Trench and Excavation Support Options And Excavation Slope Design

Eugene Washington , P.E.


Course Outline

1. General
2. Options
3. Considerations
4. Evaluations

a. Open Cut
b. Speed Shores
c. Trench Shields
d. Soldier Pile & Lagging
e. Sheet Piling
f. Modular Shoring
5. Installation of Piling
6. Trench Support selection
7. Design of trench slopes
8. Course summary

This course includes a multiple choice quiz at the end.

Learning Objective

The purpose of this course is to show the reader the importance of considering the various options of excavation and trench support methods. Safety, site, ground condition, and obstruction constraints influence the selection of any excavation support method. The cost difference between the various methods in any given application can be huge. Selecting the safe and most economical excavation support method is critical to winning and completing successful projects.

Course Introduction

The selection of any trenching method is a critical decision. If the wrong choice is made there are no good results. If the selection is too conservative at bid time, the project will be awarded to another contractor. If the choice is too aggressive, then the job faces cost overruns and potential safety problems.

This course discusses the various methods of trenching support and the considerations involved with making the support selection. The guidelines offered will illustrate the thought process of selecting and designing a safe and economical trench.

Course Content

GENERAL

The primary function of any trench support method is to protect people from caving ground. The secondary function is to provide support to nearby structures and allow equipment access to the work.

Any deep trenching should be analyzed for a comprehensive economic solution where the viable alternates are reviewed for trench method of excavation, pipe laying, backfill, schedule and obstructions. In any given project several trench support methods may be used to accommodate different conditions. There is no “one-size-fits-all” solution to the process of selecting and designing a trenching support method. The first steps are to read the plans, specifications and geotechnical reports to understand the constraints and conditions that will be encountered. Whenever possible pothole the site before bid time to get a better understanding of the soil properties. Keep in mind that most geotechnical reports are prepared for the design engineer, not the construction process. When the reports do address the construction methods they can be overly conservative by broad brushing the project with an isolated worst case or recommending impracticable solutions. Usually the reports are furnished to the contractor for “information only” and the contractor is expected to make an independent evaluation. It is critical to obtain all the information you can, including additional soil testing for cohesion and friction angle.

OPTIONS

Trenches are usually supported by one or a combination of the following methods. The normal order of preference is based on cost and productivity. Open cut by sloping the trench walls is the first choice. Shoring is selected when no other method is practicable.

1. Open cut - i.e. Sloping the trench banks to a safe angle

2. Speed Shores - i.e. Aluminum beams spread with hydraulic jacks supporting a nearly vertical trench bank

3. Trench Box or Shield - i.e. rigid frame designed to protect persons from collapsing soil.

4. Soldier pile and lagging shoring - i.e. Drilled or driven H-pile with wood or Steel Plate lagging

5. Sheet piling shoring – i.e. Interlocking steel piles

6. Modular shoring – i.e. commercial system of panels, piles and struts.

Each of the above methods has several variations that are adjusted to suit the specific conditions.

CONSIDERATIONS

There are a number of conditions that must be considered before a trench support method is selected.

1. Safety: Any method must first insure crew and public safety. There is no gain emotionally or economically in having a trench failure hurt anyone.

2. Economics: Generally an open cut is the most cost effective method of trenching if reasonable side slopes can be safely maintained, i.e. 1 to 1 +/-. When trenches exceed 30 feet or so deep it may be cheaper to micro tunnel. Shields and speed shores are the next choices if an open cut is not feasible, but the soil must stand nearly vertically long enough to excavate and backfill. The most expensive trench support methods are shoring methods such as soldier piles, sheet pile, or modular shoring.

3. Soil conditions: Open cut can be made in most soil conditions where ground water can be handled. The exceptions are oozing mud, which is really a viscous fluid, and running clean sand, which may not stand on a 1.5H to 1V slope. There are some soils, such as glacial rock flour, which are dense but unstable once disturbed.

4. Ground Water: Water flowing out of the trench wall can cause the bank to become unstable. Slow seepage usually does not cause a trench to become unstable. If it is not practicable or possible to lower the water table enough to trench, then shoring may be necessary.

