Structural Report on Monteath Mausoleum
By Kin Hui
Executive Summary
This report is to work out the structural repair and strengthening based on the findings gathered from visit of the Monteath Mausoleum in Glasgow Necropolis during January 2022. It includes my interpretation of findings which reveals some degree of building movement being taken place under circumstances as explained in the report. The collapse of rubble wall where the building sits exemplifies the ground instability. The structural intervention for the building is also examined in this report to align the principle of conservation for heritage.
(A) BACKGROUND
(1) DESCRIPTION OF SITE
Necropolis is a category A listed cemetery which was included on the list of monuments on 15 December 1970. This site also entered into the list under Historic Environment of Scotland on 1 July 1987. The main access to the site is from the direction of Glasgow Cathedral via a historical footbridge which spans across Wishart Street.
As shown in Fig. 1, Necropolis is situated on a hill with its top overlooking the city of Glasgow. The top of hill is +146m above the sea level. Beside of multifarious architectural monuments straddling over the hill, Necropolis is also a landscaped conservation area with meandering paths running in zigzag manner over its sloping terrain.
Fig. 2 indicates Monteath Mausoleum in a red dot which is located at the brow of a hill overlooking the south of the city. The mausoleum is situated near to the edge of a steep slope. The exact coordinate of the mausoleum is 55°51’41.7”N, 4°13’57.4”W. Its front entrance orients towards the eastern direction. Monteath Mausoleum is one of the largest and most decorated monuments in the Necropolis.
(2) NECROPOLIS AS VICTORIAN CEMETERY
Due to the industrial revolution in late 18th century, it witnessed an unprecedented growth in population for the city of Glasgow during the early Victorian era. The idea of building a new cemetery sprung from the poor sanitation and unhygienic conditions of the existing burial grounds in the city. It can also be considered as a product of capitalism as the Merchants’ House, the institution that owned the site at that time, saw it a lucrative undertaking to convert the hilly outcrop into a fashionable garden cemetery for the Victorian elite.
The Glasgow Necropolis was modeled on the picturesque Pere Lachaise which was the first and largest municipal cemetery of Paris built in 1804. Being named as Necropolis, the cemetery was to take on a secular theme with grand gesture to impress the visitors. Fig. 4 is a historical photo showing the panorama view of the Necropolis from its entrance location during the Victorian era. The red circled area is the Monteath Mausoleum that stood prominently on the brow of a hill.
(3) BRIEF HISTORY OF MONTEATH MAUSOLEUM
The mausoleum was designed by the Scottish architect David Cousin of Messrs. Cousin and Gale in 1842. Major Archibald Douglas Monteath was an officer of the East India Company. He shares the mausoleum with his brother, James Monteath Douglas. Albeit its intricate stone carvings in facades, one can find no mention of names of the decreased neither inside nor outside the monument.
(B) STRUCTURAL SURVEY
(4) WIND EFFECT AND GROUND CONDITIONS
As the monument is situated at the brow of the hill, it is observed that the building is subjected to strong wind current. The wind rose of Glasgow in Fig. 6 indicates the prevalent wind predominantly coming from the southwest direction. The average wind speed in the windiest month of January in Glasgow can reach 23.6km per hour. The wind velocity will increase with attitude. It has to note that vortex shedding will occur when wind flows through a circular object. As shown in Fig.6, turbulence and vortex will take place in the lee side in the wake of wind. The vortex effect is exemplified by the severe erosion of moulding on entrance porch of the monument.
The building is also situated very close to the edge of a retaining wall. A picture at Fig. 7 taken from previous student’s report from University of Strathclyde indicates the rubble wall has partially collapsed. The approximate extent of collapse is marked in the plan at Section C. At the presence, the slope is heavily covered with vegetation growth, obstructing any detail survey on the conditions of rubble wall. Due to the proximity of the mausoleum to the rubble wall, the partial collapse may be caused by the lateral pressure exerted from footings of the monument onto the retaining structure. It exhibits certain instability of the ground conditions.
(5) The BUILDING CONDITIONS
The monument is a monolithic masonry building, constructed of Giffnock sandstone. As sandstone is porous by nature, the building has tendency to absorb moisture into its fabric. Black crust formation is observed in both the interior and exterior. Extensive areas of decorative moulding are losing their surface which indicates accelerated erosion has been taking place at the building.
Fig. 9 is taken at façade SE2 near the entrance porch. The red arrow in the picture indicates the sunken plinth of the monument into the ground. This explains subsidence of the building at the said location.
Fig. 10 is taken at façade NW4 at the north façade. The red arrow in the picture indicates the puddle of water collected in the ground at the north façade. The adjoining footpath is sloping from higher ground toward the position of the mausoleum, causing of excessive collection of surface runoff near the plinth.
