Excavator Sizing in Pipe Applications

Introduction

The volume excavated from a trench is dictated by the width, depth and length. In water/sewer installations, an excavator digs and then stops to place, pipe, bedding, etc. Each "dig / place pipe" process will be called a section for this estimating program

OSHA dictates that soil be classified as "Stable Rock", "A", "B", or "C" for any trench deeper than 5 feet. Each soil classification has a "slope" assigned that protects the worker from cave-in and dramatically increases the volume of material to be removed from the trench. Observe the slope angles in the drawings and note how the trench gets wider at the top for various depths and slope angles. If bucket capacity and excavator cycle times are known, digging cycles and excavation time can be calculated.
	SOIL TYPE "A" SLOPING REQUIREMENTS The maximum allowable slope for type "A" soil is a ratio of 3/4 to 1 or 53 degrees from the horizontal surface plane. The amount of material excavated to attain the slope is less than Type C or Type B soil.

	Type C Soil (Simple slope) All simple slope excavations 20 feet or less in depth shall have a maximum allowable slope of 1.5:1
OSHA allows worker protection by use of trench boxes. Soil classification and matching slopes are still required, but the starting point of the slope can be 18 inches (1.5 ft.)(457 mm) below the top of the trench box
The drawing represents a class "C" slope, but the "A" and "B" profiles must conform to their respective slope requirements.

Observe the work area inside the trench box, narrow, smaller capacity buckets are necessary due the need to place the bucket inside the walls of the box (fine grading, pipe placement, bedding).
Observe how the trench box reduces dig time through lower volumes of material. As you use the program, you'll see how the use of a box can reduce expenses and speed up projects. A trench box can also allow use of smaller excavator than may be necessary without a box.
The length of pipe and length of trench box are normally matched if possible. Two eight foot (2.4 m) sections of pipe in a 20 foot (6.1 m) long box would leave adequate room at the ends of the box. This pipe to trench box match usually dictates the dig length before placing the pipe or moving to do another section.
Contractors usually have a goal for number of sections or total feet per day they wish to average. 15 sections per day times 20 feet (6.1 m) per dig would result in a goal of 300 feet (91.4 m) per day.
Input various lengths and sections to calculate various goals. Observe the affect on cubic yards, cycles and time spent digging each day. Note how much time a trench box saves over a trench without a box! Note that the times are for digging only. The total job must include time to final grade, bed, and place the pipe. Some depths and widths require excessive dig times for a day's work.
Input a time for the total process of fine grading the trench bottom, placing and bedding the pipe. Input the total hours to be worked each day (exclude lunch breaks). Input the number of minutes to work in each hour.
Two data lines are calculated; The total hours the contractor would have to work to meet the Trench Goal The feet of trench that can be placed within the input time limits
Note: Remember that the goal is usually for an average daily production rate. Every job has varying depths. Shallow trenches averaged with deep trenches over a total job can meet the goal.
Bucket widths are usually the limiting production factor on water/sewer jobs. Widths are limited to control the amount of bedding material in the simplest trenches or the need to work inside the width of a trench box.
A coupler system can provide a production increase by using a large bucket at the top of a trench and then switching to the narrow or restricted width at the bottom.
The bucket chosen for the first portion of this exercise is automatically transferred to the coupler calculation area. It is used for calculating the bottom portion of the trench.
Input the depth, (from the bottom of trench) to be dug by the narrow bucket Input the size (cu. yd. or cu. meter) of the bucket to be used for the top of the trench.
Compare the volume dug, cycles, time and feet of trench to the data calculated with one bucket Consider the dollar savings to the entire pipe crew for each hour (daily) gained by swapping buckets, the savings to the crew easily offsets the price of a coupler and second bucket.
Excavator size to this point has been subtle. It is assumed the user understands the need to match the bucket sizes with an excavator arm and the bucket forces to handle it. Matching of the trench depth as well as the reach and lift to handle the trenchbox is also assumed.
The program will now match the soil classifications and top trench width to another factor of excavator sizing that can be calculated. This is the placement and volume of material a given size of excavator can put beside the trench.