hello Carmela
maybe because these two vectors (Wt1 and coh1) were row and not column oriented , so I changed that and so far it works on my side
here the two minor changes I did :
Wt1 = Wt1(:); % force Wt1 to be column oriented
coh1 = coh1(:); % force coh1 to be column oriented
updated code below
clc
%input values in the excel file 'Terzaghi'
N1 = xlsread('Terzaghi.xlsx','Sheet1','G5:G35'); %SPT-N value
Da1 = xlsread('Terzaghi.xlsx','Sheet1','E5:E35'); %1st depth in the excel
Db1 = xlsread('Terzaghi.xlsx','Sheet1','F5:F35'); %2nd depth in the excel
Wl1 = xlsread('Terzaghi.xlsx','Sheet1','C2'); %water level
[~,strings] = xlsread('Terzaghi.xlsx','Sheet1','H5:H35'); %Soil classification
S1 = strings;
%values of interpolation for friction angle
x1 = [4, 10, 30, 50];
y1 = [30, 35, 40, 45];
%interpolation algorithm for friction angle
fric1 = interp1(x1,y1,N1,'linear','extrap');
%calculation of Nc, Ny, Nq
a1 = exp((0.75*pi-(deg2rad(fric1)/2)) .* tan(deg2rad(fric1)));
Nq1 = (a1.^2) ./ (2 .* (cos((deg2rad(45) + deg2rad(fric1/2)))) .^2);
Nc1 = (Nq1 - 1) .* cot(deg2rad(fric1));
Ny1 = (Nq1 - 1) .* tan(deg2rad(1.4 .* fric1));
%values of interpolation for moist unit weight
b1 = [14.9, 17.3, 20.4, 22];
%values of interpolation for submerged unit weight
c1 = [8.6, 9.4, 10.5, 11.8];
%interpolation for unit weight
mask = Db1 <= Wl1;
Wt1(mask) = interp1(x1,b1,N1(mask),'linear','extrap');
Wt1(~mask) = interp1(x1,c1,N1(~mask),'linear','extrap');
Wt1 = Wt1(:); % force Wt1 to be column oriented
%computation for cohesion
mask = ismember(S1, {'GS', 'S', 'G'});
coh1(mask) = 0;
coh1(~mask) = 0.06 .* 101.325 .* N1(~mask);
coh1 = coh1(:); % force coh1 to be column oriented
xlswrite('Terzaghi.xlsx',fric1,'Sheet1','I5:I35'); %will compute for the friction angle
xlswrite('Terzaghi.xlsx',Nq1,'Sheet1','J5:J35'); %will compute for Nq
xlswrite('Terzaghi.xlsx',Nc1,'Sheet1','K5:K35'); %will compute for Nc
xlswrite('Terzaghi.xlsx',Ny1,'Sheet1','L5:L35'); %will compute for Ny
xlswrite('Terzaghi.xlsx',coh1,'Sheet1','N5:N35'); %will compute for coh1
xlswrite('Terzaghi.xlsx',Wt1,'Sheet1','M5:M35'); %will compute for unit weight
