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SQUID Evaluates Bottom of Drilled Shafts

Posted on September 9, 2019June 4, 2021 by marketing epc in General Solutions

SQUID Evaluates Bottom of Drilled Shafts

By Brent Robinson, PhD, P.E, Vice President, Pile Dynamics, Inc.

Pile Dynamics Inc. new equipment named Shaft Quantitative Inspection Device (SQUID) provides measured load-penetration curves at the base of drilled shafts and bored piles. It provides construction professionals quick and reliable information about the cleanliness of the hole and the strength of the interface between a bearing layer and the base of a drilled shaft. SQUID is lowered into the borehole by attaching it to the drill stem or Kelly Bar after bottom clean-out by the contractor. SQUID testing can be completed within a few minutes, minimally interrupting construction.

SQUID, shown in Figure 1, is configured with three standard-size cone penetrometers (10 cm²) to calculate the force required to penetrate the bearing layer, and three displacement sensors to measure the distance the cone penetrates, starting from the top of a debris layer. The displacement near each penetrometer is measured by a high accuracy displacement transducer attached to a plate that bears on the debris layer.

The SQUID Analyzer provides the output and remains in the hands of the inspector a safe distance from the shaft excavation. Load and penetration measurements can be displayed in real time with a wired connection from the SQUID to a wireless transmitter at the top of the hole. The wire can also be replaced with an on-board wireless transmitter that collects data from several tests at the shaft bottom, and transmits that data upon return to the surface back to the SQUID Analyzer.

To determine debris thickness, the engineer defines a threshold force or tip resistance. The debris layer is defined when the measured load-penetration curves exceeds the threshold force.

On a recent site, drilled shafts of approximately 1 m in diameter were drilled to rock, and the bottoms tested with a SQUID for debris thickness. Site specifications required debris layer thickness measurements of less than 12.5 mm (1/2 inch) over 50% of the shaft bottom. Figure 2 from the SQUID software report illustrates significant increases in Penetrometer Force (x-axis) with minimal penetration (y-axis). The identified force threshold was crossed at 5, 6 and 32 mm, respectively. Because more than two measurements of the three measurements were less than 12.5 mm, it is theorized that the third penetrometer penetrated into a groove caused by the drilling teeth, the onsite inspectors accepted the shaft. SQUID measurements proceeded to the next foundation element.

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For shafts requiring end bearing, the measurements may also be used to confirm that cone tip resistance measurements on production shafts are similar to or exceed cone tip resistance measurements on shafts that are subjected to static or dynamic load testing. This verification, in conjunction with site soil exploration and observations during drilling, will provide designers with further confidence in their designs or a justification to shorten shafts. SQUID is the only device that measures parameters that could be used for the assessment of the material at the bottom of the shaft without the need of mobilizing geotechnical drilling rigs to the site.

In summary, force and displacements measured with SQUID can be used to determine cone resistance and compare it with other parameters to quantitively and qualitatively assess the soil conditions at the bottom of the shaft. A SQUID installation and demonstration video, along with further details are available on PDI’s YouTube page here.

SQUID is the latest addition to Pile Dynamics extensive line of quality assurance and quality control systems for the deep foundations industry.

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Taking advantage of technology in building site monitoring

Posted on August 30, 2019August 30, 2019 by marketing epc in General Solutions

Genetec - Taking advantage of technology in building site monitoring

By Jean-Philippe Deby, Business Development Director - Europe at Genetec

Building sites are, by their nature, transitory. They’re a project set out for a defined period of time for a team of contractors to come in, complete, and leave. To fit in with that temporary view of the project as a whole, facilities are often treated in the same way. Rather than having purpose-built toilets, builders expect the plastic porcelain and gush of blue goop that goes with it. Similarly, instead of a café or kitchen that might be installed in an office for workers, contractors more often than not will be given a temporary fridge and a kettle; if they’re lucky they might even get a food truck. In all of these situations these temporary fixtures make logical, logistical and financial sense. These features were designed to be transportable, to be used between jobs and cost much less than installing a more fixed infrastructure. One might therefore assume that security and video surveillance would be treated in the same way, but that is far from the case.

