may prove unacceptable in a built-up environment.
Vibration may prove unacceptable due to presence of sensitive structures, utility installations or machinery nearby.
Piles cannot be easily driven in sites with restricted headroom.
Excess pore water pressure may develop during driving resulting in false set of the piles, or negative skin friction on piles upon dissipation of excess pore water pressure.
Length of precast concrete piles may be constrained by transportation or size of casting yard.
Heavy piling plant may require extensive site preparation to construct a suitable piling platform in sites with poor ground conditions.
Underground obstructions cannot be coped with easily.
For driven cast-in-place piles, the fresh concrete is exposed to various types of potential damage, such as necking, ground intrusions due to displaced soil and possible damage due to driving of adjacent piles.
Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of installation.
Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endbearing resistance that occurs in practice usually cannot be reliably predicted. In the installation of piles, either displacement or replacement of the ground will predominate.
A classification system based on the degree of ground displacement during pile installation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of piles and reflects the fundamental effect of pile construction on the ground which in turn will have a pronounced influence on pile performance.
Such a classification system is therefore considered to be the most appropriate.
Piles are classified into the following four types :
(b) Small-displacement piles, which include rolled steel sections such as H-piles and open-ended tubular piles.
However, these piles will effectively become largedisplacement piles if a soil plug forms.
(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.
(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.
There are a number of foundation types available for geotechnical engineers.
1. Shallow foundations
Shallow foundations are the cheapest and most common type of foundations (Fig.1).
Shallow foundations are ideal for situations, when the soil immediately below the footing is strong enough to carry the building loads. In some situations soil imme-diately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.
Fig.1. Shallow foundations
2. Mat foundations
Mat foundations are also known as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed in a large area (Fig.2).
Fig.2. Mat foundations
3. Pile foundations
Piles are used when bearing soil is at a greater depth. In such situations, the load has to be transferred to the bearing soil stratum (Fig.3).
Fig.3. Pile foundations
4. Caissons
Caissons are nothing but larger piles. Instead of a pile, a group few large caissons can be utilized. In some situations, caissons could be the best alternative (Fig.4).
Fig.4. Caissons
5. Foundation selection criteria
Normally, all attempts are made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settle-ment when considering shallow foundations.
The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long-term. Immediate and long-term settlements should be computed (Fig.5).
Fig.5. Different foundation types
The Fig.5 shows a shallow foundation, mat foundation, pile group, and a caisson. A geotechnical engineer needs to investigate the feasibility of designing a shallow foundation due to its cheapness and ease of construction.
In the previous situation, it is clear that a weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in soil due to loading of the footing also needs to be computed.
If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles can be installed as shown in the figure ending in the bearing stratum. In this situation, one needs to be careful of the second weak layer of soil below the bearing stratum.
Piles could fail due to punching into the weak stratum (Fig.6).The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles (Fig.7).
Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. Settling soil would drag the piles down with it. This is known as negative skin friction or down drag.
Fig.6. Punching failure (soil punching into the weak soil beneath due to pile load)
Stone has been used for many years to protect embankments, levees, river banks, and engineered features against erosion and in the construction of dams, breakwaters, and other large structures. Advantages over other materials and designs are often contingent on low cost of large-scale production and processing of stone and placement on the structure.
1. Slope Protection
a. Slope protection generally means the engineered feature composed of large-stone material constructed as a relatively thin overlay on a slope otherwise vulnerable to erosion. A bedding layer is usually included. The large-stone material is commonly called riprap. At the heart of some riprap design is the characteristic of physical flexibility. Riprap adjusts to minor flank erosion or undercutting and continues to function in its protective role.
b. One key consideration in slope protection is stone size, and the specifications for riprap should be detailed in regard to median size, gradation, and allowable tolerances. The cost advantage may be even greater where gradation requirements allow quarry-run material to be used. Even here, the importance of well defined specifications must be made clear since there are still limitations on oversize or undersize components that may require at least some separation processing.
At the other extreme from quarry-run material is riprap of narrow size range for manual placement in a keyed or fitted-stone arrangement. This labor-intensive and costly method of protecting slopes is practiced only rarely today but may be encountered in maintenance of features constructed years ago.
