Shoring Suite is a software package that contains four modules that can be linked together: Shoring, EarthPres, Surcharge, and Heave. All are sophisticated design and analysis tools developed by experienced engineers and professors. The program has been widely used by engineers, contractors, universities, and government agencies nationwide and overseas.
For more information on EarthPres, Surcharge, and Heave, click here. The program can link to EarthPres and Surcharge modules so that the data from these two programs can be directly imported into Shoring. The program shows diagrams of pressures, shear, moment, and deflection.
It calculates the moment of the piles and selects the five most suitable piles for you. The program not only presents the major properties of these piles but also calculates the top deflection of each pile.
For braced systems, the program supports calculation for wale and strut. The following table lists approximate values. Please note that these conversion tables are approximate. They can be used by characterizing the soil as being either predominately granular or cohesive. If possible, the conversion of the penetration index N Value should be checked by performing laboratory or in-site tests.
It is dependent on the soil structure and the interaction or movement with the retaining system. Due to many variables, shoring problems can be highly indeterminate. Therefore, it is essential that good engineering judgment be used. Active and passive earth pressures are the two stages of stress in soils which are of particular interest in the design or analysis of shoring systems. Active pressure is the condition in which the earth exerts a force on a retaining system and the members tend to move toward the excavation.
Passive pressure is a condition in which the retaining system exerts a force on the soil. Since soils have a greater passive resistance, the earth pressures are not the same for active and passive conditions. When a state of oil failure has been reached, active and passive failure zones, approximated by straight planes, will develop as shown in the following figure level surfaces depicted.
The well known earth pressure theories of Rankine and Coulomb provide expressions for the active and passive pressure for a soil mass at a state of failure. For a true fluid the ratio would be l. The next step is to determine the value of the earth pressure coefficient K. This is accomplished by utilizing the known soil properties and the accepted theories, formulas, graphs and procedures that are available.
Earth pressure coefficients may also be calculated by acceptable soil mechanics formulas. Two of the more well known authors are Rankine and Coulomb. If a large wall friction value can develop, the Rankine Theory is not correct and will give less conservative results.
Rankine's theory is not intended to be used for determining earth pressures directly against a wall friction angled does not appear in equations above. The theory is intended to be used for determining earth pressures on a vertical plane within a mass of soil. The accuracy for Coulomb wil1 diminish with increased depth. For passive pressures the Coulomb formula can also give inaccurate results when there is a large back slope or wall friction angle. These conditions should be investigated and an increased factor of safety considered.
The difference between the Log-Spiral curved failure surface and the straight line failure plane can be large and on the unsafe side for Coulomb passive pressures especially when wall friction exceeds. Ka may be read directly from the curves using the lower portion of FIGURE 8 whereas Kp must be multipliedby a reduction factor R located at the top of the figure.
Rankine is conservative relative to other methods. Except for the passive condition when is greater than approximately Coulomb is conservative relative to Log-Spiral. These methods developed as refinements to one another; each in its turn accounting for more variables and thereby requiring increasing levels of analytic complexity.
Because of the cohesive property of clay there will be no lateral pressure exerted in the at-rest condition up to some height at the time the excavation is made.
However, with time, creep and swelling of the clay will occur and a lateral pressure will develop. This coefficient takes thel - characteristics of clay into account and will always give a positive lateral pressure.
For a 20' deep excavation the movement needed at the top of the excavation would amount to 0. A slope of 1. This pressure is represented by a lateral earth pressure diagram. External loadings which affect total lateral pressures must be considered. External loads consist of surcharges and hydrostatic pressure. The design pressure diagram will be a summation of the basic soil pressures, surcharges, and hydrostatic pressure.
The type of shoring has to be identified. The shape of the soil pressure distribution diagram depends upon the type of soil to be encountered and the amount of shoring movement that can be permitted. A shoring system can be restrained fixed, or flexible. For example, a tieback sheet pile wall converts from a cantilever to a flexible restrained systemwhen the tiebacks are stressed.
A true fixed system is unusual in shoring work. No movement of the earth retained can occur in a fixed system.