Fig. 11 is taken at the door threshold of the entrance porch. It illustrates the interior ground is lower than the external. This will lead to the interior prone to water ingress from the outside.
From the pictures in Fig. 9 to 11, we can tell it will be highly likely the soil underneath the building to be saturated with water. The compression of saturated soil under pressure leads to soil consolidation. It will cause reduction of volume of the soil and subsequently the building settlement. The freeze and thaw of water in soil during the winter season further exacerbates the situation.
Fig. 12 shows gap of roofing tiles near southeast façade. The displacement of roofing tiles may possibly reflect movement of the main building taking place. This poses the danger of falling object from high level and precaution should be exercised for any future site inspection of interior of the building.
Fig. 13 The sketch wall section shows the portion of outer wall sits under the ring beam which supports the lower pitched roof. It follows the typical wall construction of masonry building in Glasgow with ashlars facing stone on the exterior and coarse rubble wall to the interior.
(C) STRUCTURAL ANALYSIS
(6) LEANING OF BUILDING
The monument appears to lean towards the southeast direction based on visual inspection. Theoretically, it can be due to problem of soil consolidation and coupled with the lateral pressure caused by the wind. The wind effect on the monument shall be further validated by Computational Fluid Dynamic (CFD) simulation. The complication of vortex shedding needs to be noted. The cross section at Fig. 15 illustrates graphically the hypothesis on overturning and differential settlement. The outer masonry wall provides the lever to resist the overturning. It will lead to the resultant force acting off the center of footing that in turn causes the eccentric loading to the footing as encircled. Subject to further ground investigation and monitoring, it may require some sort of strengthening to the affected footing to prevent further deterioration of the situation. On the other hand, the movement of the structure will lead to uplifting on the opposite side. The internal stress undergone by the building explains why there occur gaping joints on the masonry at the roof eave of the lantern.
(7) MAPPING OF CRACK
In Fig. 16, it is noted there is crack at high level between facades NW1 and SW1, indicating some sort of movement in the structure. There is discernible misalignment of edge surface of the eave which becomes an area of structural concern.
(8) STRUCTURAL BEHAVIOUR
The loading path is shown in red arrows. Starting from the top, the loading of lantern at inner tube is transmitted downward to the inner ring of columns as well as to the stone purlins. Force to the purlins lead to outward thrust which is restrained by the ring beam. The ring beam is supposed to hold the structure together. It is supported by the outer masonry wall which takes loading further down to the footing.
Fig.18 shows two different arrangements of inner ring of columns. The actual columns arrangement follows the pattern on the left-hand side. It has to note that this parallel arrangement of columns will increase the risk of eccentricity occurrence when the thrust of the arches above not falling onto the centroid of the columns. The preferred configuration is the radial arrangement on the right-hand side with column centres to align the radial axis. Therefore, the arrangement of inner ring of columns creates an inherent weakness to the structural behaviour of the monument.
(D) INVESTIGATION
(9) GROUND INVESTIGATION
The underlying structural problems come fundamentally from the underground conditions. It calls for investigation into what the type of soil is underneath the building. Expertise from geotechnical engineer shall be sought on matter relating to soil mechanics. Ground investigation is crucial to gain thorough understanding on the soil properties. There are two ways of ground investigation. First, trial pits shall be carried out at the perimeter of the building to the level where the foundations sit. It will help to examine the foundation conditions of the building that could very useful if it is revealed in later stage that strengthening of footings are to be required. For Monteath Mausoleum, the problem is that the foundation system may be of spread footing in lieu of raft footing. If it is the former case, spreading footing resting on weak soil without any ground beam will prone for differential settlement. Therefore, the extent of excavation shall be carried out to find out the conditions of substructure. Extreme care and close site inspection should be exercised during the excavation of the pits or trenches to ensure no damage or any movement to be induced by the excavation. Excavation must be properly strutted. The second method is by drilling of boreholes. It has to bear in mind it will be an expensive undertaking and sufficient funding have to be sought prior to works of boreholes. Logging will facilitate soil samples to be obtained for laboratory testing. This will also help to establish the level where the bearing strata sits and will be also useful for future planning of works on structural strengthening. As the building is situated near the edge of a slope, it needs to find out whether the slope is a rock slope or not. This will be crucial to determine stability of the slope at where the building sits.