2. Training Structures
River training structures are relatively short, linear features constructed near the bank of a channel to control the pattern and velocity of flow. Examples are spur dikes and groins perpendicular to the bank and vane dikes more or less parallel. Rock training structures may be unzoned or earth-cored but are always designed for low cost, sometimes at the sacrifice of longevity. Groins are also used along coastlines to control sea drift currents and sand deposition. Coastal jetties train the flow of river outlets and may secondarily function as groins and breakwaters.
3. Retention Dikes
Retention dikes are designed to impound saturated materials such as from dredging. Elaborate, zoned designs are sometimes used where the waste is contaminated and the stability and the control of leachates are of high priority. Where wave attack is predictable, two or more zones of large stone are commonly incorporated, with the most critical usually being the outer armor.
As designs prescribe larger and larger stones, the problems of quality and cost increase dramatically. Smaller sized stone materials are usually more than adequate to remain stable and support the superimposed layers. Emphasis may then be redirected from high-quality stone to quantity and the need to provide large volumes of core stone.
4. Breakwaters and Jetties
a. Large breakwaters and jetties provide the outstanding examples of construction with large stone. The term rubble mound, though somewhat entrenched in the technical literature, seems inappropriate to describe these engineered structures which are commonly designed with several massive zones of different materials.
The special demands for protection against ocean wave attack lead to the ultimate among numerous possible requirements, that is very large, dense, and durable individual stones. At some large size, different for each project, the cost of constructing armor with stones exceeds that of construction with man-made concrete units such as tetrapods and dolosse. For projects near this cutoff, an especially thorough investigation of stone sources is needed. Quarries at great distance are often included, and the cost of transportation and handling must be estimated carefully.
b. Structures in deep water present special construction problems. Placement is partly remote and obscure so that quality control, quality assurance, and measurement for payment are comparably more difficult than in construction above water.
5. Zoned Embankments
a. Construction of zoned rockfill dams has constituted the largest use of large stone in some CE districts. The demands regarding material quality are usually different than for rubble mounds or even for retention dikes, and
weak rock can be used. Instead, the focus is on the immense volumes required.
b. Much of the stone material can come from excavations required as part of the project, particularly, excavations for spillways and outlet works. Quarries or stone borrow areas may be needed at least as supplementary sources. An accurate estimate of processing requirements is essential to assure sufficient volumes where suitable deposits are thin or otherwise in short supply or the material quality is marginal.
The attendant blasting, handling, and placement sometimes degrade some of the material and can cause shortfall in acceptable stone.
6. Other Uses
Other uses of large-stone material are subordinate as a group to those identified above. Nevertheless, special uses can be significant.
a. Energy Dissipators. Energy dissipators may be constructed from derrick stone placed in plunge pools to reduce erosion of foundation soil or bedrock. The high-energy environment may preclude long service life without
additions of fresh stone.
b. Structure Protection. Stone placed around the upstream end of a bridge pier is an example of protection of structures. Stone placed to stabilize and protect dikes or breakwaters primarily composed of concretefilled caissons is another example.
c. Masonry. Masonry construction is included in this manual for completeness. The high cost of this labor-intensive method largely disfavors its use today, but stone masonry still exists in CE structures and facilities and occasionally can be repaired with essentially new masonry construction.
Dimensional and cut stone for masonry are produced in a relatively few, specialized quarries.
d. Landscaping Stone. Landscaping stone is not part of an engineered structure or feature as such, but it essentially completes some CE projects. Guidance in selecting stone and in estimating or inspecting stone work for landscaping is potentially useful on a broad scale.
Project functions and their overall social, environmental, and economic effects may influence the hydraulic design of the spillway. Optimization of the hydraulic design and operation requires an awareness by the designer of the reliability, accuracy, sensitivity, and possible variances of the data used.
The ever-increasing importance of environmental considerations requires that the designer maintain close liaison with other disciplines to assure environmental and other objectives are satisfied in the design.
Spillway Function
The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood-control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as
the only flood-control facility.
A powerhouse should not be considered as a reliable discharge facility when considering the safe conveyance of the spillway design flood past the dam. A terminal structure to provide energy dissipation is usually provided at the downstream end of the spillway. The degree of energy dissipation provided is dependent upon the anticipated use of the spillway and the extent of damage that will occur if the terminal structure capacity is exceeded.
The standard project flood is a minimum value used for terminal structure design discharge. The designer must keep in mind that damage to the dam structure that compromises the structural integrity of the dam is not acceptable. Acceptance of other damages should be based on an economic evaluation of the extent of damage considering the extremely infrequent flood causing the damage.
