What Is A Monitoring Survey And How Do They Work?
Article By: Tom Ayre | Last Update: November 2020
In this article, we will be explaining monitoring survey services. We know that the subject is often confusing to people outside of the industry, hence the need for this article.
Reading this article will give you a good grounding in important aspects of monitoring surveys. This could potentially help you with:
- Saving money when appointing a monitoring survey company
- Understanding exactly what a monitoring survey is
- Protecting your property / neighbouring property from unfair claims.
Let’s get started.
What is a monitoring survey?
A monitoring survey is undertaken when there is a requirement to check a building or structure for movement over time. The procedure involves repeatedly surveying fixed locations (monitoring points) at set time intervals such as hourly, weekly, yearly etc.
The movement is reported to the client in the form of monitoring reports.
Monitoring survey schemes have a variety of techniques, levels of details and varying types of equipment, examples of these include:
- Fixed position monitoring – Identifying a set location and monitoring that position.
- Tilt monitoring – Is a wall leaning in a certain direction.
- Crack monitoring – Is a crack opening or closing?
Over the course of a monitoring survey scheme, a movement profile can be observed with actions being undertaken if movement exceeds certain levels. The levels are usually set by the client’s Structural Engineer or Party Wall Surveyor. The limits of movement generally depends on how much of an issue a movement event would cause.
How are monitoring surveys undertaken?
Fixed Position Monitoring
A fixed position monitoring survey is the method of monitoring a set location on a wall / structure and observing movement in the horizontal and vertical planes over time.
Fixed position monitoring is used for a number of projects ranging from small basement projects to large infrastructure projects such Crossrail tunnel project under London.
The monitoring project always starts with the initial setup of the monitoring targets and local survey control network. The targets are fixed to the structure in the pre-designated locations utilising either glue or drilled screws.
Without a fixed monitoring point to resurvey each time the consistency will be poor or non-existent. Fixed points are installed on to the structure and usually consist of survey prisms or reflective target stickers. When using prisms they are typically secured using screws drilled into walls or using high strength glue.
A local survey control network also needs to be installed. This involves placing survey targets (prisms or reflective survey points) away from the site. These are used by the total station as a way of calculating it’s precise position each time a monitoring survey needs to be undertaken.
A monitoring survey is undertaken using a piece of equipment called a total station. A total station is a high accuracy optical instrument that is used to measure distances and angles. Total stations are usually mounted on tripod legs whereby the operator can look through the lens and record survey points. In some cases, automation is required If so a total station can be left in a safe location and operated remotely.
At each survey interval, the total station has to be set up using the local control network. Once set up accurately, the monitoring survey process can be undertaken. Each monitoring point is precisely surveyed using the total station and recorded to the instrument.
Once this data has been reached the office, it is uploaded to the monitoring survey software which can generate data to illustrate movement (or lack of) over time.
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Tilt Sensor Monitoring
A tilt sensor precisely measures the angles and reports these back to a central hub. Tilt sensors are typically installed on to a wall / structure and can monitor tilt in multiple directions.
The tilt sensor can report these readings back at fixed intervals such as each minute, hour, day etc. A tilt angle limit can be set which can then trigger an alert to the monitoring survey company / client.
An important element to bear in mind is that tilt sensors monitor angles in degrees and not movement in say, millimetres. It is possible to translate the angular tilt into an approximate movement using trigonometry. For a tilt sensor mounted to a wall you would need to know the approximate point of rotation and height of the sensor to achieve this.
Tilt sensors are typically installed using wall fixings. Modern sensors have in built sim cards which can report the movement values back without communication cabling. They also have long term battery life’s and depending on the reporting interval, can sometimes last years.
Tilt sensors are often used in locations where access is difficult such as:
- Enclosed areas
- River walls
Crack monitoring is the simple process of monitoring crack in wall / structure for movement. Depending on what the situation is a crack may grow in width or close up. Typically there are 2 ways of monitoring a crack for movement:
- Crack gauges
- Demec Studs
Crack gauges come in a variety of different products designed for varying applications. The basic premise is that you glue or mechanically fix the gauge either side of a crack using glue or mechanical fixings. Over time the crack opens and closes, the movement is then illustrated on the gauge.
Crack gauges are very simple to setup and a fairly cheap method of monitoring cracks. You can quickly understand the movement of the crack as there are no further tools required, it just requires you to read the gauge!
Monitoring studs (often referred to as Demec Studs) area another simple method of monitoring movement of cracks over time. 2 Demec studs are securely glued either side of a crack. Once the glue has set, the distance between the Demec studs can be recorded using a precise calliper.
The Demec studs have little groves in their centres to ensure that the calliper can precisely locate the measuring hole each time. Modern digital callipers can typically give you a very accurate reading. Depending on how much money is spent, readings can be as small as 0.01 mm
When are monitoring surveys commissioned?
A monitoring survey process is usually undertaken on the advice of a professional such as a Structural Engineer or Party Wall Surveyor. If a building or structure is a risk of movement then a monitoring survey is usually commissioned.
Typically monitoring surveys are commissioned when undertaking heavy construction works next to existing buildings. Some examples include:
- Piling works
- Basement excavation
A monitoring survey scheme can also be commissioned where an insurance claim is being checked.
How can monitoring survey data be used?
Monitoring survey data is usually presented in report and graph format. The reports will outline movement over time. It’s common to see reports showing movement compared to the previous set of readings as well as the initial set of readings.
Graphs can also be produced to show the movement in a graphical sense. Graphs can help show when movement started occurring.
Movement monitoring surveys are an good way to help prove or disprove that movement within a structure has been undertaken. This can help with future claims either made for or against.
Diligent contractors ensure that they minimise movement as a result of construction however claims from neighbours can still arise.
The movement monitoring reports produced can demonstrate movement or lack of over time. This can be used in counter claim situations where by neighbours insist that damage has occurred.
Another purpose of monitoring surveys is from a safety perspective. If a building is moving during construction works then this can be identified. If a movement exceeds a certain level then this can be flagged. The potential source of the movement can be found and mitigated against prior to any problems that could potentially endanger.