An example of a fixed system would be a concrete or concrete slurry wall with tiebacks locked off at a value in excess of design load which causes the wall to exert pressure on the contained soil.
This complex type of shoring has been used for excavations for large buildings adjacent to existing structures. The triangle represents the distribution of the equivalent fluid pressure. Triangular pressure diagrams are used only for flexible type shoring systems. For restrained systems it has been determined by research and actual tests that the shape of the earth pressure diagram will approximate a trapezoid. Note that for a flexible system the arching will not take place and the active earth pressure is properly taken as an equivalent fluid with a triangular pressure distribution.
The location of the apparent point of fixity, PC, sometimes referred to as the point of contraflexure will be affected by the type of soil and configuration of the system. Lateral soil pressure is normally thought of as increasing uniformally with depth. The first pressure diagram conceptualized is a triangle. However, a triangle pressure diagram configuration is generally used only for flexible support systems.
A variety of other pressure diagrams are more appropriate for other than flexible conditions and the selected choice will depend upon soil type and the designed system. Geotechnical authors differ on theories for shape of the lateral pressure distribution.
It may be necessary to compare different methods to confirm that the method used by the designer adequately fits the conditions, and will provide for changing conditions as the work progresses.
Occasionally, the submitted design will be based on an equivalent Fluid pressure parameter Kw and may or may not include information about internal soil friction angles or the unit weight of the soil. Comparisons can be made between selected pressure diagrams based on total lateral pressure as well as the use of Kw in lieu of known soil parameters.
Alternate soil pressure diagrams may be related to the common trapezoidal diagram with pressure coefficients 'normalized' so that total lateral pressures are equal. Examples of 'normalized' pressure diagram conversions start on page Some of the more commonly accepted soil pressure diagrams are included in this chapter following the 'normalized' pressure diagrams.
Little is done in this manual relative to sheet piling systems. This horizontal pressure tends to induce vertical expansion of the soil. Equivalent PA values were computed as shown on the next sheet, so that the resultant total pressures equate to the standard trapezoidal diagram above left.
Note that the total force of the various areas are equal. If total lateral force is calculated by other means Rankine, Coulomb, Log Spiral, etc.
The general equation for active lateral pressure for clay is as follows: It is a conservative approach to consider no angle of internal friction when investigating or designing shoring systems in Clays.
Another characteristic of clays is that properties, such as cohesion and moisture content will change appreciably when the clay is exposed for extended time periods. The cohesive strength will decrease and the material will approach a cohesionless soil condition. A time period longer than one month would be considered an extended period for trench or other shoring work.
It is possible to get negative values in the basic clay formulas, Initially clays can stand unsupported to some depth. This depth is called the critical depth within the critical depth limit an active lateral pressure may or may not exist depending on other conditions, such as groundwater.
For design or analysis of earth pressure systems it is not acceptable to use negative pressure. The controlling design pressure is then determined by making comparative calculations.
Negativevalues are not to be used. The shape of the earth pressure diagram for clays varies with different authors. If it is desired to convert from the trapezoidal loading, use the. This method will always give positive values and is acceptable to any depth. Another advantage of the Stability Number Method is that it provides an indicator of when the problem of bottom heave should be investigated.
Heave is possible when the Stability Number No is greater than 6. Do not use negative pressure values. The formulas above are generally accepted for PA, A, and B. The clay will stand unsupported for a short time, but is subject to change because of the effect of weather on exposed surface, creep in the clay, loss of cohesion, dynamic load effects, etc.
For this reason negative pressure values wil1 not be used. This illustrates the point that different answers may be obtained by using an alternate acceptable analysis. When comparing Sample Problems l and 2 to Problem 3 it is noted that the highest calculated lateral earth pressure was used in the former problems but not in the latter: the reader should be made aware that the degree of accuracy is often more-dependent on proper estimates of soil strength parameters than on the method used for calculation of lateral earth pressure.
The material will approach an equivalent fluid and the correct diagram for active lateral pressure will be a triangle. Passive pressures now have to be considered for the portion of the system embedded in the ground. Steel-sheet piles, or soldier piles, are installed in to the ground a sufficient distance below the bottom of the excavation to utilize passive pressures.