Spillway Classification
Spillways are classified into four separate categories, each of which will serve satisfactorily for specific site conditions when designed for the anticipated function and discharge.
1. Overflow Spillway
This type of spillway is normally used in conjunction with a concrete gravity dam. The overflow spillway is either gated
or ungated and is an integral part of the concrete dam structure.
Figure 1 : Chief Joseph Dam overflow spillway
2. Chute Spillway
This type of spillway is usually used in conjunction with an earth- or rock-filled dam; however, concrete gravity dams also employ chute spillways. In these cases the dam is usually located in a narrow canyon with insufficient room for an overflow spillway. The chute spillway is generally located through the abutment adjacent to the dam; however, it couldbe located in a saddle away from the dam structure.
Figure 2 : Mud Mountain Dam
Figure 3 : Wynoochee Dam
3. Side Channel Spillway
This type of spillway is used in circumstances similar to those of the chute spillway. Due to its unique shape, a
side channel spillway can be sited on a narrow dam abutment. Side channel spillways generally are ungated; however, there is no reason that gates cannot be employed.
Figure 4 : Townshend Dam side channel spillway
4. Limited Service Spillway
The limited service spillway is designed with the knowledge that spillway operation will be extremely infrequent, and when operation occurs, damage may well result. Damage cannot be to the extent that it would cause a catastrophic release of reservoir water.
Gravity dams are solid concrete structures that maintain their stability against design loads from the geometric shape and the mass and strength of the concrete. Generally, they are constructed on a straight axis,
but may be slightly curved or angled to accommodate the specific site conditions.
Gravity dams typically consist of a nonoverflow section(s) and an overflow section or spillway. The two general concrete construction methods for concrete gravity dams are conventional placed mass concrete and RCC.
1. Conventional concrete dams
Conventionally placed mass concrete dams are characterized by construction using materials and techniques employed in the proportioning, mixing, placing, curing, and temperature control of mass concrete (American Concrete Institute (ACI) 207.1 R-87). Typical overflow and nonoverflow sections are shown on Figures 1 and 2.
Figure 1 : Typical dam overflow section
Figure 2 : Nonoverflow section
Construction incorporates methods that have been developed and perfected over many years of designing and building mass concrete dams. The cement hydration process of conventional concrete limits the size and rate of concrete placement and necessitates building in monoliths to meet crack control requirements.
Generally using large-size coarse aggregates, mix proportions are selected to produce a low-slump concrete that gives economy, maintains good workability during placement, develops minimum temperature rise during hydration, and produces important properties such as strength, impermeability, and durability. Dam construction with conventional concrete readily facilitates installation of conduits, penstocks, galleries, etc., within the structure.
Construction procedures include batching and mixing, and transportation, placement, vibration, cooling, curing, and preparation of horizontal construction joints between lifts.
The large volume of concrete in a gravity dam normally justifies an onsite batch plant, and requires an aggregate source of adequate quality and quantity, located at or within an economical distance of the project.
Transportation from the batch plant to the dam is generally performed in buckets ranging in size from 4 to 12 cubic yards carried by truck, rail, cranes, cableways, or a combination of these methods. The maximum bucket size is usually restricted by the capability of effectively spreading and vibrating the concrete pile after it is dumped from the bucket. The concrete is placed in lifts of 5- to 10-foot depths. Each lift consists of successive layers not exceeding 18 to 20 inches. Vibration is generally performed by large one-man, air-driven, spud-type vibrators.
Methods of cleaning horizontal construction joints to remove the weak laitance film on the surface during curing include green cutting, wet sand-blasting, and high-pressure air-water jet. Additional details of conventional concrete placements are covered in EM 1110-2-2000.
The heat generated as cement hydrates requires careful temperature control during placement of mass concrete and for several days after placement. Uncontrolled heat generation could result in excessive tensile stresses due to extreme gradients within the mass concrete or due to temperature reductions as the concrete approaches its annual temperature cycle.
Control measures involve precooling and postcooling techniques to limit the peak temperatures and control the temperature drop. Reduction in the cement content and cement replacement with pozzolans have reduced the temperature-rise potential. Crack control is achieved by constructing the conventional concrete gravity dam in a series of individually stable monoliths separated by transverse contraction joints.
