Excavation and Trench Temporary Shoring Design and Construction – 21
1. Shoring configurations
2. Box cofferdam
3. Load Diagrams
4. Box sheet calculations
5. Braced sheet calculations
6. Cantilever sheet calculations
7. Sheet pile properties
8. Cofferdam wale design
9. Box Soldier pile box calculations
10. Braced soldier pile calculations
11. Cantilevered soldier pile calculations
12. Sloping ground factors
13. Shoring installation
This course includes a multiple choice quiz at the end.
The purpose of the course is to show the reader how to design and build practical excavation shoring. The design examples show the safe and proven calculation techniques for shoring design that can be applied to most excavation shoring situations. The course explains why proper shoring construction procedures are important to creating a successful shoring system.
Sheet pile and soldier pile shoring systems are designed including the walers, struts, and lagging. Cantilever, braced and box configurations are designed and compared. The load factors are developed for partial shoring in steep temporary construction banks. Finally, the proper installation procedures are explained.
This course shows how to apply soil properties theory to actual and practical shoring design. The equations and loads are derived for designing sheet piles and soldier piles. The design examples are illustrated and show how cantilever, braced and box designs are made. The design of wales, struts, and lagging is shown. This course also shows how to design for steep temporary construction banks where conventional soil theory fails. The step by step procedure for shoring construction is explained.
This course is complemented by other courses presented on by the author. These complementary courses include “Engineering Design using Excel Worksheets”, “Trench Support Options and Slope Stability Design,” and “Understanding the Geotechnical Report.”
There are many ways to stabilize an excavation: sloping, soil nails, rock bolts, gravity walls, earth-retained walls, bins, cribs, speed shores, ring beams, and piling to name a few methods. This course will address the use of sheet piles and soldier piles with lagging as common construction shoring solutions. The use of piling is often the most cost and schedule efficient method for stabilizing excavations. Most textbooks and design manuals are long on theory, but weak on practical construction design application. In fact some of the text book theory can not be applied to actual construction situations.
Excavation shoring is a positive support system that fully stabilizes the ground for an indefinite period. There are three basic configurations for pile type shoring: cantilever, braced and box. Within the three basic configurations there are any number of variations. Z-shaped sheet piles or H-shaped soldier piles can be used. The soldier pile lagging can be timber, steel plates, or concrete panels. The steel shapes can be obtained in yield strengths of 36,000 to 70,000 psi, however there may be difficulty obtaining high strength steel shapes on short notice.
The cantilever method relies entirely on the passive resistance of the soil below the excavation line to support the excavation active load and live loads. The braced system uses internal bracing and the embedded pile to share the support of the excavation active pressure and surcharge loads. The box system relies entirely on the internal bracing to resist the excavation active and live load forces. All three systems can use sheet plies or soldier piles and lagging. Each of these shoring systems has their application.
The cantilever method is the simplest from a construction standpoint. Install the piles and/or lagging and proceed unhindered with the excavation. There is no limit to the excavation width. There are no walers and struts to interfere with other construction operations. The limitation is that a 20-foot or less excavation depth is the normal practical limit. Another disadvantage is the piles must penetrate below the excavation at least equal to the height of the excavation. The cantilever is relatively structurally inefficient and will require the most steel weight of piling.
In the right soil conditions Z-shaped sheet piles is more efficient than H-shaped soldier piles and lagging. The Z-pile will distribute the loads to the passive pressure areas more evenly and will weigh less that an equivalent H-pile system. The Z-piles can be rented for about a cent a pound per month, whereas most of the time the H-piles must be bought and salvaged at a higher cost. The disadvantages are that Z-piles can be driven into only a few soil conditions. If the ground is hard or contains cobbles, these conditions will cause damage to the piles, split interlocks, and alignment problems. Obstructions and underground utilities often prevent the use of sheet piles. On the other hand, soldier pile systems can be installed in almost any ground condition. If the ground is too hard to drive even H-piles, drilled holes are used to install the soldier pile. Usually the drilled hole is backfilled with pea gravel after the pile is installed. The pea gravel requires no compaction and allows the pile to be easily extracted when the work in the excavation is complete.
