Understanding data in construction boundary monitoring
 

In our last article we looked at pollution from construction sites and discussed how they can impact local communities from the point of noise, dust, vibration, and other pollutants that construction sites produce.

In this article we'll be exploring what is measured and what sensors are typically used for boundary construction monitoring, as well as knowing how to place the sensors.

Dust Monitoring

In the case of particulate, it's generally been the case that PM10 (particles of dust less than 10µm in size) has been the key parameter measured. Acceptable levels are typically put in place, often this is 250 micrograms per cubic metre (µg/m³) for when action should be taken. Particle counters are the measurement sensor of choice. These sensors can often measure not just PM10 but PM2.5 and PM1 simultaneously. Averages over specific time periods are calculated and its normally the 15-minute average that the site limits are set against. The inlet for the sensor will tend to be as part of a box enclosing other equipment. When mounting the sensor at the boundary of the site, if it has to go on a hoarding, make sure it is well clear of the top. Ideally the unit should be away from any obstructions to ensure that air flows don’t affect particulate readings.  Set up real time alerts and alarms based on particular levels and be proactive about your monitoring. Alerts can be set for shorter time intervals so if there are 15minute site limits, alerts can be set 5 minutes and then if a high 5 minute value is received, actions can be taken to mitigate the issue on site to reduce the dust level before it becomes a problem or exceeds the site limit.

Figure 1. Typical inlet for dust measurements

Typically with a medium and high risk construction site then there will be at least two dust monitors at key receptor points on the boundary. If you have two dust monitors you can compliment dust monitoring by using wind speed and direction data, which some dust monitors currently have built in. Using wind speed and direction data can:

- Provide information on where nuisance dust from the construction site may be going

- Identify if the construction site is contributing to a dust issue

- Give evidence in the case of a complaint about the dust source

Figure 2. Typical Ultrasonic wind speed and direction sensor

For example, if the prevailing wind is from the south west, and a dust monitor and wind speed sensor is also on the south west of the site, then having a dust sensor on the north east of the site can provide valuable information. Understanding the data between these two monitors can be crucial in establishing if a construction site is contributing to the local environment. For example, if the downwind monitor has higher particulate levels than the upwind monitor, then it's the construction site that's contributing to the dust. However, if a complaint is received from downwind of the site, and it can be demonstrated that the upwind and downwind monitors displaying similar levels, which even if they may be high, shows that the dust is not coming from construction. 

Noise Monitoring

When it comes to noise monitoring there's a lot of terminology that is used, but if acoustics is not a familiar subject, it may be alien. Limits are placed on sites based on hourly average or 10 hour averages. Noise monitoring devices will store and transmit the time history of the noise level, calculating these hourly averages. The actual parameter for the hourly average is called L_Aeq. What does this mean exactly? The ‘L’ means Level, as in a decibel level. The ‘A’ after this is the frequency weighting, so it’s a level weighted to how humans hear frequencies of noise. The ‘Eq’ part is the average, the long hand is ‘equivalent continuous sound energy level’. Understanding this last part is important: As decibels are logarithmic, every 3dB doubles the noise energy means the site is emitting twice as much. Therefore, small changes in the L_Aeq is a big difference in the output of noise and hence the potential to cause nuisance. Short but loud events can also therefore have a large impact on the hourly average.

Figure 3. Typical microphone showing the seperate foam wind protection

In the UK the 10 hour L_Aeq also needs to be reported so this is the average of the 10 hours normally from 8:00 AM until 6:00 PM. Remember again this is a logarithmic average so if you get particularly high hourly average that will have a large impact on your 10 hour value.

Data from noise monitors can be used to be more proactive so that you are not creating excessive noise in the first place. For example, noise meters will also log and transmit shorter intervals of data over, for example, 15 minutes. Setting trigger alerts to get text messages over those shorter periods will mean that if there are getting high levels, there is the opportunity to do something about it. This may be the temporarily stopping or moving a noisy activity away from certain areas if that's possible.

It's particularly important with a noise monitor to make sure it is located correctly so again you want on the boundary of a site and that often means mounting on the hoarding. Make sure that the microphone is above the level of the hoarding and not directly mounted against the hoarding, because you will also measure reflected noise from the hoarding itself. Therefore, readings could be up to three decibels elevated from what they should be, potential exceeding site limits. Measurement microphones can be delicate and therefore need protection. There will have a foam enclosure to protect it from the weather and wind, noise, and particulate, without affecting the measurements. Construction projects can go on from for years and foam can degrade so make sure that the foam is in good condition and replace it as necessary as it will be available as a spare. 

Vibration Monitoring

Ground borne vibration is normally measured on the site in order to prevent damage to surrounding buildings or nuisance to key receptors. On the construction site vibration comes particular from key activities such as piling operations so limits will be set accordingly.

Figure 4. Typical Geophone Vibration Sensor

The vibration sensor itself will be placed in a location suitable for measuring vibration, where people aren't actively moving or daily activities going on. It may have a cable coming from another unit down to the sensor at ground level and follow the manufacturers advice when placing these ground vibration sensors. Sensors should not just to be placed on top of soil, they need to be firmly mounted against something, ideally a concrete plinth that's into the ground so that the actual ground-borne vibration is being measured. The sensors itself will have three axes measurement so will independently measure the x, y and Z axis to produce different vibration levels. The values from these different axes are combined in a calculation to give an overall value called peak particle velocity or PPV for short. This is measured in millimetres per second (mm/s) and there's the overall displacement over a unit of time. Again, you can set trigger levels below your site limits so that you can proactively meant do something about the vibration if an activity is creating excessive levels. 

Summary

Site boundary monitors measure noise, dust and vibration produce a lot of data and understanding what it means is important to make the most of the data and not simply use it as a reporting tool. Using alerts triggered from exceeded levels can help construction sites be proactive, reducing complaints and conflict with nearby neighbours.