Post date: Jan 25, 2011 4:41:53 PM
Demolition works and groundwater
INTRODUCTION
In Civil Engineering, groundwater is one of the most significant elements which need to be treated appropriately to ensure construction phase or intended construction structures and infrastructures is not affected. However, till today it has been a common practice for most contractors in Malaysia to treat this element with less precaution (the most their understanding would be the installation of interlocking sheet piles to mitigate possible problems as advised by Civil and structural Consultant). This article will discuss in general the effects of demolition work on ground water.
In Layman term, ground water are liquids that flow through shallow aquifers however, it is not limited as so. Aside from that, groundwater can exist in the form of soil moisture, immobile water in very low permeability bedrock, and deep geothermal or oil formation water.
In relation to demolition work with adjacent properties still present to provide services, it would be wise for a diligent contractor to arm themselves with sufficient information in regards to the present and the correlation of readings for groundwater table and calculations/projections of flownet.
During the design stage, the Ultimate Bearing Capacity is the main parameter in designing the foundation for the structure which will subsequently exposed to changes once demolition or any major activities which could disturb/shift the equilibrium of bearing capacity and caused stresses on the adjacent/affected structures when work is progressing.
CASE STUDY; QUEEN ELIZABETH HOSPITAL
For example, For Queen Elizabeth Hospital (QEH), Kota Kinabalu, Sabah Demolition works (disclaimer – this particular article will take this particular project as an example/model for educational discussion and make it as a point of reference which have no intention to scrutinize and severe the design or work or any activity which is relevant/related to it).
Based on Soil Sheet, Kinabalu NB-10 (by Land Resources Division, England -74/741736 s); relative intimate resources when in-situ Site Investigation is not conducted. The type of soils native to the area are;
Type 7 – Klias Type (Swamps)
Parent Materials – Peat and Alluvium
Main Soil Units – Dystric Histosol; Humic Gleysol
and
Type 31 – Dalit Type (Moderate Hills: Slope > 25)
Parent Materials – Sand Stone, mudstone and alluvium
Main Soil Units – Orthic, Ferric and Gleysic Acrisol
And probably
Type 39 – Lokan Type (Very high Hills: Slope > 25)
Parent Materials – Sandstone and mudstone
Main Soil Units – Ortic Acrisol, Dystric Cambisol
These are the two type of main component of the soil which are;
Acrisol (AC) – accumulation of low activity clays in argic subsurface horizon and a by a low base saturation level.
Cambisol (CM) – hold soils with incipient soil formation and with mollic horizon overlaying subsoil with low base saturation within 1000mm depth.
If a site investigation was performed, it is best to have it as a preliminary investigation as that particular site investigation might not provide if not misleading in presenting the vital information. In regards to that and based on the slides prepared by Ir. Neoh Cheng Aik of E-Geo Consultant Sdn. Bhd., generally the best In-situ test for soil type and its parameters would be piezocone (CPTU) in comparison to others. To determine the rock type and capacity, it is reckon that the SB/PB pressuremeter would be the best with exception to hard rock which would give very doubtful readings. To compliment these tests, a set of readings from LIDAR or airborne laser scanning which later translated and simulated using ArcGIS. For more information on acquisition/translation of these features, please contact the principle of MCS.
Aside from that, please be reminded (before we continue with this article) that piezometer is one way to read/understand/compute/forecast the current behavior/parameters of the groundwater but it is not suitable or accurate (which need correlation by specialist/hydrologist) when the location to be monitored is situated very close to the sea due to barrier effect. Therefore it would be best to device other geotechnical instrumentations such as inclinometer to monitor lateral movement(s) and slope stability.
Nevertheless, unknown to most contractors, practitioners and even designers, or perhaps due to ignorance, most of the Site Investigation Reports include a section or clause (acting as disclaimer) which deny the specialist to indemnify risk(s) which originated from their reading/translation/projection. Therefore it would be wise and appropriate to take other factors into considerations as most of Malaysia Government’s project will bind a PWD203A contract which emphasize on contractor’s diligentness in executing Works and Contract and to indemnify the client from any risks and in bad events.
SOIL PROPERTIES AND EFFECTS
The low base saturation level subsoil indicates the possibility of soils with a very low pH which probably will/might effect and/or aggravate the degradation of concrete in case if it managed to penetrate through aggravated pores and voids due/as a subsequence to dilapidation of concrete cover (pores are result of curing process during concrete casting or technically known as plastic shrinkage); and/or cracks as a result of punching shear on column or stressed/failed ground beams.