The braced system has the advantage of a more efficient structural support system than a cantilever method. There is no limit on depth of excavation, since multiple layers of walers and struts can be added as needed. One disadvantage is that the walers and struts may interfere with other construction operations. The excavation may have to be staged by excavating to just below a waler elevation and the wale system installed. Then the excavation is completed with the walers and struts in the way.
Box shoring system can often be designed to be very efficient structurally. The walers can be placed to reduce the unsupported pile lengths. In weak soils with low internal friction angle boxes are effective, as the box does not rely on the passive resistance of the soil. Another advantage is that the piles do not extend below the excavation. If there is a hard rock layer close to the bottom of the excavation, the box eliminates the need to penetrate the rock for support. If the soil is oozing mud the toe of the box can be extended into the mud to control the bottom heave of the excavation. The major disadvantage is the excavation and backfill is often required in at least three stages to install and remove the waler systems. Also there is that much more installation work and interference. Before selecting a particular system, several alternatives should be considered.
Failure of shoring systems can be catastrophic if improperly designed. For this reason it is recommended that a conservative loading be applied to the critical structural members of the system. Struts are the most vulnerable for several reasons. First, struts are almost always columns in high compression. If a column fails in compression this can be with little or no warning because the failure deflection compounds the failing moment very quickly. Struts are also exposed to construction loads and impact as the crane moves objects in and out of the shoring pit. If the impact on a strut cripples it, there can be an almost instantaneous collapse.
Wales and piles will generally fail in bending or web crippling. In either case there is usually time to take corrective, if expensive action. In any event it is usually not worth the risk to under design excavation shoring. Shoring commonly must be modified during construction due to unexpected interference and ground condition. Failed shoring can take several weeks and hundreds of thousands of dollars to fix, as well as risking lives, and property.
Lets select a box cofferdam as our example because as it is built it will experience all of the three basic configurations. The example cofferdam is to surround a bridge pier footing that is 18 feet wide by 30 feet long and 5 feet thick. The footing is to be founded on rock that is 40 feet below the original ground surface. Assume the rock prevents any penetration of the piling. The over burden sand is suitable for sheet piles.
The first stage is to first install the sheets using the wales as driving guides. Then excavate slightly below the bottom of the upper wale and install the upper wale. During this activity the sheets are resisting the soil pressure in cantilever.
During stage 2 excavation and lower waler installation, the passive soil pressure supports the toe of the sheets and the upper waler supports the top of the sheets, so the braced configuration is being used. Once the lower waler is in place the cofferdam is fully supported by the walers and the box configuration is created.
The first major step in the design process is to select loading configurations. Assume the sand is relatively compact and dewatered. The sand in situ density is Gs = 100 pcf and has an internal friction angle of F = 30 degrees. There are several methods of calculating active and passive soil pressures. The recommended configurations vary by soil type and support method. I found that over the years that a simple solution works as well as any. The reason for the use of a simple configuration is most soil conditions are not constant within the supported ground, live loads can be point loads close to the top of the piles and nobody really knows what the exact soils loads will be. I use a universal configuration that is slightly conservative. But I have not had a major structural failure in 30 years and of all sorts shoring and cofferdam configurations.
In rigid cases the active soil pressure is calculated as: Pa = KaCaGsH where Ka = 0.8 for rigid configurations, Ca = (1-SinF)/(1+SinF), F = the soil internal friction angle, Gs = soil density in pcf, and
H = the depth of excavation. A 72-psf live load is applied in addition to the active soil loading.