2. Roller-compacted concrete (RCC) gravity dams
The design of RCC gravity dams is similar to conventional concrete structures. The differences lie in the construction methods, concrete mix design, and details of the appurtenant structures. Construction of an RCC dam is a relatively new and economical concept.
Economic advantages are achieved with rapid placement using construction techniques that are similar to those employed for embankment dams. RCC is a relatively dry, lean, zero slump concrete material containing coarse and fine aggregate that is consolidated by external vibration using vibratory rollers, dozer, and other heavy equipment.
In the hardened condition, RCC has similar properties to conventional concrete. For effective consolidation, RCC must be dry enough to support the weight of the construction equipment, but have a consistency wet enough to permit
adequate distribution of the past binder throughout the mass during the mixing and vibration process and, thus,
achieve the necessary compaction of the RCC and prevention of undesirable segregation and voids.
Pile foundations have been used as load carrying and load transferring systems for many years.
In the early days of civilisation, from the communication, defence or strategic point of view villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling.
Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones.
In 1740 Christoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. Steel piles have been used since 1800 and concrete piles since about 1900.
The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel driven machines.
More recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation.
Footings are understood formed by a rigid rectangular base of stone or concrete of dimensions : width B and length L, in which the ratio of L/B will not exceed 1.5.
The foundation structure will support the column load. The bearing capacity of the footing may be stimated, and its dimensions selected ; thereafter, a forcast of the settlement is made.
To illustrate the case pf footing foundations, consider a building with nine columns (Fig 1) supported on isolated footings. In this case, the footings will work independently of each other. Therefore, it is required that the differential settlements between footings will not exceed the allowable total and differential settlement requirements.
Figure 1: Isolated Footings
The differential settlements may be reduced selecting properly the area of the footings, and at times, using the stiffness of the superstructure.
From the structural point of view, however, the superstructure should not be allowed to take high secondary stresses induced bu the differential settlements of the footinhs, except in very special casees.
Singles footing foundations, in general, will be user only in soils of low compressibility and in structures where the differential settlements between columns may be controlled by the superstructure flexibility, or including in the design og the building joints or hinges that will take the differential settlements and/or rotations, respectively, without damaging the construction.
Continuous Footings
When it is necessary to control within certain limits the magnitude of differential settlements between columns supported on footings, and when soil deposits of medium or low compressibility atre encountered, it is recommended to use continuous footings. They may be defined as resisting elements joining columns together by foundation beams.
Continious footings are arraged by joining two or more columns together with beams.
The vertical differential displacements may be controlled via beam stiffness (Fig 2). The selection of the foundation beams, either running in one direction or the other along columns rows, depends largely on the layout of the column loads, and othe functional requirements concerning the structural and architectural design of the project.
Figure 2: Continuous Footings
For heavier loads, and when the project calls foor stiffness in both directions (namely, along column rows A, B and C also rows 1,2 and 3), the foundations is given stiffness with beams in both directions (Fig 3).
Figure 3: Continuous Footings
In this case, it may be observed that the footing slabs will cover practically all the foundation. This type of foundation using continuous footings is advantageous in soils of medium compressibility, where it is necessary to control differential movements between colomns.
The foundation beams are designed with the necessary stiffness to fulfill the differential settlements requirements.
Raft Foundation
When the loads are so largee that continious footings will occupy close to 50% of projected area of the building, it is more economical to use a continuous mat covering the entire area, as shown in Fig 4. The total load in this case may be assumed uniformly distributed in the area covered by the building.
The soil reaction is determined on the basis of a safe bearing capacity. The total and differential settlements may be investigated considering the stiffness of tthe raft or foundation slab is a matter of economy, compatible with the allowable differential settlements.
Figure .4 : Mat Foundations
Flexibility is important to obtain economy ; however, restrictions in differential vertical displacements between colmuns may call for certain slab stiffness, either by making it thicker or by placing foundation beams joining columns rows.
The beams can be designed with the required stiffness to reduce differential displacements.
This type of foundation may be used generally in soil deposits of medium compressibility ; however, in certain instances, the surface raft foundation may be used in soils of high and very high compressibility, where large total settlements may be allowed.
This type of foundation may be used efficiently in reduucing differential settlement.
Compensated Foundations
In soil deposits of medium, high and very high compressibility and low bearing capacity, compensated foundations are indicated.