Typically, on-site surveillance has consisted of a security guard, motion activated lights to alert them, and a basic local CCTV network so they could get a view of different areas and investigate as needed. The presence of a security guard does come with its benefits. Security guards can ease the stress load of managing site access. They can also deter thieves and vandals, and can protect against fires – particularly as building sites generally don’t have fire safety systems installed. However, this comes with a series of limitations and setbacks.

The first apparent factor is the task of employing a person for any job. When a staff member is off work sick for a day, office colleagues might face the inconvenience and have to alter their schedules to accommodate. However, if that employee is a security officer employed to guard a site, it can cause major disruptions, particularly for night shifts. Along with the unpredictability of an employee having to miss a shift due to illness or personal time off, the job comes with a level of tedium, and while the majority of the job is boring – being present, alert, and ready to act is essential. As a result, the role of a security guard comes with a very high turnover rate, with reported rates of up to 200% – which can lead to the extra expense of hiring and training new employees.

Chief amongst these factors is cost. An on-site security guard can often cost upwards of £1,000 per week. This may seem manageable, but across multiple sites the costs can stack up. By contrast a temporary CCTV solution, made up of cameras on wheels and temporary towers can be much more economically efficient when scaling up. Hiring an extra security guard or two is more expensive than installing a couple of extra cameras that are still manned by the same monitoring station. While a strong argument can be built on the foundation of cost savings alone, there are many other benefits that show why managed security systems are the best option for a construction site.

While there may be an obvious benefit of having a human guard to monitor and scare off trespassers, in the modern day their presence is easily replicated through monitoring in a controlled environment with experts looking at every angle. Sophisticated sensors use passive infrared detection to monitor movement, but unlike older motion activated lights that may be set off by debris or animals; the system will know if it is a trespasser and alert the monitoring station. The trespasser will then be alerted with an initial voice challenge through a connected voice over IP (VoIP) communication speaker system. The criminal will not know whether this voice is coming from a security guard who is based in the building or if they are some miles away.

SQUID

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The remote operator also has the ability to turn the lights on as an additional method of scaring away criminals. If that is not enough to deter them, then a police unit will be alerted and the visual likeness of the trespasser will be captured, encrypted, and stored securely. Rather than relying on one security guard, a team of experts based in a controlled monitoring station are able to work together to stop intrusions, trespassing, and other crimes from taking place.

Access control is another very important aspect to a construction site’s operations. Monitoring who is going in and out of a site is essential for site managers – both in terms of security and safety. Particularly for more high profile buildings where having too many or too few people working can result in lost time or efficiency, site managers cannot simply rely on the old methods of clocking in and out. A sophisticated system of smart cards that hold ‘access credential’ information, combined with biometric data such as fingerprint or facial identification can help managers accurately know which employees and visitors are on-site, and make sure if everyone is accounted for. In a worst case scenario such as a building collapse or fire, a fool proof access control system will give a manager instant insight into who is and isn’t on site, like a modern-day muster station on a ship, allowing them to accurately account for the presence and safety of all workers.

A secure access control system also makes thievery harder. If the only way in and out of a site without setting off alarms is by tapping in and identifying with a fingerprint scan, the only people who would have any access to any materials or equipment left overnight would be employees. This means that it’s easier for managers to keep a track of anything coming in and out of the site. Essentially, access control combined with a connected security system gives site managers a full view of everything going on at the site and, in the event of anything untoward, allows them to see what happened, how and why.

Methods of construction are constantly evolving. From architects moving away from pen and pencil towards computer-aided design (CAD) performed on desktops, tablets and mobile devices, to the workers themselves using safer and more efficient tools. Managing the safety and access to building sites is also evolving. By updating the approach to security systems through temporary CCTV solutions that are connected to a remote monitoring station and intelligent analytics to quickly identify and remove threats, managers can save time and money. Equally, with an access control system, managers now have the ability to monitor the in-out access and traffic of employees, vendors, and guests in order to manage resources accordingly. This shift to take advantage of internet protocol (IP) technology that offers a unified video surveillance (VMS) and access control model, will help building sites to be operated safely, securely and profitably.