In the flexible case the active soil pressure at the sheet pile tip is Pa = CaGs (H+Y), where: Ca = (1-SinF)/ (1+SinF), Gs = soil density in pcf, and H= the depth of excavation and Y is the required pile penetration below the excavation for passive soil resistance. For the active case in both rigid and flexible cases an additional live load of 72 psf is added to the face of the piling. The 72 psf is recommended by Cal-Trans and is derived from a vertical uniform industrial loading of 240 psf times 0.3 to translate an active horizontal load through the soil.
The reason that the flexible cantilever load configuration is taken as a hydraulic pressure is that the sheet piles will deflect enough for the soil to settle and activate cohesion and internal friction. The braced and box configurations are too stiff to allow the soil to move enough to fully activate internal friction. The walers and struts must absorb the extra pressure the soil does not absorb. For this reason, a trapezoidal load configuration is appropriate.
Now we are ready to start designing the example. At this point I highly recommend using a computer to run the design calculations. The process often requires a series of iterations before a final design can be completed. You may wish to incorporate beams or piles that are readily available or already owned. Below is the computer work sheet for the sheet pile box case. Simple statics solves the waler loads.
Please note that the walers are placed at the optimum location. The reason this works is the sheet piles are lightly loaded. The next example down (braced configuration for stage 2 excavation) shows that the sheets with a 48% higher bending stress. In this example no change of sheet pile selection is needed because of the light bending stress.
The US Steel method allows the use of an additional load reduction factor of 0.75 (0.8×0.75 = 0.6) when applied to sheeting or lagging. This extra load reduction factor should never be applied to the wales and struts. Mt is the cantilever moment taken about the upper wale. Mc is the center span maximum moment where shear is equal to zero. X is the calculation to locate the distance from zero shear to the bottom of the 0.2H triangle load. Mb is the cantilever moment about the lower wale.
This example is given as symmetrical. However, the equations that are presented by the worksheet example are written to solve an asymmetrical wale placement. Due to obstructions and/or concrete lift heights, it may be advantageous to select an asymmetrical arrangement. The construction inside the shoring or cofferdam is the controlling criteria for the design arrangement of the shoring.
Note that no effort is made to check bearing or shear in the pile. In 30 years of cofferdam and shoring design I have never found a case where shear or bearing came even close to being a critical design consideration.
The braced configuration is calculated for the intermediate stage of construction where the upper waler is in place and the excavation is completed to just below the lower wale elevation. The lower waler is installed and then the excavation is completed. Note that the sheet piles are under higher stress than when the box shoring is complete. Also note that the required length of the sheet piles is less than a foot shorter than the 40-foot required length. Very often the intermediate construction stages will actually control the shoring design.
All the calculations are made in EXCEL worksheets. The worksheets are linked so that when changes are made to the box configuration all the parameters are automatically installed in the braced and cantilevered intermediate situations. Another major advantage of using EXCEL is that the direct solution of the pile penetration (Y) for the braced and cantilever situations requires cube exponents. These are extensive equations are tedious to solve by hand. The goal seek function in EXCEL solves the Y dimension in seconds. Also, note that there are “OK” statements in the worksheets. These are actually “IF” statements.
If the pile section modulus is less than the calculated minimum, the statement “NO GOOD” will appear.
The final check is for the sheet pile cantilever when the stage 1 excavation is just below the upper wale and before the wale installation. The components of the load diagram are:
This diagram is difficult to calculate, so to make the problem readily solvable a slight modification is made. The “R” passive triangle is extended to the toe elevation of the sheet pile and the “Q” toe passive is extended to included the added area of the extended “R” area. By making by making that math model change the problem is easily solved and the overlapping areas of R and Q cancel each other to zero. Now the math model is as such:
P is the active soil load. N is the surcharge live load. R and Q are the soil passive resisting loads.
