What Is A Monitoring Survey And How Do They Work?
In this guide we explain what a monitoring survey is, why they are useful and how they are implemented.
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.
Table of Contents
What is a monitoring survey?
Monitoring surveys are a precise method of surveying when there is a requirement to repeatedly and accurately check a building / structure for movement over time. Monitoring surveys are commissioned for a number of difference schemes including such as basement construction, tunneling, demolition and insurance claims.
A monitoring survey could check for a number of different things such as:
- Movement in height / position.
- Opening or closing of a crack.
- Angular tilt of wall.
The monitoring procedure involves repeatedly surveying elements at set time intervals such as hourly, weekly, yearly etc. Depending on the project requirements monitoring can sometimes be automated utilising remotely logged systems that can automatically measure over time.
The movement is reported to the client in the form of monitoring reports and graphs. The aim of the reports / graphs is to give a clear indication of what is occuring.
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.
Types of monitoring surveys
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.
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
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
Money saving tips
Saving money when possible whilst undertaking a project is important. Below we explain how this can be done when commissioning a monitoring survey company.
Reduce the amount of targets/locations
When your Structural Engineer or Party Wall Survey is looking to commission a survey they usually don’t consider the company who has to undertake the works. Complicated monitoring survey proposals that require multiple total station setup locations add a lot of complexity and time to a job.
Keeping things simple
Simplifying what is required can often make little difference to the effectiveness of a monitoring survey and can unlock considerable savings if the following elements are considered:
- Can the survey company easily set up with access to their offsite control network?
- Can we reduce the number of monitoring points?
- Are the monitoring points safe and easy to install?
Increase the monitoring interval
Often a surveying company will quote for a monitoring survey job based on a per-visit basis. A simple way of reducing the overall cost of a monitoring survey is to simply reduce the amount of surveys.
- Can a weekly specification be pushed to a 2 weekly interval?
- Can we do less surveys whilst less intense work is being undertaken on site.
- Is it possible to pause the survey work whilst the site is temporarily shut down?
Check their experience
The experience of your monitoring survey team has the possibility of significantly increasing your costs. An incompetent survey company can incorrectly report results and show general inconsistencies. It is important you check the following aspects of a survey company prior to appointment.
- Experience in undertaking monitoring surveys
- Level of insurance for the works.
- What is and isn’t included in their price.
- Reviews from others (Google, Trustpilot ETC).