undercutting and backfilling

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UNDERCUTTING &BACKFILLING UNSTABLESUBGRADE SOILS

Technical Paper

ESTIMATE THE COST OF

AN ESTIMATORS GUIDE TO POLICIES,PROCEDURES, AND STRATEGIES

submitted by Brian P. Stumph CPE

1) Introduction A. Main CSI Division B. Speci c Sub-Division Code and Name C. Brief Description of Subject Matter 2) Types and Methods of Measurement 3) Factors Affecting TakeOff and Pricing

A. Small Quantities vs. Large Quantities B. Geographic Locations C. Seasonal Effect on Work

4) Overview of Labor, Material, Equipment, andIndirect Costs

5) Special Risk and Considerations 6) Ratios and Analysis 7) Miscellaneous Pertinent Information 8) Sample Sketch 9) Sample TakeOff and Pricing

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Brian P. Stumph is a seniorestimator with W.G. Yates & SonsConstruction, an ENR Top 50general contractor serving primarilythe southeastern United States.Brian holds a B.S. in ConstructionEngineering Technology from theUniversity of Southern Mississippi.He has worked in the constructionindustry for over fteen years asan estimator, project managerand owner of Earth SpecialtiesConstruction, LLC. His primaryarea of estimating expertise isheavy/civil construction.


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ESTIMATE THE COST OF: Undercutting and Backflling Unstable Subgrade Soils

INTRODUCTIONWhether required by the project speci cations,or needed due to unforeseen site conditions,

the undercutting and back lling of unstablesoils is a common sitework activity. This paper will show an estimator how to quantifyvolumes of undercut / back ll excavation andhow to estimate the cost of labor, materials,and equipment needed to perform the work.

A. Main CSI Division:Construction Speci cations Institute 2004Master FormatDivision 31 Earthwork

B. CSI Subdivision:Construction Speci cation Institute 2004

Master Format31 23 13 Subgrade Preparation31 23 16 Excavation31 23 23 Fill

C. Description:What is undercutting and back lling ofunstable soils?

Unstable soils are soils encountered on a sitethat do not provide a suitable base to properlysupport new structures or pavement. Un-dercutting is the removal of existing soils.Back lling is the replacing of the soil removedduring undercutting process. Undercutting and

back lling are usually performed in conjunc -tion with each other. The work is performedwith heavy equipment such as hydraulic exca-vators, bulldozers and dump trucks.

The need to undercut can occur for variousreasons. Some of the more common reasonsthat precipitate the need for undercutting and

back lling are:

1. The in situ soils may have high moisturecontent; therefore, the soil will not provide astable foundation for construction.

2. The in situ soil may be composed ofhighly expansive clays that are unstable dueto the potential for volume change.

3. The designing engineer may want toreplace low bearing capacity soils underneathfoundations with soils that will have higher

bearing capacity.

4. Otherwise stable soils become weak andunstable due to poor drainage and damagecaused by construction equipment; a result of

poor site management.

5. Prolonged wet conditions may cause soilsto become unstable, thus causing a need toreplace the wet soil with stable material.

Above items one, two, and three, are condi-

tions that will usually be addressed in a proj-ects plans, speci cations, and geotechnicalreports. The designing engineer will some-times specify areas and depth of undercut and

replacement that needs to occur. The engineermay require a certain strata of soil be removedto a certain depth and have the contractor relyon the geotechnical report information to makethe determination to what extent the undercutand back ll should occur. Items four and ve,above, are situations that occur during con-struction and usually are estimated as a changecondition. The need to remove and replaceunstable soils on a construction project is atypical activity regardless of project size.

TYPES AND METHODSOF MEASURMENTS

Undercutting and back lling volumes aremeasured in cubic yards. The estimator should

be very speci c in stating the manner by whichthe cubic yard cost was estimated when ex-

pressing a cubic yard volume of earthwork. Acubic yard measurement in earthwork can haveseveral different meanings. Unlike concreteestimating where a cubic yard is simply lengthx width x depth divided by twenty seven,earthwork expands on the simple cubic yard-age equation. The earthwork estimator mustunderstand the difference between loose cubicyardage and compacted cubic yardage.