R = GpY^2/2
Q = GpY(H + 2Y)/2
P = Ga(H+ Y)^2/2
N = We(H + Y)
R = Q + P + N or
Q = R – P – N
Y = 2(R – P – N)/Gp(H + 2Y)
Lp = H + Y
The sum of the moments about the pile tip is Msum = PLp/3 + NLp/2 + QZ/3 – RZ/3 = 0
The EXCEL computer software really helps make the solution easily calculated. Install the equations as formulas in the cells and address the criteria cells. All of the equation cells should tie together as needed. The Msum equation cell addresses a cell that is number D. To start the process number D is an arbitrary number. On the menu tool bar is Tools. In Tools is Goal Seek. Click on Goal Seek. The top box is for the formula cell, enter the cell and click on the cell containing the Msum formula, that cell address will appear in the box. The middle box is for the target number, in this case zero (0). The bottom box is for the address of the D number cell. Enter the cell and click of the D cell. Hit the keyboard Enter button or click on OK in the icon. When the calculation is complete it will ask for OK again. The design of a sheet pile cantilever wall can be done in just a few minutes.
This example shows that the sheet piles are easily strong enough and long enough to make the first stage of excavation with no redesign needed for this example. The above examples used a PZ22 sheet pile section. It is one of the lightest and weakest Z-type sheet piles. There are a number of manufactures of sheet piling, but they all are variations on the basic Z-shape. Below is a catalog cut from Bethlehem showing their size selection. These piles sizes can be mixed in the same sheet pile wall because the interlock is the same for all the piles. The pile interlocks from different manufactures are different and are not designed to fit each other’s piles. Sheet piles are usually shipped and driven in pairs.
While this design example uses PZ22 sheet piles, note that the PLZ23 sheet pile weighs only 0.6 psf more than the PZ22 sheet pile but is 67% stronger and 140% stiffer. The PLZ23 will tend to drive straighter and is less likely to be deflected by minor soil changes and occasional small cobbles. This cofferdam has about 4,000 sf of sheet piles, so adding 0.6 psf adds only 2,400 lbs. At $0.10 per pound, that adds only about $240 to the cost of the cofferdam. If there is a concern that the soil conditions will cause sheet pile damage, it is better to use a heavier sheet pile. Sheet piles are designed to be used in sands and gravels that are not cemented or very dense. Sheet piles will penetrate clays that are not too stiff. Generally when the penetration test of the soil exceeds about 20 blows per foot the sheet piles will likely experience some damage. Sheet piles are easily forced off alignment and split the interlocks open when cobbles or obstructions are encountered. When sheet piles must be used to form a watertight cofferdam and the soil conditions are unsuitable, some times the piles can be advanced using an expensive dig and drive procedure that keeps the excavation just below the toe of the sheet pile.
Now that the wale locations have been defined to accommodate the sheet piles and intermediate construction stages, the wale design can proceed. This example does not include struts. Although struts can be used to reduce overall wale system weight and beam sizes, they get in the way of the other construction activities in the cofferdam. The clear opening should be at least 15 feet on each side to allow clam or backhoe buckets to work below the wale system. When there are multiple wale layers, the struts should always be spaced directly over each other to allow vertical crane access to the work. When possible, struts should be arranged out side of the new structure. When struts must pass through a structure, blockouts, form interference, rebar modifications and pour backs are usually needed. Because struts cause interference with other activities, it is recommended to minimize their use or space as far apart as possible.
This example uses a simple box wale. The load distribution is such:
The example here determines the plan dimensions of the wale length and width as a function of the sheet pile pair length plus two corner piece sizes. Unless the shoring must be water tight, corner pieces are optional. From a practical standpoint corner pieces help control the alignment of the sheet piling while they are being threaded and driven.
Note that the program calculates the wales as a combined beam and column. The radius of gyration for the strong axis is used because the sheet piles restrain the compression flange of the beams. The bearing of the lengthwise beam is taken at the center of the widthwise beam. The shear caused by the widthwise beam is placed on the tension flange of the lengthwise beam and used to reduce the total beam moment. The compression forces are calculated using the entire width and length of the walers. Once the wale beams size and properties are entered all calculation is performed automatically.
