Blog

Trial Pits

IMG_6258
IMG_6261
IMG_6239
IMG_6266
IMG_6273
previous arrow
next arrow

Trial pits are defined by ISO-22475-1 Geotechnical investigation and testing – Sampling methods and groundwater measurements as ‘open excavations constructed to determine the ground conditions in situ, recover samples or carry out field testing’ and should form part of any initial phase or preliminary investigations.

They can be used for geotechnical design, SUDS design, contamination analysis via waste acceptance criteria testing and chemical analysis via pH and sulphate testing.

They are used to quickly investigate the ground at a larger scale and where access allows, field tests can be completed such as field density of coarse soils using a spade or geological pick or undrained shear strength of soft fine soils using a vane. However, typically class 4 samples are taken for particle size distribution tests.

Depths are normally limited to three to five metres depending on soil / rock type, groundwater levels, excavator size, etc. Information derived from boreholes include occurrence of cobbles and boulders, stratigraphy and recognised geological units, stability of walls and groundwater observations.

Trial pits naturally cause more ground disturbance than boreholes or rotary cores and locations must be carefully planned in order to avoid undermining of future formation levels.

They are normally left exposed for a period of 20 minutes to monitor groundwater levels before backfilling with the arisings and compacted in layers.

Safe T Cert

Screenshot 2025-01-02 145452

In December 2024, Substruck was awarded Grade A status in the Safe T Cert Management System for the ninth consecutive year. Safe T Cert is a certification scheme designed for certifying the Safety Management Systems of contractors working in the construction industry. The Scheme’s objective is to improve health and safety management by providing objective standards and certifying those contractors whose safety management systems have been assessed; the A grade only being awarded to the top 10% of Irish contractors.

At Substruck, safety, health and well-being is an integral part of each job and this award demonstrates this. The annual audit helps us to identify areas which need improvement and provides us with a set of targets to be implemented prior to the next audit. This keeps us on our toes and moving in the right direction.

For 2025, we plan on concentrating on the following five areas:

  • continuation of mental health initiatives and raising awareness,
  • improved application of the hierarchy of control for vibration,
  • frequency of site inspections and toolbox talks with the setting out of minimum requirements,
  • incident and defect recording for all aspects of our work and
  • housekeeping at our offices and warehouse facilities in Little Island.

A huge positive for 2024 for the roll out of health surveillance to all employees.  

Horse manure in Blarney Street

IMG_6444
IMG_6446
IMG_6443
previous arrow
next arrow

Depending on the project specification and ground conditions, the engineer may be content with drillers descriptions and SPTs for the borehole logs. Therefore, the descriptions will not be checked against laboratory tests.  However, if subsequent lab tests were to indicate a soil different from that visually described, BS5930 Code of Practice for Ground Investigations recommends that the description should not be discarded on that account but should be preserved as a record of the observers opinion.

So how can we be certain of the accuracy of field descriptions without lab tests? Well that will depend on the training and experience of the driller with in-house lab tests completed from time to time to ensure the accuracy of the logs. Annex A of ISO14688-1 Identification and Classification of Soil acknowledges this where it encourages persons to compare their descriptions to laboratory tests to check the accuracy of their fieldwork.

Anyhow, this question was posed of late between team members when apparent horse manure was found during sampling and how we knew that the sample was in fact horse manure and how were we going to record it in the drillers logs. Funny enough, the drillers assistant, a farmer, demonstrated the fact through the hairs and barley oats which had passed through the animal; barley oats being the staple diet of the working horse. The site was originally an old bakery in the early part of the 20th century and the bread was distributed across the city via horse and cart. One sample picked up the interface of the made ground and boulder clay perfectly.

Self-weight in Turners Cross

IMG_5956
IMG_5960
previous arrow
next arrow

Here at Substruck, we love ground investigation. At this property in Cork City, we were asked to complete six number dynamic probes and one number dynamic windowless sample as part of a subsidence investigation in a residential property. Dynamic probing is a first cousin of the Standard Penetration Test (SPT). Most international research in dynamic testing has been completed in relation to the SPT.