Quantity surveys are done in compacted cubicyardage. Quantities can be derived by usingthe average end area method of computation.To estimate quantities using the average endarea method, the estimator should divide thearea to be undercut into a grid. The smaller thegrid, the more accurate the estimate. Once thegrid area is determined, the existing eleva-tions and the proposed elevations should becompared to determine the depth of excavationat each corner of the grid. The resulting depthsshould then be averaged. The average depthis then multiplied by the area of the grid anddivided by twenty seven to determine the cubicyard volume. When undercutting is speci ed

to be done in a prescribed area, the grid areamay be substituted with the footprint of thearea to be undercut. (See example takeoffsheets).

The resulting cubic yardage is compacted orin place cubic yardage. The estimator shouldreview project bid documents to determine ifcompacted or loose measure is the appropri-ate pricing approach. Often unit prices forthe undercut are requested by the truck loador loose vehicle measure. To determine loosequantities, the compacted quantity is adjusted

by a swell factor. The amount of volume

change between compacted and loose willvary depending on material type. Granularsoils, such as sand, will swell less whenexcavated than clay soils. The estimator

should consult the project geotechnicalreport or consult a geotechnical engineer tohelp determine the appropriate swell factor.Historical cost data is also a good source forhelp in determining swell factors.

The estimator should determine if the costestimate should be based on compactedquantities or loose quantities. A companyscost tracking system, historical cost data,how disposal fees are paid, how the back llmaterial is purchased, as well as an owners

pricing requirements will help the estima-tor make this determination. Under mostcircumstances, it is recommended that theestimator stay consistent with his estimatingapproach once the decision to estimate theitem as compacted measure or loose mea-sure has been established. Often undercut-ting and back lling unit prices are requiredto be submitted with a project bid or as achange order for unforeseen site condi-tions. Care should be taken to make sure the

basis of the cost estimate is the same as theowners request.

FACTORS AFFECTINGTAKEOFF AND PRICING

A. Small Quantities vs.Large Quantities

Typically large quantities will have a lowerunit cost than small quantity projects. Largequantities allow the contractor to utilize la -

bor and equipment at optimum productivity.Indirect costs become less of a percentage oftotal cost on large quantity projects than onsmall quantity projects.

B. Geographic LocationThe need to remove and replace unstablesoils is not unique to any one region. Thedistance from the project site to the undercut

soil disposal area as well as the distancefrom the replacement soil site to the under-cut site are important factors when deter-mining cost. The cost of hauling material isdetermined by the distance from the projectsite and the time needed to haul the material.Generally, the shorter the distance, the lowerthe haul cost. The estimator should alsoconsider the travel time when determininghaul cost. In some cases, a slightly longerhaul distance may have a lower cost thana shorter haul distance due to the travel time.


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C. Seasonal Effect on WorkWeather will have an effect on the under-cutting and back lling operation. Sinceundercutting and back lling go together,seasonal weather patterns in uence each

part of the operation equally. Undercuttingwill be affected if there is no way to accessdisposal areas or the material becomes toowet to haul. The back lling will be evenmore affected. During wet periods, the areaswhere the back ll material is obtained will

become wet and saturated causing the soil tohave a high moisture content. Wet soils willnot reach required compaction and producea suitable ll. Conversely, during dry sea -sons, back ll soils may become to dry andwill need to be hydrated in order to meetcompaction requirements. Temperature willalso affect the effort required to properlycompact the back ll. Undercutting may belimited during the winter months becausespeci cations do not allow the material to

be placed while the ground is frozen. It ishelpful for the estimator to review histori-cal weather data in order to understand theweather may potentially impact the project.