The connection between the wales is also designed with a bolted and a welded alternate. The welded alternate has the advantage of cutting and fitting the wales to match the in place sheet pile alignment. The bolted connection has the advantage of shop fabrication and easy field assembly. Moment resisting connections can also be designed, but they are usually too expensive to be efficient for temporary work.
The welded connection merely welds the web of the widthwise beam to the flange of the lengthwise beam, with both webs in the same horizontal plane. As a precaution and added stability, usually the flanges of the beam are given a single pass fillet weld. The bolted connection requires that a plate be welded to the widthwise beam and holes drilled in the flange of the lengthwise beam. Individual preferences and field conditions dictate the choice of connection type.
The final check is for web bearing. Web crippling often determines the beam selection. The other solution is to design stiffeners at the bearing points. Stiffeners are expensive so they should be designed only when no other solution is justified.
The footprint of the shown cofferdam example is only slightly bigger than the footing of the pier. This is done to show that the shoring should be designed as small as possible. In this example the sheet pile can be used as the footing form, while minimizing shoring material cost. Also, excavation and backfill are minimized. When the shoring is installed to allow construction of vaults and basement structures you must be sure to allow enough clearance to the walls so that access for forming operations can be maintained. Usually 3 or 4 feet is considered a minimum clearance from the inside wale to a wall face. In addition, sufficient clearance should be allowed for ladder or stair tower access.
The next set of examples is the same three box, braced, and cantilever as above except soldier piles and lagging is designed. Soldier pile and lagging has several advantages over sheet piles. Soldier piles can be driven or placed in drilled holes. This allows the installation of soldier piles into nearly any soil conditions. Lagging can be steel plates or timber boards, which allows the soil to be shored around obstructions such as underground utilities. The spacing of the piles is flexible so that underground and overhead obstructions can be avoided.
The design principle is nearly the same, except the passive bearing area for soldier pile is much smaller. The major difference is that the soil active loads and surcharge live loads are not carried below the design excavation elevation. The first step is to identify what a reasonable passive influence area the soldier pile will generate.
This diagram has been applied to a large number of actual construction applications. The only once did a problem occur. While trenching across a riverbed, the dewatering system partially failed and the sandy soil liquefied. The cantilevered H-piles rotated enough to require strutting across the trench with timbers. While no structural failure occurred, the piles and steel plate lagging were being used to form a pipeline reinforced concrete encasement and the rotation was starting to encroach on the design encasement dimensions.
The first example is for the soldier pile box configuration. The design principle and method is very similar to the sheet piles. The notable differences are the pile spacing, B, is added to the equations and no additional active soil load reduction factor is applied to the piles, as they are considered main structural members of the system. In addition, the design for the lagging is included as part of the work sheet. The additional active soil load reduction factor is applied to the lagging and the span width is reduced by the H-pile flange width.
The wale design method is the same as the one used for the sheet piles. The only difference is that the walers can be designed as a series of point loads generated by each pile or an average uniform load. Even the situation depicted below shows the equivalency. There are notable exceptions, which will be discussed later in this course.
The next example is the soldier pile braced configuration. Here another element is introduced. Because the penetration of the pile below the excavation is much greater for soldier piles than for sheet piles for the same shoring configuration, the passive soil internal friction angle and soil density is often significantly different from the soil generating the active soil pressure. For comparison purposes, the same 30-degree soil internal friction angle is kept constant. The toe penetration for the sheet pile is only 5.79 feet while the penetration for the soldier pile is 10.46 feet, nearly twice as deep.
