9.5 Excavations

Question 1

The main hazards associated with excavations are: 12

Answer 1

 Ground movement / collapse of excavation sides  Surcharging  Ground and surface water  Stability of adjacent structures and services  Underground services  Overhead lines  People falling into excavations  Plant falling into excavations  Materials falling into excavation  Toxic gas and oxygen deficiency  Biological and chemical health hazards  Noise and vibration.

Question 2

Ground movement / collapse of excavation sides controls

Answer 2

Ground conditions should be regularly inspected by a competent person. The excavation should be battered or stepped, or supported.

Question 3

Surcharging occurs when

Answer 3

a load applied to the ground by plant, stored materials, spoil or structures increases the horizontal load on the excavation sides, causing them to collapse.

Question 4

SurchargingControls

Answer 4

 Barriers and signs to keep vehicles away.  Use of stop blocks for tipping vehicles.  Materials stored away from excavation (min 1.5m or > depth of excavation).

Question 5

Ground and surface water Controls

Answer 5

Water is channelled into sumps and removed using hydraulic pumps.

Question 6

Stability of adjacent structures and services Controls

Answer 6

 Shoring of structures.  Supporting of services.

Question 7

Underground services Controls

Answer 7

 Checking of existing plans.  Use of cable avoidance tools (CAT).  Appropriate digging techniques (not directly over services, hand tools in immediate vicinity – see below).

Question 8

Overhead lines Controls

Answer 8

Control plant movement beneath and near to overhead power lines (see Element C8).

Question 9

People falling into excavations Controls

Answer 9

 Provision of a substantial barrier, including guard rail, intermediate rail and toe boards.  Provision of safe access.

Question 10

Plant falling into excavations Controls

Answer 10

 Barriers and signs to keep vehicles away.  Use of stop blocks for tipping vehicles.

Question 11

Materials falling into excavations Controls

Answer 11

Materials stored away from excavation (min 1.5m or > depth of excavation).

Question 12

Toxic gas and oxygen deficiency Hazards

Answer 12

 Combustion gases from petrol or diesel engine equipment can collect within trenches.  Ground such as chalk or limestone in contact with acidic groundwater can liberate carbon dioxide.  Glauconitic sand (such as Thanet sand) can oxidise, causing oxygen deficiency.  Gases such as methane or hydrogen sulphide can seep into excavations from contaminated ground or damaged services.

Question 13

Toxic gas and oxygen deficiency Controls

Answer 13

 Keep generators clear of excavations.  Ground condition surveys.  Oxygen and gas monitoring.  Respiratory protective equipment (RPE).  Emergency arrangements.

Question 14

Biological and chemical health hazards

Answer 14

 Leptospirosis (Weil’s disease) from working near contaminated water.  Sewage from damaged sewers or sewer connections poses a risk of gastroenteritis.  Tetanus from a dirty penetrating wound.  Contaminated ground could contain asbestos, coal-tar residues, lead alkyls or mercurial compounds.

Question 15

Biological and chemical health hazardsControls

Answer 15

 Check site history.  Soil sampling.  Suitable PPE – overalls, gloves, shoes.  Welfare facilities – washing.  First aid arrangements.

Question 16

Noise and vibrationControls

Answer 16

 Specify low noise / vibration equipment and maintaining it in good order.  Limit time spent using equipment.  Suitable PPE (see Element B6).

Question 17

Use of 360o excavators After ensuring that the right excavator has been selected for the job there are five main precautions to be implemented:

Answer 17

(1) Exclusion: Most excavator related deaths involve a person working in the vicinity of the excavator rather than the driver. Fencing, bunting or other suitable barriers should be used to create and maintain a pedestrian exclusion area. (2) Clearance: Plant with minimal tail swing should be used if slewing in a confined area. Clearance of over 0.5m needs to be maintained between any part of the machine, particularly the ballast weight, and the nearest obstruction. (3) Visibility: The driver should have a good all round view and the excavator should be equipped with adequate visibility aids to ensure drivers can see areas where people may be at risk from the operation of the machine. (4) Signaller: To direct excavator operation and any pedestrian movements from a safe position. (5) Bucket attachment: Quick hitches can be used to secure buckets to the excavator arm. Check that you are able to implement and manage any quick hitch used. A number of deaths have occurred in recent years when the bucket has fallen from the machine. (See notes on ‘quick hitches’ later)

Question 18

There are three categories of worker who must be trained and competent regarding the excavator hazards and precautions:

Answer 18

(1) Drivers: should be trained, competent and authorised to operate the specific excavator. Training certificates from recognised schemes help demonstrate competence and certificates should be checked for validity. (2) Signallers: should be trained, competent and authorised to direct excavator movements and, where possible, provided with a protected position from which they can work in safety. (3) Pedestrians: should be instructed in safe pedestrian routes on site and the procedure for making drivers aware of their presence.

Question 19

The need for excavation can be avoided at the design stage by using trenchless technology for pipe laying, or pipe repair and rehabilitation.

Answer 19

Horizontal directional drilling uses a curved trajectory to enable a pipeline to pass under an obstacle, such as river or road, without the need for excavations at either end, as with horizontal boring techniques Impact moling uses a pneumatic piercing tool to create a bore hole. The approach compacts and displaces soil rather than excavating and removing it, therefore minimising disruption and ground reinstatement. Pipe bursting techniques can be used to replace an existing, damaged pipeline with a new pipeline of the same (or slightly larger) diameter. The old pipeline can be burst in-situ, or removed piece by piece and broken up as it is pushed out of the tunnel.

Question 20

Part 4 of the Construction (Design and Management) Regulations 2007 require that certain places of construction work are to be inspected by a competent person. (a) Identify: (i) when statutory inspections of supports excavations must be carried out; 3 marks (ii) the information that should be recorded in a statutory excavation report. 5 marks (b) Outline the particular features of an excavation that could result in it being considered unsafe. 12 marks

Answer 20

(a) (i) Under the Regulations, an inspection of an excavation must be carried out at the start of every shift before work commences, after an event that may have affected the stability of the excavation and following an unintentional fall of material. (ii) Information that should be recorded in the inspection report includes:  the name and position of the person making the report  the name and address of the person on whose behalf the work was carried out  a description of the place of work inspected including the plant and equipment in use  the date and time of the inspection  the details of any matter identified that could lead to risk, the action to be taken to prevent the risk and the details of any further action necessary. (b) Reasons that could lead to an excavation being considered unsafe such as:  its proximity to adjoining structures  the inadequacy or incomplete nature of the means of support for the type of soil where the excavation was sited  damage to or the dislodgement of the supports provided such as metal sheets or walings  the presence of excess water either from damaged pipes or land drains  damaged services and the presence of fallen rock, earth or other materials in the excavation  a failure to provide adequate means of access and egress  the presence of spoil or materials placed too close to the edge of the excavation  a failure to provide stop blocks despite the use of vehicles in close proximity to the excavation  the presence of gas or fumes, or biological or chemical hazards  a failure to provide adequate barriers and warning signs to prevent persons from approaching and falling into the excavation.