5. Underground obstructions: These can be utilities, boulders, trash, logs or any large object that impedes excavation and/or pile driving. If obstructions are common the trench production will be slowed for any support system. Working around utilities will slow the work. Any or a series of obstacles can prevent the effective use of shields, sheet piles and modular shoring.

6. Overhead Obstructions: Power lines, bridges and trees can create major problems for crane and backhoe operations. These low overheads can hamper or prevent pile-driving operations.

7. Right of Way: A narrow right of way may preclude an open cut sloped trench and force a shoring solution to allow access to the work.

8. Adjacent structures: A shoring system is often required to protect building foundations.

9. Environmental: In urban areas noise, vibration and exhaust constraints can limit trenching methods and equipment. Animal habitats and nesting seasons can limit schedules and construction methods.

EVALUATIONS

Below is a comparison of the trench supports methods commonly used. Each method has advantages and disadvantages that must be considered before the selection is finalized.

OPEN CUT TRENCH

If there is sufficient right of way, open cut trenches can be used in almost any soil condition. Generally a sloped open cut excavation is the most cost and schedule effective method of trenching. When the trench is very deep and/or expensive backfill materials are required, then a vertical cut at the toe of the slope supported by shoring may be effective. Ground water and weak lower layers may force partial shoring or flattening of the excavation slopes.

Open cut trench advantages:

1. Allows continuous excavation, laying and backfilling operations.

2. Minor breakdowns usually do not cause delays to all activities.

3. The open trench needs only the design of the cut bank slope. OSHA guidelines can be used, although cost saving usually will result if the bank slope is checked and designed by a registered engineer for a steeper slope. Sloping the excavation is the simplest method to design and use.

4. Because there are no additional support operations and equipment, it is the economical choice.

5. The open trench provides easy access to the work because equipment and construction materials are minimized.

6. The open cut method is suitable for most ground conditions, except for oozing mud and running sands.

Open cut trench disadvantages:

1. The slope of the bank requires more excavation and backfill volume than the other options.

2. The only bank support is the strength of the soil. If drying, flooding, or change of soil properties weakens the soil, then sloughing and collapse can happen with little or no warning.

3. The sloped banks require a wider work area.

4. The bank slopes may force the use of larger equipment because the distance to reach into the trench is increased and a greater volume of soil must be excavated and backfilled.

SPEED SHORES

Speed shores is a system where support for nearly vertical trench walls is accomplished by using hydraulic rams which force beams and sheeting against the trench wall. This can be a very efficient support system. However, they have limited application. The trench must be cut smooth and nearly vertically and be able to safely stand long enough to install and remove the shores until the trench is backfilled. The soils must have significant cohesion and no significant ground water for speed shores to be effectively used. Speed shores can be used in trenches up to 30 feet deep and 15 feet wide. Pipe sizes of more than about 3 to 4 feet in diameter precludes the use of speed shores because the pipe will interfere with the lowest ram strut. For large diameter pipes and “A Frame” can be used. The "A" Frame is a heavy hydraulic shore that is so heavy that it requires a crane or backhoe to set and extract. Speed shores adds 1 or 2 people to crew to handle the shores. The speed shores interfere with the pipe laying and backfilling operations. The installation and removal of shores must be carefully orchestrated in order to maintain pipe-laying productivity.

Speed Shore advantages:

1. The shores provide a positive support to the trench walls.

2. Having nearly vertical trench walls minimizes the amount of excavation and backfill.

3. No special equipment is needed to install the shores.

4. Having vertical trench walls minimizes the work area width.

5. Seed shores are economical to rent and easy to install.

Speed shore disadvantages:

1. The trench must be able to stand nearly vertically until the shores can be installed.

2. The shores interfere with installation of the pipe and backfill operations.

3. The normal limit of use is 30 feet of trench depth.

4. The practicable trench width limit is about 15 feet.

5. The pipe outside diameter is limited to no more than about 40 inches. This due to the fact that the bottom cylinder can be no higher than 4 feet above the bottom of the trench.