This soldier pile braced example is often used in trench shoring. When it is necessary to shore several thousand feet of trench, such as an underground pipeline in a city street, it is advantageous to be able to cycle the shoring. The procedure is install the soldier piles just ahead of the trench excavation, pipe laying and backfilling operation. As soon as the backfill is complete enough, the wale system is removed and cycled ahead, leaving the soldier piles and lagging in cantilever. The trench backfill is completed and the soldier pile are extracted and reinstalled ahead of the trench excavation operation. This method greatly reduces the amount of shoring material that must be purchased. We have had past costs that were less than $5.00 per square foot for such trench shoring systems.
The major key to this type of cycling operation is the wale system. The wales and struts must be designed to be easily placed, extracted and transported past the trenching operation. Because the wales and struts are going to be subjected to brut force tactics of removal while still under loading from the soldier piles, the wales and struts should be heavy compact steel sections such as W14 shapes. The other advantage to using the heavy W14 shapes in lieu of lighter W36 or W30 beams is the trench will be significantly narrower and still allow clearance to lower the pipe lengths past the wales. To accomplish goal the author has employed a wale, strut configuration designated the “H” where two wales, and a single strut makes up a shoring component.
The top of the piles were slotted to catch a chain link. Short sections of chain was attached to each end of the wales and slipped into the pile slots to prevent the wales from falling into the trench excavation. Clip plates were welded to the struts so they would space and lock the wales. Stiffeners were welded in the wales opposite the strut flanges to accept all the point loading on only one strut flange. The top of the piles will alternate between passive and active loading depending on the movement of the construction equipment. For this reason the strut is designed to have the entire compression load eccentrically applied to the strut flange, with the strut web bearing on the wale web. This induces a high bending moment in the horizontal plane of the strut. The reason the “H” configuration is stable is that the passive resistance of the soil is about 10 times higher than the active soil load.
The strut is first removed with a backhoe or crane while under compression load. Substantial lifting forces are required to overcome the friction between the strut and wale, so an end cap plate was welded to each end of the strut to minimize deformation of the beams. The heavy wale stiffeners help minimize damage to the wales. The wales are then easily lifted out and all three pieces were loaded on a flatbed truck and taken ahead to be used as a driving template for the soldier pipes. This procedure does not delay the pipe installation and worked very successfully to set eleven-foot outside diameter pipe in a twenty-six foot deep trench. The wale system was place at the original ground level to reduce the strut load and eliminate a two-stage excavation process. The example below produces nearly 400,000 pound of strut compression. A pair of 100 ton hydraulic jacks with pipe legs can be placed wale to wale on each side of the strut to reduce the strut compression so that the strut can be easily removed.
The final design configuration is the cantilever soldier pile. Because the toe penetration of the pile is approximately 1.5 times the cantilever height, the option of using different active and passive soil densities and internal friction angles is written into the program. Again, the same soil properties are used for comparison with sheet piles. The toe penetration for soldier piles is about 1.5 times that needed for sheet piles because soldier pile generate less passive soil bearing area. The EXCEL Goal Seek command is used to calculate the pile toe penetration, D.
It is common to encounter situations where shoring the partial height of the excavation in needed. A full depth excavation may extend beyond the right-of-way or a nearby structure must be protected. One of the problems is the temporary construction sloped bank above the pile is usually steeper than the soil friction angle. The slope stability is maintained by the soil cohesion. This slope can be designed using the friction circle BANK-EXC program written by the author of this course.
The problem is the prevailing Rankine and Culmann methods of calculating the increased active soil load induced by the weight of the slope completely fail when the slope exceeds the internal friction angle of the soil. The formulas attempt to perform a square root of a negative number, so the formula results are meaningless. There are several ways of proportioning the increased active load, but the author successfully uses the simple method illustrated below.