Although the case of most clay with high Sodium (Natrium; Na) content is usually within alkaline region, it is good to remember that for other type of clay with Calcium (Ca) and Aluminum (Al) content/ions are probably acidic. Therefore it is best to determined the pH level as well during the site investigation prior to demolition work to ensure that if there is a case of possible cracks, the risk of increasing acidity level is eliminated/lowered so that the reinforcement bars embedded by alkaline concrete and its cover remain passive (at adjacent/close-by reinforced concrete structures).
Aside from pH level in clay, based on a paper presented by S.Horpibulsuk and R. Rachan during the 15th Southeast Asian Geotechnical Society Conference, 2004 (in regards to compressibility and permeability characteristics of Bangkok clayey soils), clay types of soils are almost similar in Atterberg’s limit (range 29 to 97) and plastic limit (range 13 to 31); the shear strengths are different. Therefore it is best to determine the current bearing capacity of the soil because generally low water contents (in clays) will have higher effective stress which resulting in higher strength and vice versa. Deviation in capacity will affect the initial and intended design, and if not halt current functions; emphasize more on this issue if it leads to degradation/reduction in bearing capacity thus lead to other unwanted possibilities of shears, stresses and failures.
Nevertheless, based on classification of parent material, sandstone is the type of rock which retains a small part of the intergranular pore space while it is important to remember the second openings (joints and fractures, along with bedding planes) contain and transmit most of the groundwater in sandstone. In the other hand, mudstone is a fine grained sedimentary rock whose original constituents were clays and mud.
The two different types of soil will also contribute toward the movement of ground water where further analysis should perform to indicate it in flownet. The allowance also should be made by integrating in other hydro-geological features. Aside from that, groundwater would likely be affected by surface water within the project site and its adjacent drainage systems. This shall not be discussed in detail as it would take detailed explanations which are appropriate to be discuss in length with geotechnical specialists.
PENETRATION AND INGRESS OF GROUND WATER DUE TO DESIGN CRITERION
Apart from knowing that the building is exposed to the ingress of groundwater which would affect the efficiency of the intended design capacity, the existing building probably was not designed in accordance to BS 8102-1990; Code of Practice for Protection of Structures against water from the ground and also in the light of BS 8007-1987 (Code of Practice for Design of concrete structures for retaining aqueous liquids). Putting into considerations that based on Clause 3.2.4 (water-resisting forms of construction), it is highly possible that QEH structure(s) would not have been constructed with Type B initially and Type C in tackling groundwater issues.
Type B (Structurally integral protection) – to prevent water penetration (BS 8007) or to minimized water penetration (BS 8110) by choosing the appropriate grade of basement use. The transmission of water vapor may not be wholly prevented.
Type C (Drained protection) – a construction with structural concrete which includes diaphragm walls and/or masonry wall to minimize the ingress of water. Any moisture which does find its way into the basement is channeled, collected and discharged within the cavity created through the addition of an inner skin to both wall and floor.
Although it would be possible for QEH to perform waterproofing work at the later stage, probably with the selection of bituminous type; the later micro cracks due to creep may not be treated instantaneously because visually it would only appear when have been much aggravated (this could only be resolved by applying Polyurethane waterproofing and to be prevented during renovation with a selection of chemicals to react and close these capillaries/pores/voids which probably will exist during demolition work as results of settlements and stresses applied when groundwater begin to shift from equilibrium). During this period, groundwater probably has penetrated the permeable structures and had done much damage to the structures.
When groundwater risk(s) are not taken into consideration, the chances of structure to fail would increase despite of allowance and tolerance which probably have been projected by designers based on BS 6954:1988 (Tolerances for Building) and general clauses viz;-
· Clause 4.1 Induced deviations
· Clause 4.2 Inherent deviations **
· Clause 5 Consequences of dimensional variability**
The latter said code would not be use because it is highly likely the designer would not run a statistical analysis in simulate the values/parameters for their design instead would solely design based on the code of practice at that time. Aside from that, although this particular code is important in decision making in deciding on designs, it is not even spelled out or relative to the JKR Standard Specifications for Building Works 2005.
Where structure stresses appeared the only solutions left would be of the EN1504 and it is highly recommended most of the reinstatement/rehabilitation works would be rebinding via epoxy resin injection if not worst, restrengthening via Carbon Fiber Reinforced Polymer (CFRP) by using plates and strips.
TEMPORARY WORKS AND REMOVAL WORKS AND THEIR EFFECTS
Although it is unknown to us, assuming the driving length of 200mm square reinforced concrete piles previously as part of the structure’s foundation is to set and in equilibrium with groundwater. Say, these (end bearing) piles were driven and entered the pits or open joints of rocks. This would likely caused the pile to bend and twist, thus will lead to lower bearing during service period of the building. It would be more likely if there are karstic structures (limestone) underneath the intended project. Disregard the possibilities of degradation up to 25% in bearing capacity; during the demolition stage where the existing piles from the old structure probably need to be removed will lead toward another unforeseen possibilities. Extracting of the existing bended piles may lead toward the cracking and fracturing of the rocks thus might likely to expose possible capillaries if not aquifer(s) for the groundwater to move within the working area.