OVERVIEW OF LABOR,MATERIAL, EQUIPMENT,AND INDIRECT COSTS

A. Overview of Labor

The labor needed for undercutting and back-lling is largely determined by the number

of pieces of equipment needed to performthe task. Each piece of equipment will re-quire an operator. In addition to number ofequipment operators required, the estimatorshould consider the following :

1. Will the task require additional personnelfor traf c control on roads or streets?

2. Will labor be needed to check grade orcross section the area?

3. How much labor is needed for cleanup?

4. Will labor be needed to document loadcounts or receive load tickets?

5. Will labor be needed to route haul ve-hicles around the site?

The productivity of equipment and the rateat which back ll material can be deliveredto the site will determine production ratesof labor. The estimator should determineappropriate base wage rates for each laborclassi cation once the labor requirementsare determined. Wages paid to currentemployees, historical cost data and local

wage rate surveys are a source of wage rateinformation. As a rule of thumb, excavatorand bulldozer operators are paid higher wagesthan compactor, tractor and truck operators.

Common laborers will be the lowest paidworkers. The estimate should also include thecost of federal taxes, state taxes, workers com-

pensation insurance, general liability insuranceand any fringe bene ts. The estimator shouldreview the speci cations, and understand theDavis Bacon wage requirements when estimat-ing for federal projects.

B. Overview of MaterialThe estimator should evaluate both the mate-rial taken from the undercut, and the materialneeded for back lling to determine the costfor either disposing of the existing material (inthe case of an undercut) or purchasing suitablematerial (in the case of a back ll). Material

produced from the undercut should be evalu-ated to determine if it will be acceptable foruse elsewhere onsite. If the material will notmeet the speci cation for use elsewhere on thesite, or if there is not suf cient area availablefor onsite disposal, it must be removed to anoffsite location. Usually, the contractor must

pay a disposal fee to deposit the excess mate-rial to an offsite location. The cost of dispos-ing of the undercut material should be includedin the cost estimate. The disposal fee will be

based on either a cross-section measure orloose vehicle measure. The estimator shouldunderstand the method of measurement that

will be used to determine disposal fees. Theestimator should include the cost of the laborand equipment needed in his estimate if thecontractor is required to spread or stockpile theundercut material in lieu of paying a disposalfee. If the material from undercut is to remainonsite, the cost of labor and equipment neededto spread the material onsite should be ac-counted for in the estimate.

Required back ll material may come fromeither onsite or offsite sources. Projectspeci cations will dictate the type of materialrequired for back ll. The supplier of the mate -rial will normally set the basis for determininghow quantities for payment will be measured.Material for back ll will either be sold bytruck load (loose vehicle measure), weight,or by cross-section measure. The materialsupplier will either deliver the material to thesite or will require the contractor to arrange fordelivery.

To estimate a delivery rate, the estimator needsto know the volume capacity of the truck haul-ing the material, the trucks hourly equipmentrate (including operators labor rate), and thedistance/travel time to and from the site. Thecost per yard can be determined by multiply-

ing the hourly equipment rate times the traveltime. Divide this total by the volume capacityof the truck. For example: the hourly rate ofa truck is sixty dollars ($60.00), travel time is

thirty minutes (.5 hrs.) and the trucks volumecapacity is twenty cubic yards (loose vehiclemeasure =20 CY LVM). The cubic yard haulcost would be as follows:

$60.00 x .5 hrs

20 CY LVM= $1.50 per CY LVM

The haul rate per load will be: 20 CY LVM x$1.50 per CY LVM = $30.00 per load.

C. Overview of EquipmentThe basic equipment needed to performundercutting and back lling is an excava -tor, bulldozer, hauling unit (scraper, off-roadtruck, or highway truck) and compactors. Theestimator should also consider if equipmentwill be needed to maintain haul routes onsiteor sweep and clean public roadways. Often atractor and disk will be needed to help dry andcondition the back ll material.

The number and size of the equipment will be determined by the desired production rate.Historical cost data and manufacturers datafor a machines productivity is a good sourceof information for estimating the productivityof a piece of equipment. Invaluable sourcesfor information on equipment productivity are

eld supervisors and operators. Since eld personnel are using the machines daily, theywill usually have a good perspective regardingwhat equipment is best suited for a particular

project.