The soil triangle above the piles and over the soil failure triangle are treated as a surcharge, where:
The area of the surcharge is As = [Lp^2][Tan(Af)]^2/2S and the area of the soil failure triangle is:
Ap = [Lp^2]Tan(Af)/2. Now the slope factor is calculated as: Ks = 1 + As/Ap = 1 + Tan(Af)/S. For bank with a 1 to 1 slope and a soil friction angle of 30 degrees the slope factor calculates to Ks = 1.58. This is then induced to the active pressure calculation so that Ga = KsCaGs or KsKaCaGs. This calculation can easily be installed in the programs shown previously.
A similar proportioning can be done for the passive soil resistance in a sloped bank.
The slope of the passive failure plane is taken as Fp = 45 + F/2. Where F is the soil internal friction angle.
X/(Se+X/tanFp) = 1 X = SeTanFp/(Se + TanFp)
Kp = X/TanFp. Kp is the ratio of passive failure plane to the excavation slope over the projected failure plane to a horizontal plane projected from the intersection of the excavation slope and the piles.
Kp = Se/(Se + Tan Fp)
For a passive excavation slope of Se equals 1 and a soil internal friction angle of 30 degrees, Kp = 0.366. This passive slope factor can be readily installed in the previously shown EXCEL work sheets. One caution is that EXCEL reads angles in radians, so the angles must first be converted by an EXCEL formula cell as: =RADIANS(Fp) or directly in the trigonometry formula cell as: =TAN(RADIANS(Fp))
Once the time is invested in writing the programs into the computer, the solution is always there for quick reference and use. Often a number of iterations are needed to satisfy all the various design criteria. It is also common to find changed conditions or unexpected obstructions. Delays in construction are very costly, often costing several thousand dollars every hour. It is important to be able to provide fast and reliable solutions when the unexpected is encountered. The computer allows the design of shoring in minutes. By hand, these same designs sometimes consumed weeks. Now, I have been able to give answers to field questions during the initial telephone conversations by entering the appropriate work sheet, change the design parameters and in seconds solve the problem. This computer speed saves us many thousands of dollars every year. The computer also makes it easy to explore ideas and alternatives that would be nearly impossible only a few years ago.
The above programs are specifically applied to sandy and gravelly soils above the water table. Sands and gravels usually have internal friction angle between 30 and 40 degrees and corresponding densities of 90 to 130 pcf. Below the ground water table, the active load can be taken as a hydraulic pressure of the water density plus one-half of the soil active pressure. This usually only is applicable to sheet pile cofferdam that must be watertight. Soldier pile and lagging
systems are so free draining the ground water head is not a design concern. Generally, construction dewatering lowers the ground water table so that it is not a shoring design concern. When cofferdams are placed in open water usually a tremie seal is required to counteract buoyancy and seal the bottom of the cofferdam.
systems are so free draining the ground water head is not a design concern. Generally, construction dewatering lowers the ground water table so that it is not a shoring design concern. When cofferdams are placed in open water usually a tremie seal is required to counteract buoyancy and seal the bottom of the cofferdam.
Clay soils can generate hydraulic active soil loads of about 100 pcf when saturated with friction angles of zero to 30 degrees. Even when the clay moisture content is only slightly above optimum, the soil can slowly creep and induce very high active pressures on the shoring system. Some silt, such as glacial till have been known to suddenly liquefy when disturbed. Considerable caution is justified when designing shoring that is to be installed in clays or silts.
The successful construction of shoring depends greatly on adhering to proper procedures and sequences. The designer and builder must understand that exacting tolerances can not be maintained, with deflections and misalignments are measured in inches or feet. Piles are easily deflected off line by rocks and changing soil conditions. Even improper installation methods can result in major damage.
Vibratory pile driving hammers have largely replaced impact type driving hammers. The vibratory hammer is faster, quieter, and is less likely to cause damage to the piles. Drilled holes for a soldier pile is the preferred methods of installation when the soil contains cobbles or is too hard to allow pile driving. Predrilled holes can also facilitate the installation of sheet piles.
The first step to shoring installation is making a driving template. This is particularly important when sheet piles are used. Someone must help thread the interlocks. The designed wales can usually be used to guide the initial installation.