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Comprehensive concrete strength testing with Schmidt hammers

Posted on August 6, 2019August 30, 2019 by marketing epc in Concrete Testing, General Solutions

Comprehensive concrete strength testing with Schmidt hammers

By Isaak Tsalicoglou, January 8, 2019,10:00 am CEST

Concrete is the most widely used construction material in the world, thanks to its high compressive strength, durability, long life, and fire-resistant properties. Every year, millions of tons of concrete are used in large construction projects including dams, bridges, buildings, and roads.

In this interview with David Corbett, Product and Application Expert, we learn about the challenge of estimating the strength of concrete, why the economics and insights of destructive testing of concrete structures don’t scale, and of alternatives that are more cost-effective, efficient, and robust.

Dave, tell us about concrete strength – why is it important?

David Corbett: The compressive strength of concrete is a critical property for the safety of concrete structures and the general public. It is one of the key parameters specified for new constructions to ensure the designed service life. As such, evaluating in-situ strength is an important measure of concrete quality. Structures built with concrete of insufficient quality are more susceptible to corrosive agents that eventually lead to deterioration or even collapse, with catastrophic consequences.

What determines the strength of concrete?

The concrete strength is determined primarily by the “mix design” – this is the specification of the components it contains and under which conditions it will perform as designed. While generally we can assume that this carefully designed “recipe” will be used correctly by competent engineers, this is only half of the story. The other half is the actual execution on the construction site. For example, was more water added on site to ease the workability of the concrete? How well was the concrete compacted after pouring? I could go on, but you get the general idea: the final in-situ compressive strength is dependent on the mix design and the working practices of the construction crew. Because the latter can be highly variable, quality assurance of concrete strength is absolutely important for the safety of the structure and the public.

And what exactly does the compressive strength imply for the quality of the concrete?

Assessing the in-situ strength ensures that the concrete meets the design specification, i.e. that it possesses the required strength for the final structure to be safe, durable, and meet any required regulations. This assessment is not only possible for newly constructed structures, but also for older concrete structures or for those undergoing modifications.

How has the compressive strength of concrete structures been estimated traditionally?

The traditional method is the destructive testing of concrete cores. Coring involves cutting cylinders of concrete – the “cores” – from various locations of the structure. The compressive strength of the cores is then tested using a compression testing machine in a lab, off-site.

And what is so bad about coring?

I wouldn’t say coring is bad per se, though it does feature significant drawbacks. To start off, coring is in fact a precise method of evaluating the compressive strength of a concrete structure, when done correctly. That’s because, after all, you are removing parts of the actual structure and testing them destructively in the lab. However, by its very nature, coring is also a labor-intensive and messy activity – and, if done excessively, it would leave a structure locally weakened, with a compromised performance.

Does this mean that coring can only be used to a limited extent?

Indeed! Plainly put: extracting cores on-site and crushing them off-site doesn’t scale, neither in terms of effort, nor in terms of the extent to which it can be done to a structure. Conducting a complete assessment of concrete strength using coring alone is impossible; not only would the structure – or what remains of it – look like Swiss cheese afterwards, but it would also take forever and cost too much to do so.

Still, could you do this traditional testing while on-site to save time?

Not really. After extraction, the cores need to be taken to a lab with a compression testing machine. In other words: with traditional destructive methods you can’t just walk up to a concrete structure and have a testing result, an insight of its condition, in a matter of minutes or even seconds.

So what’s a way out of this problem, then?

The pragmatic, clever solution is to complement or even replace concrete coring with non-destructive testing methods for assessing in-situ strength. In fact, using such methods can aid engineers in selecting their coring patterns and performing assessments in a way that is more efficient and cost-effective. That’s why this “conditional coring” approach is recommended by major standards and guidelines institutions.