TRENCH BOX or SHIELD

A shield is a safety cage for the crew to safely work in a trench. The shield does not support the trench walls; it is only there to protect the people in the event of a cave in. It is designed to be dragged along the trench using the backhoe or a crane. The function of a shield is to allow the trench wall to be steepened to reduce the amount of excavation, backfill and right of way needed. The shields are very versatile in that they can be used in a wide range of soil conditions. They are heavy and cumbersome to use in tight quarters. Shield become much less efficient when underground utilities are common. The shield must be lifted over the utilities or the utilities must be cut and restored later. Shield can also cause the pipe to shift off line and grade as it is being dragged forward. When a shield is trapped by a cave-in, it can be a major effort to extract the shield. The shield size should matched to the configurations of the trench and the trenching equipment. If the shield is too heavy, the backhoe or crane selected for the job may not be able to efficiently move it. If the shield is too small it may require many extra moves and not provide sufficient space to allow the crews to bed and set the pipe as well as such activities as dewatering. The shield should be at least 4 feet longer than a joint of pipe. The shield should extend at least 18 inches above the top of any slide prone bank. Generally shields can be economically rented and are much less expensive than fixed shoring systems.

Trench box (Shield) advantages:

1. Provides positive protection for the people in the trench.

2. The trench walls can be steepened to the point that only short-term stability is needed.

3. The excavation and backfill quantities are reduced.

4. The work area can be narrower.

5. A shield can be used in most ground conditions

Trench box (Shield) disadvantages:

1. The box can be cumbersome to handle and may require additional equipment to move.

2. Trench production is usually slowed by the efforts to advance the shield.

3. The trench walls must be stable enough to operate equipment near the top of the trench.

4. The shield does not provide positive support to the trench wall unless it is “dug in”. Digging in is the process of excavating inside the box and pushing it down. Extraction of a dug in box can be very difficult due to the soil friction. The dig-in process is slow and often causes damage to the equipment because it is a brute force operation.

5. A box is difficult to use effectively if there are a lot of underground utilities or other obstructions.

SOLDIER PILE & LAGGING

Soldier pile and lagging is part of a family of fixed shoring systems to support trench walls. This family includes sheet piles and modular systems. These can be cantilevered, braced or tied back to provide ground support. Shoring systems are much more expensive than trenching with shields or speed shores. All other alternatives should be considered before shoring is employed. Soldier piling is listed here first not because of cost, but it is the most flexible system of the shoring family. The piles can easily be placed to avoid utilities and obstructions. The lagging is usually steel plates or timber. The piles can be driven or placed in drilled holes. In soft ground, the piles can be pushed into the ground by the backhoe excavator. The piling can cantilever at least 15 feet and a braced system to depths of over 100 feet.

The cost of the soldier pile and lagging installation cost vary between $5.00 and $25.00 per square foot depending on reuse and complexity. If the soil is sandy the best way to install the piles is a vibro hammer. Stiff clay may require an impact hammer. Steel plate lagging is less labor intensive to install than timber lagging. Steel plates are usually ¾” to 1” thick so the cost is $8.00 to $12.00 per square foot while 4x12 timber cost about $3.00 per square foot, but timber requires about 0.10 labor hours per square foot to install and remove. Steel plates can be reused any number of times, but timber can not be easily salvaged for reuse. Timber has the advantage of being able to easily box around utilities. Often a combination of steel plate and timber is used in the trench as conditions dictate. Steel plates become difficult to handle when they are more than about 25 feet long. While cantilever soldier piles eliminate the need for bracing, their length is more than twice the trench depth. This adds weight to piles and increases the risk of increasing penetration problems. Usually wales and struts can be used with less total steel weight than cantilevered piling. When bracing is used it is best if the struts are spaced so that the pipe joint lengths can be easily threaded into the trench. Soldier pile and lagging systems are not water tight, but will help to control minor ground water problems.

Soldier pile and lagging advantages:

1. This method provides positive trench wall support.

2. Soldier pile and lagging can be used in almost any soil condition.

3. The piles can be spaced at odd intervals to miss utilities or obstructions both overhead and underground.

4. Excavation and backfill quantities are minimized.

5. Access to the trench is maximized.

6. A minimum of right of way is needed.

7. The shoring can help to control ground water.

Soldier pile and lagging disadvantages:

1. The shoring materials are expensive to buy and install.

2. Additional equipment and material at the site congest the work site.

3. Additional crew operations of install and extract can cause logistic delays.

4. Trench production is often slowed to the rate of shoring installation.

5. The installation and extraction can cause undue noise and vibration pollution, especially if impact pile hammers are used.