This type of foundation requires a monolithic box foundation, as shown in Fig 5. When the water table is close to the ground surface, water proofing is necessary to use the buoyancy effect in designing the foundation.
Figure .5 : Compensated Foundation
In the design of compensated foundations, it should be borne in mind that the soil should be consedered as a material of two phases, namely : a solid and liquid phase.
Therefore, in a compensated foundation, the compensation is made by adding two effects :
Substitution of the submerged weight of solids.
The buoyancy effect by weight of liquid displaced.
Both effects are used to equalize the total weight of the building. The volume of the concrete box forming the foundation strructure annd basements will displace a weight of liquid that, according to Archimedes’ principal, will contribute in floating the foundation up to this value, reducing the load applied to the solid phase.
The load taken by the solid phase will, however, deform the soil because of the change in effective stresses induced in the soil structure. It shoul be investigated from the point of view of bearing capacity of the soil and total and differentioal settlements, as previously discussed for other foundations.
Compensated Foundations with Friction Piles
When a compensated foundation as described is not sufficient to support the load with the allowable total settlement, in spite of desigin the foundation with sufficient stiffness to avoid detrimental differential settlements within the foundation itself, friction piles may be used in addition to the concept of compensation.
This case may be present in deposits of high or very high compressibility extending to great depth. The piles will reinforce the upper part of the soil where a higher comopressibility is encountered.
The applicability of this foundation calls for a soil that varies from very high compressibility at the upper part of the deposit, to medium or low compressibility at the bottom (Fig 6).
Figure 6: Compensated friction pile Foundation
Point Bearing Pile Foundations
When the loads to be supported are higher than those a compensated friction pile foindation can take, the nit will be required to find a deep-seated hard stratum of low to very low compressibility and high shear strenth, where piles can be driven to point bearing. One can distinguish two cases of point bearing foundations (Fig 7 and Fig 8).
The first case is recognized when the hard stratum of convenient thickness is found underlain by materials of medium compressibility. In these cases the piles should be evenly distributed as shown in Fig 7. After solving the problem of point bearing of the piles in the hard stratum, there still existas the problem of finding if the lower compressible soil stratum will have a safe bearing value, and also if the total and differential settlements will be within the allowable values specified for the foundation in question.
This type of foundation should be designed with sufficient stiffness to control differential settlements.
The second type of pile fooundation is recognized wwhen the point bearing piles rest in a firm deposit of low compressibility extending to great despth (Fig 8). In this case, it is economical to use groups of piles to solve the foundation problem.
Figure 7 : Point Bearing Piles
Figure 8 : Point Bearing Piles in groups
The columns will rest on single footings supported on the piles. The piles driven in the firm stratum develop laterad friction contributing to the mechanical properties on shear strengh of the depostigs in whttich the are driven, on the spacing of the piles, on the length of penetration into the bearing stratum, and on the state of density and confinement of such stratum.
Pier Foundations
Pier foundations are used to support very heavy loads in buried soil deposits of very low compressibility (Fir 9). Their load capacity is a function of the mechanical properties of the soil under the base of the pier, and of the confining stress of the bearing stratum. Actuelly, the bearing capacity of such an element is determined as a deep-seated isolated footing.
Figure 9 : Pier Foundation
The piers, columns-like elements cast in place, in most cases carry high loads of 500 ton or moore ; therefore, the compressibility of the deposit on which the are resting should be very low, in order that they may be recommended.
Pier shafts may be used from dimensiosn are also a function of the procedure used to perform the excavation, and of the way the hydraulic conditions are handled. The density of the material wherre these elements aare bearing may be altered during excavations if an upward water flow is produced. Specially important is the case when the material is a cohesionless fine sediment or when the cohesion is small, in which case it is necessary to perform the excavation using a pneumatic system, introducing air under sufficient high pressure to balance the flow of water toward the bottom of the excavation, preserving the natural confining and density connditions of the bearing stratum.
Usually, if precautions are taken in the installation of these elements, the settlements will be very small. The settlement, however, may be estimated knowing the stress-strain characterictics of the strata encouontered under the base of the piers. The nagative friction on these elements may take large proportions : hence, it shoul be estimated.
