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Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 96 of 147

o C.3.4.2.H Mat Foundation Budget Implications • C.3.4.3 Comparison of the Two Foundation Systems Page 113

C.3.1 Problem Statement

With poor subsurface conditions prevalent, can the foundation system be redesigned to possibly reduce cost and time spent without interfering with architects or owner needs? This problem was identified through the geotechnical reports, change orders, and project manager interviews. A lot of money, time, and energy were spent by the project team having to deal with poor subsurface conditions. An analysis communicating some potential solutions is one study of great interest to many on the Wellington Condominiums Project.

C.3.2 Proposed Solution A possible solution to the Wellington Condominiums Project is for a structural redesign of the foundation system. A structural breath will be utilized in the analysis of the comparison between the current and proposed systems. The current foundation system makes use of single slab column footings and will be challenged through the redesign of a matt slab foundation. A matt slab foundation system is proposed and will be researched to do the following possibilities:

1. Save time and money by not having to excavate as deep in rock material. 2. If footing depth can be decreased possible savings in the dewatering system

could happen. 3. Using a matt slab could reduce the strength needed for foundation concrete

and also if designed correctly act as a slab on grade. This could potentially save time and cost to the project.

C.3.3 Analysis Steps

The procedure to investigating if a matt slab foundation system would be more viable than a traditional single slab column footing is as followed:

1. Learn in more detail and have available the single slab column footing’s estimate, schedule, design documentation, specifications, and methods of construction.

2. Redesign the foundation system utilizing a matt slab foundation. Figure out how much material, cost, and time would be spent to construct.

3. Compare the two systems and create a matrix chart based on the owner requirements of which system overall is better for the project.

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Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 103 of 147

C.3.4.2.B Mat Foundation Concept Applied to Wellington Condominiums According to the Building Design and Construction Handbook by Merritt, a mat foundation is defined: as a single combined footing for an entire building unit. It is economical when building loads are relatively heavy and the safe soil pressure is small. Based on economic considerations according to the Building Design and Construction Handbook by Merritt, mat foundations are constructed for the following reasons and are judged against the Wellington Condominiums circumstances:

1. Large Individual Footings: A mat foundation is often constructed when the sum of individual footing areas exceeds about ½ of the foundation area. Wellington Condominiums Calculation:

• 259’ x 121’ Building Area = 31,339 SQ FT • 36 Footings x (20’ x 20’ Max Footing Area) = 14,400 SQ FT • (14,400 SQ FT / 31,339 SQ FT) x 100 = 45.95% < 50% SUM

From the calculation, it can be determined that approximately 46% of the footing area is below the rule of thumb value but is within consideration for mat foundation.

2. Cavities or Compressible Lenses: Mat foundation used when subsurface exploration indicates that there will be unequal settlement below the foundation due to small cavities or compressible lenses. A mat foundation would distribute the load more evenly and create better conditions for any possible settlement.

Wellington Condominiums Analysis Viewpoint:

• Cavities or Compressible Lenses have not been indicated in the geotechnical reports or have been an issue during construction.

• Minimal value has been placed on cavities or compressible lenses for this analysis.

3. Shallow Settlements: A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.

Wellington Condominiums Analysis Viewpoint:

• Differential Settlement was identified in the geotechnical report and has been a large concern to the project team.

• Large consideration has been placed on how the foundation settlements would be with a mat foundation system.

4. Unequal Distribution of loads: Large disparity in building loads acting on different areas of the foundation can be subjected to excessive differential settlement with conventional spread footings. Using a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.

Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 104 of 147

Wellington Condominiums Analysis Viewpoint:

• The column and wall loading varies and therefore different column sizes and spread footings are utilized. The largest spread footings are constructed at the corners and center of the foundation.

• Unequal distribution of loads is something to consider but is not a major focus for the design of the Wellington Condominiums mat foundation redesign analysis. Load distribution will be considered in the analysis for completeness.

5. Hydrostatic Uplift: A mat foundation could be used to resist uplift forces due to a high groundwater table. Wellington Condominiums Analysis Viewpoint:

• Groundwater at the project site is of huge concern to the project team. Extensive groundwater measures had to be in place before construction could ever begin.

• Any reduction in waterproofing or groundwater measures from the utilization of a mat slab would be of great savings in budget and schedule.

C.3.4.2.C Mat Foundation Design Background The design background for a mat foundation tends to be very complicated and requires

extensive knowledge and experience. Being said there are many articles and programs that engineers use when considering the design of a mat foundation system. Some of the design criteria outlined in the Building Design and Construction Handbook by Merritt are as followed:

1. Weight of soil excavated for the foundation decreases the pressure on the soil under the mat. If excavated soil weighs more than the building, there is a net decrease in pressure at mat level from that prior to excavation.

2. When the mat is rigid, a uniform distribution of soil pressure can be assumed and the design can be based on a statically determinant structure as shown in the Figure 7 below.

3. If the centroid of the factored loads does not coincide with the centroid of the mat area, the resulting nonuniform soil pressure should be used in the strength design of the mat.

4. Strength-design provisions for flexure, one-way and two-way shear, development length, and serviceability should conform to ACI 318 Building Code Requirements.