Interpretation of any blow count record or field test requires care and experience and it is in the grey areas where the fun starts. However, at this property, when the hammer is moving self-weight or ‘penetration without blows’ according to ISO 22476-2:2005 along the area of distress, you can be certain you have the culprit.

A soil sample was also taken to aid in the determination of soil resistance and made ground was found to a depth of three metres. In some instances, it may be difficult to categorically state that underlying soils are in fact ‘made’ without anthropogenic evidence but when pieces of glass and steel are found, proper interpretation is much easier.  This soil type or ‘non-engineered fill’ is described by BS8004:2015 as artificially deposited material with little or no controls in place during the process.

Corroding steel lentils in Douglas

main
IMG_4943
IMG_4945
New Picture (1)
previous arrow
next arrow

It is rare in Ireland where a building defect can cause the structure to rise. In the UK, heave is quite common when the moisture content rises in areas of shrinkable clay soils. This post relates to corrosion of steel lentils, particularly those constructed during the last century.

Lentils are required to provide support above the opes of the building such as windows and doors. These lentils are usually constructed in either reinforced concrete or steel – the choice of material will normally depend on the finish of the external leaf. So for buildings finished in brick, steel lentils are required as they can be designed to incorporate the brick finish. Nowadays, these lentils (see photo) are galvanised in order to provide protection for the design life of the building, usually 50-60 years.

During the last century, anti-corrosion measures were not always fit for purpose leaving the steel to inevitably come in contact with moisture and rust. Lentils which were not maintained were even more vulnerable. Unprotected steel will rust in the presence of moisture – this can result in a four to six increase in volume.

The lentils on the subject property have increased in thickness by about two-fold resulting in moderate vertical and horizontal cracking along the bonds of the brickwork on either side of the window. Remediation in the form of repair or replacement should only be completed by competent tradepersons under the direction of a consulting engineer.

Pile Failure in Mahon

IMG_2844
IMG_2838
previous arrow
next arrow

In sites of poor ground, piles can be used to transfer the loads from the building into deeper stratum. They transfer these loads via friction, end bearing or a combination of both.

If the piles were to fail, there can be very serious consequences; the most notable local example being an apartment block in Wilton which had to be demolished about ten years ago.

Piles can fail for a number of different reasons; communication is vital between all parties during the construction process, particularly on sites with variable ground conditions.

Failure modes include buckling where the slenderness ratio is high in very poor soils or punching failure where an end bearing pile ‘punches’ through a strong formation into an underlying weak formation.

This property in Cork City, constructed over twenty years ago, is suffering from foundation movement due to the failure of the pile(s) supporting the corner. A trial hole revealed a 800mm*800mm*800mm pile cap supporting a 400*400 ground beam.

The site was located on an obsolete Waulsortian limestone quarry which was made up over the years; hence the piles. Considering that Waulsortian limestone is a geohazard, it may have had something to do with the pile failure.

Floor slab dropping in Co. Kilkenny

IMG_3094
IMG_3089
IMG_3092
previous arrow
next arrow

In modern construction, most if not all internal walls are constructed on traditional strip foundations. Along with other functions, these walls provide support to the roof if it is of traditional ’cut’ construction. One key difference between a ‘cut’ roof and a ‘trussed’ roof is that the trussed roof transfers its load to the external walls only and does not need intermediate support, unlike cut roofs.

In some bungalows with trussed roofs built in the last century, the internal walls were built on the floor slab and constructed in masonry or timber stud. As very little load was applied to these walls, the theory was that strip foundations were not required. This approach has been completed in both one off and estate housing. However, this type of construction requires very good ground conditions and a high standard of construction in order for the floors to perform over the buildings intended design life.