The installation of interlocking sheet piles would be a good method in controlling erosion as well as to thwart movement of groundwater within clayey soils and also to reduce dewatering cost. There are no problems foreseen during this period of work but bear in mind that removal of these sheet piles could pose hazard(s) and with the addition hydrostatic pressure of the groundwater under clayey condition; it would be something for the contractor to take into consideration.
The demolition works could affect the groundwater after all the calculations and projections in relation to temporary stockpile of debris if it is not well taken care. The self weight of the debris itself would cause settlement to the ground itself and therefore this will also alter the flownet for the groundwater. Therefore the contractor should also take in consideration for this factor and allowed a permanent stockpile yard throughout the carting out of debris process.
These are just a few simulations of possible happenings which may lead to the changes of groundwater. The only solutions would be phased investigation and continuous monitoring of the groundwater behaviors. Again, it is good to remember QEH is situated near to the sea and it is probably subject to hydrostatic pressure distribution (barrier effects); piezometer readings need to be correlated.
CONTRACTUALLY
The mitigation plans would be appropriate in ensuring risk(s) and complications could be mitigated earlier after the ground/site investigation. How detail is our investigations and what vital parameters were acquired and their relevance? In conventional and based on international estimation, a typical project would take as low as 0.2% of the construction cost, while the dam projects would consume up to 5% of the construction cost (An introduction to Geotechnical Processes by John Woodward). How about demolition work? How much would it cost and will there be phased investigation? Will it still constitutes as low as 0.2% when the standard preliminary provision for standard government project will be in the range of 5% of the construction cost or should it be included into another provisional bill which is uncommon in most cases. What if demolition is just part of the Works/Contract?
Contractually, for this issue; it is uncommon for mitigations to be prepared or spelled out in Contract or generic Bill of Quantities (under separated bill(s)). Reviewing this particular work in the light of the JKR Standard Specifications for Building Works 2005 (JKR Specs), demolition works only spelled out in Section C, Excavation and earthwork. Under Paragraph 4, Demolition of Existing Structures and its two sub-clauses, the Contract emphasizes in determining on cart away method and which party has the rights on the debris. Alas, if it is read as part of the earthwork for the Contract, then it is clear that it will be the contractor’s responsibility to ensure all works are progressing diligently and it has been the contractor’s responsibility and liability to continue to indemnify the client and the Government.
Knowing that the risk undertaken is intangible (predictable but not able to be quantified in risk management and shall not be classified as Force Majeure, as it is proven to be able to be forecasted and projected) due to the nature of the groundwater and the limitation(s) during site investigation (without proper interpretation by specialists and phased investigations); it would be good to act as transparent as possible with their Contractor All Risk insurance policy in order that the premium will provide extra coverage in comparison to the standard content by coupling/annexing it with reports from Risk Consultants. This will mitigate the risk(s) although it’s the contractor’s liability to continue to indemnify the client by expanding the definition of “ground subsidence” as specified in Clause of Contract 18.1(a) – Taking of Insurance. Besides understanding of definitions in contract and insurance, the insurance organization may provide this in the policy but be reminded that, there is a buyout clause (as to deleverage the risk to the contractor for their willful act or negligence in executing the Works) and deficiency of Contract for the Works.
To comprehend the definition “Force Majeure” as per Clause of Contract 57 and its sub-clauses, Contractor should understand the definition of this term technically and legally.
Initially, the contractor must prove that they have nothing to do with the event which the Contract did not allow; the Contract requested the contractor to perform work diligently, exercising professional judgment and practice, and so on. Aside from that, all codes of practice are well spelled out in JKR Specs which should be considered during execution of work.
Under the second test in putting such event as Force Majeure, the contractor also show that they could have taken their best endeavor and be well prepared for such event if it is foreseeable. This relates back to the code of practices as mentioned earlier.
CONCLUSION
The groundwater issue should be well taken care through proper analysis prior to execution of work and monitored periodically to ensure that it may not affect the work. The failure in preparing proper plans and methods in handling groundwater during demolition work may complicate the Works and exposed the contractor to risk(s). The Contract and insurance which the contractor may cling for life support in an event of possible mishap will not be standing at contractor’s side unless the contractor has proved that they have taken their best endeavor contractually and in engineering aspects.