Walls designed as pure cantilevers undergo large lateral deflections. Walls may be subject to scour and erosion. Member stresses and movement increase quite rapidly with. Cantilevered sheet pile walls for shoring systems are therefore usually restricted to moderate heights of less than 15' However, very heavy sheet pile sections are now available see TABLE 19 'Sheet Pile Sections' in Chapter 8.
A few general considerations are included at the end of this section. Following is a general procedure which OSC recommends for determining an acceptable pressure distribution to use for structural analysis of shoring systems.
Classify the soil. At one extreme would be a large or complicated project for which there is a complete geotechnical soils report which will give all pertinent parameters, description of soil, ground water conditions, andrecommendations for temporary shoring loading.
The other extreme is often encountered for relatively small projects such as trenches for pipes along streets or highways - often there is no soils data included with the shoringplans. This is done by site inspection, test pits, review of other data such as log of test borings for contract or contracts within same area, etc. The less information furnished, the more conservative the review of the shoring plans must be. Determine an equivalent Kw if necessary.
Select pressure distribution pressure diagram. Develop the basic soil pressure diagram. Calculate the effect of surcharges. The Boussinesq strip loading formula may be the most useful. Equivalent surcharge loading for soil slopes above the top of the excavation is a specialized case. Railroad loading surcharges require special treatment. Sketch pressure diagrams. Compute basic soil pressures. Combine all surcharge loads including ground water effect if applicable.
Simplify the combined diagrams for analysis or design. Apply diagrams to the shoring system and make structural review. For normal short duration loading less than threemonths an overstress of 1. Allowable stresses for shoring are included in Chapter With granular soils, settlement can be expected at a distance ' from the face of the wall equaling two times the depth of l excavation.
For clay soils this distance can be as much as three times the excavation depth. Vertical wall displacements as well as wall deflections' contribute to the amount of settlement. Maximum lateral displacements for temporary suppport walls can be as much as 0. Horizontal movement of soils under buildings, roads, or other structural components generally cause more damage than vertical displacements.
Tiebackwalls usually experience the samedeformations as internally braced walls in dense cohesive sands or very stiff clays. If deformation of the wall is deemed critical, K0 should be used for design in lieu of Ka.
If settlement will be detrimental, the vertical components of tiebacks should be considered. If wall deflections are considered to be a problem, special consideration will be required for design. Lagging in soldier pile walls have a tendency to absorb more load as time progresses.
Load transfer with time will be more pronounced in cohesive soils. Subsidence may occur behind the wall if poor construction control results in voids behind the lagging. Voids behind the lagging should be backpacked so lagging is effectively tight to the soil.
Construction practices will also have a significant effect on net soil movements. Be aware that large settlement behind shoring could be an indication of bottom heave. Consultants use a variety of soil pressure diagrams, sometimes depending on recommendations made by professional geotechnical sources.
A common recommendation is that the soil pressure diagram for cantilever members should be a triangle. For single tie back or strut conditions the recommendation may include triangular soil pressure-diagram for the vertical members only especially for sheet pile type walls , whereas either the same triangular loading diagram or a separate trapezoidal pressure diagram will be recommended for loading the wales, tieback members, or for struts.
Trapezoidal soil pressure diagrams are generally shown with the active lateral pressures shown as KwH, where Kw equals pounds per cubic foot. Active values for Kw in common usage vary between 20 to 40 pounds per cubic foot.
The selection of Kw values depends on soil characteristics, site conditions, anticipated shoring configuration, and local experience. Passive lateral pressures may be shown in the form of per square foot per foot of depth. Verification of the soil characteristics, if furnished, should be made with the log of test borings closest to the site of the planned work. Groundwater will also cause an additional pressure, but it is not a surcharge load.
Examples of surcharge loads are spoil embankments adjacent to the trench, streets or highways, construction machinery or material stockpiles, adjacent buildings or structures, and railroads. With higher, irregular, or sloping embankments it will be necessary to consider all loads acting on wedges used in the Trial Wedge analysis.