The tell-tale signs for floor settlement in dwellings with trussed roof construction are normally represented by horizontal cracking or gaps at the junction of the internal walls and the ceiling. And this is certainly the case at this property in Co. Kilkenny where there is moderate cracking throughout the property at this particular location. An internal trial hole revealed that the floor was constructed on made up ground full of broken tiles and broken concrete, clearly unsuitable for any type of floor construction.

Tilt in Co. Clare

IMG_2980
IMG_2983
IMG_2988
IMG_1743
previous arrow
next arrow

Tilt is a term that is loosely used in the construction industry but it in fact a very rare occurrence.

When buildings constructed on traditional strip foundations move, the building will become distorted and cracking will occur. This cracking is usually very noticeable. However, in raft foundations, which are much stiffer, the failure mode is quite different as the whole of the building may move as one unit without any obvious signs of distortion. This is the definition of tilt. Of course, if movement progresses, occupiers will notice floors becoming more and more off level or doors swinging open.

The Building Research Establishment, a UK independent research organisation, has offered guidance in relation to acceptable levels of tilt for low rise housing but it is noticeability of 1/250 to 1/200 where problems normally begin when tilt is identified. At this point, investigation works should be completed to determine cause and monitoring considered, particularly if the tilt has surpassed 1/200. If the building has reached a tilt of 1/100, then some form of remedial action is required and feasibility comes into play. Therefore, it is important to address the concerns as quickly as possible.

This building in Co. Clare has in fact reached the value of 1/100 and underpinning works is required. The cause of the movement was identified as a leaking supply pipe in the kitchen. The design approach becomes more challenging as the building is sited in the middle of a terrace.

‘Toothing in’ in Ballinlough

IMG_1731
IMG_1743
IMG_1749
IMG_1744
previous arrow
next arrow

When building extensions, professionals differ in how the new build is or is not going to be fixed to the original structure. Nowadays, some designers require wall starters which are mechanically and / or chemically fixed to the original blockwork and the installation of a galvanised steel or UPVC movement bead at the junction between both buildings. Others just require the bead. Wall starters are quick and easy to use and allow for very minor settlement or shrinkage of the new blockwork. Of course, if the finish is in brickwork, constructing a seamless joint between both buildings takes a lot more time and effort.

Traditionally, some builders would tooth into the existing blockwork by cutting or breaking out alternate blocks and installing new blockwork to create a continuous course. However, this is very labour intensive and difficult to create a new bond with the original building.

If ever the extension was to fail, and of course the risk should be negligible, it would normally fail at this location. However, in this property, it appears the toothing in on the external leaf was completed so well that when the extension began to move it pulled beyond the toothing in and into the original structure. However, this extension is so poorly constructed, that demolition is the most economical approach.

Sinking stream in Co. Waterford

IMG_8224
IMG_8223
New Picture
New Picture (1)
IMG_8225
previous arrow
next arrow

Limestone bedrock is the most common rock found in Ireland and is formed of the disseminated fragments of shells of calcareous animals that lived on or above the sea floor about 350 million years ago.

Limestone is subject to karst which is solution by rain or groundwater. The most famous example of this is located in the Burren, Co. Clare. Karst landscapes provide many interesting features such as caves, sinking streams and enclosed depressions or sinkholes, known locally in east Waterford as ‘falls of ground’ or ‘breaking ground’. These landforms are a geohazard to construction causing subsidence if not adequately addressed during design and construction.

One limestone type, known as Waulsortian, can be particularly subject to karstification. This rock can be found from Farnanes, heading east through Ballincollig, Cork city and Midleton as far as the coast and from Rathcormack through Tallow and onto Dungarvan. It also crops up (pardon the pun) in other areas such as Cloyne and Youghal (see light blue shading in GSI mapping). The photos of the sinking steam were taken near Cappoquin during ground investigation works for a new domestic dwelling.

Subsidence can occur in many ways such as unidentified sinkholes or punching failure of driven piles. It is also a geohazard for our poor probing rods where we slide along a pinnacle during probing until refusal is met at the base. If you live in any of these areas, contact Substruck for a professional approach to ground investigation.