The estimator should consider the produc-tion capacity of the equipment and pair/groupthe equipment to maximize productivity. Forexample, the cost of a bulldozer capable ofspreading 40 loads of dirt per hour should not

be paired with an excavator capable of makingonly 5 loads per hour. This lack of ef ciencywill result in a skewed estimate unit cost thatmay not accurately re ect the actual cost to

perform the work.Equipment cost used in an estimate will typi-cally come from a companys equipment ratesheet. If rental equipment is to be used thevendor can help the estimator establish anhourly rate that can be used in the estimate.When using rented equipment, the estimatorshould not overlook the cost of fuel and main-tenance that will be required.


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D. OVERVIEW OF

INDIRECT COST

In addition to the direct cost of labor, theindirect cost of material and equipment should

be accounted for in the estimate. The primaryindirect cost incurred will be as follows:

1. Crew Forman . The cost of theforman can either be part of the overall project general conditions or as

part of the crew cost associated withdoing the work.

2. Mobilization Cost . The estimatorshould determine how to allocatemobilization cost. If the undercuttingand back lling is part of the massexcavation on the project, and theequipment used will be utilized forother items of work, then mobilizationcost for the equipment may notcontribute signi cantly to the unit

price. However, if a small quantity ofundercutting and back lling isrequired after equipment isdemobilized, the mobilization costto perform the work will become asigni cant cost item. Accounting formobilization should be done at somelevel of the estimate.

3. Surveying Cost . If the work is to becross-sectioned to determine quantitiesfor payment, then the cost of the eldmeasurements and calculations should

be included.

Other indirect cost will be for items such assmall tools, safety supplies, and temporaryfacilities. The estimator should keep in mindthat the indirect cost for the work can be verysigni cant for small quantity projects. On verysmall quantity project the indirect cost could

be more than the direct cost.

SPECIAL RISK

CONSIDERATIONS

Any earthmoving operation will have animpact on the environment; therefore, theestimator should be certain to account for thecost of environmental controls in the estimate.Silt control will be the primary environmentalissue associated with undercutting and back ll -ing. At the start of a project, silt fences andother erosion control measures are usuallyinstalled on the site. Usually the existingerosion control measures are suf cient to ac -commodate any undercutting and back llingoperation. The estimator must include the costfor proper erosion control measures, if needed,in the undercut material disposal area. Theseareas should have erosion control measuresinstalled that meet all applicable local require-ments. Another environmental concern isthe tracking of dirt and debris onto publicroadways when hauling dirt on and off the site.Roadway cleanup cost should be included inthe estimate.

The estimator should always consider andinclude the cost of safety measures in the es-timate. The presence of underground utilitiesis a safety and production concern. In areaswhere there are utilities present, productionrates may need to be lowered to allow time forunderground utility location. In some cases,it may be appropriate to increase/adjust thenumber of workers in the estimate if the crewis required to locate the utilities before excava-tion. The cost of barricades and sloping thesides of an excavation, if needed, should also

be included in the estimate.

RATIOS AND ANAYLSIS

A site visit by the estimator is invaluable whendetermining the amount of undercutting and

back lling that is required, and the cost of performing the work. Being physically presenton the site allows the estimator to correlate site

plans and geotechnical reports with the actualconditions. This correlation will help the esti-mator understand the terrain and how it affectsaccess to and movement around the projectsite. The estimator should drive the proposedhaul routes before completing the estimate.Driving the routes will enable the estimator toverify haul time/distance.

The estimator should always check his esti-mate for accuracy. When checking the accu-racy of an estimate requiring undercutting and

back lling, the estimator should be mindful ofthe following:

1. Usually the cost of undercutting will be less than the cost of back lling.

2. The labor cost should be less than theequipment cost.

3. Converting a compacted quantity esti-mate to a loose measure quantityestimate will increase the quantitywhile decreasing the unit cost.

MISCELLANOUS

PERTINENT

INFORMATION

The estimator should always verify thatthe back ll material source has appropriateenvironmental permits. The contractor can face

penalties for purchasing materials from non- permitted locations. Likewise, care should betaken when choosing disposal areas for the un-dercut material. Disposing of material on sitesthat do not have appropriate permits can resultin nes and penalties for the contractor(s).


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