(10) COMPUTATIONAL FLUID DYNAMIC (CFD) and HERITAGE BUILDING INFORMATION MODEL (HBIM)
Wind velocity will increase with attitudes. Besides, as mentioned in section 4, the vortex shedding will be a likely phenomenon taking place at the circular structure. The complexity of wind effect on the building can be simulated by CFD. This will give a clear picture regarding distribution of wind pressure and the wind pattern acting on the building. It will help to define the future mitigation measure by tree planting. The build-up of 3d model in computer with software,e.g. Revit., accompanied with a physical model of the mausoleum will throw better understanding of the structural behaviour for global structural analysis prior to any intervention. We note that laser scanning has already been performed by third party on the mausoleum, and the sketches in this report are based on point cloud data obtained from the laser scanning which is presented in appendix.
(11) OTHER NON-DESTRUCTIVE TESTING
Infra-red scanning is one of the non-destructive testings that will be usually performed on built heritage. It will help to trace the route and extent of water penetration to the building fabrics. As Monteath Mausoleum is commonly found with open joints on the external wall, infra-red shall be carried out to ascertain water migration and the degree of moisture trapped inside the building fabric.
Despite being a slightly destructive technique, flat jack testing shall be carried out under supervision by a qualified structural engineer on the masonry facade. It will yield reliable data on flexural stress, or any eccentricity load undergone in the masonry construction.
(E) INTERVENTION
Philosophy in conservation embraces the concept of value. Alois Riegl, the Austrian art historian, touched upon the issue of antique value, historic value and intentional commemorative value. In other words, one needs to approach in the perspective of authenticity and integrity. Any intervention shall not be damaging to the subject artifact or the built heritage. Repair works need to be kept to minimum required to stabilize the monument. The nature of work shall be reversible. Element of new addition shall be identifiable and compatible with the existing.
(12) FOUNDATION CONSOLIDATION
Prior to proceeding with measures on foundation consolidation, the following is to be ascertained first. We need to find out whether the crack and subsidence are signs of continuing movement. Sometimes, they may be of long-past movements. If it is the latter, the movement shall be accommodated without any intervention. As commented by A.R. Powdys, an authoritative on conservation from SPAB, examination for suspected foundation trouble should be made as much to prove that no work is required. Alterations to foundations are liable to cause as many new defects as the old ones they cure [1]. Therefore, monitoring on the crack and subsidence shall be carried for a reasonable period. Settlement pegs can be set up for ground monitoring of differential settlement. Tell-tale over gaping joints will detect for degree of movement in the structure over time. To avoid damaging the stone, tell-tale of surface mount with screw fixing is not recommended. Instead, electronic gauge with connection to wifi will provide the convenience in taking measurement, and serve the purpose of avoiding physical damage through mounting of devices. Plumb line and total station device will help to check the verticality of the building. As the monument is currently tilted in one direction, regular monitoring of verticality will convey the picture of differential settlement. For the underground, level of water table in soil shall be monitored through the use of piezometer.
For foundation consolidation, the expertise from structural engineer is to be sought. It shall be done via underpinning. By enlargement, deepening or use of levers and anchorages, the performance of foundation can be improved. Mini piles shall be an optional method. The disadvantage with this method is that driving the mini piles into the ground will cause vibration which is undesirable to the fragile conditions of the mausoleum. As such, bored piles are more preferred than driven piles. Another method is to cast a post-tensioning reinforced concrete ring around the base of the wall. We have to bear in mind that full underpinning is always costly and, therefore, partial underpinning will be favourable when it is short of source of funding. Soil grouting is another approach to consolidate the ground to prevent further settlement.
(13) SCAFFOLDING
Dampness is the major reason that accounts for the decay of the monument. The installation of scaffolding to provide shelter will facilitate the drying process of the building. As Monteath Mausoleum is situated in a prominent location in Necropolis, one should pay particular attention to the aesthetic of scaffolding. The scaffolding will stay for quite a lengthy period that merits good design.
In this report, reference to the scaffold for Silver Pavilion in Kyoto is made (Fig. 19). The use of horizontal truss system that made of tubular steel will provide for long span across the monument without the need of too many vertical supports which will obstruct the view of the monument. Besides, due to the monument is situated at the edge of slope, very limited space on the ground will be available for the support of scaffolding. Therefore, the truss configuration will be a good solution for the case of Monteath Mausoleum (Fig. 21). One needs to specify the requirement of structural calculation to be submitted by contractor on the design of scaffolding to the approval of structural engineer.
(14) DRAINAGE
The mausoleum is also plagued by surface run-off being collected near its plinth which is the significant cause for soil consolidation. Therefore, the provision of good drainage to surface run-off of the ground is very important. It is advisable to provide channels along the perimeter of the building at ground for collection of rainwater. Channel covers shall be made of stone pebbles on stainless steel strainer for visual integration with the monument and the surroundings.
Appendix - Point Cloud by Third Party
