ESTIMATION 1

Lesson 4 - Excavation

Excavation

Excavation is the biggest unknown element in construction and therefore has a high risk for additional costs in terms of variations.

Estimating the cost of excavation is highly specialized and therefore this lesson will only touch on the basic understanding

Excavation

Excavation

Excavation

Ability to read plans and specifications Understanding of surveying and civil

engineering terminology Understanding of environmental and

legal requirements Ability to estimate quantities based on

m3 i.e cut and fill

Excavation

The estimate is separated into two types of quantities General quantities – work where you can

use motorised equipment such as scrapers, hoes and loaders and it is based on production rates

Special quantities – ie anything that reduces production rates and requires extra care Rock excavation Excavation around sewer pipes, existing

buildings

Excavation

Quantities in excavation are mainly m3 The cost of excavation is on labour and

equipment (cost/hour) needed to complete the work i.e It is based on the (volume) m3 of material moved

Programme of works- time is important factor and is a huge consideration for the type of equipment you are going to use

Excavation

The most expensive equipment (cost/hr) is the most productive. However the machine with the largest capacity isn't always the best to use

Good equipment selection will help reduce costs

Excavation

Considerations when estimating excavation

Desk top study

Site visit

Excavation – Desk top study

Review plans and specifications Existing utilities lines

How accurate Phone the companies to locate the lines for

you. Need to give them advanced notice Check what permits are required

Resource consent Aborist report/permits Sediment control

Excavation

Review plans and specifications Public records might give additional

information Soil conditions Water problems Abandoned utility lines

Excavation

Setting out carried out by the engineer If not cost to get the site surveyed

Under cutting Might need to remove dirt from below the

existing finishing line if in rock Underground services – may have to cut

the trench deeper for the bedding of the pipe

Excavation

• Cut and fill Consider is there enough fill on site from the

spoil heaps. If not where will you transport it from ? Is it available?? – Snapper rock subdivision Conversely will you have to transport excess fill

off site If so.. Where will you transfer it to? How much are tipping fees?

Excavation

• Top soil requirements Is there enough on site ? Can you keep it in spoil heaps on site ? Will you have to bring additional material in?

• Special excavation Rock ? Hire of specialist equipment Removal of existing structures

Excavation

• Review utility lines – Not only new lines but connection to existing

• Review drainage, new roads, parking lots and consider the base, sub bases for estimating

• Review the building/basement structure for estimating

Excavation – site visit

AccessibilityLocation of the site –

How remote ? Are the roads undeveloped or narrow ? Can you get the equipment you want on site? Bream Bay Sub division

If you are having to bring the dirt out – what is the best route to the tipping site? – Can the trucks use that route

Excavation

If the surrounding area is heavy traffic. Will you require additional traffic management ? Do you require a wheel wash facility (Kensington Park)

Noise restriction – if the site is in a built up area are there noise restrictions

Are there any bridge weight restrictions in the area which will affect what equipment you use

Excavation

Degree of Difficulty Are there steep slopes on site that

need different equipment – slopes that are steeper than 1 in 3 will need track machines which are slower.

Other considerations are buildings, trees, underground services, overhead electricity lines.

Excavation

Surface Conditions Drainage – what happens to the water

across the site. It can’t flow into the existing roads

Sediment control is important Permit/consents are required Silt ponds Sand bags Protect existing waterways

Excavation

Surface Conditions Trees

Consents are required Do you have to protect trees If they are to be removed – who will take them?

Can they be sold for firewood? Are you allowed burning on site?

Excavation

Sub surface Conditions Geotech reports

These reports are expensive. They show the soil conditions however the bore readings taken maybe limited - Ibis Hamiliton, Snow planet

If you are working below the water table level you may have to pump the water… Where is this water going to go

Excavation

Sub surface Conditions Some site have been used as fill sites… these

areas will need to be removed and new fill imported

Soft spots can be found in any excavation… make sure you either make allowances or tag it out your tender – Ibis Rotorua

Excavation

Utilities Utility lines may not be in the correct location and

if they are hit by a digger it becomes expensive Check manholes are in the right location and they

are at the correct depth and have the right no of inlets/outlets

Check overhead lines. Can you machines get under them. Get the electricity company to put sleeves on the lines or flags – Current roadworks in Remeura.

If Utilities need to be moved – check out how much and how long it will take.

Excavation

Project sizeCheck if the site is large enough to store all the equipment and spoil heaps

Traffic ControlTraffic management systems are becoming paramount – they are very expensive

Excavation

Security Check the security issues of leaving the big

machines on site. They wont take the machines but they will take the batteries !! Bream Bay

Do you need to hire security ?? Do you have to fence the site for public

safety??