It shall be used when making an engineering analysis of all types of shoring systems. This surcharge is intended to provide for the normal construction loads imposed by small vehicles, equipment, or materials, and workmen on the area adjacent to the trench or excavation. It should be added to all basic earth pressure diagrams.
The Boussinesq Strip is one such method and can be used for all surface surcharge loads unless the load is treated as a soil embankment. There are formulas for line and point load surcharges. The strip formula can be used satisfactorily for most situations.
This is represented by a rectangular pressure diagram. The height of the original excavation is increased by an amount equal to the surcharge pressure divided by the density of the soil. Conventional analysis Rankine, Coulomb, or Log-Spiral should be used for slopes with angles equal to or less than the soil internal friction angle Irregular embankment slopes cannot rationally be converted to the conventional slope analysis method depicted above.
Irregular embankment sloping conditions should be analyzed by the Trial Wedge method. It is applicable to irregular slopes and varying soil strata. Draw AX perpendicular to back of the wall or system.
Draw AY vertical. Draw an arc from AO to AP any convenient radius. Draw rays through break points arid any selected intermediate points AB, AC, etc to intersect arc drawn in 4. Compute the individual wedge weights sea previous sheet. Plot, at a convenient scale, the accumulative wedge weights on AY, down from A Lb , 8. Draw AZ at angle 9. The arc distances from AO to each ray is equal to the arc distances from AZ to each duplicate ray.
Draw lines at an angle equal to 6 from the wedge weight points along AY to their respective rays drawn in 9.
Plot a curve intersecting the points from Establish a point of tangency "T" with a line parallel to line AY to that portion or the curve furthermost from line AY. Establish point "S" by sketching a line from point "T", parallel to the lines delineated in step Measure between points S and T, using the same scale used to plot wedge weights, to get the resultant force acting on the shoring at the angle 6.
Resolve to the appropriate pressure diagram, which will be dependent upon soil and system type. Othermeans have been employed, but as the following pages show they are incorrect and should be avoided.
The table on the following page compares the Log-Spiral method and two of the more common "shortcut" solutions. From this table it can be seen that the difference between these methods and the more theoretically correct solution Log-Spiral can be quite large depending on the parameters used. As the ratio between the embankment slope angle and the soil internal friction angle increases, so does the difference between the various methods. When this ratio approaches 0.
For all practicality when the ratio is 0. This leaves only a small range where these "shortcut " methods are of any value. Since this is not very practical and as the Log-Spiral method is quite easy to employ, these other methods will not be used for analysis or review. Trying to analyze every possible scenario would not only be time consuming but not very practical. For normal situations. The following example compares the pressure diagrams for a HS20 truck.
The depth of excavation is 10'. Add soil pressures to sum of surcharge loads to derive combined pressure diagram. This psf loading is analogous to the 72 psf pressure diagram used for minimum surcharge loading. Generally, traffic and equipment surcharge loads beyond the limits of an inclined plane rising at an angle of 1.
Other loadings due to structures, or stockpiles of soil, materials or heavy equipment will need to be considered separately. The Southern Pacific Transportation Company SPTC concluded that values given by the AREA Boussinesq Formula were not realistic, the maximum pressure was too high and occurred to on near the ground surface so they developed their own live load surcharge earth pressure curve.
Note that all major railroads now require E80 design. Surcharge pressures are listed for one foot increments of excavation to a depth of 20 feet.
For surcharges not beginning at the face of the excavation L1 subtract tabular values for distance L1 from the tabular values for L2. The Office of Structure Construction is the liason between the job and the railroad.
The review and approval procedure is the same as it is for falsework plans. A supplemental review is performed by the Office of Structure Construction in Sacramento and the shoring plans are transmitted totherailroad for their review. It is important that this procedure is followed strictly in order that we get approval in minimum time from the railroad. For normal shoring projects, the average railroad review time is about 6 weeks. It is important that plans are prepared and include additional features that railroads require such as noted clearances, horizontal and vertical.
Most business is with the major railroads, Southern Pacific Transportation Co. These have been in effect since l All of the major railroads have agreed to accept that SPTC Specifications for review of shoring systems.