Which non-destructive methods do you have in mind?

The two most popular non-destructive methods are rebound hammer testing and ultrasonic pulse velocity testing. Among these, rebound hammer testing is the non-destructive method that has established itself as the most widely used best complement to coring and crushing, as it is the fastest and most economical test to carry out. And by rebound hammer testing, I mean Schmidt hammers.

How so? What makes Schmidt hammers so popular?

Schmidt hammers are popular and established in estimating concrete strength, because they are affordable, easy to use, relatively quick – and, of course, non-destructive. Beyond that, in their latest “Live” iteration they are versatile, connected, and help increase on-site productivity thanks to automating and mistake-proofing the testing process.

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As I understand it, by crushing a core you measure the compressive strength of the concrete sample directly. How do Schmidt hammers measure strength?

There are two types of Schmidt hammers, both invented by Proceq. The first one is the Original Schmidt, which is an R-value hammer – it measures the rebound distance upon impact on the surface. The second one is the SilverSchmidt, which is a Q-value hammer – it measures the rebound velocity before and after impact. Each type has its benefits, but both types measure the surface hardness, which is correlated to compressive strength of the concrete.

Can I use a Schmidt hammer by itself, without also using coring?

Sure – in fact, using Schmidt hammers as a screening method is a popular application. By “screening” I mean that Schmidt hammers are used to check the uniformity of concrete across a section of the structure, and thus identify areas that are weaker. This uniformity testing is, however, exactly what makes Schmidt hammers so useful in combination with coring!

How so?

Imagine that you don’t have a Schmidt hammer at your disposal and must rely only on coring. Well, one question then is, where on this structure should you extract cores? And how can you minimize the number of cores you will extract? In other words, how can you be sure you are obtaining representative results from a very small sample set? And how can you minimize the damage caused to the structure by coring? Without a Schmidt hammer, you don’t have prior knowledge to select coring locations.

I think I know what you’re getting at – ideally, you would only core locations that actually deserve coring…?

Precisely! The core testing should produce results which are representative of the structure. If you choose the locations randomly, you may by chance only choose weak areas, or only choose strong areas – and end up with a lop-sided result. A Schmidt hammer survey allows the engineer to take cores over the whole range of strengths in the structure and obtain a good correlation with a minimum number of cores.

Can you really save so much time by doing so?

Absolutely. Remember: coring is messy, expensive, and labor-intensive. On top of that, you need to wait for the samples to get crushed off-site in the lab. Therefore, anything you do to cut down on the number of cores will have an immediate impact on your costs. It will also directly reduce the waiting time until you can conclude whether the structure is sound.

Is rebound hammer testing in the end only about saving time and costs?

At first sight, it might seem so, because the time and costs savings are significant, especially as the size of the structure and the uncertainty of its strength grows. However, Schmidt hammers also help to improve the quality of the overall structural assessment. That’s because, by its very nature, coring is limited in scope – and once a good correlation has been established, the rebound hammer test can provide a comprehensive assessment of the structure with far less effort and at far less cost.

Destructive and non-destructive testing methods, together?

Definitely! In fact, you are hinting at something very clever. Destructive testing provides more certainty with far higher effort – but NDT allows faster and easier testing even in areas that are inaccessible to destructive tests. Moreover, rebound and core testing data can indeed be compared and correlated to provide an overall assessment of the entire structure. This means that the validity of engineers’ insights regarding in-situ concrete strength are themselves strengthened when engineers combine coring with Schmidt hammer testing.

Quite impressive; any parting advice for civil engineers?

All civil engineers are already familiar with both Schmidt hammers and core testing, but many consider either the one or the other method in isolation. Our most advanced customers already understand the value of Schmidt hammers as a complement to coring. So, as parting advice, I invite engineers to consider and try out the combination of coring and either Original Schmidt Live or SilverSchmidt. Compared to just coring by itself, engineers can reap massive benefits in terms of cost, effort, ease of use, and timeliness and quality of insights.