Excavation

Time of year works are completed Existing and imported soils

Earthworks are carried out in the spring and summer If the weather is wet then the machines are slower,

wet and heavy soil costs more to move than dry light soils

Compaction of the soil is very important and this is affected by the moisture content.

If the soil doesn’t dry out by natural means then additives such as lime has to be used

Conversely if the soil is too dry, then water trucks have to be used

Excavation – types of machines

Excavator with back actor and front loading bucketavailable on wheels or trackCapable of trench, basement and foundation excavation. Can load spoil into trucks

Excavation

Face shovelParticularly suitable working against banks, forming cuttings and embankments

Excavation

GrabUsed to load loose material into spoil heaps

on site

Excavation

GradersLevel out deposit fill

Excavation

RollersUsed for compaction

Excavation

Compaction and bulking factors

bulking factor compaction factor

1.00 1.25 0.90

In place loose Compacted

Excavation

Bulking factors% by volume

Gravel 10%Sand 12.5%Ordinary earth 25%Clay 33%Chalk 33.3 %Rock 50%

Excavation

By hand factorsOrdinary ground has a value of 1 Type of ground multiplerSand 0.75Stiff clay 1.25Compacted gravel 1.50Soft chalk 2.50Soft rock 4.00

Excavation - calculations

Excavate to reduced levelsRequirements – excavator with a banksmanThe output of the excavator includes digging

and loading directly onto a waiting truckSay Banksman is $20.00/hrExcavator and driver is $40.00/hrHourly cost of operation – excavator $40.00

Banksman $ 20.00 TOTAL

$60.00/hr

Excavation

Excavate to reduced levels – cont’d

Output of excavator - $15m3/hr

So cost/m3 = $60.00/15 = $4.00/m3

Excavation

Excavate trenchesRequirement 90% completed by machine,

10% completed by hand. Banksman required for machine

All in hourly rate is as before - $60.00/hrAssume excavator is 9m3/hrCost for excavation = $60/9

= $6.67/m3

Excavation

Excavate trenches – cont’dHand digging – labour = $15.00/hr1 labour digs 1 m3 in 2.5 hours Cost/m3 = $15.00 x 2.5 =$37.50/m3

Total cost90% of machine cost – 90% x $6.67 = $6.0010% of hand digging – 10% x $37.50 = $3.75

TOTAL = $9.75/m3

Excavation

Disposal of waterRequirement – 2 pumps for 8 weeks of

excavationLabour – ½ a day in attendanceLabour - $15.00 /hr

Hire of a pump $150/weekCost of fuel/pump $20/week Total cost of pump $170/week

Excavation

Disposal of water – cont’dHire of pump – 2 x $170 x 8 weeks =

$2,720.00Cost of labour0.50 hour x (8 x 5) days x $15.00/hr

=$2,420.00TOTAL = $5,140.00

Excavation

Removal of excavated materialRequirement – the excavation is loaded

directly onto 10 tonne lorriesThe cost of the excavation is included in the

excavation rateFor the excavator to work efficiently, once

one truck is loaded another one comes through the gate.

Excavation

Removal of excavated material – cont’dNo of vehicles =

time taken to load + total journey time

time taken to load

Lorry and driver = $95/hrTipping fees = $ 40/load

Excavation

Removal of excavated material – cont’dWeight of earth = 1.60tonnes/m3 so a 10

tonne lorry with a full load takes 10 tonnes/1.6 = 6.5m3

Allow for bulking factor of 25%

6.5m3 = 6.5m3 x 100/125 = 5 m3

Excavation

Removal of excavated material – cont’dTime taken to load 5 m3 assume the

excavator is 9m3/hr= 5/9 = 33 minsAssume tip is 10k from siteOutward journey 15 minsTipping time 5 minsReturn journey 12 minsTotal time 32 mins

Excavation

Removal of excavated material – cont’dTherefore no of lorries needed are33 mins + 32 mins = 2 no aprox 33 mins

Cost of operation = 2 no lorries x $95/hr = $190

Excavation

Removal of excavated material – cont’dExcavator is working continously and the

quantity of excavated material removed in 1 hour is 9m3

Cost /m3 = $190/9 = $21.11/m3Tipping charge $40/loadCost/m3 = 40/5m3 = $8.00/m3Total cost/m3 = $21.11 + $8.00

= $29.11/m3

Excavation

ReferencesEstimating excavation – Deryl BurchEstimating and tendering building works –

R.C Smith