Exceptions can be made by the railroads. To obtain a waiver of this requirement would require a very unusual situation. Between 8'-6" and 10' from center of track 13' if excavation is in fill ground other than compaction controlled fill , the shoring must be a of a type that precludes the possibility of disturbance or loss of soil or base supporting the track.
This means that progressive lagging soldier beam cannot be used; driven sheet piles or concrete walls with struts placed as excavation develops would be acceptable. The sheet piles or concrete wall would have to be placed between train movements, or during temporary shut-down of track. A walkway and standard handrail is required within 13' of centerline of track.
This is for normal access of trainmen to track, not the protection of trench excavation as required by DOSH. Such walks and handrails are to be shown on the shoring plans. The soil classification is to be shown on the plans. Include groundwater conditions anticipated. Note that the railroad exempts strutted trenches -the earth pressure for such will be the minimums as required by DOSH or by calculation, using actual soil parameters; this procedure is discussed elsewhere in the manual.
For flexible systems, such as cantilevered walls, use the minimum equivalent fluid pressure of 36 pcf, or the pressures calculated from actual soil properties. An exception to AREA is the railroad live load surcharge. This live load surchargecurve will be used for all earth retaining systems Section 4. Section 5 deals with the allowable unit stresses and factors of safety. There are some differences from the policy given in this manual, however they are minor. Use controlling railroad allowable stresses when a railroad is involved.
A good clear well-engineered plan is the best way to get an early approval from the railroad. Be sure at pertinent minimum clearance dimensions are included. Railroads require that the shoring plan be prepared by a professional engineer. For this example, an alternate design would be more practical. The remainder of this problem is not presented.
The depth of sheetpiling walls below the bottom of the excavation are determined by using the difference between the passive and active pressures acting on the wall.
The theoretical depth of pile penetration below the depth of excavation is obtained by equating horizontal forces and by taking moments about an assumed bottom of piling. The theoretical depth of penetration represents the point of rotation of the piling.
Additional penetration is needed to obtain some fixity for the piling. It is not within the scope of this text to go into great detail concerning the design and analysis of sheet piling. A few of the more common situations complete with sample problems are presented on the following pages.
A more adequate. The cohesive value of clay adjacent to sheet pile walls approaches zero with the passage of time. Design and analysis for clay soil conditions must generally meet the conditions of cohesionless soil design if the sheet piling support system is to be in use for more than a month. It is possible to have negative pressure values with cohesive soils. Since cohesive soil adjacent to sheet pile walls loses its effective cohesion with the passage of time it is recommended that negative values be ignored.
Do not use negative pressure values for the analysis of sheet piling systems. Any theoretical negative values should be converted to zero. Friction: The friction value at the soil-wall interface, or adhesion between the clay and the wall, should be ignored with sheet piling walls when the walls are in close proximity to pile driving or other vibratory operations - including functional railroad tracks.
Similarly, above the depth of excavation, the cohesive value of the clay of a combined clay-sand soil should be ignored under the same circumstances. The sheetpiling will fail if this height is exceeded.
The stability number relates to kick out at the toe of the sheetpiling wall. Therefore, for design of sheetpiling walls in cohesive soils, the first step should be the investigation of the limiting height. A stability number S has been defined for this analysis as: and was derived from the net passive pressure in front of the wall in the term: Teng found that adhesion of the cohesive soil to the sheets would allow modification to the stability equation and adjusted S from 0.
A minimum stability number of 0. However, when dynamic loadings near or at the sheets is considered such as trains, pile driving operations, heavy vibrational motions, etc. Rakers: When rakers are supported on the ground the allowable soil bearing capacity for the raker footing must be considered. Cohesionless soil having small internal friction angles will have lower soil bearing capacity. Additionally, when the footings are sloped relative to the ground surface reduced soil bearing capacities will result.
For short term shoring conditions a safety factor of 2 might be used. A reduced safety factor, however, could allow greater soil settlement, which in turn would permit additional outward wall rotation.