The template wales should be marked with the proper location of every sheet pile pair interlock that touches the wale. To allow for deflection and some misalignment that will occur, it is common to build the template 4” to 6” wider than the designed size. One way to accomplish this is to band 2 or 3×12 wood planks on the outside of the walers. Special care should be taken to insure the first pair is set plumb and in the proper location, since it will act as guide for the rest of the sheet piles. One real advantage of the vibratory pile hammer is the hydraulic pile grip is used to pick the sheet pile pair from stockpile and thread the interlocks. When the sheet pile pair is properly threaded and aligned, it should be driven to the top of the template wale. A C-clamp can be used to keep the free interlock from fanning out. Sheet piles will tend to tilt along the wale because of the unsymmetrical interlock friction during the initial driving, so the top should be restrained from walking along the wale.
If there is a prevailing wind or water current, start setting the sheet piles on the center of the upwind or up-current side. Complete this up wind side installation of sheets including the corner pile by alternating from left to right when adding sheet pairs. Make sure that the corner piles are truly plump in both directions. It is a lot easier to correct misalignments as the sheets are being threaded than discovering a problem when the final closure is attempted.
Final closure should never be made at a corner. The reason for this is the corner works in both directions. If either sheet wall line is out of plumb, the sheet interlock will probably split open. The other reason to be careful in initial alignment is that this will largely define the direction the piles will take as they continue to penetrate the ground. If the interlock is started off tight and out of line, it will likely split apart as it is being driven. This will damage the pile and may require a very expensive and time consuming repair procedures.
When the sheet piles are fully in place and driven to the top of the upper template, the upper template can be removed. The pairs of sheet piles should be advanced in about five foot increments. Drive alternate pairs so that the interlock friction stays symmetrical for every pair. This will help maintain pile alignment. Constantly check the sheets for plumbness and alignment. If the sheets start to walk out of plumb or alignment, extract the sheet pair and advance the pair on each side of the problem sheets. Sometimes by working the problem sheet pair up and down a few times, the pile will realign and driving can continue. This ability to extract and drive the sheets with a vibratory hammer is a huge advantage over impact hammers, which usually only can drive the pile efficiently. If the misalignment can not be corrected and is serious enough to require additional action, the only practical solution may be to excavate to the toe of the sheets and remove the obstruction. It may even be necessary to install temporary walers at unplanned elevations. This is another reason to have the design on a computer, so you can react quickly to address the problems as they arise.
Shoring and cofferdams are rarely installed as easily as they are planned and designed. You must expect and anticipate problems that will require redesign and innovative solutions. However, it is rewarding to solve the demanding construction and knowing it will help successfully complete the project.
Excavation shoring is widely used for many reasons. The design and installation should be common knowledge for all construction engineers. This course has presented the design and construction techniques that have been successfully applied for many years.
Excavation shoring design no longer needs to be the tedious and complex procedure that was the hand design process before the widespread use of computers. In a few hours, the design programs can be written into worksheets such as EXCEL. It actually takes longer to solve most shoring problems by hand than to write them into the computer. The added advantage is the calculations are error free and many options can be analyzed very quickly.
1. Manual of Steel Construction – Allowable Stress Design, 9th edition by AISC
2. Fundamentals of Soil Mechanics, Feb 1965 by Donald W. Taylor
3. The Encyclopedia of Applied Geology, 1984 edited by Charles W. Finkl, Jnr.
4. Handbook of Heavy Construction, 2nd edition, 1971 Edited by John A. Havers and Frank W. Stubbs, Jr.
5. Engineering Design using Excel Worksheets, 2001 by Eugene G. Washington on PDHonline.org
6. Trench Support Options and Slope Stability Design, 2001 by Eugene G. Washington on PDHonline.org
7. Understanding the Geotechnical Report, 2001 by Eugene G. Washington on PDHonline.org