6. The installation and extraction process can cause surface settlement in uncompacted soils.

7. The shoring design usually requires a professional engineer stamp.

SHEET PILING

The cost of sheet piling is similar to soldier pile and lagging. However, sheet piles can be readily rented for short term and the system will weigh about ½ of a soldier pile and steel plate lagging system. The main draw back of sheet piles is they will not easily accommodate utilities and obstructions. The soil must be fairly soft because the piles are easily damaged by cobbles and ground that resists driving at more than 10 blows per foot. Sometimes a vibro hammer will effectively drive sheet pile where an impact hammer will cause damage. One advantage of sheet piles is that they can form a watertight barrier to control ground water. Where sheets must be installed in hard ground, a dig and drive is usually required to eliminate pile damage and splitting interlocks. Sheet pile will easily cantilever 15 feet and the pile length will be slightly shorter than a soldier pile.

Sheet pile shoring advantages:

1. This method provides positive trench wall support.

2. Excavation and backfill quantities are minimized.

3. Access to the trench is maximized.

4. A minimum of right of way is needed.

5. The tight sheets are very effective in helping to control ground water.

Sheet pile shoring disadvantages:

1. The piles are expensive to buy or rent and install.

2. Additional equipment and material at the site congest the work site.

3. Additional crew operations of install and extract can cause logistic delays.

4. Trench production is often slowed to the rate of shoring installation.

5. The installation and extraction can cause undue noise and vibration pollution, especially if impact pile hammers are used.

6. The installation and extraction process can cause surface settlement in uncompacted soils.

7. The shoring design usually requires a professional engineer stamp.

8. The soil conditions must be fairly soft or the sheets will be damaged while being driven.

9. Any underground obstruction more than a few inches in size can damage the pile by bending and splitting the interlocks.

10. Underground utilities will often prevent the effective use of sheet pile shoring.

MODULAR SHORING

These are commercial systems that are fairly new to the market. They are hybrid combination steel panels that interlock with a special pile and fixed struts. They are usually pushed into the excavation with the backhoe excavator. They are basically designed to be used in soft ground and confined quarters. They do not easily accommodate underground utilities. The systems are fairly expensive to rent.

Modular shoring system advantages:

1. This method provides positive trench wall support.

2. Excavation and backfill quantities are minimized.

3. Access to the trench is maximized.

4. A minimum of right of way is needed.

5. The panels are effective in helping to control ground water.

Modular shoring system disadvantages:

1. The piles are expensive to buy or rent and install.

2. Additional crew operations of install and extract can cause logistic delays.

3. Trench production is often slowed to the rate of shoring installation, which is essentially a dig and push in operation.

4. The shoring design usually requires a professional engineer stamp.

5. The soil conditions must be fairly soft for the systems to be effectively utilized.

6. Underground utilities or other obstructions can prevent the effective use of modular shoring systems.

INSTALLATION OF PILING

There are 5 basic ways to install piling. The first is to push the pile in with the backhoe. The ground must be fairly soft and often requires a dig and push operation. This can significantly slow the trench production. It also can be difficult to control the pile alignment especially if there are cobbles present. The advantages are that no special equipment is necessary and the noise level is no greater than the backhoe exhaust.

The second method is to augur a hole and drop the pile in. This adds additional equipment and the operation is fairly slow. This method works best if the soil aggregate size is less than about 9”. Usually a 24” diameter hole is drilled. Very cohesionless soils will collapse into the hole. However, hard dense soils can be augured. This is usually the most expensive method to install a pile.

The third method is pile driving impact hammers. There are several types and sizes of hammers. Air or steam driven hammers are very noisy and can easily damage the pile if a cobble or obstruction is encountered. Diesel hammers are also very noisy and dirty. They spew soot and oil from their exhaust. The impact on the pile caused by diesel hammers is slightly less than air/steam hammers, but they can quickly damage the pile. Impact hammers can drive piles into any soil the pile is capable of penetrating. Sheet piles are especially susceptible to damage from impact hammers.