Therefore, when wall deflection or rotation is not deemed critical a safety factor of 2 may be used for short term conditions. Tieback sheetpiling wall sample problems are included in the tieback chapter. Sample Problems: Sample problems are included in this chapter to demonstrate the principles of sheetpiling design for both cohesionless and for cohesive soils.
Additional soil pressure diagrams which relate to sheet piling are presented in the section on soldier piles. In most real situations there will be some sort of surcharge present. Simplifying the resulting pressure diagrams usingsound engineering judgement should not alter the results significantly and will make the problems much easier to resolve. The surcharge pressures can be added directly to the soil diagram or may be drawn separately.
Passive resistance may be initially reduced by dividing KP by 1. A comparison of results for computed depth and moment are tabulated below. This tabulation may be of help in Checking computer results. It is common to use nongravity retaining walls to retain the soil with anchors from one or more tiers providing additional lateral resistance.
Nongravity cantilevered walls may engage discrete vertical elements with structural facing elements for the retention of soil or may be of a type that uses continuous vertical wall elements that also form the structural facing.
Typical discrete vertical elements used for temporary shoring are steel piles with facing elements being timber lagging or steel plates.
A common material for continuous vertical wall. As used in this manual, nongravity cantilevered walls with discrete vertical elements will be referred to as soldier pile walls' and those walls with continuous elements will be referred to as sheet pile walls'. Nongravity cantilevered walls derive lateral resistance through embedment of vertical wall elements and support retained soil with facing elements. The discrete vertical elements typically extend deeper into the ground than the facing to provide vertical and lateral support.
The overall stability of anchored shoring systems and the required strength of its members depends on the interaction of a number of factors, such as the relative stiffness of the members, the depth of piling penetration, the stiffness and strength of the soil, the length of tiebacks, or tierods and the amount of anchor movement.
Tiedback systems can be considered flexible systems that allow active pressure to develop; however, if sufficient tieback force is applied and the shoring system is sufficiently rigid, the system may approximate a restrained system. These types of systems will normally experience more movement than would tiedback systems, and therefore would not be suitable for shoring used to protect. The design pressure diagrams, structural analysis and general design considerations detailed in this chapter are applicable to tiedback or strutted shoring systems.
The design of deadman anchors may be found in Chapter 11, "Special Conditions". Descriptions of single-tier and multi-tier tiedback shoring.
The embedment depth and the horizontal component of the tieback design force required are determined by analyzing the active, passive, and surcharge pressures acting on the piling. The higher percentage should be used when soil properties are derived from log of test borings or other soil information and not determined from laboratory or in-situ tests used specifically to determine soil strength.
Unbonded length is normally specified to start at some minimum distance past the failure plane to ensure that no portion of the bonded length falls within the failure wedge.
Accurate determination of this length depends on how well-known the soil properties are and how accurately the location of the failure plane can be predicted. To ensure that the bonded length falls beyond the failure-plane it is common practice to extend the unbonded length about 5 feet beyond the assumed failure plane. The minimum recommended unbonded length is 15 feet. Different loads are imposed on the system before and after the completion of a level of tieback anchors.
An analysis should be included for each stage of construction and an analysis may be needed for each stage of anchor removal during backfilling operations. The tension element of a tieback may be either prestressing strands or bars using either single or multiple elements. Tiebacks may be anchored against wales, piles, or anchorblocks which are placed directly on the soil.
The example problems in this chapter illustrate the use of tiebacks with several different types of shoring systems. In this diagram, a bar tendon system is shown; strand systems are similar. See Chapter 12 "Construction," for common materials and allowable stresses. Wale - These components transfer the resultant of the earth pressure from the piling to the tieback anchor. Anchors for temporary work are often anchored directly against the soldier piling through holes or slots made in the flanges, eliminating the need for wales.
Bearing stiffeners and flange cover plates are generally added to the pile section to compensate for the loss of section. A structural analysis of this cut section should always be required. Tendon - Tieback-tendons are generally the same high strength bars or strands used in prestressing structural concrete.
