Rock Excavation in civil Engineering

Construction in Rock

In civil engineering, as well as in the building industry, rock is removed to make place for new roads and railways, house foun­dations, ports and airports, trenches and pipelines, or to pro­vide material for breakwaters and concrete aggregate. These types of construction projects differ from quarrying and open pit mining, in that bench heights vary continuously. Drilling is often performed from a rugged and steep rock surface, which means that the equipment used must have good terrain travelling characteristics. Also, as several holes are drilled from one rig setup, folding booms having good coverage in various directions and planes are the preferred equipment. Atlas Copco has a complete range of drillrigs for this type of work, from heavy crawler rigs to lightweights that can be craned into position, if necessary. The whole spectrum of drilling methods is also available, from pneumatic handheld to hydraulic tophammer, COPROD and DTH.

Accurate Tendering

Before a new project is commenced, it is normal routine that a design and preinvestigation is conducted, coupled with allocation of financing. Most civil engineering projects are subject to a tendering procedure, where scope of work, bill of quantities, drawings, geological information and time schedule are disclosed, to give all interested parties standard information on which to base their bids. The con-tractor, or a consortium of contractors, makes its calculations based on evaluating equipment requirements, methods, availability of skilled labour, time schedule, local conditions and regulations, as well as cost estimates. Past experience derived from similar pro­jects plays an important role in estimating future costs. A project involves a wide range of equipment, and clever selection is important to achieve overall optimum performance. For blasthole drilling, the variation and complexity of blasting parameters might require both heavy crawler drills and hand held rock drills.

The equipment should, however, be selected to cover a wide range of appli­cations, which will improve spare parts handling and maintenance procedure, and the prospects for usage in subse­quent projects. Tophammer drilling rigs are normally the first choice, due to their flexibility, versatility and abili­ty to drill holes of small diameter.

Road Excavation

 

Figure 1 shows a typical drilling and blasting pattern for a road cut where special emphasis is placed on the final rock wall contour. Smooth blasting, or presplitting, are commonly used, and quite often specified in the tendering documents, to obtain smooth and stable side contours. Contour holes are normally not larger than 64 mm, with spacing of less than 1 m, and charging is carried out using special, light explosive to mini-mize the effect of shock waves. Initiation is frequently by detonating cord. Presplit holes are fired simultaneously ahead of the other holes, where-as smooth blasting uses the highest number of detonator delay.

Presplitting creates an artificial plane along the limits of the excava­tion, against which the subsequent main blast may break, resulting in a smooth wall, with little or no over-break. Some of the shock waves from the subsequent main blast are reflected against the presplit plane, preventing them from being transmitted into the remaining rock. This also tends to reduce ground vibrations. Drilling with high precision is of utmost importance for the final result.

Slope stabilization should normally be carried out as an integrated part of the excavation process. Horizontal drilling, anchoring, grouting, and meshing using elevated working plat­forms could cause excessive costs and delays, where they have to be conduct­ed at a later stage (figure 2).

Urban Blasting

Several precautions have to be taken when excavating in built-up areas close to population, infrastructure, buildings and installations (figure 3).

It is quite common that special blasting consultants are involved, to assist the contractor with vibration control and design of drill patterns, as well as a thorough survey of adjacent structures before the work can com­mence. To minimize ground vibra­tions, the drillholes are kept as small as possible, normally 30-50 mm diam­eter. To eliminate the risk of fly rock, the blasts are covered with heavy mats made of logs, rubber, or tyres, cut into sections and twined together with steel wires. These mats, of around 1 t weight, are energy absorbing. The gases produced by the blast ventilate through the mats without displacing them.

 

Trenching

Trenches are excavated for installation of oil, water and sewage pipelines, as well as for cables. Blasting of trenches differs from common bench blasting, because the width of the round is considerably smaller than its length. Trench blasting is defined as rounds with a width of less than 4 m.A trench blast is more constricted than a normal open pit blast, which results in higher explosives consump­tion per cubic metre of blasted rock. The diameter of the blast holes is nor­mally smaller, which provides better distribution of the explosive in the rock, and avoids excessive overbreak outside the theoretical contour. As a rule of thumb, the hole diameter should be one sixtieth of the width of the trench, so if the width is 2.0 m, the diameter of the hole should be 33 mm.

Two methods are used for trench blasting: traditional and smooth wall. In traditional trench blasting, the middle hole/holes are placed in front of the lateral holes. All holes are charged with the same amount of explosives. In smooth wall trench blasting, all holes in each row are in line. The middle hole/holes are quite heavily charged, while the edge holes have light charges.

Blasting of trenches in urban areas requires careful planning and precau­tions. A typical drilling pattern and blasting sequence is shown in figure 4 overleaf. Overall project costs and tight time schedule are of highest priority when conducting pipeline projects for oil and gas distribution in unpopulated areas, and larger blastholes are used, as shown in figure 5 overleaf.

 

Lists of words

1. bench hight – висота уступу

2. boom - стріла

3. complexity - складність

4. trenching – проходка траншей

5. tender – пропозиція

6. shock wave - ударна хвиля

7. initiation - ініціювання

8. subsequent - наступний

9. precision - точність

10. blasting – підривні роботи

11. eliminate - усувати

12. constrict - стягувати

13. sequence - послідовність

14. anchoring- закріплення

15. bid – заявка, пропонування

16. planning - планування

17. charge - заряджати

Exercises

1) Give the definitions of the following terms:

Civil engineering, drillrig, drilling method.

2) Read and give Ukrainian equivalents of the following international words:

Engineering, port, airport, trench, project, crane, spectrum, pneumatic, hydraulic, routine, detonator.

3) Match a word in column A with a definition in column В:

1. preinvestigation

2. scope of work

3. local conditions

4. blasthole drilling

5. due to flexibility

6. shock wave

7. reduce ground vibration

8. slope stabilization

9. trench blasting

10. overall project costs

 

1. Micцевi умови

2. буріння вибухових свердловин

3. об'єм робіт
4.ударна хвиля

 

5. зменшення вібрації грунту

6. стабілізащя уступів

7. попередні розрахунки

8. вибух у траншеї

9. загальні витрати по проекту

10. завдяки гнучкості

4) Complete the following sentences.

1) Before a new project is commenced.....

2) Past experience derived from.....

3) Tophammer drilling rigs are normally.....

4) Smooth blasting, or presplitting.....

5) Presplitting creates an artificial plane.....

5) Tell what you know about:

1) Construction in Rock

2) Accurate Tendering

3) Road Excavation

4) Urban Blasting

5) Trenching

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