The fourth method is vibrating hammers. These vibro hammers are very efficient at installing piles. They will usually install piles much quicker with less damage than air/steam hammers. They are also quieter and cleaner. One draw back is the soil conditions can’t be too dense or clayey. Any obstruction such as a log or large cobble will prevent pile penetration.

A fifth method is hydraulic hammers. The backhoe excavator can operate these. These can be a vibrating ram or a push ram. They are quiet and clean. Some models are designed to ride a line of sheets to work in confined areas. They are fairly new to the market place and not widely used yet.

TRENCH SUPPORT SELECTION

Below is a chart that is a guide to identify the decision making process in general terms. Additional considerations such as owner specified methods can limit options. Also, safety and economic considerations such as the needs to reduce pipe zone overbreak may influence the trench support selection.

SELECTION CHART

Once the options are identified, and then design and comparisons can be analyzed to determine the best economical combination of safety, schedule and cost. OSHA requires that any excavation over 20 feet deep to be designed by a registered engineer. If a qualified registered engineer designs any trench, then the OSHA guidelines are superceded. OSHA classifies soils by the following types: “Stable Rock”, “A soil”, “B soil” and “C soil”.

“Stable Rock” is ground conditions that must be blasted or heavily ripped in order to excavate. The rock is expected to be stable with a vertical excavation face. OSHA defines rock as a Solid Mineral Material.

Type “A soil” is competent ground that can be expected to stand on a ¾ horizontal to 1 vertical slope for an indefinite period. OHSA defines this as Cemented Soils such as Caliche and Hardpan with 1.5 tpsf or greater unconfined compressive strength.

Type “B” soils are the most common and are easily excavated with a backhoe and where running groundwater will not cause the excavation to erode or become quick. These soils will safely stand on a 1H : 1V slope to at least 20 feet of depth. OSHA defines these soils as Granular Cohesionless soils like Angular Gravel, Silt, Sandy Loam or Sandy Clay Loam with a 0.5 to 1.5 tpsf unconfined compressive strength.

Type “C” soils are very weak or where ground water will cause the excavation to become unstable. These soils are expected to stand on a 1.5H : 1V slope. OSHA defines these soils as Granular Soils like Sand, Loamy Sand and Gravel or Submerged soils.

Due to the fact that OSHA identifies all soils in only 4 types with no consideration of depths less than 20 feet; it usually a very conservative guideline that can be safely used. In most cases, huge saving can be realized by analyzing the ground conditions and designing a safe, but steeper slope configuration than OSHA recommends.

DESIGN OF TRENCH SLOPES

The first step is to determine the soil conditions. Usually the project geotechnical report will provide useful information. The site should be visited and reviewed to visually inspect the ground conditions. Whenever possible the site should be potholed to examine the soils. This information should be reviewed before bidding and start of the work.

Until recently the determination of a stable slope was guided by OSHA and educated guesswork. The reason for this is that the Friction Circle calculations required to accurately determine the steepest stable excavation slope are some of the most complex, tedious and indeterminate equations in civil engineering. The easier and approximate calculations such as Rankine and Culmann theories give inaccurate results and do not accommodate compound slopes. Few engineers or contractors have the time or expertise to apply the accepted Friction Circle Theory. Even though the soil friction angle and cohesion are readily obtained by the standard Direct Shear laboratory test it had little practical value.

The diagram below illustrates the Friction Circle Theory. This theory was developed around 1900 AD and has proven to be as accurate as any other soil stability theory. The key information is supplied by direct shear laboratory tests. The results are plotted on a scaled graph. The line extrapolated through the test points establishes the soil properties. The slope of the line is the soil friction angle. Where the line intersects the shear axis where the normal stress is zero plots the soil cohesion.



The problem is solved by statics where:

W = Soil Weight + Equipment Surcharge Surface Pressure

C = The total Soil Cohesion along the Slip Circle

P = The Passive Soil reaction

R = The Slip Circle radius

Ls = Setback from the top of cut to the Slip Circle

X = The horizontal distance from the Slip Circle focus to the W vector

A = Soil Friction Angle.