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Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 109 of 147

for water removal may create a possible backlog of additional hydrostatic pressure that cannot be forgotten about. With a possibility of increased hydrostatic pressure with the utilization of the same drainage system comes to the responsibility of the designer as to what to do. Through advice of structural engineers and contractors it was recommended that once you start constructing your foundation that the dewatering system must be in effect to reduce the possibility of high hydrostatic pressure on the foundation system. With a permanent dewatering system installed prior to construction, it will alleviate any potential risk during construction of hydrostatic pressure. The following calculation is provided as to give a rule of thumb to the structural load interaction of the Wellington Condominiums Project: Allowable Bearing Pressure of Soil Conditions: 3.5 KSF Square footage of Mat Foundation: 259’ x 121’ 31,339 SF Allowable Total Load Transferred to Soil Conditions: 109,687 K Maximum Column Load: 473 K x 36 Columns 17,028 K Maximum Wall Load: 10 K/FT x 760 FT 7600 K Total Mat Foundation Load: 24,628 K The mat foundation load is less than the allowable mat foundation load; therefore the foundation has enough strength to allow for the prevalent soil conditions. There is a 4.45 safety factor on this analysis which will allow for any additional hydrostatic pressure and uplift from the soil conditions present. With the permanent dewatering system installed prior to foundation construction and continued throughout the project; there is no additional analysis to consider.

C.3.4.2.G Mat Foundation Schedule Implications The schedule effect to the implementation of the mat foundation system is shown in Figure 10.A along with the original schedule in Figure 10.B.

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Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 111 of 147

From the schedules the important dates to take notice are highlighted in blue and listed as followed for analysis: Mat Foundation System:

• Construction: 340 Days Mon 1/16/06 – Fri 5/4/07 • Substructure: 149 Days Wed 2/22/06 – Mon 9/18/06 • Foundation and Columns: 61 Days Mon 3/13/06 – Mon 6/5/06 • Superstructure: 169 Days Tue 9/5/06 – Fri 4/27/07

Original Foundation System:

• Construction: 340 Days Mon 1/16/06 – Fri 5/4/07 • Substructure: 132 Days Wed 2/22/06 – Thu 8/24/06 • Foundation and Columns: 39 Days Mon 3/13/06 – Thu 5/4/06 • Superstructure: 186 Days Fri 8/11/06 – Fri 4/27/07

Key: Red – Schedule Push Back Green – Schedule Pull Back Some interesting results have occurred that are worth noting for the comparison between schedules. The construction of the mat foundation in comparison to the original foundation system will result in as followed:

• Construction: Entire Construction of the project will be the same start and finish date with no increase or decrease in project schedule.

• Substructure: The construction of the substructure will increase by 17 days. The substructure construction will start on the same day but finish at a later time.

• Foundation and Columns: Foundation and Columns will increase by 22 days. The foundation and column construction will start on the same day but finish at a later time.

• Superstructure: The superstructure will decrease in time of construction by 17 days. The start time will be pushed back but will finish on the same day as the original schedule.

What this data is revealing is that there is float within the schedule and the utilization of a mat foundation system will not delay the overall project. There is an increase in parts of the schedule but due to superstructure float was able to take on those extra days of construction and still finish on time. Therefore based on these observations, more analysis must be conducted further as to whether or not to utilize a mat foundation system for the Wellington Condominiums Project.

Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 112 of 147

C.3.4.2.H Mat Foundation Budget Implications The current budget for the Wellington Condominiums Foundation system is based on the following estimates: Original Estimate:

• Single Slab Column Footings: $104, 374 • Wall Strip Footings: $13,070 • Slab on Grade: $73,048 • Change Orders $253,159

Total: $443,651

Mat Foundation Estimate @ 4’ Thickness:

• 2007 RS Means Building Construction Data • Cubic Yards of Mat Foundation: 259’ x 121’ x 4’ 4,643 CY • 2 Crews (C-14C) Totaling:

o 2 Foreman o 12 Carpenters o 4 Rodmen (reinf.) o 8 Laborers o 2 Cement Finishers o 2 Gas Engine Vibrators

• Material Cost: $174/CY $807,882 • Labor Cost: $70/CY/Crew x 2 Crews = $140/CY $650,020 • Equipment: $0.38/CY/Crew x 2 Crews = $0.76/CY $3,529

Total: $1,461,431

Mat Foundation Estimate @ 3’ Thickness:

• 2007 RS Means Building Construction Data • Cubic Yards of Mat Foundation: 259’ x 121’ x 3’ 3,482 CY • 2 Crews (C-14C) Totaling:

o 2 Foreman o 12 Carpenters o 4 Rodmen (reinf.) o 8 Laborers o 2 Cement Finishers o 2 Gas Engine Vibrators

• Material Cost: $174/CY $605,868 • Labor Cost: $70/CY/Crew x 2 Crews = $140/CY $487,480

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Wellington Condominiums Exton, PA

Spring Thesis Research BUILIDING FOR THE FUTURE

Sean Flynn – Construction Management Page 114 of 147

The results have indicated that a score of 67.6% and 69.4% for the original and mat foundation system respectively. Both are indicated as an okay system but it is the mat foundation which should be selected by the owner. This provides an interesting perspective in that the mat foundation even though cost was a number one concern and was over three times the original foundation system; the mat foundation should be perused if given the correct amount of funding availability. The advantages of the mat foundation in subsurface interaction and load distribution create just enough of an advantage to spend the extra money on the system. If however other variables were to change; it could give the possibility of the original foundation system being preferred over the mat foundation system. But with the current information provided, if the amount of funding is available, the mat foundation system should be selected for the Wellington Condominiums Project.