Cut and Fill Excavation for Road Construction on Hillside Slope – Cross Section Details and Quantity Takeoff
Cut and fill excavation is a method of construction used on undulating terrain with ridges and troughs or more specifically terrain with elevated areas (hills and mountains) and low lying areas (valleys). This method is widely used in the construction of roads, railways, canals and other projects where embankments are to be built.
The valleys and ridges (or peaks) on undulating terrain can be depicted with contour lines on a topographical gradient map.
When a building a road across a hill slope, open face excavation is required. The purpose is to cut the slope to create a surface that is level or parallel with the sea level. Vehicles are capable of moving on this road as if they are on flat ground, although the road may be at higher elevation than normal ground. How much earth requires to be cut from the slope depends on the elevation of the proposed road, its width and gradient of the hill slope. The excavated material is deposited down the slope in the valleys to create manmade formations known as embankments. As much filling as possible must be obtained from the excavations. This filling is mechanically compacted to the required AASHTO density in layers not exceeding 300mm. If material from site excavations is not enough to fill the embankments, the engineer may order the contractor to import filling elsewhere. As you can see, the volume of excavations in this type of construction is not necessarily equal to the volume of total filling. There are many factors which affect the required balance since each and every project is unique.
Cut and Fill Diagram
The cut-and-fill diagram shown below is for a road built across a hill slope. About half of the road width is shared between the excavated slope and filling. This is known as a half-benched road.
The yellow region represents the cut area. This is the volume of excavated material.
The orange region represents the fill area. This is the volume of earth filling required for the formations. Note that in this diagram, the volume of filling is much larger than the excavated volume. Sometimes, extra filling has to be imported offsite by the contractor to meet the required quantity.
After the earthworks (open face excavation and filling, including grading and levelling) are done, the next step will be preparing the ground to lay the road materials. This includes compacting the subgrade, laying the subbase course and base course. The final step is laying the surface course which is composed of the binding course and wearing course (asphalt). Longitudinal surface water drains made of wet or precast concrete may be excavated for and built on one or both sides of the road.
One way of draining storm and surface rainwater from roads on hillside slope is building culverts across the road (also known as underdrains) which intercept longitudinal drains and ditches adjacent to the slope face. The culverts will empty the storm water into the valley and flood plain on the other side of the road. The diameter of culverts should be sufficiently large to drain away as much stormwater as possible. It also prevents soil erosion and high pressure back spillage caused by high rainwater intensity and small pipe diameter.
Before and After Construction Pictures
1 – The original hill slope:
2 – Hill slope after excavations and filling in lower end with earth:
3 – Finished Road Construction on Hillside Embankment.
4 – Cut and Fill Sections Superimposed on Finished Road Construction.
Example of Cut and Fill Earthworks Quantity Takeoff
BULK EXCAVATION, FILLING, ETC.
Open face excavation in earth, not exceeding 2m deep:
- Excavate over site to reduced level. [m3]
A 100m road is to be built on hill slope by cut and fill excavation. By referring to the drawings (Fig A), takeoff the quantities for excavation and filling.
Two lane road – 3.25m x 2 = 6.5m wide.
Add 500mm wide paved shoulders (sidewalks) on both sides – 0.5m x 2 = 1.0m
Total width of road = 6.5 + 1 = 7.5m
Cross section area – base/2 x height
Triangle 1 = 13,261/2 x 1,394mm = 13.261/2 x 1.394 = 9.240m2
Volume of T1 = 9.240m2 x 100m = 924m3
Triangle 2 = 13,352/2 x 2,438mm = 13.352/2 x 2.438 = 16.276m2
Volume of T2 = 16.276m2 x 100m = 1627.60m3