When these rigid elements are used in seismic regions to support loads through deposits of high and very high compressibility, it is necessary to investigate the effect ot the horizontal motion of the soil mass during earthquakes. The horizontal drift forces against the piears because of soil displacement should not be overlooked. In occasions, rigid elements have been damaged because of the strong horizontal motions produced bu the earthquakes.
Sand Pier Foundations
The solution of foundations using sand pier or sand piles is shown in Fig 10.
Figure 10 : Sand Piers
This type of foundation is used to increase the load capacity of the soil by reducing its compressibility and increasing its shear strength capacity properties. This type of pile may be used in loose or medium dense sand deposits.
The improvement of the subsoil is a function of the volume of sand introduced at the time thses elements are installed. Usually first a hole is driven in the ground, then sand is introduced and highly compacted in layers, using a heavy ram.
The sand element will take the load because of the lateral confinement given by the subsoil. The deformation of these elements may be estimated by means of the stress-strain properties of the sand ussed, considering the pier as a long sand cylinder laterally confined bu the soil. This type of foundation is only recommended in places where the cost of cement is very high, and good aggregates to fabricate concrete are difficult to obtain.
Soil nailing is a construction technique that we use to reinforce soil to make it more stable. We use this technique for slopes, excavations, retaining walls etc. to make it more stable.
In this technique, soil is reinforced with slender elements such as reinforcing bars which are called as nails. These reinforcing bars are installed into pre-drilled holes and then grouted.
These nails are installed at an inclination of 10 to 20 degrees with vertical.
Soil nailing is used to stabilize the slopes or excavations where required slopes for excavation cannot be provided due to space constraints and construction of retaining wall is not feasible. It is just an alternate to retaining wall structures.
As the excavation proceeds, the shotcrete, concrete or other grouting materials are applied on the excavation face to grout the reinforcing steel or nails. These provide stability to the steep soil slope.
Types of Soil Nailing:
There are various types of soil nailing techniques:
Grouted Soil Nailing:
In this type of soil nailing, the holes are drilled in walls or slope face and then nails are inserted in the pre-drilled holes. Then the hole is filled with grouting materials such as concrete, shotcrete etc.
Driven Nails:
Driven nailing is used for temporary stabilization of soil slopes. In this method, the nails are driven in the slope face during excavation. This method is very fast, but does not provide corrosion protection to the reinforcement steel or nails.
Self drilling Soil Nail:
In this method, the hollow bars are used. Hollow bars are drilled into the slope surface and grout is injected simultaneously during the drilling process. This method of soil nailing is faster than grouted nailing. This method provides more corrosion resistance to nails than driven nails.
Jet Grouted Soil Nail:
In this method, jets are used for eroding the soil for creating holes in the slope surface. Steel bars are then installed in this hole and grouted with concrete. It provides good corrosion protection for the steel bars (nails).
Launched Soil Nail:
In this method of soil nailing, the steel bars are forced into the soil with very high speed using compressed air mechanism. The installation of soil nails are fast, but control over length of bar penetrating the ground is difficult.
These points must be noted for installation of soil nails:
Soil Nails must penetrate beyond the slip plane into the passive zone typically for 4 to 5m.
The spacing of soil nails in horizontal or vertical direction must be related to strength of the soil. Extra soil nails should be installed at the edge of any surface being stabilized.
Soil nailing should start immediately after excavation. Any delay may lead to collapse of soil slope.
Geologists, engineers, and other professionals often rely on unique and slightly differing definitions of landslides. This diversity in definitions reflects the complex nature of the many disciplines associated with studying landslide phenomena.
For our purposes, landslide is a general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement (please see figure 1 for an example of one type of landslide).
Figure 1. This landslide occurred at La Conchita, California, USA, in 2005. Ten people were killed. (Photograph by Mark Reid, U.S. Geological Survey.)
Varying classifications of landslides are associated with specific mechanics of slope failure and the properties and characteristics of failure types; these will be discussed briefly herein.
Figure 2. A simple illustration of a rotational landslide that has evolved into an earthflow. Image illustrates commonly used labels for the parts of a landslide (from Varnes, 1978, Reference 43).
There are a number of other phrases/terms that are used interchangeably with the term “landslide” including mass movement, slope failure, and so on. One commonly hears such terms applied to all types and sizes of landslides.
Regardless of the exact definition used or the type of landslide under discussion, understanding the basic parts of a typical landslide is helpful.
Figure 2 shows the position and the most common terms used to describe the unique parts of a landslide.
