CTC-375 Construction Methods Earthwork

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Construction Order There is an order to all projects What activities get done first What activities need to be done in a specific order What activities can be done at any point in time

Start at beginning Earthwork Moving rock or soil from one location to another Processing it to meet location, elevation, density moisture content, etc Efficient earthwork requires: accurate estimating of work quantities and conditions, proper equipment, competent job mangement

Equip Selection Proper equipment has major impact on efficiency and profitablility Can equipment perform required work Also look at profitablility, other uses for equipment, return on investment, availability of parts and services, effect of downtime on other construction equip

Equipment planning Need to plan to effectively use equipment Production of equipment Production = Volume per cycle/cycles per hour Cycles per hour is based on efficiency of equip Swing angle and elevation to truck bed Soil hardness and Soil type Room to manuever

Equipment planning Cost per production unit = equip cost per working hour/equip production per hour Table 2-1 show efficiencies

Soil And Rock General Soil Characteristics Trafficability – ability of soil to support weight of vehicles under repeated traffic Controls traffic on unimproved access roads Also gives measure of how earthmoving equip will operate Primarily function of moisture conditions and soil type Loadability – how difficult to excavate and haul a soil Granular – high Compact cohesive - low

Soil And Rock Unit Soil Weight Pounds /cy Depends on soil type, moisture content, degree of compaction Relation between soil weight and bearing capacity So soil weight is used as a measure of compaction Soil weight is also a factor in hauling

Soil And Rock Moisture Content(%) = (moist wt – dry wt)/dry wt X 100 Soil sample 120# Dry weight 100# MC = (120-100)/100X100 = 20%

Soil ID Boulders Cobbles – over 3” diameter Gravel 1/4” – 3” diameter Sand 0.7mm (200 sieve) – ¼” diameter Silt 0.002 – 0.7 mm Clay less than 0.002mm Organic Matter decaying organic matter Soils classified using these types

Soil Classification Systems Unified System All material 3’’+ removed Separates soils into two main groups- Fine grained and coarse grained Table 2-2, Figure 2-1 AASHTO System 7 classes of soil Based on suitability of soil for subgrade Table 2-3 Table 2-4

Soil Volume Change Characteristics Soil Conditions Bank – material in natural state before disturbance – Bank cubic yard Loose – material that has been excavated or loaded loose cubic yard Compacted – material after compaction compacted cubic yard

Soil Volume Change Characteristics Swell Soil increases in volume when it is excavated Soil grains are loosened and air fills voids So 1 unit of soil in bank is smaller than the soil once it is excavated Swell(%) = ((weight/bank vol)/(weight/loose vol)-1)X100 Soil wt = 2800#/cy in bank Soil wt = 2000#/cy loose Swell = ((2800/2000)-1)X100 = 40%

Soil Volume Change Characteristics Shrinkage Soil decreases in volume when it is compacted Air is forced out of soil So 1 unit of soil compacted is smaller than the soil in the bank or once it is excavated Shrinkage(%) = (1-(weight/bank vol)/(weight/compacted vol))X100 Soil wt = 2800#/cy in bank Soil wt = 3500#/cy compacted Swell = (1-(2800/3500))X100 = 20%

Soil Volume Change Characteristics Load and Shrinkage Factors Need a common unit of measure for earthwork (get rid of calculations ) Can use any of the three measures Called pay measure in contract Load factor = 1/(1+swell) Shrinkage factor = 1- shrinkage Table 2-5

Spoil Banks Material removed from excavation Longer than wide – spoil bank – triangular x section Conical – spoil pile To determine the size of the bank or pile need swell and angle of repose for soil Angle of repose = angle that soil on side of bank naturally form Varies with moisture content and type Table 2-6

Spoil Banks Spoil bank Vol = X-sect area x length B = (4V/(L x tan R))^1/2 H = (B x tan R)/2 B – base width H height L length R angle of repose V volume

Example You have to store 30,000 BCY of soil in an area no wider than 80 feet How long is the spoil bank? Swell = 25%

Spoil Banks Spoil pile Vol = 1/3 p(D/2)^2 X H D = (7.64V/tan R)^1/3 H = D/2 x tan R D – diameter of base H height R angle of repose V volume

Estimating earthwork 3 types of excavations Roadways Small pit Trench Large areas Roadways Find cut and fill using cross sections Mass diagram

Estimating earthwork Pit Excavations Area X average depth Depending on size and ground may break into several geometric shapes to get volume Give bank volume

Estimating earthwork Trench Excavations V = x sectional area X length Take x sections every 50 feet and compute volumes between x sections

Estimating earthwork Large Areas Use a grid to find volume