The anchorage of the tieback tendons at the shoring members consists of bearing plates and anchor nuts for bar tendons and bearing plates, anchor head and strand wedges for strand tendons. The details of the anchorage must accommodate the inclination of the tieback relative to the face of the shoring members. Items that may be used to accomplish this are shims or wedge plates placed between the bearing plate and soldier pile or between the wale and sheet piling or soldier piles.
Also for bar tendons spherical anchor nuts with special bearing washers plus wedge washers if needed or specially machined anchor plates may be used. The tendon should be centered within the drilled hole within its bonded length. This is accomplished by the use of centralizers spacers adequately spaced to prevent the tendon from contacting the sides of the drilled hole or by installation with the use of a hollow stem auger.
Generally a neat cement grout is used in drilled holes of diameters up to 8 inches. A sand-cement mixture is used for hole diameters greater than 8 inches. An aggregate concrete mix is comnonly used in very large holes. Type I or II cement is commonly recommended for tiebacks. Type III cement may be used when high early strength is desired. Grout, with very few exceptions, should always be injected at the bottom of the drilled hole. This method ensures complete grouting and will displace any water that has accumulated in the hole.
Tieback anchor There are several different types of tieback anchors. Their capacity depends on a number of interrelated factors: Location - amount of overburden above the tieback Drilling method and drilled hole configuration Strength and type of the soil Relative density of the soil Grouting method Tendon type, size, and shape Typical shapes of drilled holes for tieback anchors are depicted in Figure High pressure grouting of psi or greater in granular soils can result in significantly greater tieback capacity then by tremie or low pressure grouting methods.
High pressure grouting is seldom used for temporary tieback systems. Regrouting of tieback anchors has been used successfully to increase the capacity of an anchor.
This method involves the placing of high pressure grout in a previously formed anchor. Regrouting breaks up the previously placed anchor grout and disperses new grout into the anchor zone; compressing the soil and forming an enlarged bulb of grout thereby increasing the anchor capacity.
Regrouting is done through a separate grout tube installed with the anchor tendon. The separate grout tube will generally have sealed ports uniformly spaced along its length which open under pressure allowing the grout to exit into the previously formed anchor.
Due to the many factors involved, the determination of anchor capacity can vary quite widely. Proof tests or performance tests of the tiebacks are needed to confirm the anchor capacity. Bond capacity is the resistance to pull out of the tieback which is developed by the interaction of the anchor grout or concrete surface with the soil along the bonded length. Included with some shoring designs there may be a Soils Laboratory report which will contain recommended value for the bond capacity to be used for tieback anchor design.
The appropriateness of the value of the bond capacity will only be proven during tieback testing. For most of the temporary shoring work normally encountered, the tieback anchors will be straight shafted with low pressure grout placement. For these conditions the following criteria can generally be used for estimating the tieback anchor capacity. Forces On The Vertical Members Tiebacks are generally inclined, therefore the vertical component of the tieback force must be resisted by the vertical member through skin friction on the embedded length of the piling in contact with the soil and by end bearing.
Problems with tiedback walls have occurred because of excessive downward wall movement. The pile capacity should always be checked to ensure that it can resist the vertical component of the tieback force. In locations where oxygen deficiency condition is possible, use a mechanical blower to provide the necessary amount of fresh air. When employees are required to be in trenches four feet deep or more, ladders extending from the floor of the trench excavation to three feet or more above the top of the trench shall be provided and located to provide means of exit without more than 25 feet of lateral travel.
Daily inspections of excavations shall be made by a competent person. If any evidence of possible cave-ins or slides is apparent, do not permit employees to work in the trench. Employees shall not work in excavations where there is accumulated water, or in excavations in which water is accumulating, unless adequate precautions have been taken to protect employees against the hazards posed by water accumulation.
The classification of the deposits shall be made based on the results of at least one visual and at least one manual analysis. Visual analysis is conducted to determine qualitative information regarding the excavation site in general, the soil adjacent to the excavation, the soil forming the sides of the open excavation, and the soil taken as samples from excavated material.
Manual analysis of soil samples is conducted to determine quantitative as well as qualitative properties of soil and to provide more information in order to classify soil properly. Cohesive material can be successfully rolled into threads without crumbling.
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