The equations to solve the statics are:

Vertical conservation of forces: Cy = W - Py

Horizontal conservation of forces: Cx = Px

Conservation of moments about the slip circle focus: C = [WX – PRsin(A)]/R

While these equations seem simple, the geometry to solve the values of the vectors and distances requires a long series of equations. Then a wide range of circle radii and setback distances must be checked. This iteration process must be analyzed for several hundred combinations of radii and setbacks to find an approximate solution. The procedure to solve these many calculations is too lengthy to detail in this course. To view the procedures to solve friction circle problems visit the PDHonline course titled: “Soil Slope Stability Analysis using the Friction Circle Method Programmed in EXCEL”.

Now any engineer or knowledgeable contractor to design a safely stable slope can readily use direct shear test data. The author has written a computer program that solves the Friction Circle equations in the time it takes to enter the parameters. The author wrote the program after it was found that some existing programs did not give consistently accurate results for typical temporary construction slopes. The program addresses compound slopes and multi soil layers with different properties.

The key soil properties that a Direct Shear test determines are the friction angle and cohesion. The friction angle can be considered a function of the soil particle shape. The Friction Angle is also the natural angle of repose of the soil when no cohesion exists. Cohesion can be considered the glue that holds the particles together. Ball bearings will not form a pile when poured onto a level plane. Ball bearings are spherical so their friction angle is zero. With no glue they will roll out in a single layered plane. Clean sand with no cohesion will form a pile with an angle of repose of about 1.5H to 1V when poured onto a level plane. The friction angle of clean sand is about 30 to 34 degrees. Clays and silt have friction angles of zero to over 30 degrees depending on saturation and consolidation as well as particle shape. Cohesion can be very weak such as water surface tension or electrostatics to strong cementation. Round cobbles in a cemented state will stand on a vertical slope indefinitely. Cubic blocks can stand vertically with little cohesion but have a high friction angle. Both of these soil properties are critical to finding the most economical safe construction slope.

Once the moist soil density, friction angle, and cohesion are known from the geotechnical report or independent tests the design process can begin. These soil properties are entered into the computer program. The next step is to test the excavation slope for stability at various excavation slopes until the desired results are obtained. The equipment surcharge is input as a uniform load per square foot. A safety factor is selected.

Estimates can now be made to determine the best trench method to use. The OSHA classification for most soils is type B at best, and some would argue for a type A classification. If OSHA were the only guideline, then most open cuts would be laid back on a 1.5H to 1V slope. That can unnecessarily increase the excavation and backfill quantities by over 100%. OSHA also makes no allowance for excavation depth. A type B soil stable at a 1H : 1V at a 20 foot depth will probably be stable at a 3/4H : 1V slope at a 10 foot depth. A quick analysis of the soil will usually show the soils are stable on much steeper slopes than OSHA recommends. It will also reduce the guesswork as to which trench method will work or be required.

While computer analysis will greatly reduce the guesswork in trench design and selection, it does not replace the due diligence of having competent individuals constantly monitor the trench for stability. Soil properties can change without any noticeable difference. Maintaining safety equipment such as ladders close to everyone working in the trench and having the excavator operator judge the soil condition as it is being excavated are important safety precautions.

Course Summary

The proper selection trenching method is critical to achieving a safe and profitable project. The author has never found that unsafe practices increased production and profit. Insisting on safety pays off in all respects. No one is harmed, full productivity is maintained without interruption and access and logistics are improved when safe practices are employed.

Usually the trench method is selected during the bid process. If the selection is too conservative and costly, someone else will win the bid. If the selection is too aggressive then the project will experience an out of pocket loss and probably safety problems. The correct and best excavation support selections are made when the soil can be quickly and accurately analyzed. The pay-off will be winning bids and realizing safe, profitable projects.

References

1. Fundamentals of Soil Mechanics, Feb 1965 by Donald W. Taylor
2. The Encyclopedia of Applied Geology, 1984 edited by Charles W. Finkl, Jnr.
3. Understanding the Geotechnical Report, 2001 by Eugene G. Washington, PE on PDHonline.org
4. Soil Slope Stability using Friction Circle Programming in EXCEL, 2001 by Eugene G. Washington on PDHonline.org
5. Practical Excavation and Trench Temporary Shoring Design and Construction, 2001 by Eugene G. Washington on PDHonline.org

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DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDHonline.org or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered professional engineer. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.